Showing 1–50 of 61 results for author: Kuschel, T

Femtosecond all-optical coherent control of spin polarization in altermagnets

Authors: Kevin Jäckel, Holger Grisk, Niklas Dornquast, Maik Gaerner, Günter Reiss, Timo Kuschel, Jakob Walowski, Markus Münzenberg

Abstract: Altermagnets constitute an emerging materials platform for spintronic technologies by combining compensated magnetic order with ferromagnet-like spin-split electronic bands. Here, we investigate the proposed d-wave altermagnetic material RuO2 using circularly polarized ultrashort laser pulses. Time-resolved magneto-optical Kerr effect measurements, which are intrinsically sensitive to surface and… ▽ More Altermagnets constitute an emerging materials platform for spintronic technologies by combining compensated magnetic order with ferromagnet-like spin-split electronic bands. Here, we investigate the proposed d-wave altermagnetic material RuO2 using circularly polarized ultrashort laser pulses. Time-resolved magneto-optical Kerr effect measurements, which are intrinsically sensitive to surface and interface states, reveal the ultrafast spin response of RuO2. In contrast to the demagnetization dynamics characteristic of conventional ferromagnets, we observe a distinct coherent contribution to the complex Kerr rotation that appears during the light-matter interaction and lasts for about 200 fs. Similar signatures have been associated with spin-momentum locking and directional band splitting in spin-split surface states of topological insulators as well as spin-orbit-coupled semiconductors. They are governed by a finite Raman coherence time. We interpret this coherent response as evidence for transient spin-polarized surface states in RuO2, consistent with the emergence of altermagnetic surface states that are directly relevant to spin-polarized transport at surfaces and interfaces. △ Less

Submitted 31 March, 2026; originally announced March 2026.

Comments: 25 pages, 5 fiures

Giant orbital magnetoresistance in the antiferromagnet CoO driven by dynamic orbital angular momentum interaction

Authors: Christin Schmitt, Sachin Krishnia, Edgar Galindez-Ruales, Luca Micus, Takashi Kikkawa, Hiroki Arisawa, Marjana Lezaic, Duc Tran, Timo Kuschel, Jairo Sinova, Eiji Saitoh, Gerhard Jakob, Olena Gomonay, Yuriy Mokrousov, Mathias Kläui

Abstract: Recent predictions of orders of magnitude larger orbital current effects compared to spin currents have attracted significant interest. However, the full potential of giant orbital currents remains to be fully harnessed, since so far, the orbital currents need to be converted into spin currents before they can interact with the static magnetization that is dominated by spin angular momentum in con… ▽ More Recent predictions of orders of magnitude larger orbital current effects compared to spin currents have attracted significant interest. However, the full potential of giant orbital currents remains to be fully harnessed, since so far, the orbital currents need to be converted into spin currents before they can interact with the static magnetization that is dominated by spin angular momentum in conventional magnets. By using a magnet dominated by orbital angular momentum, we demonstrate a more than fifty-fold enhancement in orbital Hall magnetoresistance in CoO/Cu*, compared to conventional CoO/Pt. This is found to be driven by a unique interaction between dynamic orbital angular momentum from surface oxidized Cu* (i.e., the orbital current) and the static orbital angular momentum which constitutes the magnetic moments in the antiferromagnetic insulator CoO. A distinctive scattering mechanism for orbital currents at the CoO interface leads to a sign reversal in orbital magnetoresistance in CoO/Cu* compared to CoO/Pt. Our results show how by using orbital angular momentum-dominated materials such as CoO, we can harness the benefits of giant orbital currents that have not been possible using conventional spin-dominated magnets, for orbitronics-based devices, offering unprecedented energy efficiency for operations of antiferromagnets that combine ultimate stability with THz dynamics. △ Less

Submitted 6 March, 2026; originally announced March 2026.

Journal ref: Science 393, 76 (2026)

Emergence of a spin Hall topological Hall effect in the non-collinear phase of the ferrimagnetic insulator terbium-iron garnet

Authors: Mehak Loyal, Akashdeep Akashdeep, Edoardo Mangini, Edgar Galíndez-Ruales, Maja Eich, Nan Wang, Qianqian Lan, Lei Jin, Rafal Dunin-Borkowski, Timo Kuschel, Mathias Kläui, Gerhard Jakob

Abstract: Magnetic compensation in rare-earth iron garnets (REIGs) offers a unique setting for which competing sublattice moments can give rise to non-collinear (canted) magnetic configurations, in which the sublattice magnetizations are not aligned with each other or with the external magnetic field. We show that this compensation regime can also host non-trivial magnetic textures. To explore this behavior… ▽ More Magnetic compensation in rare-earth iron garnets (REIGs) offers a unique setting for which competing sublattice moments can give rise to non-collinear (canted) magnetic configurations, in which the sublattice magnetizations are not aligned with each other or with the external magnetic field. We show that this compensation regime can also host non-trivial magnetic textures. To explore this behavior, we investigated (111)-oriented epitaxial Tb$_3$Fe$_5$O$_{12}$/Pt heterostructures across the compensation temperature region using combined transverse magneto-transport and polar Kerr microscopy. Notably, we observe a topological Hall-like signal in the vicinity of the compensation temperature, a feature often interpreted as evidence for skyrmions in the absence of direct imaging. Here, in contrast, complementary Kerr microscopy reveals instead a non-collinear multidomain state which collapses outside the compensation regime, correlating directly with the appearance and disappearance of the spin Hall topological Hall effect (SH-THE) signal. These observations cannot be accounted for by a simple multi-anomalous-Hall-effect model, ruling out common artifacts as the origin, but indicate the presence of a topologically non-trivial contribution to the Hall response. These results establish strained REIG films as a tunable platform for exploring topological responses arising from compensation-driven non-collinear ferrimagnetic phases. △ Less

Submitted 12 February, 2026; originally announced February 2026.

Comments: 15 pages, 6 figures

Journal ref: Phys. Rev. Mater. 10, 054417 (2026)

Surface-Localized Magnetic Order in RuO2 Thin Films Revealed by Low-Energy Muon Probes

Authors: Akashdeep Akashdeep, Sachin Krishnia, Jae-Hyun Ha, Siyeon An, Maik Gaerner, Thomas Prokscha, Andreas Suter, Gianluca Janka, Günter Reiss, Timo Kuschel, Dong-Soo Han, Angelo Di Bernardo, Zaher Salman, Gerhard Jakob, Mathias Kläui

Abstract: Ruthenium dioxide (RuO2) has recently emerged as a candidate altermagnet, yet its intrinsic magnetic ground state, particularly in thin films, remains debated. This study aims to clarify the nature and spatial extent of the magnetic order in RuO2 thin films grown under different conditions. Thin films of RuO2 with thicknesses of 30 nm and 33 nm are fabricated by pulsed laser deposition and sputter… ▽ More Ruthenium dioxide (RuO2) has recently emerged as a candidate altermagnet, yet its intrinsic magnetic ground state, particularly in thin films, remains debated. This study aims to clarify the nature and spatial extent of the magnetic order in RuO2 thin films grown under different conditions. Thin films of RuO2 with thicknesses of 30 nm and 33 nm are fabricated by pulsed laser deposition and sputtering onto TiO2(110) and Al2O3(1-102) substrates, respectively. Low-energy muon spin rotation/relaxation (LE-muSR) with depth-resolved sensitivity measurements is performed in transverse magnetic fields (TF) from 4 K to 290 K. The muSR data collected with a muon implantation energy of 1 keV reveal that magnetic signals originate from the near-surface region of the film (<10 nm), and the affected volume fraction is at most about 8.5%. The localized magnetic response is consistent across different substrates, growth techniques, and parameter sets, suggesting a common origin related to surface defects and dimensionality effects. The combined use of TF-muSR and the study of depth-dependent implantation with low-energy muons provides direct evidence for surface-confined, inhomogeneous static magnetic order in RuO2 thin films, helping reconcile discrepancies. These findings underscore the importance of considering reduced-dimensional contributions and motivate further investigation into the role of defects, strain, and stoichiometry on the magnetic properties of RuO2, especially at the surface. △ Less

Submitted 6 February, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Journal ref: Appl. Phys. Lett. 128, 022406 (2026)

Depth Profiling of Oxygen Migration in Ta/HfO2 Stacks During Ionic Liquid Gating

Authors: Beatrice Bednarz, Martin Wortmann, Olga Kuschel, Fabian Kammerbauer, Mathias Kläui, Andreas Hütten, Joachim Wollschläger, Gerhard Jakob, Timo Kuschel

Abstract: Ionic liquid (IL) gating has emerged as a powerful tool to control the structural, electronic, optical, and magnetic properties of materials by driving ion motion at solid interfaces. In magneto-ionic systems, electric fields are used to move ions, typically oxygen, from a donor layer into an underlying magnetic metal. Although oxygen distribution is key to enabling precise and stable control in m… ▽ More Ionic liquid (IL) gating has emerged as a powerful tool to control the structural, electronic, optical, and magnetic properties of materials by driving ion motion at solid interfaces. In magneto-ionic systems, electric fields are used to move ions, typically oxygen, from a donor layer into an underlying magnetic metal. Although oxygen distribution is key to enabling precise and stable control in magneto-ionic systems, the spatial distribution and voltage-dependence of oxygen incorporation in such nanoscale stacks remain unknown. Here, we quantify oxygen depth profiles and oxide formation in Si/ SiO2/ Ta (15)/ HfO2 (t) films after IL gating as a function of the gate voltage and HfO2 capping thickness (2 and 3 nm). X-ray reflectivity and X-ray photoelectron spectroscopy measurements revealed a threshold electric field of ~ -2.8 MV/cm to initiate oxygen migration from HfO2 into metallic Ta. The resulting Ta2O5 thickness increases linearly with gate voltage, reaching up to 4 nm at -3 V gating. Notably, the required electric field rises with oxide thickness, indicating a progressively growing barrier for thicker oxide films. The Ta/Ta2O5 interface remains atomically sharp for all gate voltages. This suggests that complete Ta2O5 layers form sequentially before further oxygen penetration, with no sign of deeper diffusion into bulk Ta. Thinner capping layers enhance oxidation, relevant for optimized stack design. Additionally, indium migration from the indium tin oxide electrode to the sample surface was observed, which should be considered for surface-sensitive applications. These insights advance design principles for magneto-ionic and nanoionic devices requiring precise interface engineering. △ Less

Submitted 6 September, 2025; originally announced September 2025.

Comments: Document containing manuscript and supporting information

Journal ref: ACS Appl. Mater. Interfaces 18, 6200 (2026)

Surface Oxidation of SnTe Analyzed by Self-Consistent Fitting of all Emission Peaks in its X-ray Photoelectron Spectrum

Authors: Martin Wortmann, Beatrice Bednarz, Negin Beryani Nezafat, Klaus Viertel, Olga Kuschel, Jan Schmalhorst, Inga Ennen, Maik Gärner, Natalie Frese, Gerhard Jakob, Joachim Wollschläger, Gabi Schierning, Andreas Hütten, Timo Kuschel

Abstract: X-ray photoelectron spectroscopy (XPS) is among the most widely used methods for surface characterization. Currently, the analysis of XPS data is almost exclusively based on the main emission peak of a given element and the rest of the spectrum is discarded. This makes quantitative chemical state analyses by peak fitting prone to substantial error, especially in light of incomplete and flawed refe… ▽ More X-ray photoelectron spectroscopy (XPS) is among the most widely used methods for surface characterization. Currently, the analysis of XPS data is almost exclusively based on the main emission peak of a given element and the rest of the spectrum is discarded. This makes quantitative chemical state analyses by peak fitting prone to substantial error, especially in light of incomplete and flawed reference literature. However, most elements give rise to multiple emission peaks in the x-ray energy range, which are virtually never analyzed. For samples with an inhomogeneous depth distribution of chemical states, these peaks show different but interdependent ratios of signal components, as they correspond to different information depths. In this work, we show that self-consistent fitting of all emission peaks lends additional reliability to the depth profiling of chemical states by angular-resolved (AR-)XPS. We demonstrate this using a natively oxidized thin film of the topological crystalline insulator tin telluride (SnTe). This approach is not only complementary to existing depth profiling methods, but may also pave the way towards complete deconvolution of the XPS spectrum, facilitating a more comprehensive and holistic understanding of the surface chemistry of solids. △ Less

Submitted 9 April, 2025; originally announced April 2025.

Comments: main article 13 pages, 6 figures; supporting information 8 pages, 6 figures

Journal ref: Appl. Surf. Sci. 713, 164356 (2025)

Angle-Dependent Magnetoresistance Induced by Interface-Generated Spin Current in RuO$_2$/Permalloy Heterostructures

Authors: Akashdeep Akashdeep, Ewiese Mohammad Ababneh, Christin Schmitt, Edgar Galíndez-Ruales, Felix Fuhrmann, Timo Kuschel, Mathias Kläui, Vivek Pravin Amin, Gerhard Jakob

Abstract: Altermagnets, a recently discovered class of magnetic materials exhibiting ferromagnetic-like spin-split bands and antiferromagnetic-like compensated magnetic order, have attracted significant interest for next-generation spintronic applications. Ruthenium dioxide (RuO2) has emerged as a promising altermagnetic candidate due to its compensated antiparallel magnetic order and strong spin-split elec… ▽ More Altermagnets, a recently discovered class of magnetic materials exhibiting ferromagnetic-like spin-split bands and antiferromagnetic-like compensated magnetic order, have attracted significant interest for next-generation spintronic applications. Ruthenium dioxide (RuO2) has emerged as a promising altermagnetic candidate due to its compensated antiparallel magnetic order and strong spin-split electronic bands. However, recent experimental and theoretical reports also suggest that RuO2 may be non-magnetic in its ground state, underscoring the need for deeper investigations into its magnetic character. Specifically, the (100)-oriented RuO2 films are expected to generate spin currents with transverse spin polarization parallel to the Néel vector. Here, we investigate magnetotransport in epitaxial RuO2/Permalloy (Py) heterostructures to examine spin Hall magnetoresistance and interfacial effects generated in such systems. Our measurements reveal a pronounced negative angular-dependent magnetoresistance for variation of magnetic field direction perpendicular to the charge current direction. Detailed temperature-, magnetic field-, and crystallographic orientation-dependent measurements indicate that interface-generated spin current (IGSC) at the RuO2/Py interface predominantly governs the observed magnetoresistance. This shows that strong interface effects dominate over possible altermagnetic contributions from RuO2. Our results show that the role of interface-generated spin currents is crucial and should not be overlooked in studies of altermagnetic systems. A critical step in this direction is disentangling interfacial from altermagnetic contributions. The insight into interfacial contributions from altermagnetic influences is essential for the advancement of RuO2 based spintronic memory and sensing applications. △ Less

Submitted 6 February, 2026; v1 submitted 31 March, 2025; originally announced April 2025.

Journal ref: Phys. Rev. Applied 24, 054018 (2025)

Stabilizing perpendicular magnetic anisotropy with strong exchange bias in PtMn/Co by magneto-ionics

Authors: Beatrice Bednarz, Maria-Andromachi Syskaki, Rohit Pachat, Leon Prädel, Martin Wortmann, Timo Kuschel, Shimpei Ono, Mathias Kläui, Liza Herrera Diez, Gerhard Jakob

Abstract: Electric field control of magnetic properties offers a broad and promising toolbox for enabling ultra-low power electronics. A key challenge with high technological relevance is to master the interplay between the magnetic anisotropy of a ferromagnet and the exchange coupling to an adjacent antiferromagnet. Here, we demonstrate that magneto-ionic gating can be used to achieve a very stable out-of-… ▽ More Electric field control of magnetic properties offers a broad and promising toolbox for enabling ultra-low power electronics. A key challenge with high technological relevance is to master the interplay between the magnetic anisotropy of a ferromagnet and the exchange coupling to an adjacent antiferromagnet. Here, we demonstrate that magneto-ionic gating can be used to achieve a very stable out-of-plane (OOP) oriented magnetization with strong exchange bias in samples with as-deposited preferred in-plane (IP) magnetization. We show that the perpendicular interfacial anisotropy can be increased by more than a factor 2 in the stack Ta/Pt/PtMn/Co/HfO2 by applying -2.5 V gate voltage over 3 nm HfO2, causing a reorientation of the magnetization from IP to OOP with a strong OOP exchange bias of more than 50 mT. Comparing two thicknesses of PtMn, we identify a notable trade-off: while thicker PtMn yields a significantly larger exchange bias, it also results in a slower response to ionic liquid gating within the accessible gate voltage window. These results pave the way for post-deposition electrical tailoring of magnetic anisotropy and exchange bias in samples requiring significant exchange bias. △ Less

Submitted 7 June, 2024; v1 submitted 12 April, 2024; originally announced April 2024.

Comments: main manuscript and supplementary material

Journal ref: Appl. Phys. Lett. 124, 232403 (2024)

Cubic magneto-optic Kerr effect in Ni(111) thin films with and without twinning

Authors: Maik Gaerner, Robin Silber, Tobias Peters, Jaroslav Hamrle, Timo Kuschel

Abstract: In most studies utilizing the magneto-optic Kerr effect (MOKE), the detected change of polarized light upon reflection from a magnetized sample is supposed to be proportional to the magnetization $\boldsymbol{M}$. However, MOKE signatures quadratic in $\boldsymbol{M}$ have also been identified and utilized, e.g., to sense the structural order in Heusler compounds, to detect spin-orbit torques or t… ▽ More In most studies utilizing the magneto-optic Kerr effect (MOKE), the detected change of polarized light upon reflection from a magnetized sample is supposed to be proportional to the magnetization $\boldsymbol{M}$. However, MOKE signatures quadratic in $\boldsymbol{M}$ have also been identified and utilized, e.g., to sense the structural order in Heusler compounds, to detect spin-orbit torques or to image antiferromagnetic domains. In our study, we observe a strong anisotropic MOKE contribution of third order in $\boldsymbol{M}$ in Ni(111) thin films, attributed to a cubic magneto-optic tensor $\propto $ $\boldsymbol{M}^3$. We further show that the angular dependence of cubic MOKE (CMOKE) is affected by the amount of structural domain twinning in the sample. Our detailed study on CMOKE for two selected photon energies will open up new opportunities for CMOKE applications with sensitivity to twinning properties of thin films, e.g. CMOKE spectroscopy and microscopy or time-resolved CMOKE. △ Less

Submitted 19 March, 2024; v1 submitted 17 May, 2022; originally announced May 2022.

Comments: 6 pages, 3 figures, supplemental pdf

Journal ref: Phys. Rev. Applied 22, 024066 (2024)

Structural and magnetic investigation of the interfaces of $\mathrm{Fe_3O_4/MgO(001)}$ with and without NiO interlayer

Authors: Tobias Pohlmann, Florian Bertram, Jannis Thien, Jari Rodewald, Kevin Ruwisch, Timo Kuschel, Eugen Weschke, Karsten Küpper, Joachim Wollschläger

Abstract: We present an investigation on the structural and magnetic properties of the interfaces of $\mathrm{Fe_3O_4/MgO(001)}$ and $\mathrm{Fe_3O_4/NiO/MgO(001)}$ by extracting cation-selective magnetooptical depth profiles by means of x-ray magnetic reflectivity (XRMR) in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism (XMCD) data. For… ▽ More We present an investigation on the structural and magnetic properties of the interfaces of $\mathrm{Fe_3O_4/MgO(001)}$ and $\mathrm{Fe_3O_4/NiO/MgO(001)}$ by extracting cation-selective magnetooptical depth profiles by means of x-ray magnetic reflectivity (XRMR) in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism (XMCD) data. For $\mathrm{Fe_3O_4/MgO(001)}$, the magnetooptical depth profiles at the $\mathrm{Fe^{2+}_{oct}}$ and the $\mathrm{Fe^{3+}_{oct}}$ resonant energies follow exactly the structural profile, while the magnetooptical depth profile at the $\mathrm{Fe^{3+}_{tet}}$ resonance is offset by $3.2\pm1.3\,$Å from the interface, consistent with a B-site interface termination of $\mathrm{Fe_3O_4}$ with fully intact magnetic order. In contrast, for $\mathrm{Fe_3O_4/NiO(001)}$, the magnetooptical depth profiles at the $\mathrm{Fe^{2+}_{oct}}$ and the $\mathrm{Ni^{2+}}$ resonances agree with the structural profile, but the interface positions of the magnetooptical depth profiles at the $\mathrm{Fe^{3+}_{oct}}$ and the $\mathrm{Fe^{3+}_{tet}}$ resonances are laterally shifted by $3.3\pm 1.4\,$Å and $2.7\pm0.9\,$Å, respectively, not consistent with a magnetically ordered stoichiometric interface. This may be related to an intermixed $\mathrm{(Ni,Fe)O}$ layer at the interface. The magnetooptical depth profiles at the Ni $L_3$ edge reveal uncompensated magnetic moments throughout the NiO film. △ Less

Submitted 25 November, 2021; originally announced November 2021.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. B 105, 235436 (2022)

Unidirectional spin Hall magnetoresistance and spin-orbit torques in HM$_1$/Co/HM$_2$ trilayer systems

Authors: Anastasiia Moskaltsova, Denis Dyck, Jan-Michael Schmalhorst, Günter Reiss, Timo Kuschel

Abstract: We present a detailed analysis of harmonic longitudinal and Hall voltage measurements for in-plane magnetized Pt/Co/Ta and Ta/Co/Pt trilayers in reference to Pt/Co and Ta/Co bilayers. Enhancement of spin-orbit torques (SOTs) and unidirectional spin Hall magnetoresistance (USMR) is achieved by introducing the second heavy metal (HM) with the opposite sign of the spin Hall angle. The extracted SOT e… ▽ More We present a detailed analysis of harmonic longitudinal and Hall voltage measurements for in-plane magnetized Pt/Co/Ta and Ta/Co/Pt trilayers in reference to Pt/Co and Ta/Co bilayers. Enhancement of spin-orbit torques (SOTs) and unidirectional spin Hall magnetoresistance (USMR) is achieved by introducing the second heavy metal (HM) with the opposite sign of the spin Hall angle. The extracted SOT efficiencies are larger for the trilayers as compared to the bilayers, confirming the enhanced values reported for the trilayers with perpendicularly magnetized Co. The maximum effective spin Hall angle found for the Pt/Co/Ta trilayer reaches $θ_{SH}$ = 20%. The USMR of the trilayer yields up to 27% higher effect as for the respective bilayers with the largest effective USMR amplitude of -0.32 $\times$ 10$^{-5}$ for the Pt/Co/Ta trilayer at a charge current density of 10$^{7}$ A/cm$^2$. △ Less

Submitted 16 November, 2021; originally announced November 2021.

Role of NiO in the nonlocal spin transport through thin NiO films on Y$_3$Fe$_5$O$_{12}$

Authors: Geert R. Hoogeboom, Geert-Jan N. Sint Nicolaas, Andreas Alexander, Olga Kuschel, Joachim Wollschläger, Inga Ennen, Bart J. van Wees, Timo Kuschel

Abstract: In spin transport experiments with spin currents propagating through antiferromagnetic (AFM) material, the antiferromagnet is treated as a mainly passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the Néel temperature and decreasing by lowering the temperature. In order to… ▽ More In spin transport experiments with spin currents propagating through antiferromagnetic (AFM) material, the antiferromagnet is treated as a mainly passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the Néel temperature and decreasing by lowering the temperature. In order to study the role of the AFM in local and nonlocal spin transport experiments, we send spin currents through NiO of various thickness placed on Y$_3$Fe$_5$O$_{12}$. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below 100$\,$K which are unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence is similar as for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents. △ Less

Submitted 22 December, 2020; originally announced December 2020.

Journal ref: Phys. Rev. B 103, 144406 (2021)

Magnetic order of Dy$^{3+}$ and Fe$^{3+}$ moments in antiferromagnetic DyFeO$_{3}$ probed by spin Hall magnetoresistance and spin Seebeck effect

Authors: G. R. Hoogeboom, T. Kuschel, G. E. W. Bauer, M. V. Mostovoy, A. V. Kimel, B. J. van Wees

Abstract: We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in single crystal of the rare-earth antiferromagnet DyFeO$_{3}$ with a thin Pt film contact. The angular shape and symmetry of the SMR at elevated temperatures reflect the antiferromagnetic order of the Fe$^{3+}$ moments as governed by the Zeeman energy, the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya inte… ▽ More We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in single crystal of the rare-earth antiferromagnet DyFeO$_{3}$ with a thin Pt film contact. The angular shape and symmetry of the SMR at elevated temperatures reflect the antiferromagnetic order of the Fe$^{3+}$ moments as governed by the Zeeman energy, the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction. We interpret the observed linear dependence of the signal on the magnetic field strength as evidence for field-induced order of the Dy$^{3+}$ moments up to room temperature. At and below the Morin temperature of 50$\,$K, the SMR monitors the spin-reorientation phase transition of Fe$^{3+}$ spins. Below 23$\,$K, additional features emerge that persist below 4$\,$K, the ordering temperature of the Dy$^{3+}$ magnetic sublattice. We conclude that the combination of SMR and SSE is a simple and efficient tool to study spin reorientation phase transitions and sublattice magnetizations. △ Less

Submitted 21 December, 2020; originally announced December 2020.

Journal ref: Phys. Rev. B 103, 134406 (2021)

Static magnetic proximity effects and spin Hall magnetoresistance in Pt/Y$_{3}$Fe$_{5}$O$_{12}$ and inverted Y$_{3}$Fe$_{5}$O$_{12}$/Pt bilayers

Authors: Stephan Geprägs, Christoph Klewe, Sibylle Meyer, Dominik Graulich, Felix Schade, Marc Schneider, Sonia Francoual, Stephen P. Collins, Katharina Ollefs, Fabrice Wilhelm, Andrei Rogalev, Yves Joly, Sebastian T. B. Goennenwein, Matthias Opel, Timo Kuschel, Rudolf Gross

Abstract: The magnetic state of heavy metal Pt thin films in proximity to the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ has been investigated systematically by means of x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity measurements combined with angle-dependent magnetotransport studies. To reveal intermixing effects as the possible cause for induced magnetic moments in Pt, we… ▽ More The magnetic state of heavy metal Pt thin films in proximity to the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ has been investigated systematically by means of x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity measurements combined with angle-dependent magnetotransport studies. To reveal intermixing effects as the possible cause for induced magnetic moments in Pt, we compare thin film heterostructures with different order of the layer stacking and different interface properties. For standard Pt layers on Y$_{3}$Fe$_{5}$O$_{12}$ thin films, we do not detect any static magnetic polarization in Pt. These samples show an angle-dependent magnetoresistance behavior, which is consistent with the established spin Hall magnetoresistance. In contrast, for the inverted layer sequence, Y$_{3}$Fe$_{5}$O$_{12}$ thin films grown on Pt layers, Pt displays a finite induced magnetic moment comparable to that of all-metallic Pt/Fe bilayers. This magnetic moment is found to originate from finite intermixing at the Y$_{3}$Fe$_{5}$O$_{12}$/Pt interface. As a consequence, we found a complex angle-dependent magnetoresistance indicating a superposition of the spin Hall and the anisotropic magnetoresistance in these type of samples. Both effects can be disentangled from each other due to their different angle dependence and their characteristic temperature evolution. △ Less

Submitted 8 October, 2020; originally announced October 2020.

Journal ref: Phys. Rev. B 102, 214438 (2020)

Quantitative comparison of the magnetic proximity effect in Pt detected by XRMR and XMCD

Authors: Dominik Graulich, Jan Krieft, Anastasiia Moskaltsova, Johannes Demir, Tobias Peters, Tobias Pohlmann, Florian Bertram, Joachim Wollschläger, Jose R. L. Mardegan, Sonia Francoual, Timo Kuschel

Abstract: X-ray resonant magnetic reflectivity (XRMR) allows for the simultaneous measurement of structural, optical and magnetooptic properties and depth profiles of a variety of thin film samples. However, a same-beamtime same-sample systematic quantitative comparison of the magnetic properties observed with XRMR and x-ray magnetic circular dichroism (XMCD) is still pending. Here, the XRMR results (Pt L… ▽ More X-ray resonant magnetic reflectivity (XRMR) allows for the simultaneous measurement of structural, optical and magnetooptic properties and depth profiles of a variety of thin film samples. However, a same-beamtime same-sample systematic quantitative comparison of the magnetic properties observed with XRMR and x-ray magnetic circular dichroism (XMCD) is still pending. Here, the XRMR results (Pt L$_{3}$ absorption edge) for the magnetic proximity effect in Pt deposited on the two different ferromagnetic materials Fe and Co$_{33}$Fe$_{67}$ are compared with quantitatively analyzed XMCD results. The obtained results are in very good quantitative agreement between the absorption-based (XMCD) and reflectivity-based (XRMR) techniques taking into account an ab initio calculated magnetooptic conversion factor for the XRMR analysis. Thus, it is shown that XRMR provides quantitative reliable spin depth profiles important for spintronic and spin caloritronic transport phenomena at this type of magnetic interfaces. △ Less

Submitted 7 January, 2021; v1 submitted 5 October, 2020; originally announced October 2020.

Comments: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 118, 012407 (2021) and may be found at https://aip.scitation.org/doi/abs/10.1063/5.0032584

Journal ref: Appl. Phys. Lett. 118, 012407 (2021)

Enhancement in Thermally Generated Spin Voltage at Pd/NiFe$_2$O$_4$ Interfaces by the Growth on Lattice-Matched Substrates

Authors: A. Rastogi, Z. Li, A. V. Singh, S. Regmi, T. Peters, P. Bougiatioti, D. Carsten né Meier, J. B. Mohammadi, B. Khodadadi, T. Mewes, R. Mishra, J. Gazquez, A. Y. Borisevich, Z. Galazka, R. Uecker, G. Reiss, T. Kuschel, A. Gupta

Abstract: Efficient spin injection from epitaxial ferrimagnetic NiFe$_2$O$_4$ thin films into a Pd layer is demonstrated via spin Seebeck effect measurements in the longitudinal geometry. The NiFe$_2$O$_4$ films (60 nm to 1 $μ$m) are grown by pulsed laser deposition on isostructural spinel MgAl$_2$O$_4$, MgGa$_2$O$_4$, and CoGa$_2$O$_4$ substrates with lattice mismatch varying between 3.2% and 0.2%. For the… ▽ More Efficient spin injection from epitaxial ferrimagnetic NiFe$_2$O$_4$ thin films into a Pd layer is demonstrated via spin Seebeck effect measurements in the longitudinal geometry. The NiFe$_2$O$_4$ films (60 nm to 1 $μ$m) are grown by pulsed laser deposition on isostructural spinel MgAl$_2$O$_4$, MgGa$_2$O$_4$, and CoGa$_2$O$_4$ substrates with lattice mismatch varying between 3.2% and 0.2%. For the thinner films ($\leq$ 330 nm), an increase in the spin Seebeck voltage is observed with decreasing lattice mismatch, which correlates well with a decrease in the Gilbert damping parameter as determined from ferromagnetic resonance measurements. High resolution transmission electron microscopy studies indicate substantial decrease of antiphase boundary and interface defects that cause strain-relaxation, i.e., misfit dislocations, in the films with decreasing lattice mismatch. This highlights the importance of reducing structural defects in spinel ferrites for efficient spin injection. It is further shown that angle-dependent spin Seebeck effect measurements provide a qualitative method to probe for in-plane magnetic anisotropies present in the films. △ Less

Submitted 1 June, 2020; originally announced June 2020.

Comments: 18 pages, 6 figures

Journal ref: Phys. Rev. Applied 14, 014014 (2020)

Cation- and lattice-site-selective magnetic depth profiles of ultrathin $\mathrm{Fe_3O_4}$(001) films

Authors: Tobias Pohlmann, Timo Kuschel, Jari Rodewald, Jannis Thien, Kevin Ruwisch, Florian Bertram, Eugen Weschke, Padraic Shafer, Joachim Wollschläger, Karsten Kuepper

Abstract: A detailed understanding of ultrathin film surface properties is crucial for the proper interpretation of spectroscopic, catalytic and spin-transport data. We present x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic reflectivity (XRMR) measurements on ultrathin $\mathrm{Fe_3O_4}$ films to obtain magnetic depth profiles for the three resonant energies corresponding to the differ… ▽ More A detailed understanding of ultrathin film surface properties is crucial for the proper interpretation of spectroscopic, catalytic and spin-transport data. We present x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic reflectivity (XRMR) measurements on ultrathin $\mathrm{Fe_3O_4}$ films to obtain magnetic depth profiles for the three resonant energies corresponding to the different cation species $\mathrm{Fe^{2+}_{oct}}$, $\mathrm{Fe^{3+}_{tet}}$ and $\mathrm{Fe^{3+}_{oct}}$ located on octahedral and tetrahedral sites of the inverse spinel structure of $\mathrm{Fe_3O_4}$. By analyzing the XMCD spectrum of $\mathrm{Fe_3O_4}$ using multiplet calculations, the resonance energy of each cation species can be isolated. Performing XRMR on these three resonant energies yields magnetic depth profiles that correspond each to one specific cation species. The depth profiles of both kinds of $\mathrm{Fe^{3+}}$ cations reveal a $\mathrm{3.9 \pm 1~Å}$-thick surface layer of enhanced magnetization, which is likely due to an excess of these ions at the expense of the $\mathrm{Fe^{2+}_{oct}}$ species in the surface region. The magnetically enhanced $\mathrm{Fe^{3+}_{tet}}$ layer is additionally shifted about $\mathrm{3\pm 1.5~Å}$ farther from the surface than the $\mathrm{Fe^{3+}_{oct}}$ layer. △ Less

Submitted 4 May, 2020; originally announced May 2020.

Comments: 5 pages, 3 figures, supplemental pdf

Journal ref: Phys. Rev. B 102, 220411 (2020)

Advanced data analysis procedure for hard x-ray resonant magnetic reflectivity discussed for Pt thin film samples of various complexity

Authors: Jan Krieft, Dominik Graulich, Anastasiia Moskaltsova, Laurence Bouchenoire, Sonia Francoual, Timo Kuschel

Abstract: X-ray resonant magnetic reflectivity (XRMR) is a powerful method to determine the optical, structural and magnetic depth profiles of a variety of thin films. Here, we investigate samples of different complexity all measured at the Pt L$_3$ absorption edge to determine the optimal procedure for the analysis of the experimental XRMR curves, especially for nontrivial bi- and multilayer samples that i… ▽ More X-ray resonant magnetic reflectivity (XRMR) is a powerful method to determine the optical, structural and magnetic depth profiles of a variety of thin films. Here, we investigate samples of different complexity all measured at the Pt L$_3$ absorption edge to determine the optimal procedure for the analysis of the experimental XRMR curves, especially for nontrivial bi- and multilayer samples that include differently bonded Pt from layer to layer. The software tool ReMagX is used to fit these data and model the magnetooptic depth profiles based on a highly adaptable layer stack which is modified to be a more precise and physically consistent representation of the real multilayer system. Various fitting algorithms, iterative optimization approaches and a detailed analysis of the asymmetry ratio features as well as $χ^2$ (goodness of fit) landscapes are utilized to improve the agreement between measurements and simulations. We present a step-by-step analysis procedure tailored to the Pt thin film systems to take advantage of the excellent magnetic sensitivity and depth resolution of XRMR. △ Less

Submitted 17 April, 2020; originally announced April 2020.

Comments: 16 pages, 14 figures

Journal ref: J. Phys. D: Appl. Phys. 53, 375004 (2020)

Impact of the magnetic proximity effect in Pt on the total magnetic moment of Pt/Co/Ta trilayers studied by x-ray resonant magnetic reflectivity

Authors: Anastasiia Moskaltsova, Jan Krieft, Dominik Graulich, Tristan Matalla-Wagner, Timo Kuschel

Abstract: In this work, we study the influence of the magnetic proximity effect (MPE) in Pt on the total magnetic moment of thin film trilayer systems consisting of the ferromagnet (FM) Co adjacent to the heavy metals (HMs) Pt and Ta. We investigate the trilayer systems HM1/FM/HM2 with different stacking order as well as a reference bilayer without any MPE. X-ray resonant magnetic reflectivity (XRMR) is a p… ▽ More In this work, we study the influence of the magnetic proximity effect (MPE) in Pt on the total magnetic moment of thin film trilayer systems consisting of the ferromagnet (FM) Co adjacent to the heavy metals (HMs) Pt and Ta. We investigate the trilayer systems HM1/FM/HM2 with different stacking order as well as a reference bilayer without any MPE. X-ray resonant magnetic reflectivity (XRMR) is a powerful tool to probe induced magnetism, especially when buried at interfaces in a multilayer. By using XRMR, we are able to obtain magnetic depth profiles of the structural, optical and magnetic parameters. By fitting the experimental data with a Gaussian-like magnetooptic profile taking the structural roughness at the interface into account, we can extract the magnetic moment of the spin-polarized layer. Comparing the obtained moments to the measured total moment of the sample, we can determine the impact of the MPE on the total magnetic moment of the system. Such information can be critical for analyzing spin transport experiments, including spin-orbit torque and spin Hall angle measurements, where the saturation magnetization $M_s$ has to be taken into account. Therefore, by combining magnetization measurements and XRMR methods we were able to get a complete picture of the magnetic moment distribution in these trilayer systems containing spin-polarized Pt. △ Less

Submitted 19 January, 2020; originally announced January 2020.

Journal ref: AIP Advances 10, 015154 (2020)

Thermal Conductivity of Oxide Tunnel Barriers in Magnetic Tunnel Junctions Measured by Ultrafast Thermoreflectance and Magneto-optic Kerr Effect Thermometry

Authors: Hyejin Jang, Luca Marnitz, Torsten Huebner, Johannes Kimling, Timo Kuschel, David G. Cahill

Abstract: Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be manipulated by a temperature gradient, which can be utilized for spintronic and spin caloritronic applications. Evaluation of the thermally induced phenomena requires knowledge of the temperature differences across the oxide tunnel barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to accurately me… ▽ More Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be manipulated by a temperature gradient, which can be utilized for spintronic and spin caloritronic applications. Evaluation of the thermally induced phenomena requires knowledge of the temperature differences across the oxide tunnel barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to accurately measure thermal properties of an oxide tunnel barrier consisting of only a few atomic layers. In this work, we experimentally interrogate the temperature evolutions in Ru/oxide/FM/seed/MgO (oxide=MgO, MgAl2O4; FM=Co, CoFeB; seed=Pt, Ta) structures having perpendicular magnetic anisotropy using ultrafast thermometry. The Ru layer is optically thick and heated by ultrafast laser pulses; the subsequent temperature changes are monitored using thermoreflectance of Ru and magneto-optic Kerr effect (MOKE) of the FM layers. We independently measure the response times of Co and CoFeB magnetism using quadratic MOKE and obtain τem=0.2 ps for Co and 2 ps for CoFeB. These time scales are much shorter than the time scale of heat transport through the oxide tunnel barrier, which occurs at 10-3000 ps. We determine effective thermal conductivities of MgO and MgAl2O4 tunnel barriers in the range of 0.4-0.6 W m-1 K-1, comparable to an estimate of the series conductance of the Ru/oxide and oxide/FM interfaces and an order of magnitude smaller than the thermal conductivity of MgO thin films. We find that the electron-phonon thermal conductance near the tunnel barrier is only a factor of 5-12 larger than the thermal conductance of the oxide tunnel barrier. Therefore, the drop in the electronic temperature is approximately 20-30% larger than the drop in the phonon temperature across the tunnel barrier. △ Less

Submitted 7 December, 2019; originally announced December 2019.

Comments: 31 pages, 5 figures

Journal ref: Phys. Rev. Applied 13, 024007 (2020)

Asymmetric modification of the magnetic proximity effect in Pt/Co/Pt trilayers by the insertion of a Ta buffer layer

Authors: Ankan Mukhopadhyay, Sarathlal Koyiloth Vayalil, Dominik Graulich, Imran Ahamed, Sonia Francoual, Arti Kashyap, Timo Kuschel, P S Anil Kumar

Abstract: The magnetic proximity effect in top and bottom Pt layers induced by Co in Ta/Pt/Co/Pt multilayers has been studied by interface sensitive, element specific x-ray resonant magnetic reflectivity. The asymmetry ratio for circularly polarized x-rays of left and right helicity has been measured at the Pt $L_3$ absorption edge (11567 eV) with an in-plane magnetic field ($\pm158$ mT) to verify its magne… ▽ More The magnetic proximity effect in top and bottom Pt layers induced by Co in Ta/Pt/Co/Pt multilayers has been studied by interface sensitive, element specific x-ray resonant magnetic reflectivity. The asymmetry ratio for circularly polarized x-rays of left and right helicity has been measured at the Pt $L_3$ absorption edge (11567 eV) with an in-plane magnetic field ($\pm158$ mT) to verify its magnetic origin. The proximity-induced magnetic moment in the bottom Pt layer decreases with the thickness of the Ta buffer layer. Grazing incidence x-ray diffraction has been carried out to show that the Ta buffer layer induces the growth of Pt(011) rather than Pt(111) which in turn reduces the induced moment. A detailed density functional theory study shows that an adjacent Co layer induces more magnetic moment in Pt(111) than in Pt(011). The manipulation of the magnetism in Pt by the insertion of a Ta buffer layer provides a new way of controlling the magnetic proximity effect which is of huge importance in spin-transport experiments across similar kind of interfaces. △ Less

Submitted 27 November, 2019; originally announced November 2019.

Comments: 7 pages, 9 figures

Journal ref: Phys. Rev. B 102, 144435 (2020)

Vectorial observation of the spin Seebeck effect in epitaxial NiFe$_2$O$_4$ thin films with various magnetic anisotropy contributions

Authors: Zhong Li, Jan Krieft, Amit Vikram Singh, Sudhir Regmi, Ankur Rastogi, Abhishek Srivastava, Zbigniew Galazka, Tim Mewes, Arunava Gupta, Timo Kuschel

Abstract: We have developed a vectorial type of measurement for the spin Seebeck effect (SSE) in epitaxial NiFe$_2$O$_4$ thin films which have been grown by pulsed laser deposition on MgGa$_2$O$_4$ (MGO) with (001) and (011) orientation as well as CoGa$_2$O$_4$ (011) (CGO), thus varying the lattice mismatch and crystal orientation. We confirm that a large lattice mismatch leads to strain anisotropy in addit… ▽ More We have developed a vectorial type of measurement for the spin Seebeck effect (SSE) in epitaxial NiFe$_2$O$_4$ thin films which have been grown by pulsed laser deposition on MgGa$_2$O$_4$ (MGO) with (001) and (011) orientation as well as CoGa$_2$O$_4$ (011) (CGO), thus varying the lattice mismatch and crystal orientation. We confirm that a large lattice mismatch leads to strain anisotropy in addition to the magnetocrystalline anisotropy in the thin films using vibrating sample magnetometry and ferromagnetic resonance measurements. Moreover, we show that the existence of a magnetic strain anisotropy in NiFe$_2$O$_4$ thin films significantly impacts the shape and magnitude of the magnetic-field-dependent SSE voltage loops. We further demonstrate that bidirectional field-dependent SSE voltage curves can be utilized to reveal the complete magnetization reversal process, which establishes a vectorial magnetometry technique based on a spin caloric effect. △ Less

Submitted 12 May, 2019; v1 submitted 14 February, 2019; originally announced February 2019.

Comments: 5 pages, 4 figures

Journal ref: Appl. Phys. Lett. 114, 232404 (2019)

Spincaloritronic measurements: a round robin comparison of the longitudinal spin Seebeck effect

Authors: A. Sola, V. Basso, M. Kuepferling, M. Pasquale, D. Meier, G. Reiss, T. Kuschel, T. Kikkawa, K. Uchida, E. Saitoh, H. Jin, S. Boona, S. Watzman, J. Heremans, M. B. Jungfleisch, W. Zhang, J. E. Pearson, A. Hoffmann, H. W. Schumacher

Abstract: The rising field of spin caloritronics focuses on the interactions between spin and heat currents in a magnetic material; the observation of the spin Seebeck effect opened the route to this branch of research. This paper reports the results of a round robin test performed by five partners on a single device highlighting the reproducibility problems related to the measurements of the spin Seebeck c… ▽ More The rising field of spin caloritronics focuses on the interactions between spin and heat currents in a magnetic material; the observation of the spin Seebeck effect opened the route to this branch of research. This paper reports the results of a round robin test performed by five partners on a single device highlighting the reproducibility problems related to the measurements of the spin Seebeck coefficient, the quantity that describes the strength of the spin Seebeck effect. This work stimulated the search for more reproducible measurement methods through the analysis of the systematic effects. △ Less

Submitted 3 December, 2018; originally announced December 2018.

Comments: PDFLaTeX, 8 pages, 8 figures

Journal ref: IEEE Trans. Instrum. Meas. 68, 1765 (2019)

Comment on "Optical detection of transverse spin-Seebeck effect in permalloy film using Sagnac interferometer microscopy"

Authors: Johannes Kimling, Timo Kuschel

Abstract: Measuring spin currents and spin accumulations is challenging. This is mainly because of their small size and the lack of a standard spin detector comparable to conventional ampere meters or voltage detectors. It is all the more pleasing to hear about new methods for detecting spin currents and spin accumulations. A recent example is the paper of McLaughlin et al. [Phys. Rev. B 95, 180401 (2017)],… ▽ More Measuring spin currents and spin accumulations is challenging. This is mainly because of their small size and the lack of a standard spin detector comparable to conventional ampere meters or voltage detectors. It is all the more pleasing to hear about new methods for detecting spin currents and spin accumulations. A recent example is the paper of McLaughlin et al. [Phys. Rev. B 95, 180401 (2017)], who report novel measurements of the transverse spin-Seebeck effect (TSSE) in metals by optical means using a Sagnac interferometer microscope. However, because no standard procedures or reference measurements are available for verification, critical discussions are crucial for evaluation of such novel experimental approaches. In our comment, we estimate the size of the effect observed by McLaughlin et al. To illustrate that the estimated size is exceptionally large, we use a comparison with another spin-caloritronic effect: the spin-dependent Seebeck effect. Such a large TSSE is in contradiction with previous experiments that could not resolve a TSSE signal. Therefore, we suggest that signal measured by McLaughlin et al. is not or not only caused by the TSSE. △ Less

Submitted 31 October, 2018; originally announced October 2018.

Journal ref: Phys. Rev. B 99, 106401 (2019)

Tunnel magneto-Seebeck effect

Authors: Timo Kuschel, Michael Czerner, Jakob Walowski, Andy Thomas, Hans Werner Schumacher, Günter Reiss, Christian Heiliger, Markus Münzenberg

Abstract: The interplay of charge, spin and heat transport is investigated in the fascinating research field of spin caloritronics, the marriage of spintronics and thermoelectrics. Here, many new spin-dependent thermal transport phenomena in magnetic nanostructures have been explored in the recent years. One of them is the tunnel magneto-Seebeck (TMS) effect in magnetic tunnel junctions (MTJs) that has larg… ▽ More The interplay of charge, spin and heat transport is investigated in the fascinating research field of spin caloritronics, the marriage of spintronics and thermoelectrics. Here, many new spin-dependent thermal transport phenomena in magnetic nanostructures have been explored in the recent years. One of them is the tunnel magneto-Seebeck (TMS) effect in magnetic tunnel junctions (MTJs) that has large potential for future nanoelectronic devices, such as nanostructured sensors for three-dimentional thermal gradients, or scanning tunneling microscopes driven by temperature differences. The TMS describes the dependence of the MTJ's thermopower on its magnetic cofiguration when a thermal gradient is applied. In this review, we highlight the successful way from first observation of the TMS in 2011 to current ongoing developments in this research area. We emphasize on different heating techniques, material designs, applications, and additional physical aspects such as the role of the thermal conductivity of the barrier material. We further demonstrate the efficient interplay between ab initio calculations and experiments within this field, as this has led, e.g., to the detection of large TMS ratios in MTJs with half-metallic Heusler electrodes. △ Less

Submitted 6 November, 2018; v1 submitted 16 August, 2018; originally announced August 2018.

Comments: 19 pages, 18 figures. Topical Review Article. Please sent a mail, if you have any comments or suggestions

Journal ref: J. Phys. D: Appl. Phys. 52, 133001 (2019)

Impact of magnetic moment and anisotropy of Co$_\textrm{1-x}$Fe$_\textrm{x}$ thin films on the magnetic proximity effect of Pt

Authors: Panagiota Bougiatioti, Orestis Manos, Olga Kuschel, Joachim Wollschläger, Martin Tolkiehn, Sonia Francoual, Timo Kuschel

Abstract: We present a systematic study of the magnetic proximity effect in Pt, depending on the magnetic moment and anisotropy of adjacent metallic ferromagnets. Element-selective x-ray resonant magnetic reflectivity measurements at the Pt absorption edge (11565$\,$eV) are carried out to investigate the spin polarization of Pt in Pt/Co$_\textrm{1-x}$Fe$_\textrm{x}$ bilayers. We observe the largest magnetic… ▽ More We present a systematic study of the magnetic proximity effect in Pt, depending on the magnetic moment and anisotropy of adjacent metallic ferromagnets. Element-selective x-ray resonant magnetic reflectivity measurements at the Pt absorption edge (11565$\,$eV) are carried out to investigate the spin polarization of Pt in Pt/Co$_\textrm{1-x}$Fe$_\textrm{x}$ bilayers. We observe the largest magnetic moment of (0.72$\,\pm\,$0.03)$\, μ_\textrm{B}$ per spin polarized Pt atom in Pt/Co$_\textrm{33}$Fe$_\textrm{67}$, following the Slater-Pauling curve of magnetic moments in Co-Fe alloys. In general, a clear linear dependence is observed between the Pt moment and the moment of the adjacent ferromagnet. Further, we study the magnetic anisotropy of the magnetized Pt which clearly adopts the magnetic anisotropy of the ferromagnet below. This is depicted for Pt on Fe(001) and on Co$_\textrm{50}$Fe$_\textrm{50}$(001), which have a 45$^{\circ}$ relative rotation of the fourfold magnetocrystalline anisotropy. △ Less

Submitted 25 July, 2018; v1 submitted 24 July, 2018; originally announced July 2018.

Enhanced magnon spin transport in NiFe$_2$O$_4$ thin films on a lattice-matched substrate

Authors: J. Shan, A. V. Singh, L. Liang, L. Cornelissen, A. Gupta, B. J. van Wees, T. Kuschel

Abstract: We investigate magnon spin transport in epitaxial nickel ferrite (NiFe$_2$O$_4$, NFO) films grown on magnesium gallate spinel (MgGa$_2$O$_4$, MGO) substrates, which have a lattice mismatch with NFO as small as 0.78%, resulting in the reduction of antiphase boundary defects and thus in improved magnetic properties in the NFO films. In the nonlocal transport experiments, enhanced signals are observe… ▽ More We investigate magnon spin transport in epitaxial nickel ferrite (NiFe$_2$O$_4$, NFO) films grown on magnesium gallate spinel (MgGa$_2$O$_4$, MGO) substrates, which have a lattice mismatch with NFO as small as 0.78%, resulting in the reduction of antiphase boundary defects and thus in improved magnetic properties in the NFO films. In the nonlocal transport experiments, enhanced signals are observed for both electrically and thermally excited magnons, and the magnon relaxation length ($λ_m$) of NFO is found to be around 2.5 $μ$m at room temperature. Moreover, at both room and low temperatures, we present distinct features from the nonlocal spin Seebeck signals which arise from magnon polaron formation. Our results demonstrate excellent magnon transport properties (magnon spin conductivity, $λ_m$ and spin mixing conductance at the interface between Pt) of NFO films grown on a lattice-matched substrate that are comparable with those of yttrium iron garnet. △ Less

Submitted 24 July, 2018; originally announced July 2018.

Comments: 5 pages, 3 figures

Journal ref: Appl. Phys. Lett. 113, 162403 (2018)

Nonlocal magnon spin transport in yttrium iron garnet with tantalum and platinum spin injection/detection electrodes

Authors: Jing Liu, Ludo J. Cornelissen, Juan Shan, Bart J. van Wees, Timo Kuschel

Abstract: We study the magnon spin transport in the magnetic insulator yttrium iron garnet (YIG) in a nonlocal experiment and compare the magnon spin excitation and detection for the heavy metal paramagnetic electrodes platinum (Pt|YIG|Pt) and tantalum (Ta|YIG|Ta). The electrical injection and detection processes rely on the (inverse) spin Hall effect in the heavy metals and the conversion between the elect… ▽ More We study the magnon spin transport in the magnetic insulator yttrium iron garnet (YIG) in a nonlocal experiment and compare the magnon spin excitation and detection for the heavy metal paramagnetic electrodes platinum (Pt|YIG|Pt) and tantalum (Ta|YIG|Ta). The electrical injection and detection processes rely on the (inverse) spin Hall effect in the heavy metals and the conversion between the electron spin and magnon spin at the heavy metal|YIG interface. Pt and Ta possess opposite signs of the spin Hall angle. Furthermore, their heterostructures with YIG have different interface properties, i.e. spin mixing conductances. By varying the distance between injector and detector, the magnon spin transport is studied. Using a circuit model based on the diffusion-relaxation transport theory, a similar magnon relaxation length of ~ 10 μm was extracted from both Pt and Ta devices. By changing the injector and detector material from Pt to Ta, the influence of interface properties on the magnon spin transport has been observed. For Ta devices on YIG the spin mixing conductance is reduced compared with Pt devices, which is quantitatively consistent when comparing the dependence of the nonlocal signal on the injector-detector distance with the prediction from the circuit model. △ Less

Submitted 30 March, 2018; originally announced March 2018.

Comments: 7 pages, 4 figures

Journal ref: J. Phys. D: Appl. Phys. 51, 224005 (2018)

Thermal conductivity of thin insulating films determined by tunnel magneto-Seebeck effect measurements and finite-element modeling

Authors: Torsten Huebner, Ulrike Martens, Jakob Walowski, Markus Münzenberg, Andy Thomas, Günter Reiss, Timo Kuschel

Abstract: In general, it is difficult to access the thermal conductivity of thin insulating films experimentally just by electrical means. Here, we present a new approach utilizing the tunnel magneto-Seebeck effect (TMS) in combination with finite-element modeling (FEM). We detect the laser-induced TMS and the absolute thermovoltage of laser-heated magnetic tunnel junctions with 2.6 nm thin barriers of MgAl… ▽ More In general, it is difficult to access the thermal conductivity of thin insulating films experimentally just by electrical means. Here, we present a new approach utilizing the tunnel magneto-Seebeck effect (TMS) in combination with finite-element modeling (FEM). We detect the laser-induced TMS and the absolute thermovoltage of laser-heated magnetic tunnel junctions with 2.6 nm thin barriers of MgAl$_2$O$_4$ (MAO) and MgO, respectively. A second measurement of the absolute thermovoltage after a dielectric breakdown of the barrier grants insight into the remaining thermovoltage of the stack. Thus, the pure TMS without any parasitic Nernst contributions from the leads can be identified. In combination with FEM via COMSOL, we are able to extract values for the thermal conductivity of MAO ($0.7$ W/(K$\cdot$m)) and MgO ($5.8$ W/(K$\cdot$m)), which are in very good agreement with theoretical predictions. Our method provides a new promising way to extract the experimentally challenging parameter of the thermal conductivity of thin insulating films. △ Less

Submitted 7 February, 2018; v1 submitted 6 February, 2018; originally announced February 2018.

Journal ref: J. Phys. D: Appl. Phys. 51, 224006 (2018)

Anomalous Nernst effect on the nanometer scale: Exploring three-dimensional temperature gradients in magnetic tunnel junctions

Authors: Ulrike Martens, Torsten Huebner, Henning Ulrichs, Oliver Reimer, Timo Kuschel, Ronnie R. Tamming, Chia-Lin Chang, Raanan I. Tobey, Andy Thomas, Markus Münzenberg, Jakob Walowski

Abstract: Localized laser heating creates temperature gradients in all directions and thus leads to three-dimensional electron flux in metallic materials. Temperature gradients in combination with material magnetization generate thermomagnetic voltages. The interplay between these direction-dependent temperature gradients and the magnetization along with their control enable to manipulate the generated volt… ▽ More Localized laser heating creates temperature gradients in all directions and thus leads to three-dimensional electron flux in metallic materials. Temperature gradients in combination with material magnetization generate thermomagnetic voltages. The interplay between these direction-dependent temperature gradients and the magnetization along with their control enable to manipulate the generated voltages, e.g. in magnetic nanodevices. We identify the anomalous Nernst effect (ANE) generated on a nanometer length scale by micrometer sized temperature gradients in magnetic tunnel junctions (MTJs). In a systematic study, we extract the ANE by analyzing the influence of in-plane temperature gradients on the tunnel magneto-Seebeck effect (TMS) in three dimensional devices. To investigate these effects, we utilize in-plane magnetized MTJs based on CoFeB electrodes with an MgO tunnel barrier. Due to our measurement configuration, there is no necessity to disentangle the ANE from the spin Seebeck effect in inverse spin-Hall measurements. The temperature gradients are created by a tightly focused laser spot. The spatial extent of the measured effects is defined by the MTJ size, while the spatial resolution is given by the laser spot size and the step size of its lateral translation. This method is highly sensitive to low voltages and yields an ANE coefficient of $K_N\approx 1.6\cdot 10^{-8}\,\mathrm{V/TK}$ for CoFeB. In general, TMS investigations in MTJs are motivated by the usage of otherwise wasted heat in magnetic memory devices for read/write operations. Here, the additionally generated ANE effect allows to expand the MTJs' functionality from simple memory storage to nonvolatile logic devices and opens new application fields such as direction dependent temperature sensing with the potential for further downscaling. △ Less

Submitted 12 January, 2018; originally announced January 2018.

Journal ref: Commun. Phys. 1, 65 (2018)

Electrical transport and optical band gap of NiFe$_\textrm{2}$O$_\textrm{x}$ thin films

Authors: Panagiota Bougiatioti, Orestis Manos, Christoph Klewe, Daniel Meier, Niclas Teichert, Jan-Michael Schmalhorst, Timo Kuschel, Günter Reiss

Abstract: We fabricated NiFe$_\textrm{2}$O$_\textrm{x}$ thin films on MgAl$_2$O$_4$(001) substrates by reactive dc magnetron co-sputtering varying the oxygen partial pressure during deposition. The fabrication of a variable material with oxygen deficiency leads to controllable electrical and optical properties which would be beneficial for the investigations of the transport phenomena and would, therefore,… ▽ More We fabricated NiFe$_\textrm{2}$O$_\textrm{x}$ thin films on MgAl$_2$O$_4$(001) substrates by reactive dc magnetron co-sputtering varying the oxygen partial pressure during deposition. The fabrication of a variable material with oxygen deficiency leads to controllable electrical and optical properties which would be beneficial for the investigations of the transport phenomena and would, therefore, promote the use of such materials in spintronic and spin caloritronic applications. We used several characterization techniques in order to investigate the film properties, focusing on their structural, magnetic, electrical, and optical properties. From the electrical resistivity measurements we obtained the conduction mechanisms that govern the systems in high and low temperature regimes, extracting low thermal activation energies which unveil extrinsic transport mechanisms. The thermal activation energy decreases in the less oxidized samples revealing the pronounced contribution of a large amount of electronic states localized in the band gap to the electrical conductivity. Hall effect measurements showed the mixed-type semiconducting character of our films. The optical band gaps were determined via ultraviolet-visible spectroscopy. They follow a similar trend as the thermal activation energy, with lower band gap values in the less oxidized samples. △ Less

Submitted 6 August, 2017; originally announced August 2017.

Journal ref: J. Appl. Phys. 122, 225101 (2017)

Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes

Authors: Alexander Boehnke, Ulrike Martens, Christian Sterwerf, Alessia Niesen, Torsten Huebner, Marvin von der Ehe, Markus Meinert, Timo Kuschel, Andy Thomas, Christian Heiliger, Markus Münzenberg, Günter Reiss

Abstract: Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck volt… ▽ More Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some μV, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co$_2$FeAl and Co$_2$FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B based junctions. △ Less

Submitted 20 November, 2017; v1 submitted 3 July, 2017; originally announced July 2017.

Comments: 9 pages, 4 figures

Journal ref: Nat. Commun. 8, 1626 (2017)

Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB based magnetic tunnel junctions by variation of the MgAl$_2$O$_4$ and MgO barrier thickness

Authors: Torsten Huebner, Ulrike Martens, Jakob Walowski, Alexander Boehnke, Jan Krieft, Christian Heiliger, Andy Thomas, Günter Reiss, Timo Kuschel, Markus Münzenberg

Abstract: We investigate the influence of the barrier thickness of Co$_{40}$Fe$_{40}$B$_{20}$ based magnetic tunnel junctions on the laser-induced tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm, we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm barrier thickness. This maximum is independently measured for two barrier materials, namely MgAl$_2$O$_4$ and M… ▽ More We investigate the influence of the barrier thickness of Co$_{40}$Fe$_{40}$B$_{20}$ based magnetic tunnel junctions on the laser-induced tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm, we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm barrier thickness. This maximum is independently measured for two barrier materials, namely MgAl$_2$O$_4$ and MgO. Additionally, samples with an MgAl$_2$O$_4$ barrier exhibit a high thermovoltage of more than 350$μ$V in comparison to 90$μ$V for the MTJs with MgO barrier when heated with the maximum laser power of 150mW. Our results allow for the fabrication of improved stacks when dealing with temperature differences across magnetic tunnel junctions for future applications in spin caloritronics, the emerging research field that combines spintronics and themoelectrics. △ Less

Submitted 26 June, 2017; originally announced June 2017.

Journal ref: Phys. Rev. B 96, 214435 (2017)

Nonlocal magnon-polaron transport in yttrium iron garnet

Authors: L. J. Cornelissen, K. Oyanagi, T. Kikkawa, Z. Qiu, T. Kuschel, G. E. W. Bauer, B. J. van Wees, E. Saitoh

Abstract: The spin Seebeck effect (SSE) is observed in magnetic insulator|heavy metal bilayers as an inverse spin Hall effect voltage under a temperature gradient. The SSE can be detected nonlocally as well, viz. in terms of the voltage in a second metallic contact (detector) on the magnetic film, spatially separated from the first contact that is used to apply the temperature bias (injector). Magnon-polaro… ▽ More The spin Seebeck effect (SSE) is observed in magnetic insulator|heavy metal bilayers as an inverse spin Hall effect voltage under a temperature gradient. The SSE can be detected nonlocally as well, viz. in terms of the voltage in a second metallic contact (detector) on the magnetic film, spatially separated from the first contact that is used to apply the temperature bias (injector). Magnon-polarons are hybridized lattice and spin waves in magnetic materials, generated by the magnetoelastic interaction. Kikkawa et al. [Phys. Rev. Lett. \textbf{117}, 207203 (2016)] interpreted a resonant enhancement of the local SSE in yttrium iron garnet (YIG) as a function of the magnetic field in terms of magnon-polaron formation. Here we report the observation of magnon-polarons in \emph{nonlocal} magnon spin injection/detection devices for various injector-detector spacings and sample temperatures. Unexpectedly, we find that the magnon-polaron resonances can suppress rather than enhance the nonlocal SSE. Using finite element modelling we explain our observations as a competition between the SSE and spin diffusion in YIG. These results give unprecedented insights into the magnon-phonon interaction in a key magnetic material. △ Less

Submitted 14 June, 2017; originally announced June 2017.

Comments: 5 pages, 6 figures

Journal ref: Phys. Rev. B 96, 104441 (2017)

Negative spin Hall magnetoresistance of Pt on the bulk easy-plane antiferromagnet NiO

Authors: Geert R. Hoogeboom, Aisha Aqeel, Timo Kuschel, Thomas T. M. Palstra, Bart J. van Wees

Abstract: We report on spin Hall magnetoresistance (SMR) measurements of Pt Hall bars on the antiferromagnetic NiO(111) single crystal. An SMR with a sign opposite of conventional SMR is observed over a wide range of temperatures as well as magnetic fields stronger than 0.25T. The negative sign of the SMR can be explained by the alignment of magnetic moments being almost perpendicular to the external magnet… ▽ More We report on spin Hall magnetoresistance (SMR) measurements of Pt Hall bars on the antiferromagnetic NiO(111) single crystal. An SMR with a sign opposite of conventional SMR is observed over a wide range of temperatures as well as magnetic fields stronger than 0.25T. The negative sign of the SMR can be explained by the alignment of magnetic moments being almost perpendicular to the external magnetic field within the easy plane (111) of the antiferromagnet. This correlation of magnetic moment alignment and external magnetic field direction is realized just by the easy-plane nature of the material without the need of any exchange coupling to an additional ferromagnet. The SMR signal strength decreases with increasing temperature, primarily due to the decrease in Néel order by including fluctuations. An increasing magnetic field increases the SMR signal strength as there are less domains and the magnetic moments are more strongly manipulated at high magnetic fields. The SMR is saturated at an applied magnetic field of $6$~T resulting in a spin-mixing conductance of $\sim10^{18}~ Ω^{-1}$m$^{-2}$, which is comparable to that of Pt on insulating ferrimagnets such as yttrium iron garnet. An argon plasma treatment doubles the spin-mixing conductance. △ Less

Submitted 9 June, 2017; originally announced June 2017.

Journal ref: Appl. Phys. Lett. 111, 052409 (2017)

Quantitative analysis of the influence of keV He ion bombardment on exchange bias layer systems

Authors: Nicolas David Müglich, Gerhard Götz, Alexander Gaul, Markus Meyl, Günter Reiss, Timo Kuschel, Arno Ehresmann

Abstract: The mechanism of ion bombardment induced magnetic patterning of exchange bias layer systems for creating engineered magnetic stray field landscapes is still unclear. We compare results from vectorial magneto-optic Kerr effect measurements to a recently proposed model with time dependent rotatable magnetic anisotropy. Results show massive reduction of rotational magnetic anisotropy compared to all… ▽ More The mechanism of ion bombardment induced magnetic patterning of exchange bias layer systems for creating engineered magnetic stray field landscapes is still unclear. We compare results from vectorial magneto-optic Kerr effect measurements to a recently proposed model with time dependent rotatable magnetic anisotropy. Results show massive reduction of rotational magnetic anisotropy compared to all other magnetic anisotropies. We disprove the assumption of comparable weakening of all magnetic anisotropies and show that ion bombardment mainly influences smaller grains in the antiferromagnet. △ Less

Submitted 18 April, 2017; originally announced April 2017.

Comments: 5 figures

Journal ref: New J. Phys. 20, 053018 (2018)

Quantitative disentanglement of spin Seebeck, proximity-induced and intrinsic anomalous Nernst effect in NM/FM bilayers

Authors: Panagiota Bougiatioti, Christoph Klewe, Daniel Meier, Orestis Manos, Olga Kuschel, Joachim Wollschläger, Laurence Bouchenoire, Simon D. Brown, Jan-Michael Schmalhorst, Günter Reiss, Timo Kuschel

Abstract: We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe$_2$O$_{\textrm{4-x}}$ ($4\geq x \geq 0$) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect (ANE) in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to… ▽ More We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe$_2$O$_{\textrm{4-x}}$ ($4\geq x \geq 0$) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect (ANE) in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating NiFe$_2$O$_4$ to metallic Ni$_{33}$Fe$_{67}$. A proximity-induced ANE could only be identified in the metallic Pt/Ni$_{33}$Fe$_{67}$ bilayer in contrast to Pt/NiFe$_2$O$_{\rm x}$ ($x>0$) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity. △ Less

Submitted 16 February, 2017; originally announced February 2017.

Journal ref: Phys. Rev. Lett. 119, 227205 (2017)

Longitudinal spin Seebeck coefficient: heat flux vs. temperature difference method

Authors: A. Sola, P. Bougiatioti, M. Kuepferling, D. Meier, G. Reiss, M. Pasquale, T. Kuschel, V. Basso

Abstract: The determination of the longitudinal spin Seebeck effect (LSSE) coefficient is currently plagued by a large uncertainty due to the poor reproducibility of the experimental conditions used in its measurement. In this work we present a detailed analysis of two different methods used for the determination of the LSSE coefficient. We have performed LSSE experiments in different laboratories, by using… ▽ More The determination of the longitudinal spin Seebeck effect (LSSE) coefficient is currently plagued by a large uncertainty due to the poor reproducibility of the experimental conditions used in its measurement. In this work we present a detailed analysis of two different methods used for the determination of the LSSE coefficient. We have performed LSSE experiments in different laboratories, by using different setups and employing both the temperature difference method and the heat flux method. We found that the lack of reproducibility can be mainly attributed to the thermal contact resistance between the sample and the thermal baths which generate the temperature gradient. Due to the variation of the thermal resistance, we found that the scaling of the LSSE voltage to the heat flux through the sample rather than to the temperature difference across the sample greatly reduces the uncertainty. The characteristics of a single YIG/Pt LSSE device obtained with two different setups was $(1.143\pm0.007)\cdot 10^{-7}$ Vm/W and $(1.101\pm0.015)\cdot 10^{-7}$ Vm/W with the heat flux method and $(2.313\pm0.017)\cdot 10^{-7}$ V/K and $(4.956\pm0.005)\cdot 10^{-7}$ V/K with the temperature difference method. This shows that systematic errors can be considerably reduced with the heat flux method. △ Less

Submitted 10 November, 2018; v1 submitted 12 January, 2017; originally announced January 2017.

Comments: PDFLaTeX, 10 pages, 6 figures

Journal ref: Sci. Rep. 7, 46752 (2017)

Nonlocal magnon spin transport in NiFe$_2$O$_4$ thin films

Authors: J. Shan, P. Bougiatioti, L. Liang, G. Reiss, T. Kuschel, B. J. van Wees

Abstract: We report magnon spin transport in nickel ferrite (NiFe$_2$O$_4$, NFO)/ platinum (Pt) bilayer systems at room temperature. A nonlocal geometry is employed, where the magnons are excited by the spin Hall effect or by the Joule heating induced spin Seebeck effect at the Pt injector, and detected at a certain distance away by the inverse spin Hall effect at the Pt detector. The dependence of the nonl… ▽ More We report magnon spin transport in nickel ferrite (NiFe$_2$O$_4$, NFO)/ platinum (Pt) bilayer systems at room temperature. A nonlocal geometry is employed, where the magnons are excited by the spin Hall effect or by the Joule heating induced spin Seebeck effect at the Pt injector, and detected at a certain distance away by the inverse spin Hall effect at the Pt detector. The dependence of the nonlocal magnon spin signals as a function of the magnetic field is closely related to the NFO magnetization behavior. In contrast, we observe that the magnetoresistance measured locally at the Pt injector does not show a clear relation with the average NFO magnetization. We estimate the magnon spin relaxation length to be 3.1 $\pm$ 0.2 $μ$m in the investigated NFO samples. △ Less

Submitted 13 April, 2017; v1 submitted 8 January, 2017; originally announced January 2017.

Comments: 6 pages, 4 figures

Journal ref: Appl. Phys. Lett. 110, 132406 (2017)

Magnon Planar Hall Effect and Anisotropic Magnetoresistance in a Magnetic Insulator

Authors: J. Liu, L. J. Cornelissen, J. Shan, T. Kuschel, B. J. van Wees

Abstract: Electrical resistivities can be different for charge currents travelling parallel or perpendicular to the magnetization in magnetically ordered conductors or semiconductors, resulting in the well-known planar Hall effect and anisotropic magnetoresistance. Here, we study the analogous anisotropic magnetotransport behavior for magnons in a magnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$. Electrical and t… ▽ More Electrical resistivities can be different for charge currents travelling parallel or perpendicular to the magnetization in magnetically ordered conductors or semiconductors, resulting in the well-known planar Hall effect and anisotropic magnetoresistance. Here, we study the analogous anisotropic magnetotransport behavior for magnons in a magnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$. Electrical and thermal magnon injection, and electrical detection methods are used at room temperature with transverse and longitudinal geometries to measure the magnon planar Hall effect and anisotropic magnetoresistance, respectively. We observe that the relative difference between magnon current conductivities parallel and perpendicular to the magnetization, with respect to the average magnon conductivity, i.e. $|(σ_{\parallel}^{\textrm{m}}-σ_{\perp}^{\textrm{m}})/σ_{0}^{\textrm{m}}|$ , is approximately 5% with the majority of the measured devices showing $σ_{\perp}^{\textrm{m}}>σ_{\parallel}^{\textrm{m}}$. △ Less

Submitted 24 December, 2016; originally announced December 2016.

Comments: 18 pages, 16 figures

Journal ref: Phys. Rev. B 95, 140402(R) (2017)

Co-sputtered PtMnSb thin films and PtMnSb/Pt bilayers for spin-orbit torque investigations

Authors: Jan Krieft, Johannes Mendil, Myriam H. Aguirre, Can O. Avci, Christoph Klewe, Karsten Rott, Jan-Michael Schmalhorst, Günter Reiss, Pietro Gambardella, Timo Kuschel

Abstract: The manipulation of the magnetization by spin-orbit torques (SOTs) has recently been extensively studied due to its potential for efficiently writing information in magnetic memories. Particular attention is paid to non-centrosymmetric systems with space inversion asymmetry, where SOTs emerge even in single-layer materials. The half-metallic half-Heusler PtMnSb is an interesting candidate for stud… ▽ More The manipulation of the magnetization by spin-orbit torques (SOTs) has recently been extensively studied due to its potential for efficiently writing information in magnetic memories. Particular attention is paid to non-centrosymmetric systems with space inversion asymmetry, where SOTs emerge even in single-layer materials. The half-metallic half-Heusler PtMnSb is an interesting candidate for studies of this intrinsic SOT. Here, we report on the growth and epitaxial properties of PtMnSb thin films and PtMnSb/Pt bilayers deposited on MgO(001) substrates by dc magnetron co-sputtering at high temperature in ultra-high vacuum. The film properties were investigated by x-ray diffraction, x-ray reflectivity, atomic force microscopy, and electron microscopy. Thin PtMnSb films present a monocrystalline C1b phase with (001) orientation, coexisting at increasing thickness with a polycrystalline phase with (111) texture. Films thinner than about 5 nm grow in islands, whereas thicker films grow layer-by-layer, forming a perfect MgO/PtMnSb interface. The thin PtMnSb/Pt bilayers also show island growth and a defective transition zone, while thicker films grow layer-by-layer and Pt grows epitaxially on the half-Heusler compound without significant interdiffusion. △ Less

Submitted 22 December, 2016; originally announced December 2016.

Comments: 5 pages, 4 figures

Journal ref: Phys. Status Solidi (Rapid Res. Lett.) 11, 1600439 (2017)

Impact of strain and morphology on magnetic properties of Fe$_3$O$_4$/NiO bilayers grown on Nb:SrTiO$_3$(001) and MgO(001)

Authors: Olga Kuschel, Nico Pathé, Tobias Schemme, Kevin Ruwisch, Jari Rodewald, Ralph Buß, Florian Bertram, Timo Kuschel, Karsten Kuepper, Joachim Wollschläger

Abstract: We present a comparative study on the morphology and structural as well as magnetic properties of crystalline Fe$_3$O$_4$/NiO bilayers grown on both MgO(001) and SrTiO$_3$(001) substrates by reactive molecular beam epitaxy. These structures are investigated by means of x-ray photoelectron spectroscopy, low energy electron diffraction, x-ray reflectivity and diffraction as well as vibrating sample… ▽ More We present a comparative study on the morphology and structural as well as magnetic properties of crystalline Fe$_3$O$_4$/NiO bilayers grown on both MgO(001) and SrTiO$_3$(001) substrates by reactive molecular beam epitaxy. These structures are investigated by means of x-ray photoelectron spectroscopy, low energy electron diffraction, x-ray reflectivity and diffraction as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) is only 0.8%, it is exposed to a lateral lattice mismatch of -6.9% if grown on SrTiO$_3$. In the case of Fe$_3$O$_4$, the misfit strain on MgO(001) and SrTiO$_3$(001) amounts to 0.3% and -7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films have been varied between 5 and 20nm. While NiO films are well ordered on both substrates, Fe$_3$O$_4$ films grown on NiO/SrTiO$_3$ exhibit a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grow pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO$_3$ the NiO films show strong strain relaxation. Fe$_3$O$_4$ films exhibit also strong relaxation even for films of 5nm thickness on both NiO/MgO as well as on NiO/SrTiO$_3$. The magnetite layers on both substrates show a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in $\left\langle100\right\rangle$ directions. The coercive field is strongly enhanced for magnetite grown on NiO/SrTiO$_3$ due to higher density of structural defects, compared to magnetite grown on NiO/MgO. △ Less

Submitted 5 June, 2018; v1 submitted 14 November, 2016; originally announced November 2016.

Comments: 11 pages, 7 figures

Journal ref: Materials 11, 1122 (2018)

Quantitative separation of the anisotropic magnetothermopower and planar Nernst effect by the rotation of an in-plane thermal gradient

Authors: Oliver Reimer, Daniel Meier, Michel Bovender, Lars Helmich, Jan-Oliver Dreessen, Jan Krieft, Anatoly S. Shestakov, Christian H. Back, Jan-Michael Schmalhorst, Andreas Hütten, Günter Reiss, Timo Kuschel

Abstract: A thermal gradient as the driving force for spin currents plays a key role in spin caloritronics. In this field the spin Seebeck effect (SSE) is of major interest and was investigated in terms of in-plane thermal gradients inducing perpendicular spin currents (transverse SSE) and out-of-plane thermal gradients generating parallel spin currents (longitudinal SSE). Up to now all spincaloric experime… ▽ More A thermal gradient as the driving force for spin currents plays a key role in spin caloritronics. In this field the spin Seebeck effect (SSE) is of major interest and was investigated in terms of in-plane thermal gradients inducing perpendicular spin currents (transverse SSE) and out-of-plane thermal gradients generating parallel spin currents (longitudinal SSE). Up to now all spincaloric experiments employ a spatially fixed thermal gradient. Thus anisotropic measurements with respect to well defined crystallographic directions were not possible. Here we introduce a new experiment that allows not only the in-plane rotation of the external magnetic field, but also the rotation of an in-plane thermal gradient controlled by optical temperature detection. As a consequence, the anisotropic magnetothermopower and the planar Nernst effect in a permalloy thin film can be measured simultaneously and reveal a phase shift, that allows the quantitative separation of the thermopower, the anisotropic magnetothermopower and the planar Nernst effect. △ Less

Submitted 28 September, 2016; originally announced September 2016.

Comments: Main Manuscript: 9 pages, 9 figures; Supplementary Informations: 9 pages, 8 figures

Journal ref: Sci. Rep. 7, 40586 (2017)

Influence of yttrium iron garnet thickness and heater opacity on the nonlocal transport of electrically and thermally excited magnons

Authors: Juan Shan, Ludo J. Cornelissen, Nynke Vlietstra, Jamal Ben Youssef, Timo Kuschel, Rembert A. Duine, Bart J. van Wees

Abstract: We studied the nonlocal transport behavior of both electrically and thermally excited magnons in yttrium iron garnet (YIG) as a function of its thickness. For electrically injected magnons, the nonlocal signals decrease monotonically as the YIG thickness increases. For the nonlocal behavior of the thermally generated magnons, or the nonlocal spin Seebeck effect (SSE), we observed a sign reversal w… ▽ More We studied the nonlocal transport behavior of both electrically and thermally excited magnons in yttrium iron garnet (YIG) as a function of its thickness. For electrically injected magnons, the nonlocal signals decrease monotonically as the YIG thickness increases. For the nonlocal behavior of the thermally generated magnons, or the nonlocal spin Seebeck effect (SSE), we observed a sign reversal which occurs at a certain heater-detector distance, and it is influenced by both the opacity of the YIG/heater interface and the YIG thickness. Our nonlocal SSE results can be qualitatively explained by the bulk-driven SSE mechanism together with the magnon diffusion model. Using a two-dimensional finite element model (2D-FEM), we estimated the bulk spin Seebeck coefficient of YIG at room temperature. The quantitative disagreement between the experimental and modeled results indicates more complex processes going on in addition to magnon diffusion and relaxation, especially close to the contacts. △ Less

Submitted 3 August, 2016; originally announced August 2016.

Comments: 16 pages, 11 figures

Journal ref: Phys. Rev. B 94, 174437 (2016)

Picosecond spin Seebeck effect

Authors: Johannes Kimling, Gyung-Min Choi, Jack T. Brangham, Tristan Matalla-Wagner, Torsten Huebner, Timo Kuschel, Fengyuan Yang, David G. Cahill

Abstract: We report time-resolved magneto-optic Kerr effect measurements of the longitudinal spin Seebeck effect driven by an interfacial temperature difference between itinerant electrons and magnons. The measured time-evolution of spin accumulation induced by laser-excitation indicates transfer of angular momentum across Au/Y$_3$Fe$_5$O$_{12}$ and Cu/Y$_3$Fe$_5$O$_{12}$ interfaces on a picosecond time-sca… ▽ More We report time-resolved magneto-optic Kerr effect measurements of the longitudinal spin Seebeck effect driven by an interfacial temperature difference between itinerant electrons and magnons. The measured time-evolution of spin accumulation induced by laser-excitation indicates transfer of angular momentum across Au/Y$_3$Fe$_5$O$_{12}$ and Cu/Y$_3$Fe$_5$O$_{12}$ interfaces on a picosecond time-scale. The product of spin-mixing conductance and interfacial spin Seebeck coefficient determined is of the order of $10^8$ A m$^{-2}$ K$^{-1}$. △ Less

Submitted 2 August, 2016; originally announced August 2016.

Comments: 16 pages, 4 figures, 1 table

Journal ref: Phys. Rev. Lett. 118, 057201 (2017)

Grain size correlated rotatable magnetic anisotropy in polycrystalline exchange bias systems

Authors: Nicolas David Müglich, Gerhard Götz, Alexander Gaul, Markus Meyl, Günter Reiss, Timo Kuschel, Arno Ehresmann

Abstract: Angular resolved measurements of the exchange bias field and the coercive field are a powerful tool to distinguish between different competing magnetic anisotropies in polycrystalline exchange bias layer systems. No simple analytical model is as yet available, which considers time dependent effects like enhanced coercivity arising from the grain size distribution of the antiferromagnet. In this wo… ▽ More Angular resolved measurements of the exchange bias field and the coercive field are a powerful tool to distinguish between different competing magnetic anisotropies in polycrystalline exchange bias layer systems. No simple analytical model is as yet available, which considers time dependent effects like enhanced coercivity arising from the grain size distribution of the antiferromagnet. In this work we expand an existing model class describing polycrystalline exchange bias systems by a rotatable magnetic anisotropy term to describe grain size correlated effects. Additionally, we performed angular resolved magnetization curve measurements using vectorial magnetooptic Kerr magnetometry. Comparison of the experimental data with the proposed model shows excellent agreement and reveals the ferromagnetic anisotropy and properties connected to the grain size distribution of the antiferromagnet. Therefore, a distinction between the different influences on coercivity and anisotropy becomes available. △ Less

Submitted 27 May, 2016; v1 submitted 10 May, 2016; originally announced May 2016.

Comments: 9 pages, 9 figures

Journal ref: Phys. Rev. B 94, 184407 (2016)

Comparison of laser induced and intrinsic tunnel magneto-Seebeck effect in CoFeB/MgAl$_2$O$_4$ and CoFeB/MgO magnetic tunnel junctions

Authors: Torsten Huebner, Alexander Boehnke, Ulrike Martens, Andy Thomas, Jan-Michael Schmalhorst, Günter Reiss, Markus Münzenberg, Timo Kuschel

Abstract: We present a comparison of the tunnel magneto-Seebeck effect for laser induced and intrinsic heating. Therefore, Co$_{40}$Fe$_{40}$B$_{20}$/MgAl$_2$O$_4$ and Co$_{25}$Fe$_{55}$B$_{20}$/MgO magnetic tunnel junctions have been prepared. The TMS ratio of 3\,\% in case of the MAO MTJ agrees well with ratios found for other barrier materials, while the TMS ratio of 23\,\% of the MgO MTJ emphasizes the… ▽ More We present a comparison of the tunnel magneto-Seebeck effect for laser induced and intrinsic heating. Therefore, Co$_{40}$Fe$_{40}$B$_{20}$/MgAl$_2$O$_4$ and Co$_{25}$Fe$_{55}$B$_{20}$/MgO magnetic tunnel junctions have been prepared. The TMS ratio of 3\,\% in case of the MAO MTJ agrees well with ratios found for other barrier materials, while the TMS ratio of 23\,\% of the MgO MTJ emphasizes the influence of the CoFe composition. We find results using the intrinsic method that differ in sign and magnitude in comparison to the results of the laser heating. The intrinsic contributions can alternatively be explained by the Brinkman model and the given junction properties. Especially, we are able to demonstrate that the symmetric contribution is solely influenced by the barrier asymmetry. Thus, we conclude that the symmetry analysis used for the intrinsic method is not suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect. △ Less

Submitted 1 July, 2016; v1 submitted 26 April, 2016; originally announced April 2016.

Journal ref: Phys. Rev. B 93, 224433 (2016)

Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited NiFe2O4 and on Fe of Various Thicknesses Investigated by XRMR

Authors: Timo Kuschel, Christoph Klewe, Panagiota Bougiatioti, Olga Kuschel, Joachim Wollschläger, Laurence Bouchenoire, Simon D. Brown, Jan-Michael Schmalhorst, Daniel Meier, Günter Reiss

Abstract: The longitudinal spin Seebeck effect is detected in sputter-deposited NiFe2O4 films using Pt as a spin detector and compared to previously investigated NiFe2O4 films prepared by chemical vapor deposition. Anomalous Nernst effects induced by the magnetic proximity effect in Pt can be excluded for the sputter-deposited NiFe2O4 films down to a certain limit, since x-ray resonant magnetic reflectivity… ▽ More The longitudinal spin Seebeck effect is detected in sputter-deposited NiFe2O4 films using Pt as a spin detector and compared to previously investigated NiFe2O4 films prepared by chemical vapor deposition. Anomalous Nernst effects induced by the magnetic proximity effect in Pt can be excluded for the sputter-deposited NiFe2O4 films down to a certain limit, since x-ray resonant magnetic reflectivity measurements show no magnetic response down to a limit of 0.04 μB per Pt atom comparable to the case of the chemicallydeposited NiFe2O4 films. These differently prepared films have various thicknesses. Therefore, we further studied Pt/Fe reference samples with various Fe thicknesses and could confirm that the magnetic proximity effect is only induced by the interface properties of the magnetic material. △ Less

Submitted 25 February, 2016; originally announced February 2016.

Comments: 4 pages, 4 figures

Journal ref: IEEE Trans. Magn. 52, 4500104 (2016)

From Fe$_3$O$_4$/NiO bilayers to NiFe$_2$O$_4$-like thin films through Ni interdiffusion

Authors: O. Kuschel, R. Buß, W. Spiess, T. Schemme, J. Wöllermann, K. Balinski, T. Kuschel, A. T. N'Diaye, J. Wollschläger, K. Kuepper

Abstract: Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni$^{2+}$ ions out of NiO into Fe$_3$O$_4$ ultrathin films resulting in off-stoichiometric nickelferrite-like thin layers. We synthesized epitaxial Fe$_3$O$_4$/NiO bil… ▽ More Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni$^{2+}$ ions out of NiO into Fe$_3$O$_4$ ultrathin films resulting in off-stoichiometric nickelferrite-like thin layers. We synthesized epitaxial Fe$_3$O$_4$/NiO bilayers on Nb-doped SrTiO$_3$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400\,$^{\circ}$C, 600\,$^{\circ}$C, and 800\,$^{\circ}$C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogenous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFe$_2$O$_4$. Further evidence comes from low energy electron diffraction patterns indicating a spinel type structure at the surface after annealing. Site and element specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni$^{2+}$ and Fe$^{3+}$ ions and the Fe$^{3+}$ in tetrahedral coordination. We find a quite low coercive field of 0.02\,T, indicating a rather low defect concentration within the thin ferrite films. △ Less

Submitted 18 February, 2016; originally announced February 2016.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. B 94, 094423 (2016)

Detection of DC currents and resistance measurements in longitudinal spin Seebeck effect experiments on Pt/YIG and Pt/NFO

Authors: Daniel Meier, Timo Kuschel, Sibylle Meyer, Sebastian T. B. Goennenwein, Liming Shen, Arunava Gupta, Jan-Michael Schmalhorst, Günter Reiss

Abstract: In this work we investigated thin films of the ferrimagnetic insulators YIG and NFO capped with thin Pt layers in terms of the longitudinal spin Seebeck effect (LSSE). The electric response detected in the Pt layer under an out-of-plane temperature gradient can be interpreted as a pure spin current converted into a charge current via the inverse spin Hall effect. Typically, the transverse voltage… ▽ More In this work we investigated thin films of the ferrimagnetic insulators YIG and NFO capped with thin Pt layers in terms of the longitudinal spin Seebeck effect (LSSE). The electric response detected in the Pt layer under an out-of-plane temperature gradient can be interpreted as a pure spin current converted into a charge current via the inverse spin Hall effect. Typically, the transverse voltage is the quantity investigated in LSSE measurements (in the range of μV). Here, we present the directly detected DC current (in the range of nA) as an alternative quantity. Furthermore, we investigate the resistance of the Pt layer in the LSSE configuration. We found an influence of the test current on the resistance. The typical shape of the LSSE curve varies for increasing test currents. △ Less

Submitted 3 January, 2016; originally announced January 2016.

Comments: 4 pages, 2 figures

Journal ref: AIP Advances 6, 056302 (2016)