Diffusion of Mn interstitials in (Ga,Mn)As epitaxial layers

Magnetic properties of thin (Ga,Mn)As layers improve during annealing by out-diffusion of interstitial Mn ions to a free surface. Out-diffused Mn atoms participate in the growth of a Mn-rich surface layer and a saturation of this layer causes an inhibition of the out-diffusion. We combine high-resolution x-ray diffraction with x-ray absorption spectroscopy and a numerical solution of the diffusion problem for the study of the out-diffusion of Mn interstitials during a sequence of annealing steps. Our data demonstrate that the out-diffusion of the interstitials is substantially affected by the internal electric field caused by an inhomogeneous distribution of charges in the (Ga,Mn)As layer.Comment: 11 pages, 5 figure

Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional Mn_Ga dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mn-doping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band.Comment: 22 pages, 14 figure

Ultrashort spin–orbit torque generated by femtosecond laser pulses

To realize the very objective of spintronics, namely the development of ultra-high frequency and energy-efficient electronic devices, an ultrafast and scalable approach to switch magnetic bits is required. Magnetization switching with spin currents generated by the spin–orbit interaction at ferromagnetic/non-magnetic interfaces is one of such scalable approaches, where the ultimate switching speed is limited by the Larmor precession frequency. Understanding the magnetization precession dynamics induced by spin–orbit torques (SOTs) is therefore of great importance. Here we demonstrate generation of ultrashort SOT pulses that excite Larmor precession at an epitaxial Fe/GaAs interface by converting femtosecond laser pulses into high-amplitude current pulses in an electrically biased p-i-n photodiode. We control the polarity, amplitude, and duration of the current pulses and, most importantly, also their propagation direction with respect to the crystal orientation. The SOT origin of the excited Larmor precession was revealed by a detailed analysis of the precession phase and amplitude at different experimental conditions

Spontaneous anomalous Hall effect arising from an unconventional compensated magnetic phase in a semiconductor

The anomalous Hall effect, commonly observed in metallic magnets, has been established to originate from the time-reversal symmetry breaking by an internal macroscopic magnetization in ferromagnets or by a non-collinear magnetic order. Here we observe a spontaneous anomalous Hall signal in the absence of an external magnetic field in an epitaxial film of MnTe, which is a semiconductor with a collinear antiparallel magnetic ordering of Mn moments and a vanishing net magnetization. The anomalous Hall effect arises from an unconventional phase with strong time-reversal symmetry breaking and alternating spin polarization in real-space crystal structure and momentum-space electronic structure. The anisotropic crystal environment of magnetic Mn atoms due to the non-magnetic Te atoms is essential for establishing the unconventional phase and generating the anomalous Hall effect.Comment: 34 pages, 14 figure

Altermagnetic lifting of Kramers spin degeneracy

Lifted Kramers spin-degeneracy has been among the central topics of condensed-matter physics since the dawn of the band theory of solids. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology to topological quantum matter. Traditionally, lifted Kramers spin-degeneracy has been considered to originate from two possible internal symmetry-breaking mechanisms. The first one refers to time-reversal symmetry breaking by magnetization of ferromagnets, and tends to be strong due to the non-relativistic exchange-coupling origin. The second mechanism applies to crystals with broken inversion symmetry, and tends to be comparatively weaker as it originates from the relativistic spin-orbit coupling. A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic, that allows for lifting the Kramers spin degeneracy without net magnetization and inversion-symmetry breaking. Here we provide the confirmation using photoemission spectroscopy and ab initio calculations. We identify two distinct unconventional mechanisms of lifted Kramers spin degeneracy generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization. Our observation of the altermagnetic lifting of the Kramers spin degeneracy can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors, that have been either identified recently or perceived for many decades as conventional antiferromagnets

Inertial displacement of a domain wall excited by ultra-short circularly polarized laser pulses.

Domain wall motion driven by ultra-short laser pulses is a pre-requisite for envisaged low-power spintronics combining storage of information in magnetoelectronic devices with high speed and long distance transmission of information encoded in circularly polarized light. Here we demonstrate the conversion of the circular polarization of incident femtosecond laser pulses into inertial displacement of a domain wall in a ferromagnetic semiconductor. In our study, we combine electrical measurements and magneto-optical imaging of the domain wall displacement with micromagnetic simulations. The optical spin-transfer torque acts over a picosecond recombination time of the spin-polarized photo-carriers that only leads to a deformation of the initial domain wall structure. We show that subsequent depinning and micrometre-distance displacement without an applied magnetic field or any other external stimuli can only occur due to the inertia of the domain wall

Identifying the octupole antiferromagnetic domain orientation in Mn3NiN by scanning anomalous Nernst effect microscopy

The intrinsic anomalous Nernst effect in a magnetic material is governed by the Berry curvature at the Fermi energy and can be realized in non-collinear antiferromagnets with vanishing magnetization. Thin films of (001)-oriented Mn3NiN have their chiral antiferromagnetic structure located in the (111) plane facilitating the anomalous Nernst effect unusually in two orthogonal in-plane directions. The sign of each component of the anomalous Nernst effect is determined by the local antiferromagnetic domain state. In this work, a temperature gradient is induced in a 50 nm thick Mn3NiN two micrometer-size Hall cross by a focused scanning laser beam, and the spatial distribution of the anomalous Nernst voltage is used to image and identify the octupole macrodomain arrangement. Although the focused laser beam width may span many individual domains, cooling from room temperature to the antiferromagnetic transition temperature in an in-plane magnetic field prepares the domain state, producing a checkerboard pattern resulting from the convolution of contributions from each domain. These images together with atomistic and micromagnetic simulations suggest an average macrodomain of the order of 1 μm2

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 antiferromagnetic domains is one of the key prerequisites for understanding 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∘ switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent 90∘ 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-based technique of x-ray photoemission electron microscopy using x-ray magnetic linear dichroism

Altermagnetic lifting of Kramers spin degeneracy

Lifted Kramers spin degeneracy (LKSD) has been among the central topics of condensed-matter physics since the dawn of the band theory of solids1,2. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology3–7 to topological quantum matter8–14. Traditionally, LKSD has been considered to originate from two possible internal symmetry-breaking mechanisms. The first refers to time-reversal symmetry breaking by magnetization of ferromagnets and tends to be strong because of the non-relativistic exchange origin15. The second applies to crystals with broken inversion symmetry and tends to be comparatively weaker, as it originates from the relativistic spin–orbit coupling (SOC)16–19. A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic20,21, that allows for LKSD without net magnetization and inversion-symmetry breaking. Here we provide the confirmation using photoemission spectroscopy and ab initio calculations. We identify two distinct unconventional mechanisms of LKSD generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization20–23. Our observation of the altermagnetic LKSD can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors20,21, that have been either identified recently or perceived for many decades as conventional antiferromagnets21,24,25

Antiferromagnetic domain wall memory with neuromorphic functionality

Acknowledgements: This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through Project-ID 314695032 – SFB 1277 (Subprojects A010) “Emergente relativistische Effekte in der Kondensierten Materie”, Project-ID452301518 “Investigation of quench switching of antiferromagnets with high spatial and temporal resolution” and by the European Union’s Horizon 2020 research and innovation program under the Marie Sk lodowska- Curie Grant Agreement No. 861300 “Cold Opto-Magnetism for Random Access Devices”. The work had also the support from the Czech Science Foundation within the Project GACR 21-28876J.AbstractAntiferromagnetic materials have unique properties due to their alternating spin arrangements. Their compensated magnetic order, robust against external magnetic fields, prevents long-distance crosstalk from stray fields. Furthermore, antiferromagnets with combined parity and time-reversal symmetry enable electrical control and detection of ultrafast exchange-field enhanced spin manipulation up to THz frequencies. Here we report the experimental realization of a nonvolatile antiferromagnetic memory mimicking an artificial synapse, in which the reconfigurable synaptic weight is encoded in the ratio between reversed antiferromagnetic domains. The non-volatile memory is “written” by spin-orbit torque-driven antiferromagnetic domain wall motion and “read” by nonlinear magnetotransport. We show that the absence of long-range interacting stray magnetic fields leads to very reproducible electrical pulse-driven variations of the synaptic weights.</jats:p