Showing papers on "Antiferromagnetism published in 2016"

Electrical switching of an antiferromagnet.

Abstract: Antiferromagnets are hard to control by external magnetic fields because of the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization. However, relativistic quantum mechanics allows for generating current-induced internal fields whose sign alternates with the periodicity of the antiferromagnetic lattice. Using these fields, which couple strongly to the antiferromagnetic order, we demonstrate room-temperature electrical switching between stable configurations in antiferromagnetic CuMnAs thin-film devices by applied current with magnitudes of order 106 ampere per square centimeter. Electrical writing is combined in our solid-state memory with electrical readout and the stored magnetic state is insensitive to and produces no external magnetic field perturbations, which illustrates the unique merits of antiferromagnets for spintronics.

Ising-Type Magnetic Ordering in Atomically Thin FePS3.

Abstract: Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order–disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with TN ∼ 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering.

Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic

Abstract: A single-phase multiferroic material is constructed, in which ferroelectricity and strong magnetic ordering are coupled near room temperature, enabling direct electric-field control of magnetism. Materials that exhibit coupled ferroelectric and magnetic ordering are attractive candidates for use in future memory devices, but such materials are rare and typically exhibit their desirable properties only at low temperatures. Julia Mundy and colleagues now describe and successfully implement a strategy for building artificial layered materials in which ferroelectricity and magnetism are both present, and coupled near room temperature. Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism1,2. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism3. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms4,5,6,7,8,9,10,11,12,13,14,15, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications2. Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO3—the geometric ferroelectric with the greatest known planar rumpling16—we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially—from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)1. Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering.

Monolayer MXenes: promising half-metals and spin gapless semiconductors

Abstract: Half-metals and spin gapless semiconductors are promising candidates for spintronic applications due to the complete (100%) spin polarization of electrons around the Fermi level. Based on recent experimental and theoretical findings of graphene-like monolayer transition metal carbides and nitrides (also known as MXenes), we demonstrate using first-principles calculations that monolayers Ti2C and Ti2N exhibit nearly half-metallic ferromagnetism with the magnetic moments of 1.91 and 1.00μB per formula unit, respectively, while monolayer V2C is a metal with unstable antiferromagnetism, and monolayer V2N is a nonmagnetic metal. Interestingly, under a biaxial strain, there is a phase transition from a nearly half-metal to truly half-metal, spin gapless semiconductor, and metal for monolayer Ti2C. Monolayer Ti2N is still a nearly half-metal under a suitable biaxial strain. Large magnetic moments can be induced by the biaxial tensile and compressive strains for monolayer V2C and V2N, respectively. We also show that the structures of these four monolayer MXenes are stable according to the calculated formation energy and phonon spectrum. Our investigations suggest that, unlike monolayer graphene, monolayer MXenes Ti2C and Ti2N without vacancy, doping or external electric field exhibit intrinsic magnetism, especially the half-metallic ferromagnetism and spin gapless semiconductivity, which will stimulate further studies on possible spintronic applications for new two-dimensional materials of MXenes.

Antiferromagnetic Spin Seebeck Effect

Abstract: We report on the observation of the spin Seebeck effect in antiferromagnetic MnF_{2}. A device scale on-chip heater is deposited on a bilayer of MnF_{2} (110) (30 nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF_{2} (110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF_{2} through the inverse spin Hall effect. The low temperature (2-80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel to the easy axis of the MnF_{2} thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

Ultrathin nanosheets of CrSiTe3: a semiconducting two-dimensional ferromagnetic material

Abstract: Finite range ferromagnetism and antiferromagnetism in two-dimensional (2D) systems within an isotropic Heisenberg model at non-zero temperature were originally proposed to be impossible. However, recent theoretical studies using an Ising model have shown that 2D magnetic crystals can exhibit magnetism. Experimental verification of existing 2D magnetic crystals in this system has remained exploratory. In this work we exfoliated CrSiTe3, a bulk ferromagnetic semiconductor, to mono- and few-layer 2D crystals onto a Si/SiO2 substrate. Raman spectra indicate good stability and high quality of the exfoliated flakes, consistent with the computed phonon spectra of 2D CrSiTe3, giving strong evidence for the existence of 2D CrSiTe3 crystals. When the thickness of the CrSiTe3 crystals is reduced to a few layers, we observed a clear change in resistivity at 80–120 K, consistent with theoretical calculations of the Curie temperature (Tc) of ∼80 K for the magnetic ordering of 2D CrSiTe3 crystals. The ferromagnetic mono- and few-layer 2D CrSiTe3 indicated here should enable numerous applications in nano-spintronics.

Strong interplay between stripe spin fluctuations, nematicity and superconductivity in FeSe

Abstract: In iron-based superconductors the interactions driving the nematic order (that breaks four-fold rotational symmetry in the iron plane) may also mediate the Cooper pairing. The experimental determination of these interactions, which are believed to depend on the orbital or the spin degrees of freedom, is challenging because nematic order occurs at, or slightly above, the ordering temperature of a stripe magnetic phase. Here, we study FeSe (ref. )-which exhibits a nematic (orthorhombic) phase transition at Ts = 90 K without antiferromagnetic ordering-by neutron scattering, finding substantial stripe spin fluctuations coupled with the nematicity that are enhanced abruptly on cooling through Ts. A sharp spin resonance develops in the superconducting state, whose energy (∼4 meV) is consistent with an electron-boson coupling mode revealed by scanning tunnelling spectroscopy. The magnetic spectral weight in FeSe is found to be comparable to that of the iron arsenides. Our results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.

Spin and Charge Resolved Quantum Gas Microscopy of Antiferromagnetic Order in Hubbard Chains

Abstract: The repulsive Hubbard Hamiltonian is one of the foundational models describing strongly correlated electrons and is believed to capture essential aspects of high temperature superconductivity. Ultracold fermions in optical lattices allow for the simulation of the Hubbard Hamiltonian with a unique control over kinetic energy, interactions and doping. A great challenge is to reach the required low entropy and to observe antiferromagnetic spin correlations beyond nearest neighbors, for which quantum gas microscopes are ideal. Here we report on the direct, single-site resolved detection of antiferromagnetic correlations extending up to three sites in spin-$1/2$ Hubbard chains, which requires an entropy well below $s^*=\ln(2)$. Finally, the simultaneous detection of spin and density opens the route towards the study of the interplay between magnetic ordering and doping in various dimensions.

Giant facet-dependent spin-orbit torque and spin Hall conductivity in the triangular antiferromagnet IrMn3

Abstract: There has been considerable interest in spin-orbit torques for the purpose of manipulating the magnetization of ferromagnetic elements for spintronic technologies. Spin-orbit torques are derived from spin currents created from charge currents in materials with significant spin-orbit coupling that propagate into an adjacent ferromagnetic material. A key challenge is to identify materials that exhibit large spin Hall angles, that is, efficient charge-to-spin current conversion. Using spin torque ferromagnetic resonance, we report the observation of a giant spin Hall angle θ SH eff of up to ~0.35 in (001)-oriented single-crystalline antiferromagnetic IrMn 3 thin films, coupled to ferromagnetic permalloy layers, and a θ SH eff that is about three times smaller in (111)-oriented films. For (001)-oriented samples, we show that the magnitude of θ SH eff can be significantly changed by manipulating the populations of various antiferromagnetic domains through perpendicular field annealing. We identify two distinct mechanisms that contribute to θ SH eff : the first mechanism, which is facet-independent, arises from conventional bulk spin-dependent scattering within the IrMn 3 layer, and the second intrinsic mechanism is derived from the unconventional antiferromagnetic structure of IrMn 3 . Using ab initio calculations, we show that the triangular magnetic structure of IrMn 3 gives rise to a substantial intrinsic spin Hall conductivity that is much larger for the (001) than for the (111) orientation, consistent with our experimental findings.

Spin Nernst effect of magnons in collinear antiferromagnets

Abstract: In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around the $\mathrm{\ensuremath{\Gamma}}$ point and the $K$ point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. Possible material candidates are discussed.

Kitaev exchange and field-induced quantum spin-liquid states in honeycomb α -RuCl 3

Abstract: Large anisotropic exchange in 5d and 4d oxides and halides open the door to new types of magnetic ground states and excitations, inconceivable a decade ago. A prominent case is the Kitaev spin liquid, host of remarkable properties such as protection of quantum information and the emergence of Majorana fermions. Here we discuss the promise for spin-liquid behavior in the 4d5 honeycomb halide α-RuCl3. From advanced electronic-structure calculations, we find that the Kitaev interaction is ferromagnetic, as in 5d5 iridium honeycomb oxides, and indeed defines the largest superexchange energy scale. A ferromagnetic Kitaev coupling is also supported by a detailed analysis of the field-dependent magnetization. Using exact diagonalization and density-matrix renormalization group techniques for extended Kitaev-Heisenberg spin Hamiltonians, we find indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field. Our results offer a unified picture on recent magnetic and spectroscopic measurements on this material and open new perspectives on the prospect of realizing quantum spin liquids in d5 halides and oxides in general.

Multiple-stable anisotropic magnetoresistance memory in antiferromagnetic MnTe

Abstract: Commercial magnetic memories rely on the bistability of ordered spins in ferromagnetic materials. Recently, experimental bistable memories have been realized using fully compensated antiferromagnetic metals. Here we demonstrate a multiple-stable memory device in epitaxial MnTe, an antiferromagnetic counterpart of common II–VI semiconductors. Favourable micromagnetic characteristics of MnTe allow us to demonstrate a smoothly varying zero-field antiferromagnetic anisotropic magnetoresistance (AMR) with a harmonic angular dependence on the writing magnetic field angle, analogous to ferromagnets. The continuously varying AMR provides means for the electrical read-out of multiple-stable antiferromagnetic memory states, which we set by heat-assisted magneto-recording and by changing the writing field direction. The multiple stability in our memory is ascribed to different distributions of domains with the Neel vector aligned along one of the three magnetic easy axes. The robustness against strong magnetic field perturbations combined with the multiple stability of the magnetic memory states are unique properties of antiferromagnets. Contrary to ferromagnets, antiferromagnets possess no net magnetic moment, which has limited their applicability as magnetic memory media. Here, the authors demonstrate a heat-assisted multiple-stable memory based on epitaxial thin films of antiferromagnet MnTe with three-fold symmetric anisotropy.

Magnetic Ordering-Induced Multiferroic Behavior in [CH3NH3][Co(HCOO)3] Metal-Organic Framework.

Abstract: We present the first example of magnetic ordering-induced multiferroic behavior in a metal–organic framework magnet. This compound is [CH3NH3][Co(HCOO)3] with a perovskite-like structure. The A-site [CH3NH3]+ cation strongly distorts the framework, allowing anisotropic magnetic and electric behavior and coupling between them to occur. This material is a spin canted antiferromagnet below 15.9 K with a weak ferromagnetic component attributable to Dzyaloshinskii–Moriya (DM) interactions and experiences a discontinuous hysteretic magnetic-field-induced switching along [010] and a more continuous hysteresis along [101]. Coupling between the magnetic and electric order is resolved when the field is applied along this [101]: a spin rearrangement occurs at a critical magnetic field in the ac plane that induces a change in the electric polarization along [101] and [10-1]. The electric polarization exhibits an unusual memory effect, as it remembers the direction of the previous two magnetic-field pulses applied. Th...

Enhanced spin pumping efficiency in antiferromagnetic IrMn thin films around the magnetic phase transition

Abstract: We report measurement of a spin pumping effect due to fluctuations of the magnetic order of IrMn thin films. A precessing NiFe ferromagnet injected spins into IrMn spin sinks, and enhanced damping was observed around the IrMn magnetic phase transition. Our data was compared to a recently developed theory and converted into interfacial spin mixing conductance enhancements. By spotting the spin pumping peak, the thickness dependence of the IrMn critical temperature could be determined and the characteristic length for the spin-spin interactions was deduced.

Emerging magnetism and anomalous Hall effect in iridate-manganite heterostructures.

Abstract: Strong Coulomb repulsion and spin–orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Initial attempts to investigate systems, where both of these fundamental interactions are comparably strong, such as 3d and 5d complex oxide superlattices, have revealed properties that only slightly differ from the bulk ones of the constituent materials. Here we observe that the interfacial coupling between the 3d antiferromagnetic insulator SrMnO3 and the 5d paramagnetic metal SrIrO3 is enormously strong, yielding an anomalous Hall response as the result of charge transfer driven interfacial ferromagnetism. These findings show that low dimensional spin–orbit entangled 3d–5d interfaces provide an avenue to uncover technologically relevant physical phenomena unattainable in bulk materials. Whilst superlattices containing thin films of 5d transition metal oxides are expected to yield strong interfacial coupling, only weak effects have been observed. Here, the authors report strong coupling between 3d SrMnO3 and 5d SrIrO3due to the interplay of strong Coulomb and spin orbit interactions.

Magnetic structure and magnon dynamics of the quasi-two-dimensional antiferromagnet FePS3

Abstract: Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of FePS3, an S = 2 Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of k(M) = [01 1/2] and the moments are collinear with the normal to the ab planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were J(1) = 1.46, J(2) = -0.04, and J(3) = -0.96 meV. The single-ion anisotropy is large, Delta = 2.66 meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, J' = -0.0073 meV, proving that FePS3 is a very good approximation to a two-dimensional magnet.

Mn2C monolayer: a 2D antiferromagnetic metal with high Néel temperature and large spin-orbit coupling.

Abstract: To realize antiferromagnetic spintronics in the nanoscale, it is highly desirable to identify new nanometer-scale antiferromagnetic metals with both high Neel temperature and large spin–orbit coupling In this work, on the basis of first-principles calculation and particle swarm optimization (PSO) global structure search, we demonstrate that a two-dimensional Mn2C monolayer is an antiferromagnetic metal with a Mn magnetic moment of ∼3μB Mn2C monolayer has an anti-site structure of MoS2 sheet with carbon atoms hexagonally coordinated by neighboring Mn atoms Remarkably, the in-plane carrier mobility of 2D Mn2C is highly anisotropic, amounting to about 47 000 cm2 V−1 s−1 in the a′ direction, which is much higher than that of MoS2 monolayer The Neel temperature of Mn2C monolayer is high up to 720 K Due to strong spin–orbit coupling in plane, the magnetic anisotropy energy of Mn2C monolayer is larger than those of pure metals, such as Fe, Co, and Ni These advantages render 2D Mn2C sheet with great potential applications in nanometer-scale antiferromagnetic spintronics

Diffusive magnonic spin transport in antiferromagnetic insulators

Abstract: It has been shown recently that a layer of the antiferromagnetic insulator (AFI) NiO can be used to transport spin current between a ferromagnet (FM) and a nonmagnetic metal (NM). In the experiments one uses the microwave-driven ferromagnetic resonance in a FM layer to produce a spin pumped spin current that flows through an AFI layer and reaches a NM layer where it is converted into a charge current by means of the inverse spin Hall effect. Here we present a theory for the spin transport in an AFI that relies on the spin current carried by the diffusion of thermal antiferromagnetic magnons. The theory explains quite well the measured dependence of the voltage in the NM layer on the thickness of the NiO layer.

Strain-Induced Isostructural and Magnetic Phase Transitions in Monolayer MoN2

Abstract: The change of bonding status, typically occurring only in chemical processes, could dramatically alter the material properties. Here, we show that a tunable breaking and forming of a diatomic bond can be achieved through physical means, i.e., by a moderate biaxial strain, in the newly discovered MoN2 two-dimensional (2D) material. On the basis of first-principles calculations, we predict that as the lattice parameter is increased under strain, there exists an isostructural phase transition at which the N–N distance has a sudden drop, corresponding to the transition from a N–N nonbonding state to a N–N single bond state. Remarkably, the bonding change also induces a magnetic phase transition, during which the magnetic moments transfer from the N(2p) sublattice to the Mo(4d) sublattice; meanwhile, the type of magnetic coupling is changed from ferromagnetic to antiferromagnetic. We provide a physical picture for understanding these striking effects. The discovery is not only of great scientific interest in e...

Thermal spin dynamics of yttrium iron garnet

Abstract: The magnetic insulator yttrium iron garnet can be grown with near perfection and is therefore and ideal conduit for spin currents. It is a complex material with 20 magnetic moments in the unit cell. In spite of being a ferrimagnet, YIG is almost always modeled as a simple ferromagnet with a single spin wave mode. We use the method of atomistic spin dynamics to study the temperature evolution of the full spin wave spectrum, in quantitative agreement with neutron scattering experiments. The antiferromagnetic or optical mode is found to suppress the spin Seebeck effect at room temperature and beyond due to thermally pumped spin currents with opposite polarization to the ferromagnetic mode.

High temperature spin-polarized semiconductivity with zero magnetization in two-dimensional Janus MXenes

Abstract: Searching for two-dimensional (2D) materials with room-temperature magnetic order and high spin-polarization is essential for the development of next-generation nanospintronic devices. A new class of 2D magnetic materials with high Neel temperature, fully compensated antiferromagnetic order (zero magnetization) and completely spin-polarized semiconductivity is proposed for the first time. Based on the density functional theory calculations, we predict these properties for asymmetrically functionalized MXenes (Janus Cr2C) – Cr2CXX′ (X, X′ = H, F, Cl, Br, OH). The valence and conduction bands in these materials are made up of opposite spin channels and they can behave as bipolar magnetic semiconductors with zero magnetization. A Neel temperature as high as 400 K has been found for Cr2CFCl, Cr2CClBr, Cr2CHCl, Cr2CHF, and Cr2CFOH materials. Remarkably, the spin carrier orientation and induced transition from bipolar magnetic semiconductors to half-metal antiferromagnets can be easily controlled by electron or hole doping. The band gap of Janus MXenes can be effectively tuned by the selection of a pair of chemical elements/functional groups terminating the upper and the lower surfaces. The spin-polarized semiconductivity with zero magnetism is preserved when MXenes are put on the SiC(0001) support. The results presented herein open a new road towards the construction of 2D high-temperature spin-polarized materials with antiferromagnetism potentially suitable for spintronic applications.

Competing antiferromagnetism in a quasi-2D itinerant ferromagnet: Fe3GeTe2

Abstract: Fe3GeTe2 is known as an air-stable layered metal with itinerant ferromagnetism with a transition temperature of about 220 K. From our extensive dc and ac magnetic measurements, we have determined that the ferromagnetic layers of Fe3GeTe2 actually order antiferromagnetically along the c-axis below 152 K. The antiferromagnetic state was further substantiated by theoretical calculation to be the ground state. A magnetic structure model was proposed to describe the antiferromagnetic ground state as well as competition between antiferromagnetic and ferromagnetic states. Fe3GeTe2 shares many common features with pnictide superconductors and may be a promising system in which to search for unconventional superconductivity.

Monolayer MXenes: promising half-metals and spin gapless semiconductors

Abstract: Half-metals and spin gapless semiconductors are promising candidates for spintronic applications due to the complete (100%) spin polarization of electrons around the Fermi level. Based on recent experimental and theoretical findings of graphene-like monolayer transition metal carbides and nitrides (also known as MXenes), we demonstrate from first-principles calculations that monolayer Ti2C and Ti2N exhibit nearly half-metallic ferromagnetism with the magnetic moments of 1.91 and 1.00 UB per formula unit, respectively, while monolayer V2C is a metal with instable antiferromagnetism, and monolayer V2N is a nonmagnetic metal. Interestingly, under a biaxial strain, there is a phase transition from nearly half-metal to truly half-metal, spin gapless semiconductor, and metal for monolayer Ti2C. Monolayer Ti2N is still a nearly half-metal under a proper biaxial strain. Large magnetic moments can be induced by the biaxial tensile and compressive strains for monolayer V2C and V2N, respectively. We also show that the structures of these four monolayer MXenes are stable according to the calculated formation energy and phonon spectrum. Our investigations suggest that, unlike monolayer graphene, monolayer MXenes Ti2C and Ti2N without vacancy, doping or external electric field exhibit intrinsic magnetism, especially the half-metallic ferromagnetism and spin gapless semiconductivity, which will stimulate further studies on possible spintronic applications for new two-dimensional materials of MXenes.

Coupled magnetic, structural, and electronic phase transitions in FeRh

Abstract: The B2-ordered intermetallic magnetic compound FeRh exhibits a thermodynamically first-order phase transition in the vicinity of room temperature that makes it a highly intriguing subject for both fundamental and applied study. On heating through the transition the magnetic character changes from antiferromagnetic to ferromagnetic order with an accompanying large increase in the electrical conductivity and an abrupt expansion in the lattice structure. Accompanying these effects is a very large entropy change comprising both magnetic and lattice contributions. As well as being driven by temperature, these coupled phase transitions may be driven by the application or removal of a magnetic field, or, because of the extremely strong lattice-spin interactions present in this compound, by an applied strain (pressure), and combinations thereof. In addition to these driving factors, the transition temperature can also be tuned by both compositional and finite size effects. Building from historical work on bulk forms of FeRh, the effects of extrinsic and intrinsic parameter variation on the coupled magnetic, structural, and electronic phase transitions are reviewed here, with special attention directed to phenomena that manifest themselves in thin films. Overall, the rich manner in which the physical properties of FeRh change at the phase transition has potential for a wide range of technological applications in areas such as thermally-assisted magnetic recording media, CFC-free magnetic cooling, sensors for energy management, and novel spintronic devices.

Theory of the spin Seebeck effect in antiferromagnets

Abstract: The spin Seebeck effect (SSE) consists in the generation of a spin current by a temperature gradient applied in a magnetic film. The SSE is usually detected by an electric voltage generated in a metallic layer in contact with the magnetic film resulting from the conversion of the spin current into charge current by means of the inverse spin Hall effect. The SSE has been widely studied in bilayers made of the insulating ferrimagnet yttrium iron garnet (YIG) and metals with large spin-orbit coupling such as platinum. Recently the SSE has been observed in bilayers made of the antiferromagnet $\mathrm{Mn}{\mathrm{F}}_{2}$ and Pt, revealing dependences of the SSE voltage on temperature and field very different from the ones observed in YIG/Pt. Here we present a theory for the SSE in structures with an antiferromagnetic insulator (AFI) in contact with a normal metal (NM) that relies on the bulk magnon spin current created by the temperature gradient across the thickness of the AFI/NM bilayer. The theory explains quite well the measured dependences of the SSE voltage on the sample temperature and on the applied magnetic field in $\mathrm{Mn}{\mathrm{F}}_{2}/\mathrm{Pt}$.

Two-dimensional magnetic boron

Abstract: We predict a two-dimensional (2D) antiferromagnetic (AFM) boron (designated as $M$-boron) by using ab initio evolutionary methodology. $M$-boron is entirely composed of ${\mathrm{B}}_{20}$ clusters in a hexagonal arrangement. Most strikingly, the highest valence band of $M$-boron is isolated, strongly localized, and quite flat, which induces spin polarization on either cap of the ${\mathrm{B}}_{20}$ cluster. This flat band originates from the unpaired electrons of the capping atoms and is responsible for magnetism. $M$-boron is thermodynamically metastable and is the first magnetic 2D form of elemental boron.

Room-temperature spin–orbit torque in NiMnSb

Abstract: Materials that crystalize in diamond-related lattices, with Si and GaAs as their prime examples, are at the foundation of modern electronics. Simultaneously, the two atomic sites in the unit cell of these crystals form inversion partners which gives rise to relativistic non-equilibrium spin phenomena highly relevant for magnetic memories and other spintronic devices. When the inversion-partner sites are occupied by the same atomic species, electrical current can generate local spin polarization with the same magnitude and opposite sign on the two inversion-partner sites. In CuMnAs, which shares this specific crystal symmetry of the Si lattice, the effect led to the demonstration of electrical switching in an antiferromagnetic memory at room temperature. When the inversion-partner sites are occupied by different atoms, a non-zero global spin-polarization is generated by the applied current which can switch a ferro-magnet, as reported at low temperatures in the diluted magnetic semiconductor (Ga,Mn)As. Here we demonstrate the effect of the global current-induced spin polarization in a counterpart crystal-symmetry material NiMnSb which is a member of the broad family of magnetic Heusler compounds. It is an ordered high-temperature ferromagnetic metal whose other favorable characteristics include high spin-polarization and low damping of magnetization dynamics. Our experiments are performed on strained single-crystal epilayers of NiMnSb grown on InGaAs. By performing all-electrical ferromagnetic resonance measurements in microbars patterned along different crystal axes we detect room-temperature spin-orbit torques generated by effective fields of the Dresselhaus symmetry. The measured magnitude and symmetry of the current-induced torques are consistent with our relativistic density-functional theory calculations.

Weyl points in the ferromagnetic Heusler compound Co2MnAl

Abstract: The anomalous Hall conductivity (AHC) in some ferromagnetic and antiferromagnetic Heusler compounds was theoretically and experimentally found to be exceptionally large. For the case of ferromagnetic Co2MnAl we here argue that the large AHC is connected with the appearance of Weyl points near the Fermi energy. We find four Weyl points slightly above the Fermi edge. We describe our analysis for a magnetization being in the (110)-direction. For the possible (100)-direction we find at least four Weyl points, too. We predict that Co2MnGa also possesses Weyl points near or at the Fermi energy.