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The existence of the V3+ ion orbital moment is the open issue of the nature of magnetism in the van der Waals ferromagnet VI3. The huge magnetocrystalline anisotropy in conjunction with the significantly reduced ordered magnetic moment compared to the spin-only value provides strong but indirect evidence of a significant V orbital moment. We used the unique capability of X-ray magnetic circular dichroism to determine the orbital component of the total magnetic moment and provide for the first time a direct proof of an exceptionally sizable orbital moment of the V3+ ion in VI3. Our ligand field multiplet simulations of the XMCD spectra in synergy with the results of DFT calculations agree with the existence of two V sites with different orbital occupations and therefore different OM magnitudes in the ground state.

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature TS2 of the second structural transition associated with the order-order magnetic phase transition. Below TS2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered an exclusive domain of antiferromagnets., Accepted in Journal of Physical Chemistry Letters

Ever since its first observation, the microscopic origin of ultrafast magnetization dynamics has been actively debated. Even more questions arise when considering composite materials featuring a combination of intrinsic and proximity-induced magnetic moments. Currently, it is unknown whether the specific ultrafast dynamics of different sublattices in the popular ferromagnets consisting of 3d (Co, Fe) and 4d, 5d (Pd, Pt) transition metals are playing a crucial role in various effects, including all-optical magnetization switching. Here we investigate the element-specific dynamics of Co-Pt alloys on femtosecond and picosecond time scales using magneto-optical spectroscopy in the extended ultraviolet (EUV) region. Our results reveal that despite the proximity-induced nature of the magnetization of Pt atoms, the two sublattices in the alloy can have different responses to the optical excitation featuring distinct demagnetization rates. Additionally we show that it is important to consider the modification of magnetic anisotropy in opto-magnetic experiments as the vast majority of them are sensitive only to a single projection of the magnetic moment on the predefined axis, which may lead to experimental artifacts.

Physical review / B 105(3), 035124 (2022). doi:10.1103/PhysRevB.105.035124, Co$_2$-based Heusler compounds are promising materials for spintronics applications due to their high Curie temperature, large spin polarization, large magnetization density, and exotic transport properties. In the present paper, we report the anomalous Hall effect (AHE) in a polycrystalline Co$_2$FeAl Heusler compound using combined experimental and theoretical studies. The Rietveld analysis of high-resolution synchrotron x-ray diffraction data reveals a large degree (∼50%) of antisite disorder between Fe and Al atoms. The analysis of anomalous transport data provides the experimental anomalous Hall conductivity (AHC) about 227 S/cm at 2 K with an intrinsic contribution of 155 S/cm, which has nearly constant variation with temperature. The detailed scaling analysis of anomalous Hall resistivity suggests that the AHE in Co$_2$FeAl is governed by the Berry phase driven intrinsic mechanism. Our theoretical calculations reveal that the disorder present in the Co$_2$FeAl compound enhances the Berry curvature induced intrinsic AHC., Published by Inst., Woodbury, NY

Ultrafast demagnetization induced by femtosecond laser pulses in thin metallic layers is caused by the outflow of spin-polarized hot electron currents describable by the superdiffusive transport model. These laser-generated spin currents can cross the interface into another magnetic layer and give rise to magnetization dynamics in magnetic spin valves with noncollinear magnetizations. To describe ultrafast transport and spin dynamics in such nanostructures we develop here the superdiffusive theory for general noncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Ru multilayer system with noncollinear Ni and Fe magnetic moments and analyze how the ultrafast demagnetization and spin-transfer torque depend on the noncollinearity. We employ ab initio calculations to compute the spin- and energy-dependent transmissions of hot electrons at the interfaces of the multilayer. Taking into account multiple electron scattering at interfaces and spin mixing in the spacer layer we find that the laser-induced demagnetization of the Ni layer and magnetization change of the Fe layer strongly depend on the angle between their magnetizations. Similarly, the spin-transfer torques on the Ni and Fe layers and the total spin momentum absorbed in the Ni and Fe layer are found to vary markedly with the amount of noncollinearity. These results suggest that changing the amount of noncollinearity in magnetic multilayers one can efficiently control the hot electron spin transport, which may open a way toward achieving fast, laser-driven spintronic devices., Comment: 14 pages, 9 figures

The antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure is a promising material for the AFM spintronics. The resistivity measurements indicate the presence of defects about whose types and concentrations is more speculated as known. We confirmed vacancies on Mn or Cu sublattices and Mn$_{\rm Cu}$ and Cu$_{\rm Mn}$ antisites as most probable defects in CuMnAs by our new ab initio total energy calculations. We have estimated resistivities of possible defect types as well as resistivities of samples for which the X-ray structural analysis is available. In the latter case we have found that samples with Cu- and Mn-vacancies with low formation energies have also resistivities which agree well with the experiment. Finally, we have also calculated exchange interactions and estimated the N\'eel temperatures by using the Monte Carlo approach. A good agreement with experiment was obtained., Comment: 6 pages, 2 figures, presented on JEMS2018, to be published in JMMM

We investigate theoretically the combined effect of phonons and magnons caused by finite temperatures on the electrical resistivity of nonstoichiometric half-Heusler NiMnSb alloy. The coherent potential approximation within the alloy analogy model is employed for an efficient treatment of chemical impurities, atomic displacements, and magnetic disorder. Spin fluctuations of local Mn moments are described by two models: (i) uncompensated disordered local moment approach and (ii) tilting of the moments. The calculated resistivity agrees with experimental data, the agreement is good up to 600 K. We show that a strong magnetic disorder leads to a violation of the Matthiessen’s rule for the resistivity. We also discuss the spin polarization of the electrical current which exceeds 90% at room temperature but it is dramatically reduced by the magnetic disorder for higher temperatures approaching the Curie point ( T C = 730 K).

Recently discovered ferromagnetism of the layered van der Waals material ${\mathrm{VI}}_{3}$ attracts much research attention. Despite substantial progress, in the following important aspects no consensus has been reached: (i) a possible deviation of the easy axis from the normal to the ${\mathrm{VI}}_{3}$ layers, (ii) a possible inequivalence of the V atoms, (iii) the value of the V magnetic moments. The theoretical works differ in the conclusions on the conduction nature of the system, the value, and the role of the V orbital moments. To the best of our knowledge there are no theoretical works addressing issues (i) and (ii) and only one work dealing with the reduced value of the V moment. By combining the symmetry arguments with density functional theory (DFT) and $\mathrm{DFT}+U$ calculations we have shown that the antidimerization distortion of the crystal structure reported by Son et al. [Phys. Rev. B 99, 041402(R) (2019)] must lead to the deviation of the easy axis from the normal to the ${\mathrm{VI}}_{3}$ layers in close correlation with the experimental results. The antidimerization accompanied by breaking the inversion symmetry leads to the inequivalence of the V atoms. Our $\mathrm{DFT}+U$ calculations result in large value $\ensuremath{\sim}0.8\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}$ of the V orbital moments of the V atoms leading to reduced total V moment in agreement with a number of experimental results and with the physical picture suggested by Yang et al. [Phys. Rev. B 101, 100402(R) (2020)]. We obtained large intra-atomic noncollinearity of the V spin and orbital moments revealing strong competition between effects coursed by the onsite electron correlation, spin-orbit coupling, and interatomic hybridization since pure intra-atomic effects lead to collinear spin and orbital moments. Our calculations confirm the experimental results of strong magnetoelastic coupling revealing itself in the strong dependence of the magnetic properties on the distortion of the atomic structure.

Evolution of magnetism in single crystals of the van der Waals compound $\mathrm{V}{\mathrm{I}}_{3}$ in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at ${T}_{1}=54.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{2}=53\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{C}}=49.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{FM}}=26\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively, have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature ${T}_{\mathrm{C}}$ and the transition between two different ferromagnetic phases ${T}_{\mathrm{FM}}$, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. ${T}_{\mathrm{C}}$ was found to be almost unaffected by pressures up to 0.6 GPa whereas ${T}_{\mathrm{FM}}$ increases rapidly with increasing pressure and reaches ${T}_{\mathrm{C}}$ at a triple point at \ensuremath{\approx} 0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is significantly reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. First-principles calculations of $\mathrm{V}{\mathrm{I}}_{3}$ under pressure allow us to ascribe the evolution of ${T}_{\mathrm{C}}$ with pressure to the reduction of interplanar distance, including the observed slope change at 0.6 GPa. These calculations also describe the associated band gap closing, showing that with a modest compression the material would become metallic. Overall, the large pressure range covered corresponds to a significant change of interplanar interactions. The obtained data thus allow us to shed light on how does the transition between the three-dimensional (3D) and quasi-2D system affect magnetic interactions in the system.

Knowledge of the magnetic ordering temperature is one of the fundamental physical properties characterizing material behavior. Magnetism is not only related to the magnitude of magnetic moments within specific magnetic ordering, but it also influences other physical properties. In the present work, we determine magnetic ordering temperatures for high-entropy alloys (HEAs), with examples of the Al- and Mo-based Cantor alloy derivatives. Nowadays, HEAs represent a promising class of materials in the sense of mechanical engineering. Regarding the paramagnetic state of the well-known and well-explored Cantor alloy, we deal with its Mo and Al derivatives, which offer higher possible ordering temperatures. We discuss the differences between $p$-type and $d$-type substitution as well as their influence on the magnetic behavior. We determine the ordering temperatures based on the ab initio calculated magnetic exchange interactions in terms of the mean-field approximation, random-phase approximation, and Monte Carlo simulations. Based on the thorough description of magnetic pair exchange interactions, a particular composition's influence on the achieved critical temperatures is described.

We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferromagnets by studying the action of the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density functional theory and atomistic spin dynamics simulations, we show how a circularly polarized laser pulse can switch the order parameter of the antiferromagnet within a few hundred femtoseconds. This nonthermal switching takes place on an elliptical path, driven by the staggered magnetic moments induced by the inverse Faraday effect and leading to reliable switching between two perpendicular magnetic states., Comment: 6 pages, 6 figures

Recent experiments on a single crystal of ${\mathrm{U}}_{2}{\mathrm{Ni}}_{2}\mathrm{Sn}$ established a strong uniaxial magnetic anisotropy and an unusual magnetic structure that is antiferromagnetic both inside the U layers and between the layers. We apply fully relativistic first-principles calculations to investigate various magnetic configurations of the system. The calculations confirmed both the character of the magnetic anisotropy and the type of the ground-state magnetic structure. The study of the electron density of states shows that the lowest energy of the ground-state structure is related to the doubling of the magnetic unit cell and corresponding reduction of the Brillouin zone, leading to the formation of the energy gaps in the electronic structure. The mapping of the energies of the magnetic structures on the Heisenberg Hamiltonian of interacting atomic moments leads to a surprising result that only one of the interatomic exchange interactions is antiferromagnetic. We explain how this one antiferromagnetic interaction competing with several ferromagnetic interactions leads to the simultaneous presence of two types of antiferromagnetic behavior. We provide a simple mean-field estimate of the N\'eel temperature and conclude that short-range magnetic order, neglected on the mean-field level, may play an important role at temperatures around the critical temperature. We demonstrate the existence of self-consistent noncollinear magnetic states that limits the accuracy of treating ${\mathrm{U}}_{2}{\mathrm{Ni}}_{2}\mathrm{Sn}$ as a pure Ising magnet. We apply $\mathrm{GGA}+U$ method to examine the influence of the Hubbard parameter $U$ on the magnetic moments and energies of the magnetic configurations. An estimation of the influence of the contraction of the lattice on the magnetic properties is reported.

Manipulation of magnetic domain walls via a helicity-independent laser pulse has recently been experimentally demonstrated and various physical mechanisms leading to domain wall dynamics have been discussed. Spin-dependent superdiffusive transport of hot electrons has been identified as one of the possible ways how to affect a magnetic domain wall. Here, we develop a model based on superdiffusive spin-dependent transport to study the laser-induced transport of hot electrons through a smooth magnetic domain wall. We show that the spin transfer between neighboring domains can enhance ultrafast demagnetization in the domain wall. More importantly, our calculations reveal that when the laser pulse is properly focused on to the vicinity of the domain wall, it can excite sufficiently strong spin currents to generate a spin-transfer torque that can rapidly move the magnetic domain wall by several nanometers in several hundreds of femtoseconds, leading to a huge nonequilibrium domain wall velocity.

The effect of Cu substitution on the stability of the CuMnAs tetragonal phase was studied both experimentally and by ab initio calculations. Polycrystalline samples with various compositions Cu$_{1+x}$Mn$_{1-x}$As $(x = 0 - 0.5)$ were synthetized. The tetragonal phase of CuMnAs is found to be stabilized by substituting Mn by Cu in the amount of $x$ ~ 0.1 or higher. This observation is supported by ab initio calculations of the total energy of the tetragonal and orthorhombic phases; with increasing Cu content the tetragonal phase is favoured. Small variations of composition thus allow to grow selectively one of these two phases with distinct and unique features for antiferromagnetic spintronics. Measurements of magnetic susceptibility and differential scanning calorimetry have shown that the tetragonal Cu$_{1+x}$Mn$_{1-x}$As has an antiferromagnetic behaviour with the maximum N\'eel temperature $T_N$ = 507 K for the highest Mn content samples, decreasing with the decreasing Mn content.

Doping ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ by magnetic ions represents an interesting problem since it may break the time-reversal symmetry needed to maintain the topological insulator character. Mn dopants in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory has focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by a van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T}_{\mathrm{C}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T}_{\mathrm{C}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.

4f spin to phonon coupling is crucial for ultrafast spin-wave generation and magnetization dynamics in rare-earth metals., Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin–lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.

A new prototype tunnel junction with AgBr based barrier is proposed in this study. Electronic and magnetic properties of Fe/AgBr/Fe (001) heterostructures are investigated on the basis of first-principles and ballistic conductance calculations. The interlayer exchange coupling and electronic transport properties are analyzed as functions of the AgBr barrier thickness. The results show that Fe magnetism at Fe/AgBr (001) interfaces is enhanced as compared with the bulk material. A very strong direct tunnelling of Fe Δ1 states is observed. The minority-spin channel is characterized by resonant tunnelling of interfacial states that are forming at Fe/AgBr (001) interfaces. Spin-polarization of the tunnelling current is positive and can exceed 99%. Tunnelling magnetoresistance ratios of about 8000% are evidenced for the junctions with clean interfaces. Fe/AgBr (001) coherent interfaces could act as nearly ideal symmetry-filters. In the presence of interfacial disorder the tunnelling magnetoresistance ratios decrease dramatically. The exchange coupling between Fe electrodes oscillates as a function of the barrier thickness.

Ab initio calculations based on the fully relativistic Dirac approach and incorporating chemical disorder and temperature-induced atomic displacements (phonons) are presented. The tight-binding linear muffin-tin orbital method is used, and the multicomponent coherent potential approximation deals with the chemical disorder and phonons on the same level. Electrical resistivities calculated without and with spin–orbit interaction are compared, and the anomalous Hall effect for magnetic alloys is investigated. The developed technique is tested on pure nickel and on random binary Cu–Ni alloys. The calculated results are found to be in good agreement with experimental and other theoretical data. The combined effect of phonons and spin disorder, simulated within the disordered local moment state, has also been studied. The results confirm the validity of Matthiessen’s rule in a wide range of temperatures for the electrical resistivity of pure nickel.

Femtosecond laser pulses can induce ultrafast demagnetization as well as generate bursts of hot-electron spin currents. In trilayer spin valves consisting of two metallic ferromagnetic layers separated by a nonmagnetic one, hot-electron spin currents excited by an ultrashort laser pulse propagate from the first ferromagnetic layer through the spacer, reaching the second magnetic layer. When the magnetizations of the two magnetic layers are noncollinear, this spin current exerts a torque on magnetic moments in the second ferromagnet. Since this torque is acting only within the subpicosecond timescale, it excites coherent high-frequency magnons, as recently demonstrated in experiments. Here, we calculate the temporal shape of the hot-electron spin currents using the superdiffusive transport model and simulate the response of the magnetic system to the resulting ultrashort spin-transfer torque pulse by means of atomistic spin-dynamics simulations. Our results confirm that the acting spin-current pulse is short enough to excite magnons with frequencies beyond $1\phantom{\rule{0.16em}{0ex}}\mathrm{THz}$, a frequency range out of reach for current-induced spin-transfer torques. We demonstrate the formation of thickness-dependent standing spin waves during the first picoseconds after laser excitation. In addition, we vary the penetration depth of the spin-transfer torque to reveal its influence on the excited magnons. Our simulations clearly show a suppression effect of magnons with short wavelengths already for penetration depths in the range of $1\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, confirming experimental findings reporting penetration depths below $2\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$.

We have measured the hot-electron induced demagnetization of a [Co/Pt]2 multilayer in M(x nm)/Cu(100 nm)/[Co(0.6 nm)/Pt(1.1 nm)]2 samples depending on the nature of the capping layer M and its thickness x. We found out that a Pt layer is more efficient than [Co/Pt]X, Cu or MgO layers in converting IR photon pulses into hot-electron pulses at a given laser power. We also found out that the maximum relative demagnetization amplitude is reached for M(x) = Pt (7 nm). Our experimental results show qualitative agreement with numerical simulations based on the superdiffusive spin transport model. We concluded that the maximum relative demagnetization amplitude, which corresponds to the highest photon conversion into hot-electrons, is an interplay between the IR penetration depth and the hot-electron inelastic mean free path within the capping layer., Comment: 10 pages, 3 figures

The interest in Fe-chalcogenide unconventional superconductors is intense after the critical temperature of FeSe was reported enhanced by more than one order of magnitude in the monolayer limit at the interface to an insulating oxide substrate. In heterostructures comprising interfaces of FeSe with topological insulators, additional interesting physical phenomena is predicted to arise e.g. in form of {\it topological superconductivity}. So far superconductive properties of Fe-chalcogenide monolayers were mostly studied by local scanning tunneling spectroscopy experiments, which can detect pseudo-gaps in the density of states as an indicator for Cooper pairing. Direct macroscopic transport properties which can prove or falsify a superconducting phase were rarely reported due to the difficulty to grow films with homogeneous material properties. Here we report on a promising growth method to fabricate continuous carpets of monolayer thick FeSe on molecular beam epitaxy grown Bi$_2$Se$_3$ topological insulator thin films. In contrast to previous works using atomically flat cleaved bulk Bi$_2$Se$_3$ crystal surfaces we observe a strong influence of the high step-edge density (terrace width about 10~nm) on MBE-grown Bi$_2$Se$_3$ substrates, which significantly promotes the growth of coalescing FeSe domains with small tetragonal crystal distortion without compromising the underlying Bi$_2$Se$_3$ crystal structure., 11 pages, 8 figures

Multiprincipal element alloys, called high-entropy alloys represent a promising group of materials. They possess unique mechanical or electrical properties, which provide a wide range of potential applications. In general, mechanical or electrical properties of a material are influenced by the magnetic behavior. Therefore, in our work, we are exploring magnetic behavior of so-called ``Cantor alloy'' CoCrFeMnNi and its molybdenum based derivatives. Mo alloys were studied not only to carefully describe their properties but also to probe the magnetic behavior of the parent alloy by adding a nonmagnetic element. Based on ab initio calculations using the TB-LMTO-ASA (tight-binding linear-muffin-tin-orbital atomic-sphere approximation) method within CPA (coherent-potential approximation), we have found the ground-state magnetic structures of a particular alloy. We deal with various magnetic structures including complex structures beyond the simple FM or DLM phases. We show the influence of the presence of a particular element on the magnetic properties. It includes, e.g., magnitudes of magnetic moments or preferred magnetic phases. The calculations were extended by studying the magnetic exchange interaction employing the Liechtenstein formula. We clearly show the contribution of each element to the magnetism as a function of the composition or crystal structure. We provide a thorough description of magnetic behavior in the mentioned compounds.

Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering, we gain a detailed understanding of the complex nonequilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wave-vector-resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy, a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond time scale. We determine the evolving nonequilibrium phonon occupations, which we find to deviate markedly from thermal occupations.

Magnetotransport constitutes a useful probe to understand the interplay between electronic band topology and magnetism in spintronic devices. A recent theory of Lu and Shen [Phys. Rev. Lett. 112, 146601 (2014)] on magnetically doped topological insulators predicts that quantum corrections $\mathrm{\ensuremath{\Delta}}\ensuremath{\kappa}$ to the temperature dependence of conductivity can change sign across the Curie transition. This phenomenon has been attributed to a suppression of the Berry phase of the topological surface states at the Fermi level, caused by a magnetic energy gap. Here, we demonstrate experimentally that $\mathrm{\ensuremath{\Delta}}\ensuremath{\kappa}$ can reverse its sign even when the Berry phase at the Fermi level remains unchanged. The contradictory behavior to theory predictions is resolved by extending the model by Lu and Shen to a nonmonotonic temperature scaling of the inelastic scattering length showing a turning point at the Curie transition.

Noncollinear magnetic moments in antiferromagnets (AFM) lead to a complex behavior of electrical transport, even to a decreasing resistivity due to an increasing temperature. Proper treatment of such phenomena is required for understanding AFM systems at finite temperatures; however first-principles description of these effects is complicated. With ab initio techniques, we investigate three models of spin fluctuations (magnons) influencing the transport in AFM CuMnAs; the models are numerically feasible and easily implementable to other studies. We numerically justified a fully relativistic collinear disordered local moment approach and we present its uncompensated generalization. A saturation or a decrease of resistivity caused by magnons, phonons, and their combination (above approx. 400 K) was observed and explained by changes in electronic structure. Within the coherent potential approximation, our finite-temperature approaches may be applied also to systems with impurities, which are found to have a large impact not only on residual resistivity, but also on canting of magnetic moments from the AFM to the ferromagnetic (FM) state., 25 pages, 8 figures, 2 tables; accepted version to JMMM

Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction—the underlying magnetoelastic stress—can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the sub-picosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization., Although magnetostriction is universal in magnetic materials, understanding its microscopic origin remains challenging. Here the authors use X-ray and ultrafast electron diffraction to separate the material’s sub-picosecond spin and lattice responses and reveal the magnetoelastic stress generated by demagnetization.

A thorough investigation is presented of the magnetic and structural properties of Mn-doped Bi2Se3 topological insulators grown by molecular beam epitaxy on top of insulating BaF2 (111) substrates. The magnetic properties have been studied in the temperature range from 2 K to 300 K in magnetic fields up to 7 T. The systems were further characterized by means of high-resolution X-ray diffraction, electron-microprobe analysis, and X-ray photoemission spectroscopy. Samples with the atomic concentration of Mn up to about 0.06 exhibit an almost perfect crystalline structure while, for higher Mn concentrations, diffuse scattering from defects is observed. Photoemission results suggest a localized non-metallic Mn 3d5 ground state which is weakly or intermediately coupled to the Bi2Se3 environment. The exchange interaction between the Mn moments leads to a ferromagnetic phase at low temperatures with a roughly linear relation between the Curie temperature and the atomic concentration of Mn.

The structural stability and ground state electronic properties of Fe/AgCl/Fe (0 0 1) junctions with c (2 × 2) Fe/AgCl (0 0 1) heterointerfaces are studied by means of ab␣initio surface Green’s function technique for surface and interfaces. The spin-resolved ballistic conductances across Fe/AgCl/Fe (0 0 1) heterojunctions are determined by using Kubo–Landauer approach. Total energy calculations show that sharp Fe/AgCl (0 0 1) interfaces are possible. The interfacial charge transfer is interface sensitive and generates electrostatic interactions that contribute to the stabilization of Fe/AgCl (0 0 1) interfaces. The magnetism of interfacial Fe layers is robust but is responsive to the interfacial structure. The exchange coupling between Fe electrodes across AgCl barriers shows unexpected oscillations with respect to the spacer thickness that may be related in part with the amount of metal induced gap states in AgCl spacers. A powerful Δ 1 spin filter effect is evidenced across AgCl barriers. Depending on the interface structure, large magnetoresistance ratios, above 3100%, are predicted. Interfacial interdiffusion may affect the magnetoresistive properties. The lower lattice mismatch at c (2 × 2) Fe/AgCl (0 0 1) interfaces combined with the strong Δ 1 direct tunnelling across AgCl barriers make Fe/AgCl (0 0 1) interfaces attractive in the context of magnetoelectronics.

Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and nonselective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of $\ensuremath{\approx}100\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$. We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting nonconducting materials for achieving coherent THz-magnetization reversal.

“The technical support from SLAC Accelerator Directorate, Technology Innovation Directorate, LCLS laser division and Test Facility Division is gratefully acknowledged. We thank S.P. Weathersby, R.K. Jobe, D. McCormick, A. Mitra, S. Carron and J. Corbett for their invaluable help and technical assistance. Research at SLAC was supported through the SIMES Institute which like the LCLS and SSRL user facilities is funded by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-76SF00515. The UED work was performed at SLAC MeV-UED, which is supported in part by the DOE BES SUF Division Accelerator & Detector R&D program, the LCLS Facility, and SLAC under contract Nos. DE-AC02-05-CH11231 and DE-AC02-76SF00515. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.”and“Work at BNL was supported by DOE BES Materials Science and Engineering Division under Contract No: DE-AC02-98CH10886. J.C. would like to acknowledge the support from National Science Foundation Grant No. 1207252. E.E.F. would like to acknowledge support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES) under Award No. DE-SC0003678.”This has been corrected in both the PDF and HTML versions of the Article.

Ultrafast laser excitation of a metal causes correlated, highly nonequilibrium dynamics of electronic and ionic degrees of freedom, which are however only poorly captured by the widely-used two-temperature model. Here we develop an out-of-equilibrium theory that captures the full dynamic evolution of the electronic and phononic populations and provides a microscopic description of the transfer of energy delivered optically into electrons to the lattice. All essential nonequilibrium energy processes, such as electron-phonon and phonon-phonon interactions are taken into account. Moreover, as all required quantities are obtained from first-principles calculations, the model gives an exact description of the relaxation dynamics without the need for fitted parameters. We apply the model to FePt and show that the detailed relaxation is out-of-equilibrium for picoseconds., Comment: 6 pages, 5 figures

Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser pulse is accompanied by a nonthermal spin-polarized current of hot electrons. These spin currents are studied here theoretically in a spin valve with noncollinear magnetizations. To this end, we introduce an extended model of superdiffusive spin transport that enables the treatment of noncollinear magnetic configurations, and apply it to the perpendicular spin valve geometry. We show how spin-transfer torques arise due to this mechanism and calculate their action on the magnetization present, as well as how the latter depends on the thicknesses of the layers and other transport parameters. We demonstrate that there exists a certain optimum thickness of the out-of-plane magnetized spin-current polarizer such that the torque acting on the second magnetic layer is maximal. Moreover, we study the magnetization dynamics excited by the superdiffusive spin-transfer torque due to the flow of hot electrons employing the Landau-Lifshitz-Gilbert equation. Thereby we show that a femtosecond laser pulse applied to one magnetic layer can excite small-angle precessions of the magnetization in the second magnetic layer. We compare our calculations with recent experimental results.

The finite-temperature electrical transport properties depending on the spin are essential for spintronics research focused on developing devices that should operate not only in the conditions of low temperatures. In this study we present a theoretical approach incorporating both chemical and temperature-induced disorder within the coherent potential approximation and the tight-binding linear muffin-tin orbital method, and the linear response theory is used to obtain spin-resolved electrical conductivity. Both nonmagnetic and magnetic materials are studied from the first principles in a wide temperature range. It was found, with neglected magnetic disorder, that vertex corrections to the total conductivity and spin-flip contributions to the conductivity are small; therefore, the spin-resolved coherent conductivities can be used to describe spin-dependent electrical transport. The developed formalism is applied to pure nonmagnetic platinum and to ferromagnetic random Cu-Ni alloys. For the latter system, the spin polarization of the current is nearly constant in the examined temperature range.

We compare the magnetic and electronic configuration of single Mn atoms in molecular beam epitaxy (MBE) grown Bi2Se3 thin films, focusing on electron paramagnetic (ferromagnetic) resonance (EPR and FMR, respectively) and superconducting quantum interference device (SQUID) techniques. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) reveal the expected increase of disorder with increasing concentration of magnetic guest atoms, however, Kikuchi patterns show that disorder consists majorly of μm-scale 60° twin domains in the hexagonal Bi2Se3 structure, which are promoted by the presence of single unclustered Mn impurities. Ferromagnetism below TC ~ (5.4±0.3) K can be well described by critical scaling laws M ( T ) ~ ( 1 − T / T C ) β with a critical exponent β = ( 0.34 ± 0.2 ) , suggesting 3D Heisenberg class magnetism instead of e.g. 2D-type coupling between Mn-spins in van der Waals gap sites. From EPR hyperfine structure data we determine a Mn2+ (d5, S = 5/2) electronic configuration with a g-factor of 2.002 for −1/2 → +1/2 transitions. In addition, from the strong dependence of the low temperature FMR fields and linewidth on the field strength and orientation with respect to the Bi2Se3 (0001) plane, we derive magnetic anisotropy energies of up to K1 = −3720 erg/cm3 in MBE-grown Mn-doped Bi2Se3, reflecting the first order magneto-crystalline anisotropy of an in-plane magnetic easy plane in a hexagonal (0001) crystal symmetry. We observe an increase of K1 with increasing Mn concentration, which we interpret to be correlated to a Mn-induced in-plane lattice contraction. Across the ferromagnetic-paramagnetic transition the FMR intensity is suppressed and resonance fields converge the paramagnetic limit of Mn2+ (d5, S = 5/2).

Structural, electronic and spin-dependent transport properties of Fe/NaCl/Fe (0 0 1) tunnel junctions with c(2 × 2) structure are studied by means of first-principles calculations. Several Fe/NaCl (0 0 1) interfaces are considered and their stability is analyzed. The interfacial charge transfer has an important role in stabilizing Fe/NaCl (0 0 1) interfaces. Interfacial iron magnetic moments are enhanced over the bulk value. Fe induced gap states are observed in barrier layers on both Na and Cl ions. Small exchange couplings with exponential decays as function of the barriers thickness are evidenced. Resonant tunnelling plays a major role on the spin-polarized transport properties. High magnetoresistance ratios comparable to Fe/MgO/Fe junctions are predicted. Total energy calculations show that interfacial interdiffusion is not favourable but the disorder at interfaces may strongly affect the electronic transport properties. Absence of interfacial interdiffusion together with the small lattice mismatch between Fe and NaCl layers are important advantages of Fe/NaCl/Fe (0 0 1) junctions for the purpose of spintronic applications.

Unraveling the origin of ultrafast demagnetization in multisublattice ferromagnetic materials requires femtosecond x-ray techniques to trace the magnetic moment dynamics on individual elements, but this could not yet be achieved in the hard x-ray regime. We demonstrate here the first ultrafast demagnetization dynamics in the ferromagnetic heavy 5d-transition metal Pt using circularly-polarized hard x-rays at an x-ray free electron laser (XFEL). The decay time of laser-induced demagnetization of L10-FePt is determined to be τ Pt = 0.61 ± 0.04 ps using time-resolved x-ray magnetic circular dichroism at the Pt L3 edge, whereas magneto-optical Kerr measurements indicate the decay time for the total magnetization as τ total < 0.1 ps . A transient magnetic state with a photo-modulated ratio of the 3d and 5d magnetic moments is demonstrated for pump–probe delays larger than 1 ps. We explain this distinct photo-modulated transient magnetic state by the induced-moment behavior of the Pt atom and the x-ray probing depth. Our findings pave the way for the future use of XFELs to disentangle atomic spin dynamics contributions.

Optical manipulation of magnetization with femtosecond laser pulses has opened a new paradigm shift in magnetization and spin dynamics. Since the very first demonstration of femtosecond laser-induced demagnetization in 1996 by Beaurepaire et al., optical manipulation of magnetization has developed into a highly active area of research that has been growing at a breathtaking pace over the last decades. Scientific highlights include the discovery of femtosecond timescale laser-induced demagnetization, all-optical generation of coherent spin waves, photo-induced generation of magnetization, and many others. Undoubtedly, the most appealing is the recent demonstration of all-optical magnetization reversal and opto-magnetic recording with femtosecond laser pulses. All-optical magnetization switching (AOS) is emerging as a novel magnetic recording technology, its potential being fully recognized by the magnetic recording industry, which enlisted the AOS process on its roadmap toward ultrafast and ultrahigh densities magnetic recording beyond 1 Tb/in2. Here we review experimental work, models developed to explain these experiments, and introduce relevant theoretical concepts needed to construct these models. Validity of different involved methods and approximations is critically examined, employing ab initio calculation results where possible. Proposed explanations of ultrafast magnetization dynamics involve different underlying physical mechanisms, the last section of the review provides a discussion of the possible contributions of these mechanisms. We note that the ultrafast magnetism field has become very vast and the present review cannot exhaustively describe all the works related to laser-induced magnetization dynamics.

We uncover the multiple mechanisms underlying laser-driven demagnetization in cobalt using a tabletop high harmonic source. Ultrafast magnon excitation, as well as a transient reduction of the exchange splitting, are both important on femtosecond timescales.

We present time-resolved measurements exploring the THz-induced magnetization dynamics as function of the driving field strength in the ferromagnetic thin film samples Co, Fe and Ni. The experimental results are excellently reproduced by ab-initio calculations.

Electronic, magnetic, and transport properties of the antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure, promising for the AFM spintronics, are studied from first principles using the Vienna ab-initio simulation package. We investigate the site-occupation of sublattices and the lattice parameters of three competing phases. We analyze the factors that determine which of the three conceivable structures will prevail. We then estimate formation energies of possible defects for the experimentally prepared lattice structure. Mn$_{\rm Cu}$- and Cu$_{\rm Mn}$-antisites as well as Mn$\leftrightarrow$Cu swaps and vacancies on Mn or Cu sublattices were identified as possible candidates for defects in CuMnAs. We find that the interactions of the growing thin film with the substrate and with vacuum as well as the electron correlations are important for the phase stability while the effect of defects is weak. In the next step, using the tight-binding linear muffin-tin orbital method for the experimental structure, we estimate transport properties for systems containing defects with low formation energies. Finally, we determine the exchange interactions and estimate the N\'eel temperature of the AFM-CuMnAs alloy using the Monte Carlo approach. A good agreement of the calculated resistivity and N\'eel temperature with experimental data makes possible to draw conclusions concerning the competing phases., Comment: 9 pages, 5 figures, accepted Phys. Rev. B

The excitation of a ferromagnetic film by a femtosecond laser pulse causes an unexpectedly fast quenching of the film's magnetization on subpicosecond time scales. The microscopic physical mechanisms responsible for this remain a scientific puzzle. The authors employ femtosecond extreme ultraviolet pulses produced by high harmonic generation to follow how the magnetization of a thin cobalt film evolves after the excitation by a 40-fs laser pulse. By measuring the time-, energy-, and angle-resolved magneto-optical response of the Co films across the ${M}_{2,3}$ absorption edge, they obtain a set of time-lapsed magnetic asymmetry spectra, which contain a wealth of information about the different mechanisms at work. When combined with advanced ab initio magneto-optical calculations, they identify two dominant contributions: first, a transient reduction of exchange splitting, and second, magnon excitation. This work thus distinguishes between two fundamental models of magnetism, the Stoner and Heisenberg models, which ascribe magnetization dynamics to an exchange splitting reduction and spin wave excitations, respectively.

We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is $\ensuremath{\sim}100$ fs. An effective electronic Gr\"uneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.

We present a first-principles study of the electronic, magnetic, and transport properties of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ doped with Mn atoms in substitutional $({\mathrm{Mn}}_{\mathrm{Bi}})$ and interstitial van der Waals gap positions $({\mathrm{Mn}}_{i})$, which act as acceptors and donors, respectively. The effect of native ${\mathrm{Bi}}_{\mathrm{Te}}$- and ${\mathrm{Te}}_{\mathrm{Bi}}$-antisite defects and their influence on calculated electronic transport properties is also investigated. We have studied four models representing typical cases, namely, (i) ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ with and without native defects, (ii) ${\mathrm{Mn}}_{\mathrm{Bi}}$ defects with and without native defects, (iii) the same, but for ${\mathrm{Mn}}_{i}$ defects, and (iv) the combined presence of ${\mathrm{Mn}}_{\mathrm{Bi}}$ and ${\mathrm{Mn}}_{i}$. It has been found that lattice relaxations around ${\mathrm{Mn}}_{\mathrm{Bi}}$ defects play an important role for both magnetic and transport properties. The resistivity is strongly influenced by the amount of carriers, their type, and by the relative positions of the Mn-impurity energy levels and the Fermi energy. Our results suggest strategies to tune bulk resistivities and also clarify the location of Mn atoms in samples. Calculations indicate that at least two of the considered defects have to be present simultaneously in order to explain the experimental observations, and the role of interstitials may be more important than expected.

Magnetic anisotropies of random body-centered tetragonal Fe–Co alloys are studied by means of relativistic ab initio electronic structure calculations. Special attention is paid to the values and trends of magnetic moments and magnetic anisotropy energies as well as to their sensitivity to current approximations employed. It is found that the high uniaxial anisotropy energy K u, predicted for alloys with 60 % of Co by several authors, is reduced significantly in the coherent potential approximation describing the chemical disorder in real alloys. On the other hand, the four-component Dirac formalism leads to enhanced values of the K u as compared to a simple perturbative treatment of the spin-orbit interaction. Similar behavior is found for the orbital magnetic moments, the values of which are systematically smaller than the measured ones irrespective of the theoretical approach used.