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Topologically protected magnetic textures, such as skyrmions, half-skyrmions (merons) and their antiparticles, constitute tiny whirls in the magnetic order. They are promising candidates for information carriers in next-generation memory devices, as they can be efficiently propelled at very high velocities using current-induced spin torques. Antiferromagnets have been shown to host versions of these textures, which have gained significant attention because of their potential for terahertz dynamics, deflection free motion, and improved size scaling due to the absence of stray field. Here we show that topological spin textures, merons and antimerons, can be generated at room temperature and reversibly moved using electrical pulses in thin film CuMnAs, a semimetallic antiferromagnet that is a testbed system for spintronic applications. The electrical generation and manipulation of antiferromagnetic merons is a crucial step towards realizing the full potential of antiferromagnetic thin films as active components in high density, high speed magnetic memory devices., Comment: 11 pages, 4 figures

Recent research works have shown that the magnetic order in some antiferromagnetic materials can be manipulated and detected electrically, due to two physical mechanisms: Neel-order spin-orbit torques and anisotropic magnetoresistance. While these observations open up opportunities to use antiferromagnets for magnetic memory devices, different physical characterization methods are required for a better understanding of those mechanisms. Here we report a magnetic field induced rotation of the antiferromagnetic Neel vector in epitaxial tetragonal CuMnAs thin films. Using soft x-ray magnetic linear dichroism spectroscopy, x-ray photoemission electron microscopy, integral magnetometry and magneto-transport methods, we demonstrate spin-flop switching and continuous spin reorientation in antiferromagnetic films with uniaxial and biaxial magnetic anisotropies, respectively. From field-dependent measurements of the magnetization and magnetoresistance, we obtain key material parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, spin-flop and exchange fields., Comment: 26 pages, 6 figures

Ultra-fast dynamics, insensitivity to external magnetic fields, or absence of magnetic stray fields are examples of properties that make antiferromagnets of potential use in the development of spintronic devices. Similar to their ferromagnetic counterparts, antiferromagnets can store information in the orientations of the collective magnetic order vector. However, also in analogy to ferromagnets, the readout magnetoresistivity signals in simple antiferromagnetic films have been weak and the extension of the electrical reorientation mechanism to optics has not been achieved. Here we report reversible and reproducible quenching of an antiferromagnetic CuMnAs film by either electrical or ultrashort optical pulses into nano-fragmented domain states. The resulting resistivity changes approach 20\% at room temperature, which is comparable to the giant magnetoresistance ratios in ferromagnetic multilayers. We also obtain a signal readout by optical reflectivity. The analog time-dependent switching and relaxation characteristics of our devices can mimic functionality of spiking neural network components.

The effects of current induced N\'eel spin-orbit torques on the antiferromagnetic domain structure of epitaxial Mn$_2$Au thin films were investigated by X-ray magnetic linear dichroism - photoemission electron microscopy (XMLD-PEEM). We observed current induced switching of AFM domains essentially corresponding to morphological features of the samples. Reversible as well as irreversible N\'eel vector reorientation was obtained in different parts of the samples and the switching of up to 30 % of all domains in the field of view of 10 $\mu$m is demonstrated. Our direct microscopical observations are compared to and fully consistent with anisotropic magnetoresistance effects previously attributed to current induced N\'eel vector switching in Mn$_2$Au., Comment: 5 pages, 4 figures

The antiferromagnetic to ferromagnetic phase transition in B2-ordered FeRh is imaged in laterally confined nanopatterned islands using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. The resulting magnetic images directly detail the progression in the shape and size of the FM phase domains during heating and cooling through the transition. In 5 um square islands this domain development during heating is shown to proceed in three distinct modes: nucleation, growth, and merging, each with subsequently greater energy costs. In 0.5 um islands, which are smaller than the typical final domain size, the growth mode is stunted and the transition temperature was found to be reduced by 20 K. The modification to the transition temperature is found by high resolution scanning transmission electron microscopy to be due to a 100 nm chemically disordered edge grain present as a result of ion implantation damage during the patterning. FeRh has unique possibilities for magnetic memory applications; the inevitable changes to its magnetic properties due to subtractive nanofabrication will need to be addressed in future work in order to progress from sheet films to suitable patterned devices.

Antiferromagnets have a number of favourable properties as active elements in spintronic devices, including ultra-fast dynamics, zero stray fields and insensitivity to external magnetic fields . Tetragonal CuMnAs is a testbed system in which the antiferromagnetic order parameter can be switched reversibly at ambient conditions using electrical currents . In previous experiments, orthogonal in-plane current pulses were used to induce 90 degree rotations of antiferromagnetic domains and demonstrate the operation of all-electrical memory bits in a multi-terminal geometry . Here, we demonstrate that antiferromagnetic domain walls can be manipulated to realize stable and reproducible domain changes using only two electrical contacts. This is achieved by using the polarity of the current to switch the sign of the current-induced effective field acting on the antiferromagnetic sublattices. The resulting reversible domain and domain wall reconfigurations are imaged using x-ray magnetic linear dichroism microscopy, and can also be detected electrically. The switching by domain wall motion can occur at much lower current densities than those needed for coherent domain switching., Comment: 8 pages, 4 figures

Using x-ray magnetic circular and linear dichroism techniques, we demonstrate a collinear exchange coupling between an epitaxial antiferromagnet, tetragonal CuMnAs, and an Fe surface layer. A small uncompensated Mn magnetic moment is observed which is antiparallel to the Fe magnetization. The staggered magnetization of the 5nm thick CuMnAs layer is rotatable under small magnetic fields, due to the interlayer exchange coupling. This allows us to obtain the x-ray magnetic linear dichroism spectra for different crystalline orientations of CuMnAs in the (001) plane., Comment: 10 pages, 3 figures

We investigate the role of lithographically-induced strain relaxation in a micron-scaled device fabricated from epitaxial thin films of the magnetostrictive alloy Fe81Ga19. The strain relaxation due to lithographic patterning induces a magnetic anisotropy that competes with the magnetocrystalline and shape induced anisotropies to play a crucial role in stabilising a flux-closing domain pattern. We use magnetic imaging, micromagnetic calculations and linear elastic modelling to investigate a region close to the edges of an etched structure. This highly-strained edge region has a significant influence on the magnetic domain configuration due to an induced magnetic anisotropy resulting from the inverse magnetostriction effect. We investigate the competition between the strain-induced and shape-induced anisotropy energies, and the resultant stable domain configurations, as the width of the bar is reduced to the nanoscale range. Understanding this behaviour will be important when designing hybrid magneto-electric spintronic devices based on highly magnetostrictive materials.

Recent breakthroughs in electrical detection and manipulation of antiferromagnets have opened a new avenue in the research of non-volatile spintronic devices. Antiparallel spin sublattices in antiferromagnets, producing zero dipolar fields, lead to the insensitivity to magnetic field perturbations, multi-level stability, ultra-fast spin dynamics and other favorable characteristics which may find utility in fields ranging from magnetic memories to optical signal processing. However, the absence of a net magnetic moment and the ultra-short magnetization dynamics timescales make antiferromagnets notoriously difficult to study by common magnetometers or magnetic resonance techniques. In this paper we demonstrate the experimental determination of the Neel vector in a thin film of antiferromagnetic CuMnAs which is the prominent material used in the first realization of antiferromagnetic memory chips. We employ a femtosecond pump-probe magneto-optical experiment based on magnetic linear dichroism. This table-top optical method is considerably more accessible than the traditionally employed large scale facility techniques like neutron diffraction and X-ray magnetic dichroism measurements. This optical technique allows an unambiguous direct determination of the Neel vector orientation in thin antiferromagnetic films utilized in devices directly from measured data without fitting to a theoretical model., Comment: 10 pages, 4 figures

The magnetic order in antiferromagnetic (AF) materials is hard to control with external magnetic fields. However, recent advances in detecting and manipulating AF order electrically have opened up new prospects for these materials in basic and applied spintronics research. Using x-ray magnetic linear dichroism microscopy, we show here that staggered effective fields generated by electrical current can induce reproducible and reversible modification of the antiferromagnetic domain structure in microdevices fabricated from a tetragonal CuMnAs thin film. The current-induced domain switching is inhomogeneous at the submicron level. A clear correlation between the average domain orientation and the anisotropy of the electrical resistance is demonstrated., Comment: 5 pages, 4 figures

The physical properties of epitaxial films can fundamentally differ from those of bulk single crystals even above the critical thickness. By a combination of non-resonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we show that epitaxial (111)-BiFeO3 films support sub-micron antiferromagnetic domains, which are magneto-elastically coupled to a coherent crystallographic monoclinic twin structure. This unique texture, which is absent in bulk single crystals, should enable control of magnetism in BiFeO3 film devices via epitaxial strain., Comment: 5 pages, 3 figures

We report current-induced domain wall motion (CIDWM) in Ta\Co20Fe60B20\MgO nanowires. Domain walls are observed to move against the electron flow when no magnetic field is applied, while a field along the nanowires strongly affects the domain wall motion direction and velocity. A symmetric effect is observed for up-down and down-up domain walls. This indicates the presence of right-handed domain walls, due to a Dzyaloshinskii-Moriya interaction (DMI) with a DMI coefficient D=+0.06 mJ/m2. The positive DMI coefficient is interpreted to be a consequence of boron diffusion into the tantalum buffer layer during annealing. In a Pt\Co68Fe22B10\MgO nanowire CIDWM along the electron flow was observed, corroborating this interpretation. The experimental results are compared to 1D-model simulations including the effects of pinning. This advanced modelling allows us to reproduce the experiment outcomes and reliably extract a spin-Hall angle {\theta}SH=-0.11 for Ta in the nanowires, showing the importance of an analysis that goes beyond the currently used model for perfect nanowires., Comment: 22 pages, 5 figures

Using X-ray photoelectron emission microscopy we have observed the coexistence of ferromagnetic and antiferromagnetic phases in a (3 at.%)Pd-doped FeRh epilayer. By quantitatively analyzing the resultant images we observe that as the epilayer transforms there is a change in magnetic domain symmetry from predominantly twofold at lower temperatures through to an equally weighted combination of both four and twofold symmetries at higher temperature. It is postulated that the lowered symmetry Ising-like nematic phase resides at the near-surface of the epilayer. This behavior is different to that of undoped FeRh suggesting that the variation in symmetry is driven by the competing structural and electronic interactions in the nanoscale FeRh film coupled with the effect of the chemical doping disorder., Comment: 10 pages, 8 figures, 1 table

The electronic and magnetic properties of Mn coordinated to 1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have been investigated by combining scanning tunneling microscopy and X-ray magnetic circular dichroism (XMCD) performed at low temperature (3 K). When formed on Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn sites with similar single-ion anisotropy energies, but different coupling constants. Low-temperature XMCD spectra show that the local environment of the Mn centers differs appreciably for the two substrates. Multiplet structure calculations were used to derive the corresponding ligand field parameters confirming an in-plane uniaxial anisotropy. The observed interatomic coupling is discussed in terms of superexchange as well as substrate-mediated magnetic interactions., Comment: J. Phys. Chem. C 2014

Large thermal changes driven by a magnetic field have been proposed for environmentally friendly energy efficient refrigeration, but only a few materials which suffer hysteresis show these giant magnetocaloric effects. Here we create giant and reversible extrinsic magnetocaloric effects in epitaxial films of the ferromagnetic manganite La0.7Ca0.3MnO3 using strain mediated feedback from BaTiO3 substrates near a first-order structural phase transition. Our findings should inspire the discovery of giant magnetocaloric effects in a wide range of magnetic materials, and the parallel development of nanostructured bulk samples for practical applications., Comment: 32 pages, 1 Table, 5 figures, supplementary information

X-ray magnetic circular dichroism at the Er M4,5 edge is used to study the switching behavior of the hard ErFe2 layers in an epitaxial [ErFe2(70{\AA})/YFe2 (150{\AA})]{\times}25 exchange-spring superlattice. Magnetic hysteresis loops for the Er magnetization, at temperatures T < 200 K, reveal a switching behavior with a single type of irreversible switch corresponding to vertical exchange spring states. Experiments at T > 200 K reveal a crossover to a regime with two types of switching processes. Computational modelling for this system gives a semi-quantitative agreement with the experiment and reveals that the observed high temperature switching behavior is due to a spin-flop like reorientation transition. In contrast to conventional spin-flop transitions in antiferromagnets, in this exchange spring system the increase in anisotropy energy of the hard magnetic layers is overcome by the decrease in Zeeman energy of the soft layers. Computational studies also reveal the presence of transitions between vertical exchange spring and spin-flop states with a first-order character as well as continuous transitions between these states., Comment: 10 Pages 5 Figures

Magnetic frustration effects in artificial kagome arrays of nanomagnets are investigated using x-ray photoemission electron microscopy and Monte Carlo simulations. Spin configurations of demagnetized networks reveal unambiguous signatures of long range, dipolar interaction between the nanomagnets. As soon as the system enters the spin ice manifold, the kagome dipolar spin ice model captures the observed physics, while the short range kagome spin ice model fails., Comment: 4 pages, 4 figures, 1 table

High spatial resolution X-ray photo-emission electron microscopy technique has been used to study the influence of the dipolar coupling taking place between the NiFe and the Co ferromagnetic electrodes of micron sized, elliptical shaped magnetic tunnel junctions. The chemical selectivity of this technique allows to observe independently the magnetic domain structure in each ferromagnetic electrode. The combination of this powerful imaging technique with micromagnetic simulations allows to evidence that a 360 degree domain wall can be stabilized in the NiFe soft layer. In this letter, we discuss the origin and the formation conditions of those 360 degree domain walls evidenced experimentally and numerically.

The alpha-beta magneto-structural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 K to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an alpha-beta phase coexistence and, more important, for the stabilization of the ferromagnetic alpha-phase at higher temperature than in bulk. We explain the premature appearance of the beta-phase at 275 K and the persistence of the ferromagnetic alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature., Comment: 4 pages, 3 figures, accepted for publication in Physical Review Letters