Showing 1–22 of 22 results for author: Mewes, T

Vertically Graded Fe-Ni Alloys with Low Damping and a Sizeable Spin-Orbit Torque

Authors: Rachel E. Maizel, Shuang Wu, Purnima P. Balakrishnan, Alexander J. Grutter, Christy J. Kinane, Andrew J. Caruana, Prabandha Nakarmi, Bhuwan Nepal, David A. Smith, Youngmin Lim, Julia L. Jones, Wyatt C. Thomas, Jing Zhao, F. Marc Michel, Tim Mewes, Satoru Emori

Abstract: Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. In conventional devices with heavy-metal/ferromagnet bilayers, reducing the ferromagnet thickness to $\sim$1 nm enhances the SOT but dramatically increases damping. Here, we investigate an alternative approach based on a 10 nm thick single-layer ferromagnet to attain both low… ▽ More Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. In conventional devices with heavy-metal/ferromagnet bilayers, reducing the ferromagnet thickness to $\sim$1 nm enhances the SOT but dramatically increases damping. Here, we investigate an alternative approach based on a 10 nm thick single-layer ferromagnet to attain both low damping and a sizable SOT. Instead of relying on a single interface, we continuously break the bulk inversion symmetry with a vertical compositional gradient of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We find low effective damping parameters of $α_\mathrm{eff} < 5\times10^{-3}$ in the FeNi alloy films, despite the steep compositional gradients. Moreover, we reveal a sizable anti-damping SOT efficiency of $|θ_\mathrm{DL}| \approx 0.05$, even without an intentional compositional gradient. Through depth-resolved x-ray diffraction, we identify a lattice strain gradient as crucial symmetry breaking that underpins the SOT. Our findings provide fresh insights into damping and SOTs in single-layer ferromagnets for power-efficient spintronic devices. △ Less

Submitted 19 December, 2024; v1 submitted 14 June, 2024; originally announced June 2024.

Journal ref: Phys. Rev. Appl. 22, 044052 (2024)

Suppression of Spin Pumping at Metal Interfaces

Authors: Youngmin Lim, Bhuwan Nepal, David A. Smith, Shuang Wu, Abhishek Srivastava, Prabandha Nakarmi, Claudia Mewes, Zijian Jiang, Adbhut Gupta, Dwight D. Viehland, Christoph Klewe, Padraic Shafer, In Jun Park, Timothy Mabe, Vivek P. Amin, Jean J. Heremans, Tim Mewes, Satoru Emori

Abstract: An electrically conductive metal typically transmits or absorbs a spin current. Here, we report on evidence that interfacing two metal thin films can suppress spin transmission and absorption. We examine spin pumping in ferromagnet/spacer/ferromagnet heterostructures, in which the spacer -- consisting of metallic Cu and Cr thin films -- separates the ferromagnetic spin-source and spin-sink layers.… ▽ More An electrically conductive metal typically transmits or absorbs a spin current. Here, we report on evidence that interfacing two metal thin films can suppress spin transmission and absorption. We examine spin pumping in ferromagnet/spacer/ferromagnet heterostructures, in which the spacer -- consisting of metallic Cu and Cr thin films -- separates the ferromagnetic spin-source and spin-sink layers. The Cu/Cr spacer largely suppresses spin pumping -- i.e., neither transmitting nor absorbing a significant amount of spin current -- even though Cu or Cr alone transmits a sizable spin current. The antiferromagnetism of Cr is not essential for the suppression of spin pumping, as we observe similar suppression with Cu/V spacers where V is a nonmagnetic analogue of Cr. We speculate that diverse combinations of spin-transparent metals may form interfaces that suppress spin pumping, although the underlying mechanism remains unclear. Our work may stimulate a new perspective on understanding and engineering spin transport in metallic multilayers. △ Less

Submitted 7 July, 2023; v1 submitted 2 May, 2023; originally announced May 2023.

Journal ref: APL Mater. 11, 101121 (2023)

Influence of the presence of multiple resonances on material parameter determination using broadband ferromagnetic resonance spectroscopy

Authors: Prabandha Nakarmi, Tim Mewes

Abstract: The influence of the presence of multiple resonances in ferromagnetic resonance spectra on extracted material parameters is investigated using numerical simulations. Our results show that the systematic error of assuming an incorrect number of resonances for a material can lead to the extraction of material parameters that significantly deviate from any of the true material parameters. When noise… ▽ More The influence of the presence of multiple resonances in ferromagnetic resonance spectra on extracted material parameters is investigated using numerical simulations. Our results show that the systematic error of assuming an incorrect number of resonances for a material can lead to the extraction of material parameters that significantly deviate from any of the true material parameters. When noise is present in experimental ferromagnetic resonance spectra increasing the frequency range of the broadband characterization can significantly reduce the error-margins when the data is analyzed assuming the correct number of resonances present in the material. For the cases investigated in this study it was found that analyzing the data using a single resonance results in extracted gyromagnetic ratios and effective magnetization parameters that are consistent with the weighted average of the true material parameters. We further provide a cautionary example regarding the extraction of the inhomogeneous linewidth broadening and damping parameters of materials that contain an unknown number of resonances. △ Less

Submitted 18 April, 2022; originally announced April 2022.

Comments: 24 pages, 11 figures

Room-Temperature Intrinsic and Extrinsic Damping in Polycrystalline Fe Thin Films

Authors: Shuang Wu, David A. Smith, Prabandha Nakarmi, Anish Rai, Michael Clavel, Mantu K. Hudait, Jing Zhao, F. Marc Michel, Claudia Mewes, Tim Mewes, Satoru Emori

Abstract: We examine room-temperature magnetic relaxation in polycrystalline Fe films. Out-of-plane ferromagnetic resonance (FMR) measurements reveal Gilbert damping parameters of $\approx$ 0.0024 for Fe films with thicknesses of 4-25 nm, regardless of their microstructural properties. The remarkable invariance with film microstructure strongly suggests that intrinsic Gilbert damping in polycrystalline meta… ▽ More We examine room-temperature magnetic relaxation in polycrystalline Fe films. Out-of-plane ferromagnetic resonance (FMR) measurements reveal Gilbert damping parameters of $\approx$ 0.0024 for Fe films with thicknesses of 4-25 nm, regardless of their microstructural properties. The remarkable invariance with film microstructure strongly suggests that intrinsic Gilbert damping in polycrystalline metals at room temperature is a local property of nanoscale crystal grains, with limited impact from grain boundaries and film roughness. By contrast, the in-plane FMR linewidths of the Fe films exhibit distinct nonlinear frequency dependences, indicating the presence of strong extrinsic damping. To fit our in-plane FMR data, we have used a grain-to-grain two-magnon scattering model with two types of correlation functions aimed at describing the spatial distribution of inhomogeneities in the film. However, neither of the two correlation functions is able to reproduce the experimental data quantitatively with physically reasonable parameters. Our findings advance the fundamental understanding of intrinsic Gilbert damping in structurally disordered films, while demonstrating the need for a deeper examination of how microstructural disorder governs extrinsic damping. △ Less

Submitted 23 February, 2022; v1 submitted 8 September, 2021; originally announced September 2021.

Magnetism and Spin Dynamics in Room-Temperature van der Waals Magnet Fe$_5$GeTe$_2$

Authors: Laith Alahmed, Bhuwan Nepal, Juan Macy, Wenkai Zheng, Arjun Sapkota, Nicholas Jones, Alessandro R. Mazza, Matthew Brahlek, Wencan Jin, Masoud Mahjouri-Samani, Steven S. L. Zhang, Claudia Mewes, Luis Balicas, Tim Mewes, Peng Li

Abstract: Two-dimensional (2D) van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe$_5$GeTe$_2$ with a Curie temperature T$_c = 332$ K, and studie… ▽ More Two-dimensional (2D) van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe$_5$GeTe$_2$ with a Curie temperature T$_c = 332$ K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c-axis (H$\parallel$c) and the ab-plane (H$\parallel$ab), with temperatures ranging from 300 K to 10 K. We have found a sizable Landé g-factor difference between the H$\parallel$c and H$\parallel$ab cases. In both cases, the Landé g-factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe$_5$GeTe$_2$ has a damping constant comparable to Permalloy. With reducing temperature, the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe$_5$GeTe$_2$ transitions from ferromagnetic to ferrimagnetic at lower temperatures. Our experiments highlight key information regarding the magnetic state and spin scattering processes in Fe$_5$GeTe$_2$, which promote the understanding of magnetism in Fe$_5$GeTe$_2$, leading to implementations of Fe$_5$GeTe$_2$ based room-temperature spintronic devices. △ Less

Submitted 14 September, 2021; v1 submitted 24 March, 2021; originally announced March 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)

Sub-Nanosecond Spin-Transfer Torque in an Ensemble of Superparamagnetic-Like Nanomagnets

Authors: Satoru Emori, Christoph Klewe, Jan-Michael Schmalhorst, Jan Krieft, Padraic Shafer, Youngmin Lim, David A. Smith, Arjun Sapkota, Abhishek Srivastava, Claudia Mewes, Zijian Jiang, Behrouz Khodadadi, Hesham Elmkharram, Jean J. Heremans, Elke Arenholz, Gunter Reiss, Tim Mewes

Abstract: Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as is the case for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a material with weak, short-range magnetic order -- namely, a macroscopic ensemble of superparamagnetic-like Co nanomagnets. We employ element- and time-resolved X-ra… ▽ More Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as is the case for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a material with weak, short-range magnetic order -- namely, a macroscopic ensemble of superparamagnetic-like Co nanomagnets. We employ element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to directly detect sub-ns dynamics of the Co nanomagnets, excited into precession with cone angle $\geq$0.003$^{\circ}$ by an oscillating spin current. XFMR measurements reveal that as the net moment of the ensemble decreases, the strength of the spin-transfer torque increases relative to those of magnetic field torques. Our findings point to spin-transfer torque as an effective way to manipulate the state of nanomagnet ensembles at sub-ns timescales. △ Less

Submitted 6 May, 2020; originally announced May 2020.

Magnetic domain wall substructures in Pt/Co/Ni/Ir multi-layers

Authors: Maxwell Li, Arjun Sapkota, Anish Rai, Ashok Pokhrel, Tim Mewes, Claudia Mewes, Di Xiao, Marc De Graef, Vincent Sokalski

Abstract: We examine the substructures of magnetic domain walls (DWs) in [Pt/(Co/Ni)$_M$/Ir]$_N$ multi-layers using a combination of micromagnetic theory and Lorentz transmission electron microscopy (LTEM). Thermal stability calculations of Q=$\pm$1 substructures (2-$π$ vertical Bloch lines (VBLs) and DW skyrmions) were performed using a geodesic nudged elastic band (GNEB) model, which supports their metast… ▽ More We examine the substructures of magnetic domain walls (DWs) in [Pt/(Co/Ni)$_M$/Ir]$_N$ multi-layers using a combination of micromagnetic theory and Lorentz transmission electron microscopy (LTEM). Thermal stability calculations of Q=$\pm$1 substructures (2-$π$ vertical Bloch lines (VBLs) and DW skyrmions) were performed using a geodesic nudged elastic band (GNEB) model, which supports their metastability at room temperature. Experimental variation in strength of the interfacial Dzyaloshinskii-Moriya interaction (DMI) and film thickness reveals conditions under which these substructures are present and enables the formation of a magnetic phase diagram. Reduced thickness is found to favor Q=$\pm$1 substructures likely due to the suppression of hybrid DWs. The results from this study provide an important framework for examining 1-D DW substructures in chiral magnetic materials. △ Less

Submitted 17 May, 2021; v1 submitted 16 April, 2020; originally announced April 2020.

Magnetic Damping in Epitaxial Fe Alloyed with Vanadium and Aluminum

Authors: David A. Smith, Anish Rai, Youngmin Lim, Timothy Hartnett, Arjun Sapkota, Abhishek Srivastava, Claudia Mewes, Zijian Jiang, Michael Clavel, Mantu K. Hudait, Dwight D. Viehland, Jean J. Heremans, Prasanna V. Balachandran, Tim Mewes, Satoru Emori

Abstract: To develop low-moment, low-damping metallic ferromagnets for power-efficient spintronic devices, it is crucial to understand how magnetic relaxation is impacted by the addition of nonmagnetic elements. Here, we compare magnetic relaxation in epitaxial Fe films alloyed with light nonmagnetic elements of V and Al. FeV alloys exhibit lower intrinsic damping compared to pure Fe, reduced by nearly a fa… ▽ More To develop low-moment, low-damping metallic ferromagnets for power-efficient spintronic devices, it is crucial to understand how magnetic relaxation is impacted by the addition of nonmagnetic elements. Here, we compare magnetic relaxation in epitaxial Fe films alloyed with light nonmagnetic elements of V and Al. FeV alloys exhibit lower intrinsic damping compared to pure Fe, reduced by nearly a factor of 2, whereas damping in FeAl alloys increases with Al content. Our experimental and computational results indicate that reducing the density of states at the Fermi level, rather than the average atomic number, has a more significant impact in lowering damping in Fe alloyed with light elements. Moreover, FeV is confirmed to exhibit an intrinsic Gilbert damping parameter of $\simeq$0.001, among the lowest ever reported for ferromagnetic metals. △ Less

Submitted 21 July, 2020; v1 submitted 9 April, 2020; originally announced April 2020.

Comments: 28 pages, 9 figures

Journal ref: Phys. Rev. Applied 14, 034042 (2020)

Formation of zero-field Skyrmion arrays in asymmetric superlattices

Authors: Maxwell Li, Anish Rai, Ashok Pokhrel, Arjun Sapkota, Claudia Mewes, Tim Mewes, Marc De Graef, Vincent Sokalski

Abstract: We demonstrate the formation of metastable Néel-type skyrmion arrays in Pt/Co/Ni/Ir multi-layers at zero-field following \textit{ex situ} application of an in-plane magnetic field using Lorentz transmission electron microscopy. The resultant skyrmion texture is found to depend on both the strength and misorientation of the applied field as well as the interfacial Dzyaloshinskii-Moriya interaction.… ▽ More We demonstrate the formation of metastable Néel-type skyrmion arrays in Pt/Co/Ni/Ir multi-layers at zero-field following \textit{ex situ} application of an in-plane magnetic field using Lorentz transmission electron microscopy. The resultant skyrmion texture is found to depend on both the strength and misorientation of the applied field as well as the interfacial Dzyaloshinskii-Moriya interaction. To demonstrate the importance of the applied field angle, we leverage bend contours in the specimens which coincide with transition regions between skyrmion and labyrinth patterns. Subsequent application of a perpendicular magnetic field near these regions reveals the unusual situation where skyrmions with opposite magnetic polarities are stabilized in close proximity. △ Less

Submitted 11 September, 2020; v1 submitted 8 November, 2019; originally announced November 2019.

Journal ref: Appl. Phys. Lett. 117, 112403 (2020)

Conductivity-Like Gilbert Damping due to Intraband Scattering in Epitaxial Iron

Authors: Behrouz Khodadadi, Anish Rai, Arjun Sapkota, Abhishek Srivastava, Bhuwan Nepal, Youngmin Lim, David A. Smith, Claudia Mewes, Sujan Budhathoki, Adam J. Hauser, Min Gao, Jie-Fang Li, Dwight D. Viehland, Zijian Jiang, Jean J. Heremans, Prasanna V. Balachandran, Tim Mewes, Satoru Emori

Abstract: Confirming the origin of Gilbert damping by experiment has remained a challenge for many decades, even for simple ferromagnetic metals. In this Letter, we experimentally identify Gilbert damping that increases with decreasing electronic scattering in epitaxial thin films of pure Fe. This observation of conductivity-like damping, which cannot be accounted for by classical eddy current loss, is in e… ▽ More Confirming the origin of Gilbert damping by experiment has remained a challenge for many decades, even for simple ferromagnetic metals. In this Letter, we experimentally identify Gilbert damping that increases with decreasing electronic scattering in epitaxial thin films of pure Fe. This observation of conductivity-like damping, which cannot be accounted for by classical eddy current loss, is in excellent quantitative agreement with theoretical predictions of Gilbert damping due to intraband scattering. Our results resolve the longstanding question about a fundamental damping mechanism and offer hints for engineering low-loss magnetic metals for cryogenic spintronics and quantum devices. △ Less

Submitted 22 January, 2020; v1 submitted 25 June, 2019; originally announced June 2019.

Journal ref: Phys. Rev. Lett. 124, 157201 (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)

Magnetic domain wall Skyrmions

Authors: Ran Cheng, Maxwell Li, Arjun Sapkota, Anish Rai, Ashok Pokhrel, Tim Mewes, Claudia Mewes, Di Xiao, Marc De Graef, Vincent Sokalski

Abstract: It is well established that the spin-orbit interaction in heavy metal/ferromagnet heterostructures leads to a significant interfacial Dzyaloshinskii-Moriya Interaction (DMI) that modifies the internal structure of magnetic domain walls (DWs) to favor Néel over Bloch type configurations. However, the impact of such a transition on the structure and stability of internal DW defects (e.g., vertical B… ▽ More It is well established that the spin-orbit interaction in heavy metal/ferromagnet heterostructures leads to a significant interfacial Dzyaloshinskii-Moriya Interaction (DMI) that modifies the internal structure of magnetic domain walls (DWs) to favor Néel over Bloch type configurations. However, the impact of such a transition on the structure and stability of internal DW defects (e.g., vertical Bloch lines) has not yet been explored. We present a combination of analytical and micromagnetic calculations to describe a new type of topological excitation called a DW Skyrmion characterized by a $360^\circ$ rotation of the internal magnetization in a Dzyaloshinskii DW. We further propose a method to identify DW Skyrmions experimentally using Fresnel mode Lorentz TEM; simulated images of DW Skyrmions using this technique are presented based on the micromagnetic results. △ Less

Submitted 29 April, 2019; v1 submitted 7 September, 2018; originally announced September 2018.

Journal ref: Phys. Rev. B 99, 184412 (2019)

Low Gilbert damping in Co2FeSi and Fe2CoSi films

Authors: Christian Sterwerf, Soumalya Paul, Behrouz Khodadadi, Markus Meinert, Jan-Michael Schmalhorst, Mathias Buchmeier, Claudia K. A. Mewes, Tim Mewes, Günter Reiss

Abstract: Thin highly textured Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si ($0 \leq$ x $\leq 1$) films were prepared on MgO (001) substrates by magnetron co-sputtering. The magneto-optic Kerr effect (MOKE) and ferromagnetic resonance (FMR) measurements were used to investigate the composition dependence of the magnetization, the magnetic anisotropy, the gyromagnetic ratio and the relaxation of the films. The e… ▽ More Thin highly textured Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si ($0 \leq$ x $\leq 1$) films were prepared on MgO (001) substrates by magnetron co-sputtering. The magneto-optic Kerr effect (MOKE) and ferromagnetic resonance (FMR) measurements were used to investigate the composition dependence of the magnetization, the magnetic anisotropy, the gyromagnetic ratio and the relaxation of the films. The effective magnetization for the thin Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si films, determined by FMR measurements, are consistent with the Slater Pauling prediction. Both MOKE and FMR measurements reveal a pronounced fourfold anisotropy distribution for all films. In addition we found a strong influence of the stoichiometry on the anisotropy as the cubic anisotropy strongly increases with increasing Fe concentration. The gyromagnetic ratio is only weakly dependent on the composition. We find low Gilbert damping parameters for all films with values down to $0.0012\pm0.00012$ for Fe$_{1.75}$Co$_{1.25}$Si. The effective damping parameter for Co$_2$FeSi is found to be $0.0018\pm 0.0004$. We also find a pronounced anisotropic relaxation, which indicates significant contributions of two-magnon scattering processes that is strongest along the easy axes of the films. This makes thin Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si films ideal materials for the application in STT-MRAM devices. △ Less

Submitted 22 May, 2016; originally announced May 2016.

Measurements of the exchange stiffness of YIG films by microwave resonance techniques

Authors: Stefan Klingler, Andrii V. Chumak, Tim Mewes, Behrouz Khodadadi, Claudia Mewes, Carsten Dubs, Oleksii Surzhenko, Burkard Hillebrands, Andrés Conca

Abstract: Measurements of the exchange stiffness $D$ and the exchange constant $A$ of Yttrium Iron Garnet (YIG) films are presented. The YIG films with thicknesses from 0.9 $μ$m to 2.6 $μ$m were investigated with a microwave setup in a wide frequency range from 5 to 40 GHz. The measurements were performed when the external static magnetic field was applied in-plane and out-of-plane. The method of Schreiber… ▽ More Measurements of the exchange stiffness $D$ and the exchange constant $A$ of Yttrium Iron Garnet (YIG) films are presented. The YIG films with thicknesses from 0.9 $μ$m to 2.6 $μ$m were investigated with a microwave setup in a wide frequency range from 5 to 40 GHz. The measurements were performed when the external static magnetic field was applied in-plane and out-of-plane. The method of Schreiber and Frait, based on the analysis of the perpendicular standing spin wave (PSSW) mode frequency dependence on the applied out-of-plane magnetic field, was used to obtain the exchange stiffness $D$. This method was modified to avoid the influence of internal magnetic fields during the determination of the exchange stiffness. Furthermore, the method was adapted for in-plane measurements as well. The results obtained using all methods are compared and values of $D$ between $(5.18\pm0.01) \cdot 10^{-17}$T$\cdot$m$^2$ and $(5.34\pm0.02) \cdot 10^{-17}$ T$\cdot$m$^2$ were obtained for different thicknesses. From this the exchange constant was calculated to be $A=(3.65 \pm 0.38)~$pJ/m. △ Less

Submitted 25 August, 2014; originally announced August 2014.

Comments: 10 pages, 3 figures, 3 tables

Journal ref: J. Phys. D: Appl. Phys. 48 (2015) 015001 (5pp)

Structural and magnetic properties of Cr-diluted CoFeB

Authors: Yishen Cui, Manli Ding, S. Joseph Poon, T. Paul Adl, S. Keshavarz, Tim Mewes, Stuart A. Wolf, Jiwei Lu

Abstract: The crystallization process and the magnetization of Cr diluted CoFeB was investigated in both ribbon samples and thin film samples with Cr content up to 30 at. %. A primary crystallization of bcc phase from an amorphous precursor in ribbon samples was observed when the annealing temperature rose to between 421 oC and 456 oC, followed by boron segregation at temperatures between 518 oC and 573 oC.… ▽ More The crystallization process and the magnetization of Cr diluted CoFeB was investigated in both ribbon samples and thin film samples with Cr content up to 30 at. %. A primary crystallization of bcc phase from an amorphous precursor in ribbon samples was observed when the annealing temperature rose to between 421 oC and 456 oC, followed by boron segregation at temperatures between 518 oC and 573 oC. The two onset crystallization temperatures showed strong dependences on both Cr and B concentrations. The impact of Cr concentration on the magnetic properties including a reduced saturation magnetization and an enhanced coercive field was also observed. The magnetizations of both ribbon samples and thin film samples were well fitted using the generalized Slater-Pauling curve with modified moments for B (-0.94 μB) and Cr (-3.6 μB). Possible origins of the enhanced coercive field were also discussed. We also achieved a damping parameter in CoFeCrB thin films at the same level as Co40Fe40B20, much lower than the value reported for CoFeCrB films previously. The results suggest a possible advantage of CoFeCrB in reducing the critical switching current density in Spin Transfer Torque Random Access Memory (STT-RAM). △ Less

Submitted 27 September, 2013; originally announced September 2013.

Comments: 10 pages, 5 figures, 2 tables. To be published in Journal of Applied Physics

Switching Distributions for Perpendicular Spin-Torque Devices within the Macrospin Approximation

Authors: W. H. Butler, T. Mewes, C. K. A. Mewes, P. B. Visscher, W. H. Rippard, S. E. Russek, Ranko Heindl

Abstract: We model "soft" error rates for writing (WSER) and for reading (RSER) for perpendicular spin-torque memory devices by solving the Fokker-Planck equation for the probability distribution of the angle that the free layer magnetization makes with the normal to the plane of the film. We obtain: (1) an exact, closed form, analytical expression for the zero-temperature switching time as a function of in… ▽ More We model "soft" error rates for writing (WSER) and for reading (RSER) for perpendicular spin-torque memory devices by solving the Fokker-Planck equation for the probability distribution of the angle that the free layer magnetization makes with the normal to the plane of the film. We obtain: (1) an exact, closed form, analytical expression for the zero-temperature switching time as a function of initial angle; (2) an approximate analytical expression for the exponential decay of the WSER as a function of the time the current is applied; (3) comparison of the approximate analytical expression for the WSER to numerical solutions of the Fokker-Planck equation; (4) an approximate analytical expression for the linear increase in RSER with current applied for reading; (5) comparison of the approximate analytical formula for the RSER to the numerical solution of the Fokker-Planck equation; and (6) confirmation of the accuracy of the Fokker-Planck solutions by comparison with results of direct simulation using the single-macrospin Landau-Lifshitz-Gilbert (LLG) equations with a random fluctuating field in the short-time regime for which the latter is practical. △ Less

Submitted 12 February, 2012; originally announced February 2012.

Ferromagnetic resonance force microscopy on a thin permalloy film

Authors: E. Nazaretski, D. V. Pelekhov, I. Martin, M. Zalalutdinov, J. W. Baldwin, T. Mewes, B. Houston, P. C. Hammel, R. Movshovich

Abstract: Ferromagnetic Resonance Force Microscopy (FMRFM) offers a means of performing local ferromagnetic resonance. We have studied the evolution of the FMRFM force spectra in a continuous 50 nm thick permalloy film as a function of probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-film interaction force, accounting for the presence of the localized strongly no… ▽ More Ferromagnetic Resonance Force Microscopy (FMRFM) offers a means of performing local ferromagnetic resonance. We have studied the evolution of the FMRFM force spectra in a continuous 50 nm thick permalloy film as a function of probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-film interaction force, accounting for the presence of the localized strongly nonuniform magnetic field of the FMRFM probe magnet. Excellent agreement between the experimental data and the simulation results provides insight into the mechanism of FMR mode excitation in an FMRFM experiment. △ Less

Submitted 18 April, 2007; originally announced April 2007.

Comments: 9 pages, 2 figures

Journal ref: Appl. Phys. Lett. 90, 234105 (2007)

Origin of four-fold anisotropy in square lattices of circular ferromagnetic dots

Authors: G. N. Kakazei, Yu. G. Pogorelov, M. D. Costa, T. Mewes, P. E. Wigen, P. C. Hammel, V. O. Golub, T. Okuno, V. Novosad

Abstract: We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR) field $H_r$, found in a square lattice of circular Permalloy dots when the interdot distance $a$ gets comparable to the dot diameter $d$. The minimum $H_r$, along the lattice $<11>$ axes, and the maximum, along the $<10>$ axes, differ by $\sim$ 50 Oe at $a/d$ = 1.1. This anisotropy, not expected in uniformly magnetized… ▽ More We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR) field $H_r$, found in a square lattice of circular Permalloy dots when the interdot distance $a$ gets comparable to the dot diameter $d$. The minimum $H_r$, along the lattice $<11>$ axes, and the maximum, along the $<10>$ axes, differ by $\sim$ 50 Oe at $a/d$ = 1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a non-uniform magnetization $\bm(\br)$ in a dot in response to dipolar forces in the patterned magnetic structure. It is well described by an iterative solution of a continuous variational procedure. △ Less

Submitted 13 June, 2006; v1 submitted 19 February, 2006; originally announced February 2006.

Comments: 4 pages, 3 figures, revtex, details of analytic calculation and new references are added

Separation of the first- and second-order contributions in magneto-optic Kerr effect magnetometry of epitaxial FeMn/NiFe bilayers

Authors: T. Mewes, H. Nembach, M. Rickart, B. Hillebrands

Abstract: The influence of second-order magneto-optic effects on Kerr effect magnetometry of epitaxial exchange coupled FeMn/NiFe-bilayers is investigated. A procedure for separation of the first- and second-order contributions is presented. The full angular dependence of both contributions during the magnetization reversal is extracted from the experimental data and presented using gray scaled magnetizat… ▽ More The influence of second-order magneto-optic effects on Kerr effect magnetometry of epitaxial exchange coupled FeMn/NiFe-bilayers is investigated. A procedure for separation of the first- and second-order contributions is presented. The full angular dependence of both contributions during the magnetization reversal is extracted from the experimental data and presented using gray scaled magnetization reversal diagrams. The theoretical description of the investigated system is based on an extended Stoner-Wohlfarth model, which includes an induced unidirectional and fourfold anisotropy in the ferromagnet, caused by the coupling to the antiferromagnet. The agreement between the experimental data and the theoretical model for both the first- and second-order contributions are good, although a coherent reversal of the magnetization is assumed in the model. △ Less

Submitted 12 November, 2003; originally announced November 2003.

Comments: 6 pages, 7 figures, submitted to J. Appl. Phys

Induced four fold anisotropy and bias in compensated NiFe/FeMn double layers

Authors: T. Mewes, B. Hillebrands, R. L. Stamps

Abstract: A vector spin model is used to show how frustrations within a multisublattice antiferromagnet such as FeMn can lead to four-fold magnetic anisotropies acting on an exchange coupled ferromagnetic film. Possibilities for the existence of exchange bias are examined and shown to exist for the case of weak chemical disorder at the interface in an otherwise perfect structure. A sensitive dependence on… ▽ More A vector spin model is used to show how frustrations within a multisublattice antiferromagnet such as FeMn can lead to four-fold magnetic anisotropies acting on an exchange coupled ferromagnetic film. Possibilities for the existence of exchange bias are examined and shown to exist for the case of weak chemical disorder at the interface in an otherwise perfect structure. A sensitive dependence on interlayer exchange is found for anisotropies acting on the ferromagnet through the exchange coupling, and we show that a wide range of anisotropies can appear even for a perfect crystalline structure with an ideally flat interface. △ Less

Submitted 16 July, 2003; originally announced July 2003.

Comments: 7 pages, 7 figures

Probing the interface magnetism in the FeMn/NiFe exchange bias system using magnetic second harmonic generation

Authors: L. Sampaio, A. Mougin, J. Ferre, P. Georges, A. Brun, H. Bernas, S. Poppe, T. Mewes, J. Fassbender, B. Hillebrands

Abstract: Second harmonic generation magneto-optic Kerr effect (SHMOKE) experiments, sensitive to buried interfaces, were performed on a polycrystalline NiFe/FeMn bilayer in which areas with different exchange bias fields were prepared using 5 KeV He ion irradiation. Both reversible and irreversible uncompensated spins are found in the antiferromagnetic layer close to the interface with the ferromagnetic… ▽ More Second harmonic generation magneto-optic Kerr effect (SHMOKE) experiments, sensitive to buried interfaces, were performed on a polycrystalline NiFe/FeMn bilayer in which areas with different exchange bias fields were prepared using 5 KeV He ion irradiation. Both reversible and irreversible uncompensated spins are found in the antiferromagnetic layer close to the interface with the ferromagnetic layer. The SHMOKE hysteresis loop shows the same exchange bias field as obtained from standard magnetometry. We demonstrate that the exchange bias effect is controlled by pinned uncompensated spins in the antiferromagnetic layer. △ Less

Submitted 26 November, 2002; originally announced November 2002.

Comments: submitted to Phys. Rev. Lett

Journal ref: Europhys. Lett., 63 (6), pp. 819-825 (2003)