Spin transfer torques (STTs) control magnetization by electric currents, enabling a range of nano-scale spintronic applications. They can destabilize the equilibrium magnetization state by counteracting magnetic relaxation. Here we maximize the STT effect through a dedicated growth-annealing protocol for CoFeB thin films, such that magnetic anisotropies originating from the interface and shape almost cancel each other. The nearly isotropic magnets enable low-current dynamical stabilization of the magnetization in the direction opposite to an applied magnetic field, thereby realizing a spintronic analogue of the Kapitza pendulum. In an intermediate current regime, the STT drives large magnetization vector fluctuations that cover the entire Bloch sphere. The continuous variable associated with the stochastic magnetization direction may serve as a resource for probabilistic computing and neuromorphic hardware. Our results establish isotropic magnets as a platform to study as-yet-uncharted, far-from-equilibrium spin dynamics including anti-magnonics, with promising implications for unconventional computing paradigms.

The Néel, or staggered, spin-orbit torque (SOT) enables an efficient electrical manipulation of antiferromagnetic order. However, it has been tied to special crystal structures, and only its field-like component is established. We develop a theory of staggered damping-like SOT, which arises generically in synthetic antiferromagnets sandwiched by the same heavy metal on both sides. Deriving a Néel-vector Lagrangian, we obtain the domain-wall (DW) velocity in closed form: bilinear in current and in-plane field, opposite in sign for Néel and Bloch walls, and tunable through the interlayer exchange coupling. DW structure thus becomes a control knob for DW motion, while the DW motion, in turn, offers an electrical readout of its structure. Micromagnetic simulations confirm the theory. The predicted sign reversals and scaling laws of the DW velocity, together with the thresholds for the Néel-Bloch transition, give immediate experimental targets.

In this Letter, we study the domain-wall dynamics under electric current in the helimagnet MnAu 2 . We have found that the threshold electric current of the transition from a multidomain state to a single-chiral domain state in a magnetic field is much lower than that of chirality reversal from a single-chiral domain within certain ranges of temperature and magnetic field. The chirality after the transition depends on whether the magnetic field and electric current were parallel or antiparallel. Numerical calculations based on the Landau-Lifshitz-Gilbert equation reproduced the experimental observations. These results indicate that the domain walls are highly mobile in the helimagnet.

We report on epitaxial growth in thin-film synthesis of a polar magnetic semiconductor, AgCrSe2, on a lattice-matched yttria-stabilized zirconia (111) substrate by pulsed-layer deposition (PLD). By using an Ag-rich PLD target to compensate for Ag deficiency in thin films, the nucleation of impurity phases is suppressed, resulting in the c-axis-oriented and single-phase AgCrSe2 thin film. Structural analysis using x-ray diffraction and cross-sectional scanning transmission electron microscopy reveals epitaxial growth with the presence of both twisted and polar domains. Optical absorbance spectrum and magnetization measurements show an absorption edge at around 0.84 eV and a magnetic transition temperature at 41 K, respectively. These values are consistent with the reported values of direct bandgap and Néel temperature of bulk AgCrSe2, reflecting a single-phase and stoichiometric feature of the obtained film. Our demonstration of epitaxial thin-film growth of AgCrSe2 serves as a bedrock for exploration of its potential thermoelectric and spintronic functionalities at surfaces or heterointerfaces.

Flexible spintronics has opened new avenue to promising devices and applications in the field of wearable electronics. Particularly, miniaturized strain sensors exploiting the spintronic function have attracted considerable attention, in which the magnetoelasticity linking magnetism and lattice distortion is a vital property for high-sensitive detection of strain. This paper reports the demonstration that the magnetoelastic properties of Fe$_4$N can be significantly varied by partially replacing Fe with Co or Mn. The high quality Fe$_4$N film exhibits large negative magnetostriction along the [100] direction ($\lambda_{100}$) of -121 ppm while Fe$_{3.2}$Co$_{0.8}$N shows $\lambda_{100}$ of +46 ppm. This wide-range tunability of $\lambda_{100}$ from -121 to +46 across 0 allows us to thoroughly examine the correlation between the magnetoelasticity and other magnetic properties. The strong correlation between $\lambda_{100}$ and magnetic damping ($\alpha$) is found. The enhanced extrinsic term of $\alpha$ is attributable to the large two magnon scattering coming from the large magnetostriction. In addition, the density of states at the Fermi level plays a primal role to determine both $\lambda_{100}$ and the intrinsic term of $\alpha$. Thanks to the giant tunability and the bipolarity of magnetoelasticity, magnetic nitrides are candidate materials for high-sensitive spintronic strain sensors.

Current-induced domain-wall motion (CIDWM) in a synthetic antiferromagnet is a key phenomenon for developing potential high-density-packed magnetic domain-wall memory with fast operation. Here, CIDWM is reported in the antiferromagnetically-coupled two Co layers through the Ir interlayer sandwiched by the two Pt layers: Pt/Co/Ir/Co/Pt. The top and bottom Pt layers play a role for generating the spin current coming from the spin Hall effect, which gives rise to the dual spin-orbit torque (SOT) acting on the perpendicular magnetizations of the Co layers. Although a simple argument would predict that SOTs from top and bottom Pt layers cancel each other out, the dual SOT nucleates a reversed magnetic domain and drives the CIDWM effectively at current density of the order of 1011 A m-2. This study also examines the effect of antisymmetric interlayer exchange coupling (AIEC) on CIDWM. A positive correlation between the magnitude of AIEC and the domain wall velocity is found, whereas the current density required for nucleating the reversed domain shows a negative correlation with the magnitude of AIEC. These facts suggest that the existence of AIEC improves the performance of CIDWM. The present results provide a new avenue to design SOT domain wall devices based on a synthetic antiferromagnet.

The antisymmetric-type long-range exchange interactions between two ferromagnetic layers through a nonmagnetic layer, called antisymmetric interlayer exchange coupling (AIEC), have recently been discovered and attracted much attention. This paper reports that AIEC is naturally built in synthetic antiferromagnets (SyAFs) depending on the thin-film growth conditions. For SyAFs comprising Pt/Co/Ir/Co/Pt layers, two kinds of film growth parameters are examined: the effects of epitaxial growth and oblique incident of sputter-deposition. The present results indicate that spatial fluctuations in thicknesses are one of the major sources inducing AIEC, whereas epitaxial growth, which leads to sharp interfaces and uniformity in layer thicknesses, effectively suppresses unexpected AIEC. In addition, the oblique sputter deposition provides large AIEC and anisotropic distribution of AIEC direction in the case of textured films. The findings in this study provide key factors for designing AIEC and are useful for developing emerging computing technologies with artificial three-dimensional topological magnetic structures.

We observe a non-trivial oscillatory behavior in spin current transmission through antiferromagnetic epitaxial NiO(001) films, realized in Pt/NiO/CoFeB trilayers by varying the thickness of the Pt underlayer (tPt). To examine the influence of epitaxy on spin transport characteristics, we prepare both (001)-epitaxial and polycrystalline Pt/NiO heterostructures, and evaluate the spin current transmitted through the NiO layer via measurements of the thermo-spin effects. In the epitaxial samples, the spin current transmission efficiency in NiO, evaluated from the thermo-spin-effects-induced temperature amplitude, exhibits several peaks at NiO thicknesses (tNiO) of 10 and 22 nm. Strikingly, for tNiO = 10 nm, the transmission efficiency as a function of tPt exhibits an oscillation-like dependence with a period of 8–10 nm, which does not appear in polycrystalline counterparts. These results indicate that the spin current transmission in epitaxial NiO is strongly modulated by the underlying epitaxial Pt layer, despite the conventional role of Pt only as a spin current source. Our findings provide experimental insights into the complex and still unresolved mechanisms governing spin current transmission in NiO.

Among magnetic thin films with perpendicular magnetic anisotropy (PMA), L1(0)-ordered FePt has attracted significant attention because of its exceptionally strong PMA. However, the microscopic origin of its strong PMA has not been elucidated experimentally. We have investigated the contribution of the Fe 3d electrons to its magnetic anisotropy energy by angle-dependent x-ray magnetic circular dichroism at the Fe L-2,L-3 edge. By this technique, one can deduce the magnetic dipole moment m(T), which represents the anisotropic spatial distribution of spin-polarized electrons, and the orbital moment anisotropy (OMA) of Fe 3d electrons. Detected finite m(T) indicates that the spin-polarized Fe 3d electrons are distributed preferentially in the out-of-plane direction of the films. This m(T) of Fe overwhelms the positive contribution of OMA to PMA and reduces the PMA of L1(0)-ordered FePt thin films, consistent with a previous first-principles calculation. The present result implies that a large positive contribution of the non-magnetic element Pt rather than Fe governs the PMA of L1(0)-ordered FePt thin films.

We found signatures of current-induced sliding motion in helimagnetic MnAu_{2} thin films. An abrupt change in differential resistivity occurred at a threshold bias current in the helimagnetic state, whereas it was absent in the induced ferromagnetic state. Broadband voltage noise also emerged above the threshold current in the helimagnetic state. Based on the similarity to canonical charge and spin density wave systems, we ascribed the origin of these phenomena to the sliding motion of the helimagnetic structure.

Among magnetic thin films with perpendicular magnetic anisotropy (PMA), L1_0-ordered FePt has attracted significant attention because of its exceptionally strong PMA. However, the microscopic origin of its strong PMA has not been elucidated experimentally. We have investigated the contribution of the Fe 3d electrons to its magnetic anisotropy energy by angle-dependent x-ray magnetic circular dichroism at the Fe L_2,3 edge. By this technique, one can deduce the magnetic dipole moment m_T, which represents the anisotropic spatial distribution of spin-polarized electrons, and the orbital moment anisotropy (OMA) of Fe 3d electrons. Detected finite m_T indicates that the spin-polarized Fe 3d electrons are distributed preferentially in the out-of-plane direction of the films. This m_T of Fe overwhelms the positive contribution of OMA to PMA, and reduces the PMA of L1_0-ordered FePt thin films, consistent with a previous first-principles calculation. The present result implies that a large positive contribution of the non-magnetic element Pt rather than Fe governs the PMA of L1_0-ordered FePt thin films.

We report the induction of large in-plane uniaxial magnetic anisotropy for the CoFeB-based thin films by post-annealing under the magnetic field application. The comprehensive study reveals the influence of CoFeB composition and Pt addition to CoFeB on the induced magnetic anisotropy by magnetic field annealing. The addition of Pt to a CoFeB film significantly enhances the effect of magnetic field annealing, and as a result, the CoFeB–Pt films exhibit the in-plane uniaxial magnetic anisotropy energies that are one order of magnitude larger than the pristine CoFeB films. In order to understand the mechanism of magnetic anisotropy induction by post-annealing, we carried out the magnetostriction measurement for the CoFeB-based thin films. The correlation between the induced magnetic anisotropy and magnetoelastic properties is discussed.

Transition metal oxides are a platform for exploring strain-engineered intriguing physical properties and developing spintronic or flexible electronic functionalities owing to strong coupling of spin, charge and lattice degrees of freedom. In this study, we exemplify the strain-engineered magnetism of La_2/3Sr_1/3MnO_3 in freestanding and rippled membrane forms without and with process-induced strain, respectively, prepared by epitaxial lift-off technique. We find that the deposition of Pt/Ti stressor suppresses the crack formation in the lift-off process and induces a ripple structure in the La_2/3Sr_1/3MnO_3 membrane. Laser micrograph and Raman spectroscopy show a ripple period of about 30 um and a height of a few um, where alternating convex and concave structures are subjected to tensile strain of 0.6 compressive strain of 0.5 La_2/3Sr_1/3MnO_3 membrane exhibits room-temperature ferromagnetism, the macroscopic magnetic transition temperature (TC) of the rippled membrane is reduced by as large as 27 the rippled membrane reveals that the spatial variation of TC to be approximately 4 at convex and concave structures. The large reduction of macroscopic TC in the rippled membrane may be ascribed to the lattice disorders due to strain gradient. Our demonstration of tuning ferromagnetism by the ripple structure validates the high potential of the process-induced strain in epitaxial lift-off technique and paves the way for strain-mediated emerging physical properties in various transition metal oxides.

Altermagnetism presents intriguing possibilities for spintronic devices due to its unique combination of strong spin-splitting and zero net magnetization. However, realizing its full potential hinges on fabricating single-variant altermagnetic thin films. In this work, we present definitive evidence for formation of single-variant altermagnetic RuO2(101) thin films with fully epitaxial growth on Al2O3(1 1 ¯ 02) r-plane substrates, confirmed through rigorous structural analyses using X-ray diffraction, atomic-resolution transmission electron microscopy and X-ray magnetic linear dichroism. The mutual correspondence of the occupancy of oxygen atoms on the surfaces of RuO2(101)[010] and Al2O3(1 1 ¯ 02)[11 2 ¯ 0] plays a decisive role in the formation of the single-variant RuO2, which is also supported by our first-principles density functional theory calculations. We further observed spin-splitting magnetoresistance in the single-variant RuO2(101)/CoFeB bilayers, highlighting the characteristic effect of single variant on spin transport. The demonstration of single-variant RuO2(101) films marks a significant advancement in the field of altermagnetism and paves the way for exploring their potential applications.

Transition metal oxides are a platform for exploring strain-engineered intriguing physical properties and developing spintronic or flexible electronic functionalities owing to the strong coupling of spin, charge, and lattice degrees of freedom. In this study, we exemplify the strain-engineered magnetism of La2/3Sr1/3MnO3 in freestanding and rippled membrane forms without and with process-induced strain, respectively, prepared by the epitaxial lift-off technique. We find that the deposition of the Pt/Ti stressor suppresses the crack formation in the lift-off process and induces a ripple structure in the La2/3Sr1/3MnO3 membrane. Laser micrograph and Raman spectroscopy show a ripple period of about 30 μm and a height of a few μm, where alternating convex and concave structures are subjected to a tensile strain of 0.6% and a compressive strain of 0.5%, respectively. While the freestanding La2/3Sr1/3MnO3 membrane exhibits room-temperature ferromagnetism, the macroscopic magnetic transition temperature (TC) of the rippled membrane is reduced by as much as 27%. Temperature-variable Kerr microscopy observation in the rippled membrane reveals that the spatial variation of TC is ∼4% of the macroscopic TC, which coincides with the local strains at convex and concave structures. The large reduction of macroscopic TC in the rippled membrane may be ascribed to the lattice disorders due to the strain gradient. Our demonstration of tuning ferromagnetism by the ripple structure validates the high potential of the process-induced strain in the epitaxial lift-off technique and paves the way for strain-mediated emerging physical properties in various transition metal oxides.

Although the relationship between magnetostriction and magnetic damping is often described phenomenologically, their intrinsic connection remains unclear. In this Letter, we demonstrate that the magnitude of magnetic damping depends on the sign of magnetostriction in (Fe1-xCox)4N and Ni1-yCoy alloys across various compositions, consistent with experimental observations. This behavior is attributed to strain-induced changes in exchange splitting, which shift the minority-spin density of states near the Fermi level, thereby affecting both magnetostriction and damping through spin-conserving transitions. Additionally, the presence of locally degenerate orbitals plays a crucial role in determining magnetostriction. These findings suggest that magnetization dynamics and magnetostriction can be intrinsically controlled, facilitating the design of magnetic materials for applications such as flexible spintronics.

Spin caloritronics, a research field studying on the interconversion between a charge current ( J c ) and a heat current ( J q ) mediated by a spin current ( J s ) and/or magnetization (M), has attracted much attention not only for academic interest but also for practical applications. Newly discovered spin-caloritronic phenomena such as the spin Seebeck effect (SSE) have stimulated the renewed interest in the thermoelectric phenomena of a magnet, which have been known for a long time, e.g. the anomalous Nernst effect (ANE). These spin-caloritronic phenomena involving the SSE and the ANE have provided with a new direction for thermoelectric conversion exploiting J s and/or M. Importantly, the symmetry of ANE allows the thermoelectric conversion in the transverse configuration between J q and J c . Although the transverse configuration is totally different from the conventional longitudinal configuration based on the Seebeck effect and has many advantages, we are still facing several issues that need to be solved before developing practical applications. The primal issue is the improvement of conversion efficiency. In the case of ANE-based applications, a material with a large anomalous Nernst coefficient ( S ANE ) is the key for solving the issue. This review article introduces the increase of S ANE can be achieved by forming superlattice structures, which has been demonstrated for several kinds of materials combinations. The overall picture of studies on spin caloritronics is first surveyed. Then, we mention the pioneering work on the transverse thermoelectric conversion in superlattice structures, which was performed using Fe-based metallic superlattices, and show the recent studies for the Ni-based metallic superlattices and the ordered alloy-based metallic superlattices.

This paper reports the systematic study on the structure, magnetic properties and magnetoelastic properties for the Fe100-x Ga x (001) thin films epitaxially grown on the different substrates of GaAs(001) and MgO(001) using the sputtering technique. The alloy composition dependence of effective magnetoelastic coupling coefficient B eff along the FeGa [110] direction indicated that the largest magnetoelastic coupling was obtained for the Fe-Ga layer with x = 30 grown on the MgO substrate, which was evaluated to be B eff = - 9.4 x 107 erg cm-3. Considering the results of structural analysis and magnetization measurement, the different crystallite sizes depending on the kind of substrate may give rise to the different magnetoelastic coupling strengths between the Fe-Ga layers on the MgO and the GaAs. The magnetostriction along the Fe-Ga [111] direction lambda 111 was also estimated with the assumption of plausible elastic property of Fe-Ga, and showed the values comparable to the reported value of bulk Fe-Ga. This means the large magnetostriction can be obtained even for the Fe-Ga thin films epitaxially grown not only on the GaAs(001) but also on the MgO(001). The findings in this work will give a guideline for designing spintronic applications with a Fe-Ga layer exhibiting a large magnetoelastic coupling.

An antiferromagnetic metal with a two-dimensional triangular network offers a unique playground of intriguing magneto-transport properties and functionalities stemming from the interplay between conducting electrons and intricate magnetic phases. A NiAs-type CrSe is one of the candidates owing to alternate stackings of Cr and Se triangular atomic networks in its crystal structure. While the fabrication of CrSe thin films is indispensable to develop functional devices, studies on its thin-film properties have been limited to date due to the lack of metallic samples. Here, we report on the realization of metallic conductivities of CrSe thin films, which allows us to investigate their intrinsic magneto-transport properties. The metallic sample exhibits a co-occurrence of weak ferromagnetism with perpendicular magnetic anisotropy and antiferromagnetic behavior, indicating the presence of non-coplanar spin structures. In addition, control of the polarity and tilting angle of the non-coplanar spin structure is accomplished by a sign of cooling magnetic fields. The observed non-coplanar spin structure, which can be a source of emergent magnetic field acting on the conducting electrons, highlights the high potential of the triangular lattice antiferromagnet and provides a unique platform for functional thin-film devices composed of NiAs-type derivative Cr chalcogenides and pnictides.