Showing 1–27 of 27 results for author: Doerr, K

Magneto-optical evidence for single-crystal-like magnetic switching of epitaxial antiferromagnetic LaFeO3 films

Authors: A. Rieche, W. Hoppe, C. Körner, A. D. Rata, F. Weber, J. B. G. Danziger, E. M. Vocks, F. Wührl, M. Bargheer, W. Widdra, G. Woltersdorf, S. Ebbinghaus, A. Herklotz, K. Dörr

Abstract: Strained epitaxial films of the antiferromagnetic orthoferrite LaFeO3 offer a promising platform for antiferromagnetic spintronics, yet their magnetic switching behavior and domain structure have remained largely unexplored due to the small magnitude of the weak ferromagnetic moment. Here, we demonstrate that longitudinal magneto-optical Kerr effect (MOKE) measurements provide a sensitive and dire… ▽ More Strained epitaxial films of the antiferromagnetic orthoferrite LaFeO3 offer a promising platform for antiferromagnetic spintronics, yet their magnetic switching behavior and domain structure have remained largely unexplored due to the small magnitude of the weak ferromagnetic moment. Here, we demonstrate that longitudinal magneto-optical Kerr effect (MOKE) measurements provide a sensitive and direct probe of magnetic switching and domain processes in coherently strained LaFeO3 thin films grown on orthorhombic substrates. By employing DyScO3(110), GdScO3(110), and NdGaO3(110) substrates, we achieve straincontrolled, largely twin-free growth and identify the orientation of the orthorhombic c-axis through the presence or absence of a longitudinal MOKE signal. Compressively strained films exhibit large Kerr signals, rectangular hysteresis loops, and magnetic single-domain remanence over macroscopic areas. Tensile strain on orthorhombic substrates is associated with two competing structural effects on thin film orientation; in-plane magnetization has been identified in some films on GdScO3(110) by MOKE. Angle-dependent MOKE hysteresis follows the Kondorsky model, indicating domain-wall-controlled switching analogous to bulk single crystals. Kerr microscopy reveals abrupt domain nucleation and rapid domain-wall motion, with defects acting as pinning centers and governing the coercive field. Our results establish MOKE as an efficient optical tool for identifying orthorhombic orientation, probing magnetic switching of coupled weak magnetization and Neel vectors, and accessing domain dynamics in LaFeO3 films. This provides a foundation for strain-engineered orthoferrite thin films in antiferromagnetic spintronics and magnonics. △ Less

Submitted 2 June, 2026; originally announced June 2026.

Conductive domain walls in ferroelectrics as tunable coherent THz radiation source

Authors: Ramaz Khomeriki, Kathrin Dörr, Jamal Berakdar

Abstract: THz emission associated with currents in conductive domains in BiFeO$_3$ following infrared radiation is theoretically investigated. This experimentally observed phenomenon is explained by the domain wall stripes acting as metallic resonators with the oscillating charge accumulation being at the domain wall edges. The charge oscillation frequency is related to the plasma frequency inside the domai… ▽ More THz emission associated with currents in conductive domains in BiFeO$_3$ following infrared radiation is theoretically investigated. This experimentally observed phenomenon is explained by the domain wall stripes acting as metallic resonators with the oscillating charge accumulation being at the domain wall edges. The charge oscillation frequency is related to the plasma frequency inside the domain wall. The value of plasma frequency determines both the frequency and the amplitude of the emission emanating from the BiFeO$_3$ lattice. We show that for certain geometries of the domain wall structure and for specific polarization of the incident pulse the THz emission embodies a non-vanishing chirality. △ Less

Submitted 1 September, 2025; originally announced September 2025.

Efficient broadband terahertz generation by above band-gap excitation of the pyroelectric ZnSnN2

Authors: T. S. Seifert, H. Hempel, O. Gückstock, R. Schneider, Q. Remy, A. Fioretti, T. Unold, S. Michaelis de Vasconcellos, R. Bratschitsch, R. Eichberger, K. Dörr, A. Zakutayev, T. Kampfrath

Abstract: Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN2, and find that above-band-gap excitation results in the efficient formation of an ultrafast photoc… ▽ More Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN2, and find that above-band-gap excitation results in the efficient formation of an ultrafast photocurrent generating THz radiation. The resulting THz electric field spans a frequency range from below 1 to above 30 THz. Our results suggest that the photocurrent is primarily driven by an ultrafast pyroelectric effect where the photo-excited carriers screen the spontaneous electric polarization of ZnSnN2. Strong structural disorder reduces the photocarrier lifetime significantly and, thus, enables broadband operation. ZnSnN2 shows similar THz-emitter performance as the best spintronic THz emitters regarding bandwidth and amplitude. Our study unveils the large potential of pyroelectric materials as efficient and broadband THz emitters with built-in bias fields. △ Less

Submitted 11 June, 2025; originally announced June 2025.

Ultrafast electron dynamics upon above band-gap excitation in epitaxial LaFeO$_3$(001) thin films

Authors: Friederike Elisa Wührl, Antonia Rieche, Anne Oelschläger, Kathrin Dörr, Wolf Widdra

Abstract: Strong electron correlations in perovskite oxides give rise to rich and often unexpected electronic phenomena. In this study, we present a comprehensive surface-science investigation of epitaxial thin films of the charge-transfer insulator LaFeO$_3$(001). The characterization includes low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and photoemiss… ▽ More Strong electron correlations in perovskite oxides give rise to rich and often unexpected electronic phenomena. In this study, we present a comprehensive surface-science investigation of epitaxial thin films of the charge-transfer insulator LaFeO$_3$(001). The characterization includes low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and photoemission spectroscopy. We map both the occupied and unoccupied electronic states using two-photon photoemission (2PPE) spectroscopy. Furthermore, we probe electron dynamics through an ultraviolet-ultraviolet (UV-UV) pump-probe experiment, exciting electrons from hybridized O~$2p$/Fe $3d$ states to Fe minority-spin states above the band gap. Our results reveal three distinct unoccupied states, which we assign to Fe $t_{2g\downarrow}$, Fe $e_{g\downarrow}$, and La $5d$ orbitals. Notably, the conduction band minimum exhibits a biexponential decay with time constants of 39\,fs and 1100\,fs, suggesting the presence of two independent decay pathways. △ Less

Submitted 30 May, 2025; originally announced May 2025.

Polarization rotation in a ferroelectric BaTiO$_3$ film through low-energy He-implantation

Authors: Andreas Herklotz, Robert Roth, Zhi Xiang Chong, Liang Luo, Joong Mok Park, Matthew Brahlek, Jigang Wang, Kathrin Dörr, Thomas Zac Ward

Abstract: Domain engineering in ferroelectric thin films is crucial for next-generation microelectronic and photonic technologies. Here, a method is demonstrated to precisely control domain configurations in BaTiO$_3$ thin films through low-energy He ion implantation. The approach transforms a mixed ferroelectric domain state with significant in-plane polarization into a uniform out-of-plane tetragonal phas… ▽ More Domain engineering in ferroelectric thin films is crucial for next-generation microelectronic and photonic technologies. Here, a method is demonstrated to precisely control domain configurations in BaTiO$_3$ thin films through low-energy He ion implantation. The approach transforms a mixed ferroelectric domain state with significant in-plane polarization into a uniform out-of-plane tetragonal phase by selectively modifying the strain state in the film's top region. This structural transition significantly improves domain homogeneity and reduces polarization imprint, leading to symmetric ferroelectric switching characteristics. The demonstrated ability to manipulate ferroelectric domains post-growth enables tailored functional properties without compromising the coherently strained bottom interface. The method's compatibility with semiconductor processing and ability to selectively modify specific regions make it particularly promising for practical implementation in integrated devices. This work establishes a versatile approach for strain-mediated domain engineering that could be extended to a wide range of ferroelectric systems, providing new opportunities for memory, sensing, and photonic applications where precise control of polarization states is essential. △ Less

Submitted 13 December, 2024; originally announced December 2024.

Growth-sequence-dependent interface magnetism of SrIrO$_3$ - La$_{0.7}$Sr$_{0.3}$MnO$_3$ bilayers

Authors: L. Bergmann, P. Düring, S. Agrestini, A. Efimenko, S. -C. Liao, Z. Hu, P. Gargiani, C. -J. Choi, H. Baik, D. -S. Park, K. Dörr, A. D. Rata

Abstract: Bilayers of the oxide 3d ferromagnet La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and the 5d paramagnet SrIrO$_{3}$ (SIO) with large spin-orbit coupling (SOC) have been investigated regarding the impact of interfacial SOC on magnetic order. For the growth sequence of LSMO on SIO, ferromagnetism is strongly altered and large out-of-plane-canted anisotropy associated with lacking magnetic saturation up to 4… ▽ More Bilayers of the oxide 3d ferromagnet La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and the 5d paramagnet SrIrO$_{3}$ (SIO) with large spin-orbit coupling (SOC) have been investigated regarding the impact of interfacial SOC on magnetic order. For the growth sequence of LSMO on SIO, ferromagnetism is strongly altered and large out-of-plane-canted anisotropy associated with lacking magnetic saturation up to 4 T has been observed. Thin bilayer films have been grown coherently in both growth sequences on SrTiO$_3$ (001) by pulsed laser deposition and structurally characterized by scanning transmission electron microscopy (STEM) and x-ray diffraction (XRD). Measurements of magnetization and field-dependent Mn L$_{2,3}$ edge x-ray magnetic circular dichroism (XMCD) reveal changes of LSMO magnetic order which are strong in LSMO on SIO and weak in LSMO underneath of SIO. We attribute the impact of the growth sequence to the interfacial lattice structure/symmetry which is known to influence the interfacial magnetic coupling. △ Less

Submitted 7 January, 2020; originally announced January 2020.

Reversible electric-field-driven magnetization in a columnar nanocomposite film

Authors: Mohsin Rafique, Andreas Herklotz, Kathrin Doerr, Sadia Manzoor

Abstract: Magnetic hysteresis loops show a moderate perpendicular anisotropy of the magnetostrictive CFO pillars, which is related to their vertical compression. The application of an electric field to the electromechanical PMN-PT substrate produced significant and reversible changes in the magnetization due to an additional strain-induced magnetic anisotropy. This demonstrates completely reversible, room-t… ▽ More Magnetic hysteresis loops show a moderate perpendicular anisotropy of the magnetostrictive CFO pillars, which is related to their vertical compression. The application of an electric field to the electromechanical PMN-PT substrate produced significant and reversible changes in the magnetization due to an additional strain-induced magnetic anisotropy. This demonstrates completely reversible, room-temperature electric-field-assisted control of magnetization in self-assembled vertical nanocomposites of CFO and BFO. △ Less

Submitted 13 April, 2019; originally announced April 2019.

Journal ref: https://doi.org/10.1016/j.tsf.2019.06.002

Termination control of magnetic coupling at a complex oxide interface

Authors: S. Das, A. D. Rata, I. V. Maznichenko, S. Agrestini, E. Pippel, K. Chen, S. M. Valvidares, H. Babu Vasili, J. Herrero-Martin, E. Pellegrin, K. Nenkov, A. Herklotz, A. Ernst, I. Mertig, Z. Hu, K. Doerr

Abstract: Atomically flat interfaces between ternary oxides have chemically different variants, depending on the terminating lattice planes of both oxides. Electronic properties change with the interface termination which affects, for instance, charge accumulation and magnetic interactions at the interface. Well-defined terminations have yet rarely been achieved for oxides of ABO3 type (with metals A, B). H… ▽ More Atomically flat interfaces between ternary oxides have chemically different variants, depending on the terminating lattice planes of both oxides. Electronic properties change with the interface termination which affects, for instance, charge accumulation and magnetic interactions at the interface. Well-defined terminations have yet rarely been achieved for oxides of ABO3 type (with metals A, B). Here, we report on a strategy of thin film growth and interface characterization applied to fabricate the La0.7Sr0.3MnO3-SrRuO3 interface with controlled termination. Ultra-strong antiferromagnetic coupling between the ferromagnets occurs at the MnO2-SrO interface, but not for the other termination, in agreement with density functional calculations. X-ray magnetic circular dichroism measurements reveal coupled reversal of Mn and Ru magnetic moments at the MnO2-SrO interface. Our results demonstrate termination control of magnetic coupling across a complex oxide interface and provide a pathway for theoretical and experimental explorations of novel electronic interface states with engineered magnetic properties. △ Less

Submitted 28 June, 2016; originally announced June 2016.

Strain dependence of antiferromagnetic interface coupling in La0.7Sr0.3MnO3/SrRuO3 superlattices

Authors: Sujit Das, Andreas Herklotz, Eckhard Pippel, Er-Jia Guo, Diana Rata, Kathrin Dörr

Abstract: We have investigated the magnetic response of La0.7Sr0.3MnO3/SrRuO3 superlattices to biaxial in-plane strain applied in-situ. Superlattices grown on piezoelectric substrates of 0.72PbMg1/3Nb2/3O3-0.28PbTiO3(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu0HAF = 2.8 T is found to decrease by deltaHAF/delta epsilon ~ -520 mT %-1 under… ▽ More We have investigated the magnetic response of La0.7Sr0.3MnO3/SrRuO3 superlattices to biaxial in-plane strain applied in-situ. Superlattices grown on piezoelectric substrates of 0.72PbMg1/3Nb2/3O3-0.28PbTiO3(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu0HAF = 2.8 T is found to decrease by deltaHAF/delta epsilon ~ -520 mT %-1 under reversible biaxial strain mu0HAF at 80 K in a [La0.7Sr0.3MnO3(22)/SrRuO3(55)]15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers which are under as-grown tensile strain. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface, since the enhanced magnetic order of Mn spins leads to a larger net coupling of SrRuO3 layers at the interface. Strain-dependent orbital occupation in a single-ion picture cannot explain the sign of the observed strain dependence, whereas the enhanced Mn order at the interface is qualitatively in line with it. △ Less

Submitted 27 February, 2015; v1 submitted 3 November, 2014; originally announced November 2014.

Comments: 7 pages, 6 figures, submitted to Physical Review B

Mn$^{2+}$/Mn$^{3+}$ state of La$_{0.7}$Ce$_{0.3}$MnO$_3$ by oxygen reduction and photodoping

Authors: Andreas Thiessen, Elke Beyreuther, Stefan Grafström, Kathrin Dörr, Robert Werner, Reinhold Kleiner, Dieter Koelle, Lukas M. Eng

Abstract: Films of cerium-doped LaMnO$_3$, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La$_{0.7}$Ce$_{0.3}$MnO$_3$ (LCeMO) films in the da… ▽ More Films of cerium-doped LaMnO$_3$, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La$_{0.7}$Ce$_{0.3}$MnO$_3$ (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2$+$/3$+$ state, independently of the film thickness and the degree of CeO$_2$ segregation. Charge injection from the photoconductive SrTiO$_3$ substrate into the Mn e$_g$ band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron-hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping. △ Less

Submitted 6 December, 2013; v1 submitted 18 October, 2013; originally announced October 2013.

Comments: 6 pages, 2 figures

Journal ref: J. Phys.: Condens. Matter 26 (2014) 045502

Investigation of the In-Gap Electronic Structure of LaAlO3 - SrTiO3 Heterointerfaces by Soft X-ray Spectroscopy

Authors: A. Koitzsch, J. Ocker, M. Knupfer, M. C. Dekker, K. Dörr, B. Büchner, P. Hoffmann

Abstract: We investigated \LAO\ - \STO\ heterointerfaces grown either in oxygen rich or poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the Ti L$_{2,3}$ absorption edge of the valence band and Ti 2p core level spectroscopy directly monitor the impact of oxygen treatment upon the electronic structure. Two types of Ti$^{3+}$ related charge carriers are identified. One is located at t… ▽ More We investigated \LAO\ - \STO\ heterointerfaces grown either in oxygen rich or poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the Ti L$_{2,3}$ absorption edge of the valence band and Ti 2p core level spectroscopy directly monitor the impact of oxygen treatment upon the electronic structure. Two types of Ti$^{3+}$ related charge carriers are identified. One is located at the Fermi energy and related to the filling of the \STO\ conduction band. It appears for low oxygen pressure only. The other one is centered at E$_{B}$ $\approx$ 1 eV and independent of the oxygen pressure during growth. It is probably due to defects. The magnitude of both excitations is comparable. It is shown that low oxygen pressure is detrimental for the Ti - O bonding. Our results shed light on the nature of the charge carriers in the vicinity of the \LAO\ - \STO\ interface. △ Less

Submitted 20 December, 2011; v1 submitted 29 March, 2011; originally announced March 2011.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 84, 245121 (2011)

Effects of variable anisotropic-strain on the emission of neutral excitons confined in epitaxial quantum dots

Authors: J. D. Plumhof, V. Krapek, F. Ding, K. D. Jöns, R. Hafenbrak, P. Klenovsky, A. Herklotz, K. Dörr, P. Michler, A. Rastelli, O. G. Schmidt

Abstract: We study the effect of elastic anisotropic biaxial strain on the light emitted by neutral excitons confined in different kinds of semiconductor quantum dots (QDs). We find that the light polarization rotates by up to 80 degree and the excitonic fine structure splitting varies by several tens of $μ$eVs as the strain is varied. By means of a continuum model we mainly ascribe the observed effects to… ▽ More We study the effect of elastic anisotropic biaxial strain on the light emitted by neutral excitons confined in different kinds of semiconductor quantum dots (QDs). We find that the light polarization rotates by up to 80 degree and the excitonic fine structure splitting varies by several tens of $μ$eVs as the strain is varied. By means of a continuum model we mainly ascribe the observed effects to substantial changes of the hole wave function. These results show that strain-fields of a few permill magnitude are suffcient to dramatically modify the electronic structure of QDs. △ Less

Submitted 12 November, 2010; originally announced November 2010.

Comments: Submitted to Phys. Rev. Lett. 4 Pages, 3 figures, Suppl. material not submitted

Two Component Heat Diffusion Observed in CMR Manganites

Authors: J. Bielecki, R. Rauer, E. Zanghellini, R. Gunnarsson, K. Dörr, L. Börjesson

Abstract: We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe… ▽ More We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the sub-picosecond dynamics of CMR manganites in terms of a phenomenological three temperature model describing the energy transfer between the electron, lattice and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long term relaxation due to film-to-substrate heat diffusion and then develop a modified three temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the bandgap in LMO. In addition we also model the non-thermal sub-picosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO. △ Less

Submitted 26 February, 2010; v1 submitted 25 February, 2010; originally announced February 2010.

Comments: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434 v2: Abstract corrected

Journal ref: Physical Review B 81, 064434 (2010)

Lattice structure and magnetization of LaCoO3 thin films

Authors: A. D. Rata, A. Herklotz, L. Schultz, K. Doerr

Abstract: We investigate the structure and magnetic properties of thin films of the LaCoO$_{3}$ compound. Thin films are deposited by pulsed laser deposition on various substrates in order to tune the strain from compressive to tensile. Single-phase (001) oriented LaCoO$_{3}$ layers were grown on all substrates despite large misfits. The tetragonal distortion of the films covers a wide range from -2% to 2… ▽ More We investigate the structure and magnetic properties of thin films of the LaCoO$_{3}$ compound. Thin films are deposited by pulsed laser deposition on various substrates in order to tune the strain from compressive to tensile. Single-phase (001) oriented LaCoO$_{3}$ layers were grown on all substrates despite large misfits. The tetragonal distortion of the films covers a wide range from -2% to 2.8%. Our LaCoO$_{3}$ films are ferromagnetic with Curie temperature around 85 K, contrary to the bulk. The total magnetic moment is below $1μ_{B}$/Co$^{3+}$, a value relatively small for an exited spin-state of the Co$^{3+}$ ions, but comparable to values reported in literature. A correlation of strain states and magnetic moment of Co$^{3+}$ ions in LaCoO$_{3}$ thin films is observed. △ Less

Submitted 25 February, 2010; originally announced February 2010.

Comments: submitted tu European Phys. J. B

Journal ref: Eur. Phys. J. B 76, 215219 (2010)

Reversible strain effect on the magnetization of LaCoO3 films

Authors: A. Herklotz, A. D. Rata, L. Schultz, K. Doerr

Abstract: The magnetization of ferromagnetic LaCoO3 films grown epitaxially on piezoelectric substrates has been found to systematically decrease with the reduction of tensile strain. The magnetization change induced by the reversible strain variation reveals an increase of the Co magnetic moment with tensile strain. The biaxial strain dependence of the Curie temperature is estimated to be below 4K/% in t… ▽ More The magnetization of ferromagnetic LaCoO3 films grown epitaxially on piezoelectric substrates has been found to systematically decrease with the reduction of tensile strain. The magnetization change induced by the reversible strain variation reveals an increase of the Co magnetic moment with tensile strain. The biaxial strain dependence of the Curie temperature is estimated to be below 4K/% in the as-grown tensile strain state of our films. This is in agreement with results from statically strained films on various substrates. △ Less

Submitted 7 May, 2008; originally announced May 2008.

Journal ref: Phys. Rev. B 79, 092409 (2009)

Strain-induced insulator state in La_0.7Sr_0.3CoO_3

Authors: A. D. Rata, A. Herklotz, K. Nenkov, L. Schultz, K. Doerr

Abstract: We report on the observation of a strain-induced insulator state in ferromagnetic La_0.7Sr_0.3CoO_3 films. Tensile strain above 1% is found to enhance the resistivity by several orders of magnitude. Reversible strain of 0.15% applied using a piezoelectric substrate triggers huge resistance modulations, including a change by a factor of 10 in the paramagnetic regime at 300 K. However, below the f… ▽ More We report on the observation of a strain-induced insulator state in ferromagnetic La_0.7Sr_0.3CoO_3 films. Tensile strain above 1% is found to enhance the resistivity by several orders of magnitude. Reversible strain of 0.15% applied using a piezoelectric substrate triggers huge resistance modulations, including a change by a factor of 10 in the paramagnetic regime at 300 K. However, below the ferromagnetic ordering temperature, the magnetization data indicate weak dependence on strain for the spin state of the Co ions. We interpret the changes observed in the transport properties in terms of a strain-induced splitting of the Co e_g levels and reduced double exchange, combined with a percolation-type conduction in an electronic cluster state. △ Less

Submitted 5 October, 2007; originally announced October 2007.

Journal ref: Phys. Rev. Lett. 100, 076401 (2008)

Metal to insulator transition in manganites - optical conductivity changes up to 24 eV

Authors: A. Rusydi, R. Rauer, G. Neuber, M. Bastjan, I. Mahns, S. Müller, P. Saichu, B. Schulz, G. Stryganyuk, K. Dörr, G. A. Sawatzky, S. L. Cooper, M. Rübhausen

Abstract: The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in $\rm La_{1-x}Ca_{x}MnO_3$ for $\rm (x=0.3,0.2)$ was investigated by dc conductivity, ellipsometry, and VUV reflectance for energies between 0 and 24 eV. A stablized Kramers-Kronig transformation yields the optical conductivity and reveals changes in the optical sp… ▽ More The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in $\rm La_{1-x}Ca_{x}MnO_3$ for $\rm (x=0.3,0.2)$ was investigated by dc conductivity, ellipsometry, and VUV reflectance for energies between 0 and 24 eV. A stablized Kramers-Kronig transformation yields the optical conductivity and reveals changes in the optical spectral weight up to 24 eV at the metal to insulator transition. In the observed energy range, the spectral weight is conserved within $\rm 0.3 %$. The redistribution of spectral weight between low and high energies has important ramifications for the down-folding of low-energy Hamiltonians. We discuss the importance of the charge-transfer, Coulomb onsite, Jahn-Teller, and screening effects to the electronic structure. △ Less

Submitted 15 January, 2007; originally announced January 2007.

Comments: 4 pages, 3 figures

Critical properties in single crystals of Pr1-xPbxMnO3

Authors: B. Padmanabhan, H. L. Bhat, Suja Elizabeth, Sahana Roessler, U. K. Roessler, K. Doerr, K. H. Mueller

Abstract: The critical properties at the ferromagnetic - paramagnetic transition have been analysed from data of static magnetization measurements on single crystals of Pr1-xPbxMnO3, for x = 0.23 and x = 0.30. In Pr1-xPbxMnO3 the ferromagnetic ordering and the metal-insulator transition do not coincide in parts of the phase diagram. The crystal with x = 0.23 is a ferromagnetic insulator with Curie tempera… ▽ More The critical properties at the ferromagnetic - paramagnetic transition have been analysed from data of static magnetization measurements on single crystals of Pr1-xPbxMnO3, for x = 0.23 and x = 0.30. In Pr1-xPbxMnO3 the ferromagnetic ordering and the metal-insulator transition do not coincide in parts of the phase diagram. The crystal with x = 0.23 is a ferromagnetic insulator with Curie temperature Tc = 173 K, while the crystal with x =0.30 has Tc = 198 K and remains metallic up to a metal-insulator transition temperature Tmi = 235 K. The dc magnetization measurements were carried out in the field range from 0 to 5 T for an interval in the critical temperature range Tc+_10 K corresponding to a reduced temperature interval 0.003 < epsilon < 0.6. The exponents beta for spontaneous magnetization, gamma for the initial susceptibility above Tc and delta for the critical magnetization isotherm at Tc were obtained by static scaling analysis from modified Arrott plots and by the Kouvel Fisher method for the insulating crystal with composition x = 0.23. The data are well described by critical exponents similar to those expected for the Heisenberg universality class relevant for conventional isotropic magnets Systematic deviations from scaling in the data for the metallic crystal with composition x = 0.30 are demonstrated from effective critical exponents near the assumed ordering transition. The unconventional magnetic ordering in this system indicates the presence of frustrated magnetic couplings that suppresses magnetic ordering and lowers the transition temperature. △ Less

Submitted 10 October, 2006; originally announced October 2006.

Comments: 9 double column pages, 14 figures. 1 table

Influence of strain on magnetization and magnetoelectric effect in La0.7A0.3MnO3 / PMN-PT(001) (A = Sr; Ca)

Authors: C. Thiele, K. Dorr, O. Bilani, J. Rodel, L. Schultz

Abstract: We investigate the influence of a well-defined reversible biaxial strain <=0.12 % on the magnetization (M) of epitaxial ferromagnetic manganite films. M has been recorded depending on temperature, strain and magnetic field in 20 - 50 nm thick films. This is accomplished by reversibly compressing the isotropic in-plane lattice parameter of the rhombohedral piezoelectric 0.72PMN-0.28PT (001) subst… ▽ More We investigate the influence of a well-defined reversible biaxial strain <=0.12 % on the magnetization (M) of epitaxial ferromagnetic manganite films. M has been recorded depending on temperature, strain and magnetic field in 20 - 50 nm thick films. This is accomplished by reversibly compressing the isotropic in-plane lattice parameter of the rhombohedral piezoelectric 0.72PMN-0.28PT (001) substrates by application of an electric field E <= 12 kV cm-1. The magnitude of the total variable in-plane strain has been derived. Strain-induced shifts of the ferromagnetic Curie temperature (Tc) of up to 19 K were found in La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 films and are quantitatively analysed for LSMO within a cubic model. The observed large magnetoelectric coupling coefficient alpha=mu0 dM/dE <= 6 10-8 s m-1 at ambient temperature results from the strain-induced M change in the magnetic-film-ferroelectric-substrate system. It corresponds to an enhancement of mu0 DeltaM <= 19 mT upon biaxial compression of 0.1 %. The extraordinary large alpha originates from the combination of three crucial properties: (i) the strong strain dependence of M in the ferromagnetic manganites, (ii) large piezo-strain of the PMN-PT substrates and (iii) effective elastic coupling at the film-substrate interface. △ Less

Submitted 29 September, 2006; originally announced September 2006.

Comments: 15 pages, 6 figures, 1 table

Effect of Ta(5+) Substitution on the Ground State of CMR Manganites

Authors: L. Seetha Lakshmi, K. Doerr, K. Nenkov, A. Handstein, V. Sridharan, V. S. Sastry, K. -H. Mueller

Abstract: In this paper, we discuss the most deleterious effect of charge state modification at the Mn site on the ground state of the CMR manganites. In this paper, we discuss the most deleterious effect of charge state modification at the Mn site on the ground state of the CMR manganites. △ Less

Submitted 21 December, 2005; originally announced December 2005.

Comments: 4 pages; figures are included in the text (ISRS-2004, Chennai, India)

Structure, Tranport and Magnetism of La(0.67)Ca(0.33)Mn(1-x)Ta(x)O(3)

Authors: L. Seetha Lakshmi, K. Doerr, K. Nenkov, V. Sridharan, V. S. Sastry, K. -H. Mueller

Abstract: For the first time,we report the effect of pentavalent subtitution at the Mn site of La(0.67)Ca(0.33)MnO(3). The ferromagnetic -metallic ground state modifies to a cluster-glass insulator beyond x=0.03. The observed suppression in the transition temperatures (39K/at.%) is the largest reported for the Mn site subtituted manganites. We argue that charge state(valence) of the substituent has a domi… ▽ More For the first time,we report the effect of pentavalent subtitution at the Mn site of La(0.67)Ca(0.33)MnO(3). The ferromagnetic -metallic ground state modifies to a cluster-glass insulator beyond x=0.03. The observed suppression in the transition temperatures (39K/at.%) is the largest reported for the Mn site subtituted manganites. We argue that charge state(valence) of the substituent has a dominant role in affecting the ferromagnetic - metallic ground state of CMR manganites. △ Less

Submitted 13 October, 2004; originally announced October 2004.

Comments: Four pages inclusing three figures.(To appear in J.Magn.Magn.Mater. in 2005)

Rounding of a first-order magnetic phase transition in Ga doped La0.67Ca0.33MnO3

Authors: S. Roessler, U. K. Roessler, K. Nenkov, D. Eckert, S. M. Yusuf, K. Dorr, K. -H. Muller

Abstract: The effect of disorder on the critical properties of the ferromagnetic phase transition in colossal magnetoresistive manganite La0.67Ca0.33MnO3 has been studied by substituting Ga for Mn. It is found that, upon 10% Ga substitution, the peak in the specific heat at the Curie point T_C changes drastically and appears as a small anomaly. Static magnetization data analyzed in the asymptotic critical… ▽ More The effect of disorder on the critical properties of the ferromagnetic phase transition in colossal magnetoresistive manganite La0.67Ca0.33MnO3 has been studied by substituting Ga for Mn. It is found that, upon 10% Ga substitution, the peak in the specific heat at the Curie point T_C changes drastically and appears as a small anomaly. Static magnetization data analyzed in the asymptotic critical region using modified Arrott plots and the Kouvel-Fisher method give values for the critical exponents beta=0.387(6), gamma=1.362(2), and delta=4.60(3). The results show that the first-order transition in La0.67Ca0.33MnO3 becomes continuous by Ga substitution. The critical properties of the rounded transition in La0.67Ca0.33Mn0.9Ga0.1O3 suggest that the magnetic subsystem in this mixed-valent perovskite is close to that of a conventional isotropic ferromagnet belonging to the Heisenberg universality class with short-range interactions. It is concluded that the first-order magnetic transition in pure La0.67Ca0.33MnO3 is induced by fluctuations from a competing mode, which couples to the magnetic subsystem. △ Less

Submitted 19 July, 2004; originally announced July 2004.

Comments: 25 pages manuscript, 8 figures

Critical properties of the double exchange ferromagnet Nd0.4Pb0.4MnO3

Authors: M. Sahana, U. K. Roessler, Nilotpal Ghosh, Suja Elizabeth, H. L. Bhat, K. Doerr, D. Eckert, M. Wolf, K. -H. Mueller

Abstract: Results of a study of dc-magnetization M(T, H), performed on a Nd0.6Pb0.4MnO3 single crystal in the temperature range around T_C (Curie temperature) which embraces the critical region | epsilon | = |T -T_C |/T_C <= 0.05 are reported. The magnetic data analyzed in the critical region using the Kouvel-Fisher method give the values for the T_C =156.47 +/- 0.06 K and the critical exponents, beta = 0… ▽ More Results of a study of dc-magnetization M(T, H), performed on a Nd0.6Pb0.4MnO3 single crystal in the temperature range around T_C (Curie temperature) which embraces the critical region | epsilon | = |T -T_C |/T_C <= 0.05 are reported. The magnetic data analyzed in the critical region using the Kouvel-Fisher method give the values for the T_C =156.47 +/- 0.06 K and the critical exponents, beta = 0.374 +/- 0.006 (from the temperature dependence of magnetization), and gamma = 1.329 +/- 0.003 (from the temperature dependence of initial susceptibility). The critical isotherm M(T_C, H) gives delta = 4.547 +/- 0.1. Thus the scaling law gamma+beta=delta beta is fulfilled. The critical exponents obey the single scaling-equation of state M(H, epsilon) = epsilon^b f_+/- (H/epsilon^(beta + gamma)) where, f_+ for T > T_C and f_- for T< T_C. The exponent values are very close to those expected for the universality class of 3D Heisenberg ferromagnets with short-range interactions. △ Less

Submitted 2 April, 2003; originally announced April 2003.

Comments: 19 pages, including 6 figures

Ferromagnetic insulating state in manganites:55Mn study

Authors: M. M. Savosta, V. I. Kamenev, V. A. Borodin, P. Novak, M. Marysko, J. Hejtmanek, K. Dorr, M. Sahana, A. Shames

Abstract: 55Mn NMR was used to study the ferromagnetic insulating state in four different manganites. In all cases the coexistence of two types of regions possessing different NMR spectra and nuclear spin dynamics was found. In the ferromagnetic insulating clusters the hopping frequency fhop of the electron holes is slower comparing to the NMR frequency fres and two lines ascribed to 3+ and 4+ valence sta… ▽ More 55Mn NMR was used to study the ferromagnetic insulating state in four different manganites. In all cases the coexistence of two types of regions possessing different NMR spectra and nuclear spin dynamics was found. In the ferromagnetic insulating clusters the hopping frequency fhop of the electron holes is slower comparing to the NMR frequency fres and two lines ascribed to 3+ and 4+ valence states of Mn are observed. The relaxation rate increases rapidly with the increasing temperature, so that these spectra can no longer be observed for temperatures higher than approx 60 K. In ferromagnetic metallic clusters fres < fhop and the motionally narrowed spectrum is observed in a broad temperature interval, as the relaxation remains only moderately fast. △ Less

Submitted 2 August, 2002; originally announced August 2002.

Comments: 9 pages, 12 figures

Observation of minority spin character of the new electron doped manganite La_0.7Ce_0.3MnO_3 from tunneling magnetoresistance

Authors: C. Mitra, P. Raychaudhuri, K. Doerr, K. -H. Mueller, L. Schultz, P. M. Oppeneer, S. Wirth

Abstract: We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction build of an electron doped manganite (La_0.7Ce_0.3MnO_3) and a hole doped manganite (La_0.7Ca_0.3MnO_3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T_C the sign of the TMR is dependent on… ▽ More We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction build of an electron doped manganite (La_0.7Ce_0.3MnO_3) and a hole doped manganite (La_0.7Ca_0.3MnO_3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T_C the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La_0.7Ce_0.3MnO_3 is a minority spin carrier ferromagnet with a high degree of spin polarization, i.e. a transport half metal. △ Less

Submitted 11 October, 2002; v1 submitted 15 May, 2002; originally announced May 2002.

Comments: 4 pages, 3 figures, 2nd revised version

A 'p-n' diode with hole and electron-doped lanthanum manganite

Authors: C. Mitra, P. Raychaudhuri, G. Kobernik, K. Dorr, R. Pinto, K-H. Muller, L. Schultz

Abstract: The hole-doped manganite La0.7Ca0.3MnO3 and the electron-doped manganite La0.7Ce0.3MnO3 undergo an insulator to metal transition at around 250 K, above which both behave as a polaronic semiconductor. We have successfully fabricated an epitaxial trilayer (La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3), where SrTiO3 is an insulator. At room temperature, i.e. in the semiconducting regime, it exhibits asymme… ▽ More The hole-doped manganite La0.7Ca0.3MnO3 and the electron-doped manganite La0.7Ce0.3MnO3 undergo an insulator to metal transition at around 250 K, above which both behave as a polaronic semiconductor. We have successfully fabricated an epitaxial trilayer (La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3), where SrTiO3 is an insulator. At room temperature, i.e. in the semiconducting regime, it exhibits asymmetric current-voltage (I-V) characteristics akin to a p-n diode. The observed asymmetry in the I-V characteristics disappears at low temperatures where both the manganite layers are metallic. To the best of our knowledge, this is the first report of such a p-n diode, using the polaronic semiconducting regime of doped manganites. △ Less

Submitted 9 August, 2001; originally announced August 2001.

Comments: PostScript text and 2 figures, to be published in Appl. Phys. Lett.

Current-induced conductance switching in epitaxial La0.7Sr0.3MnO3/SrTiO3 multilayers

Authors: K. Doerr, K. -H. Mueller, T. Walter, M. Sahana, D. Eckert, K. Nenkov, L. Schultz, K. Brand, M. Lehmann

Abstract: We report on the non-linear in-plane electrical transport in coherently grown [La0.7Sr0.3MnO3/SrTiO3] multilayers with ultrathin (< 3 nm) single layers. Current-induced switching of the conductance, with low conductance at larger currents, is demonstrated. The conductance switching is modified under a magnetic field, resulting in an extremely large magnetoresistance of negative, or in a special… ▽ More We report on the non-linear in-plane electrical transport in coherently grown [La0.7Sr0.3MnO3/SrTiO3] multilayers with ultrathin (< 3 nm) single layers. Current-induced switching of the conductance, with low conductance at larger currents, is demonstrated. The conductance switching is modified under a magnetic field, resulting in an extremely large magnetoresistance of negative, or in a special case even positive, sign. Our results suggest a percolative nature of transport where a large local current density gives rise to a spin-polarizing and a thermal effect of current. △ Less

Submitted 29 December, 2000; originally announced December 2000.

Comments: 10 pages, 6 figures