arXiv subjects

Tomas Jungwirth

Publications and source records attributed to Tomas Jungwirth.

30 records · Page 2Linked to original sources

arXiv2020

Magneto-Seebeck microscopy of domain switching in collinear antiferromagnet CuMnAs

Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of aniferromagnetic domains is one of the key prerequisites for understading physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20 nm epilayer of uniaxial CuMnAs we observe reversible 180 deg switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent 90 deg switching of the Néel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron microscopy using X-ray magnetic linear dichroism.

physics.app-ph↗

arXiv2019

Imaging and writing magnetic domains in the non-collinear antiferromagnet Mn$_{\text{3}}$Sn

Harnessing the unique properties of non-collinear antiferromagnets (AFMs) will be essential for exploiting the full potential of antiferromagnetic spintronics. Indeed, many of the effects enabling ferromagnetic spintronic devices have a corresponding counterpart in materials with non-collinear spin structure. In addition, new phenomena such as the magnetic spin Hall effect were experimentally observed in non-collinear AFMs, and the presence of the equivalent to the ferromagnetic spin transfer torque via spin polarized currents was theoretically predicted. In spite of these developments, an interpretation of the rich physical phenomena observed in non-collinear antiferromagnets is challenging, since the microscopic spin arrangement, the magnetic domain distribution, and the domain orientations have proven notoriously difficult to access experimentally. This is all the more problematic, as imaging and writing magnetic domains is of central importance for applications. Successful imaging is a basic requirement to experimentally confirm the spin transfer torque acting on non-collinear domain walls and therefore of eminent interest. Here, we demonstrate that the local magnetic structure of the non-collinear AFM Mn3Sn films can be imaged by scanning thermal gradient microscopy (STGM). The technique is based on scanning a laser spot over the sample's surface, and recording the ensuing thermo-voltage. We image the magnetic structure at a series of different temperatures and show that at room temperature, the domain structure is not affected by the application of moderate magnetic fields. In addition to imaging, we establish a scheme for heat-assisted magnetic recording, using local laser heating in combination with magnetic fields to intentionally write domain patterns into the antiferromagnet.

cond-mat.mes-hall↗

arXiv2017

Interface-Induced Phenomena in Magnetism

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

cond-mat.mtrl-sci↗

arXiv2015

Electrical switching of an antiferromagnet

Louis Neel pointed out in his Nobel lecture that while abundant and interesting from a theoretical viewpoint, antiferromagnets did not seem to have any applications. Indeed, the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization make antiferromagnets hard to control by tools common in ferromagnets. Remarkably, Neel in his lecture provides the key which, as we show here, allows us to control antiferromagnets by electrical means analogous to those which paved the way to the development of ferromagnetic spintronics applications. The key noted by Neel is the equivalence of antiferromagnets and ferromagnets for effects that are an even function of the magnetic moment. Based on even-in-moment relativistic transport phenomena, we demonstrate room-temperature electrical switching between two stable configurations combined with electrical read-out in antiferromagnetic CuMnAs thin film devices. Our magnetic memory is insensitive to and produces no magnetic field perturbations which illustrates the unique merits of antiferromagnets for spintronics.

cond-mat.mes-hall↗

arXiv2012

Investigation of Optical Spin Transfer Torque in Ferromagnetic Semiconductor GaMnAs by Magneto-Optical Pump-and-Probe Method

We report on magnetization precession induced in (Ga,Mn)As by an optical spin transfer torque (OSTT). This phenomenon, which corresponds to a transfer of angular momentum from optically-injected spin-polarized electrons to magnetization, was predicted theoretically in 2004 and observed experimentally by our group in 2012. In this paper we provide experimental details about the observation of OSTT by a time-resolved pump-and-probe magneto-optical technique. In particular, we show that the precession of magnetization due to OSTT can be experimentally separated from that induced by the well known magnetic-anisotropy-related mechanism in a hybrid structure piezo-stressor/(Ga,Mn)As with an in situ electrical control of the magnetic anisotropy. We also illustrate that OSTT is clearly apparent in the measured dynamical magneto-optical signal in a large variety of (Ga,Mn)As samples with a Mn concentration from 3% to 9%.

cond-mat.mtrl-sci↗

arXiv2010

Spin-orbit coupling induced anisotropy effects in bimetallic antiferromagnets: A route towards antiferromagnetic spintronics

Magnetic anisotropy phenomena in bimetallic antiferromagnets Mn$_2$Au and MnIr are studied by first-principles density functional theory calculations. We find strong and lattice-parameter dependent magnetic anisotropies of the ground state energy, chemical potential, and density of states, and attribute these anisotropies to combined effects of large moment on the Mn 3$d$ shell and large spin-orbit coupling on the 5$d$ shell of the noble metal. Large magnitudes of the proposed effects can open a route towards spintronics in compensated antiferromagnets without involving ferromagnetic elements.

cond-mat.mtrl-sci↗

arXiv2009

Anisotropic magnetoresistance of spin-orbit coupled carriers scattered from polarized magnetic impurities

Anisotropic magnetoresistance (AMR) is a relativistic magnetotransport phenomenon arising from combined effects of spin-orbit coupling and broken symmetry of a ferromagnetically ordered state of the system. In this work we focus on one realization of the AMR in which spin-orbit coupling enters via specific spin-textures on the carrier Fermi surfaces and ferromagnetism via elastic scattering of carriers from polarized magnetic impurities. We report detailed heuristic examination, using model spin-orbit coupled systems, of the emergence of positive AMR (maximum resistivity for magnetization along current), negative AMR (minimum resistivity for magnetization along current), and of the crystalline AMR (resistivity depends on the absolute orientation of the magnetization and current vectors with respect to the crystal axes) components. We emphasize potential qualitative differences between pure magnetic and combined electro-magnetic impurity potentials, between short-range and long-range impurities, and between spin-1/2 and higher spin-state carriers. Conclusions based on our heuristic analysis are supported by exact solutions to the integral form of the Boltzmann transport equation in archetypical two-dimensional electron systems with Rashba and Dresselhaus spin-orbit interactions and in the three-dimensional spherical Kohn-Littinger model. We include comments on the relation of our microscopic calculations to standard phenomenology of the full angular dependence of the AMR, and on the relevance of our study to realistic, two-dimensional conduction-band carrier systems and to anisotropic transport in the valence band of diluted magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2005

Charge Hall effect driven by spin-dependent chemical potential gradients and Onsager relations in mesoscopic systems

We study theoretically the spin-Hall effect as well as its reciprocal phenomenon (a transverse charge current driven by a spin-dependent chemical potential gradient) in electron and hole finite size mesoscopic systems. The Landauer-Buttiker-Keldysh formalism is used to model samples with mobilities and Rashba coupling strengths which are experimentally accessible and to demonstrate the appearance of measurable charge currents induced by the spin-dependent chemical potential gradient in the reciprocal spin-Hall effect. We also demonstrate that within the mesoscopic coherent transport regime the Onsager relations are fulfilled for the disorder averaged conductances for electron and hole mesoscopic systems.

cond-mat.mes-hall↗

arXiv2005

Experimental observation of the spin-Hall effect in a two dimensional spin-orbit coupled semiconductor system

We report the experimental observation of the spin-Hall effect in a two-dimensional (2D) hole system with Rashba spin-orbit coupling. The 2D hole layer is a part of a p-n junction light-emitting diode with a specially designed co-planar geometry which allows an angle-resolved polarization detection at opposite edges of the 2D hole system. In equilibrium the angular momenta of the Rashba split heavy hole states lie in the plane of the 2D layer. When an electric field is applied across the hole channel a non zero out-of-plane component of the angular momentum is detected whose sign depends on the sign of the electric field and is opposite for the two edges. Microscopic quantum transport calculations show only a weak effect of disorder suggesting that the clean limit spin-Hall conductance description (intrinsic spin-Hall effect) might apply to our system.

cond-mat.mes-hall↗

arXiv2001

Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As

We present a microscopic theory of the long-wavelength magnetic properties of the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host semiconductor band structure, described by a six-band Kohn-Luttinger Hamiltonian, are taken into account. We relate our quantum-mechanical calculation to the classical micromagnetic energy functional and determine anisotropy energies and exchange constants. We find that the exchange constant is substantially enhanced compared to the case of a parabolic heavy-hole-band model.

cond-mat.mtrl-sci↗

arXiv2001

Orbital effect of in-plane magnetic field on quantum transport in chaotic lateral dots

We show how the in-plane magnetic field, which breaks time-reversal and rotational symmetries of the orbital motion of electrons in a heterostructure due to the momentum-dependent inter-subband mixing, affects weak localisation correction to conductance of a large-area chaotic lateral quantum dot and parameteric dependences of universal conductance fluctuations in it.

cond-mat.mes-hall↗

arXiv2000

First-Order Phase Transition in a Quantum Hall Ferromagnet

The single-particle energy spectrum of a two-dimensional electron gas in a perpendicular magnetic field consists of equally-spaced spin-split Landau levels, whose degeneracy is proportional to the magnetic field strength. At integer and particular fractional ratios between the number of electrons and the degeneracy of a Landau level (filling factors n) quantum Hall effects occur, characterised by a vanishingly small longitudinal resistance and quantised Hall voltage. The quantum Hall regime offers unique possibilities for the study of cooperative phenomena in many-particle systems under well-controlled conditions. Among the fields that benefit from quantum-Hall studies is magnetism, which remains poorly understood in conventional material. Both isotropic and anisotropic ferromagnetic ground states have been predicted and few of them have been experimentally studied in quantum Hall samples with different geometries and filling factors. Here we present evidence of first-order phase transitions in n = 2 and 4 quantum Hall states confined to a wide gallium arsenide quantum well. The observed hysteretic behaviour and anomalous temperature dependence in the longitudinal resistivity indicate the occurrence of a transition between the two distinct ground states of an Ising quantum-Hall ferromagnet. Detailed many-body calculations allowed the identification of the microscopic origin of the anisotropy field.

cond-mat↗