Intrinsic spin Nernst effect of magnons in a noncollinear antiferromagnet (original) (raw)
Related papers
Magnonic analog of the Edelstein effect in antiferromagnetic insulators
Physical Review B, 2020
We investigate the nonequilibrium spin polarization due to a temperature gradient in antiferromagnetic insulators, which is the magnonic analogue of the inverse spin-galvanic effect of electrons. We derive a linear response theory of a temperature-gradient-induced spin polarization for collinear and noncollinear antiferromagnets, which comprises both extrinsic and intrinsic contributions. We apply our theory to several noncentrosymmetric antiferromagnetic insulators, i.e., to a one-dimensional antiferromagnetic spin chain, a single layer of kagome noncollinear antiferromagnet, e.g., KFe3(OH)6(SO4)2, and a noncollinear breathing pyrochlore antiferromagnet, e.g., LiGaCr4O8. The shapes of our numerically evaluated response tensors agree with those implied by the magnetic symmetry. Assuming a realistic temperature gradient of 10 K/mm, we find two-dimensional spin densities of up to ∼ 10 6 /cm 2 and three-dimensional bulk spin densities of up to ∼ 10 14 /cm 3 , encouraging an experimental detection.
Magnon Spin Nernst Effect in Antiferromagnets
Physical Review Letters, 2016
We predict that a temperature gradient can induce a magnon-mediated spin Hall response in an antiferromagnet with non-trivial magnon Berry curvature. We develop a linear response theory which gives a general condition for a Hall current to be well defined, even when the thermal Hall response is forbidden by symmetry. We apply our theory to a honeycomb lattice antiferromagnet and discuss a role of magnon edge states in a finite geometry.
Transformation of spin current by antiferromagnetic insulators
It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current through the excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice, and, depending on the excitation conditions, can be either attenuated or enhanced. When the phase difference between the excited evanescent modes is close to π/2, there is an optimum AFM thickness for which the output spin current reaches a maximum, that can significantly exceed the magnitude of the input spin current. The spin current transfer through the AFM depends on the ambient temperature and increases substantially when temperature approaches the Neel temperature of the AFM layer. Progress in modern spintronics critically depends on finding novel media that can serve as effective conduits of spin angular momentum over large distances with minimum losses [1–3]. The mechanism of spin transfer is reasonably well-understood in ferromagnetic (FM) metals [4, 5] and insulators [3, 4, 6–9], but there are only very few theoretical papers describing spin current in an-tiferromagnets (AFM) (see, e.g., [10]). The recent experiments [11–13] have demonstrated that a thin layer of a dielectric AFM (NiO, CoO) could effectively conduct spin current. The transfer of spin current was studied in the FM/AFM/Pt trilayer structure (see Fig. 1). The FM layer driven in ferromagnetic resonance (FMR) excited spin current in a thin layer of AFM, which was detected in the adjacent Pt film using the inverse spin Hall effect (ISHE). It was also found in [13] that the spin current through the AFM depends on the ambient temperature and goes through a maximum near the Neel temperature T N. The most intriguing feature of the experiments was the fact that for a certain optimum thickness of the AFM layer (∼ 5 nm) the detected spin current had a maximum [11, 12], which could be even higher than in the absence of the AFM spacer [12]. The spin current transfer in the reversed geometry, when the spin current flows from the Pt layer driven by DC current through the AFM spacer into a microwave-driven FM material has been reported recently in [14]. The experiments [11–14] posed a fundamental question of the mechanism of the apparently rather effective spin current transfer through an AFM dielectric. A possible mechanism of the spin transfer through an easy-axis AFM has been recently proposed in [10]. However, this uniaxial model can not explain the non-monotonous dependence of the transmitted spin current on the AFM layer thickness and the apparent " amplification " of the spin current seen in the experiments [11, 12] performed with the bi-axial NiO AFM layer [15]. In this Letter, we propose a possible mechanism of spin current transfer through anisotropic AFM dielectrics, which may explain all the peculiarities of the experiments FIG. 1. Sketch of the model of spin current transfer through an AFM insulator based on the experiment [11]. The FM layer excites spin wave excitations in the AFM layer. The output spin current (at the AFM/Pt interface) is detected by the Pt layer through the inverse spin Hall effect (ISHE). [11, 12, 14]. Namely, we show that the spin current can be effectively carried by the driven evanescent spin wave excitations, having frequencies that are much lower than the frequency of the AFM resonance. We demonstrate that the angular momentum exchange between the spin subsystem and the AFM lattice plays a crucial role in this process, and may lead to the enhancement of the spin current inside the AFM layer. We consider a model of a simple AFM having two magnetic sublattices with the partial saturation magnetiza-tion M s. The distribution of the magnetizations of each sublattice can be described by the vectors M 1 and M 2 , |M 1 | = |M 1 | = M s. We use a conventional approach for describing the AFM dynamics by introducing the vectors of antiferromagnetism (l) and magnetism (m) [16–19]: l = (M 1 − M 2)/(2M s), m = (M 1 + M 2)/(2M s). (1) Assuming that all the magnetic fields are smaller then the exchange field H ex and neglecting the bias magnetic field, that is used to saturate the FM layer, the effective AFM Lagrangian can be written as [16, 18, 19]:
An insulating doped antiferromagnet with low magnetic symmetry as a room temperature spin conduit
Applied Physics Letters, 2020
We report room temperature long-distance spin transport of magnons in antiferromagnetic thin film hematite doped with Zn. The additional dopants significantly alter the magnetic anisotropies, resulting in a complex equilibrium spin structure that is capable of efficiently transporting spin angular momentum at room temperature without the need for a well-defined, pure easy-axis or easy-plane anisotropy. We find intrinsic magnon spin-diffusion lengths of up to 1.5 μm, and magnetic domain governed decay lengths of 175 nm for the low frequency magnons, through electrical transport measurements demonstrating that the introduction of non-magnetic dopants does not strongly reduce the transport length scale showing that the magnetic damping of hematite is not significantly increased. We observe a complex field dependence of the non-local signal independent of the magnetic state visible in the local magnetoresistance and direct magnetic imaging of the antiferromagnetic domain structure. We explain our results in terms of a varying and applied-field-dependent ellipticity of the magnon modes reaching the detector electrode allowing us to tune the spin transport. Antiferromagnetic (AFM) spintronics seeks to utilize the high-frequency dynamics, stability against magnetic perturbations and negligible stray fields in functionalizing AFM materials 1. The electrical reading 2,3 and writing 4,5 of the Néel vector n in insulating AFMs, which further benefit from reduced Joule heating, demonstrate the role AFMs can play in developing devices, for instance, for memories. The Néel vector has been shown to efficiently transport AFM magnons, quantized magnetic excitations, across long distances in the low temperature easy-axis phase of the insulating AFM hematite (α-Fe2O3) 6,7. In easy-axis AFMs, the excited magnons are circularly polarized, making them capable of transporting angular momentum, and thus information 8. In the absence of a magnetic field, magnetic anisotropy, or some other symmetry-breaking mechanism, the two magnon modes are degenerate, carrying equal and opposite values of angular momentum and no net spin transport is observed. It has been shown that the application of a spin-bias at the interface of the AFM and a heavy metal can lead to an excess of magnons with one polarization, enabling net spin transport 6-8. The efficiency of the magnon
Spin colossal magnetoresistance in an antiferromagnetic insulator
Nature materials, 2018
Colossal magnetoresistance (CMR) refers to a large change in electrical conductivity induced by a magnetic field in the vicinity of a metal-insulator transition and has inspired extensive studies for decades. Here we demonstrate an analogous spin effect near the Néel temperature, T = 296 K, of the antiferromagnetic insulator CrO. Using a yttrium iron garnet YIG/CrO/Pt trilayer, we injected a spin current from the YIG into the CrO layer and collected, via the inverse spin Hall effect, the spin signal transmitted into the heavy metal Pt. We observed a two orders of magnitude difference in the transmitted spin current within 14 K of the Néel temperature. This transition between spin conducting and non-conducting states was also modulated by a magnetic field in isothermal conditions. This effect, which we term spin colossal magnetoresistance (SCMR), has the potential to simplify the design of fundamental spintronics components, for instance, by enabling the realization of spin-current s...
Antiferromagnetic (AFM) materials with zero or vanishingly small macroscopic magnetization are nowadays the constituent elements of spintronic devices. However, the possibility to use them as active elements that show nontrivial and controllable magnetic dynamics is still discussible. In the present paper we extend the phenomenologic theory [Andreev and Marchenko, Sov. Phys. Usp. 23, 21 (1980)] of macroscopic dynamics in AFM materials for the cases typical for spin-valve devices. In particular, we consider the solidlike magnetic dynamics of AFM materials with strong exchange coupling in the presence of a spin-polarized current and give the general expression for the current-induced Rayleigh dissipation function in terms of the rotation vector for different types of AFMs. Based on the analysis of linearized equations of motion we predict the current-induced spin reorientation and AFM resonance, and find the values of critical currents in terms of AFMR frequencies and damping constants. The possibility of a current-induced spin-diode effect and second-harmonic generation in the AFM layer is also shown.
APL Materials, 2023
Antiferromagnets (AFs) are prospective for next-generation high-density and high-speed spintronic applications due to their negligible stray field and ultrafast spin dynamics, notwithstanding the challenges in detecting and manipulating AF order with no magnetization (M = 0). Among the AFs, non-collinear AFs are of particular interest because of their unique properties arising from the non-collinear spin structure and the small magnetization M. In this work, we describe the recently observed vector spin Seebeck effect in non-collinear LuFeO3, where the magneto-thermovoltage under an in-plane temperature gradient, not previously observed, is consistent with the predicted spin swapping effect. Our results shed light on the importance of the non-collinear spin structure in the emerging spin phenomena in non-collinear AFs and offer a new class of materials for AF spintronics and spin caloritronics.
Theory of spin torques and giant magnetoresistance in antiferromagnetic metals
Physical Review B, 2006
Spintronics in ferromagnetic metals is built on a complementary set of phenomena in which magnetic configurations influence transport coefficients and transport currents alter magnetic configurations. Here, we propose that corresponding effects occur in circuits containing antiferromagnetic metals. The critical current for order parameter orientation switching can be smaller in the antiferromagnetic case because of the absence of shape anisotropy and because spin torques can act through the entire volume of an antiferromagnet. We discuss possible applications of antiferromagnetic metal spintronics.
Controlling spin current polarization through non-collinear antiferromagnetism
Nature Communications, 2020
The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry....