Session M21: Spin- and Magnon-Based Phenomena in Antiferromagnetic Materials

Quantum-classical description of spin and charge pumping by dynamical noncolliner magnetic textures and the ensuing electromagnetic radiation in THz spintronics

The interaction of femtosecond (fs) light pulses with magnetic materials has been intensely studied for more than two decades to understand ultrafast demagnetization in single magnetic layer or THz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. Despite long history, microscopic understanding of ultrafast-light-driven magnets is incomplete due to numerous competing effects and with virtually no study reporting calculation of output THz radiation. This talk presents a recently developed and versatile multiscale quantum-classical formalism where conduction electrons are described by quantum master equation [1] of the Lindblad type or by time-dependent nonequilibrium Green's functions [2-8]; classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation; and incoming light is described by classical vector potential while outgoing electromagnetic radiation is computed using the Jefimenko equations for retarded electric and magnetic fields [1]. We illustrate it by application to ultrafast-light-driven bilayer of Weyl antiferromagnet Mn₃Sn with noncollinear local magnetization and SO coupling intrinsically present within Mn3Sn layer [1] which pumps charge current emitting THz radiation; as well as on magnetic-field driven annihilation of two topological solitons [8], realized as magnetic domain walls within a metallic ferromagnetic nanowire, which leads to spin wave burst observed experimentally [9], as well as our prediction [8] of additional spin pumping over ultrabroadband 0.03-27 THz frequency range and potentially electromagnetic radiation in the same range upon spin-to-charge conversion.   

Hall mass and transverse Noether spin currents in noncollinear antiferromagnets

Noncollinear antiferromagnets (AFM) in the family of Mn3X (X=Ir, Sn, Ge, Pt, etc.) have attracted considerable interest as a platform for spintronics because they both possess the general benefits of antiferromagnets and exhibit various interesting transport, magnetic, and optical properties.  Here we study the conserved Noether current associated with spin-rotation symmetry of the local spins in noncollinear kagome AFM. We found that a transverse component of the d.c. Noether spin current can be created by a longitudinal driving force associated with a propagating spin wave and is proportional to a response coefficient that we denote as the Hall (inverse) mass, reminiscent of the isotropic spin Hall conductivity. We show the effect of the Hall mass and its association with transverse spin currents by simulating spin pumping in a ferromagnet (FM)-noncollinear AFM bilayer structure. Our results shed light on the potential of noncollinear AFM in manipulating the polarization and flow of spin currents in general spintronic devices.   

Hybrid Magnonics in Hybrid Perovskite Antiferromagnets

Hybrid magnonic systems are a newcomer for pursuing coherent information processing owing to their rich quantum engineering functionalities. One prototypical example is hybrid magnonics in antiferromagnets with an easy-plane anisotropy that resembles a quantum-mechanically mixed two-level spin system through the coupling of acoustic and optical magnons. Generally, the coupling between these orthogonal modes is forbidden due to their opposite parity. In this work [1], we show that the Dzyaloshinskii–Moriya-Interaction (DMI), a chiral antisymmetric interaction that occurs in magnetic systems with low symmetry, can lift this restriction. We report that layered hybrid perovskite antiferromagnetic with an interlayer DMI can lead to a strong intrinsic magnon-magnon coupling strength up to 0.24 GHz, which is four times greater than the dissipation rates of the acoustic/optical modes. Our work shows that the DMI in these hybrid antiferromagnets holds promise for leveraging magnon-magnon coupling by harnessing symmetry breaking in a highly tunable, solution-processable layered magnetic platform.

[1] A. Comstock et al., “Hybrid Magnonics in Hybrid Perovskite Antiferromagnets”. Nature Communications 14, 1834 (2023).https://www.nature.com/articles/s41467-023-37505-w   

Determination of Dzyaloshinskii-Moriya Interaction in Hematite via Anisotropic Morin Transition Response to Magnetic Field

Hematite (α-Fe₂O₃) is a notable antiferromagnetic material with unique spin properties that spurs intense research interest in the rapidly emerging field of antiferromagnetic spintronics. One of the intriguing characteristics is the spin reorientation transition, often known as the Morin transition, at a temperature of 260 K. This phenomenon is significantly influenced by the presence of the Dzyaloshinskii-Moriya (DM) interaction. In this work, we investigate the Morin transition in (10-10)-oriented hematite under applied magnetic fields by performing transport measurements in a hematite/Pt heterostructure device. The applied field stabilizes the easy-plane phase and consequently suppresses the Morin transition, which is corroborated by magnetometry measurements. Moreover, the field-induced Morin temperature suppression depends on the magnetic field orientation with respect to the direction of the Dzyaloshinskii-Moriya vector. Based on the field-dependent anisotropic responses of the Morin temperature, we reveal the critical role of and unambiguously determine the strength of the Dzyaloshinskii-Moriya interaction in hematite.   

Phenomenological model of magnon spin transport in polar antiferromagnets

Thermally excited magnons in an antiferromagnetic insulator can give rise to a non-local inverse spin Hall voltage in a detector wire in a planar device structure. Recently, the multiferroic bismuth ferrite (BFO) has been demonstrated to exhibit electric field controlled magnon transport, offering tantalizing opportunities for electrically controllable spintronic devices. The magnetic order of BFO, however, is complex, hosting a spin cycloid tied to the ferroelectric polarization, and the physics of spin transport in BFO remains a significant question. Here, we present a phenomenological model for magnon spin transport in polar antiferromagnets, inspired by the symmetries associated with the material and the device geometry. The model summarizes the physics of magnon spin transport in a single function η representing the extent to which a given magnon mode will contribute to the inverse spin Hall voltage in the detector wire. By writing the parameters of the function η and analyzing their transformations under relevant symmetry operations, we can predict aspects of the thermally driven non-local magnon signals. By offering symmetry-enforced constraints on the inverse spin Hall voltage measurements, our results enable us to identify which symmetries are broken in the system and subsequently gain insight into the physical mechanisms of magnon spin transport. We envision that our symmetry-based phenomenological model will help us understand the spin transport in polar antiferromagnetic insulators.   

Tuning Néel temperature in Magnetoelectric B-doped Cr2O3 films

Multi-functional thin films of boron (B) doped Cr₂O₃ exhibit voltage-controlled and nonvolatile Néel vector π/2 rotation in the absence of a magnetic field. Isothermal toggling of antiferromagnetic states is demonstrated in prototype device structures at CMOS compatible temperatures between 300 K and 400 K. Néel vector rotation is detected with the help of heavy metal (Pt) Hall-bars in proximity of pulsed laser deposited B:Cr₂O₃ films. Cold neutron depth profiling (cNDP), performed at National Institute of Standards and Technology, reveals thermally activated B-accumulation at the B:Cr2O3/ vacuum interface in thin films deposited on Al2O3 substrates. This work uses cNDP and Spin Hall measurements [1] to demonstrate a shift in T_N towards higher values of ≈ 477 K, associated with the increase in B-concentration within an interfacial layer of about 50 nm.

[1] A. Mahmood et al., Adv. Physics. Res., 2023, 2300061   

Post annealing x-ray photoemission spectroscopy depth profiling investigating B-concentration in B:Cr2O3 thin films

Chromium Oxide (Cr₂O₃) is a magnetoelectric antiferromagnetic material with a Néel temperature i.e., T_N ~ 307 K. Doping Cr₂O₃ with Boron (B) increases the antiferromagnetic transition temperature (T_N). The enhancement of T_N from B-doping facilitates the operation of B:Cr₂O₃ based devices in CMOS environments. In addition, B-doping turns Cr₂O₃ into a multifunctional single-phase material which enables the  Néel vector rotation in the absence of applied magnetic field. In context of antiferromagnetic spintronics, this peculiar nonvolatile voltage driven Néel vector rotation makes B:Cr₂O₃ an extremely good candidate for ultra-low power, ultra-fast, nonvolatile memory and logic device applications. In our work we prepare 200 nm thick B:Cr₂O₃ films grown epitaxially on Al₂O₃ (0001) substrates by using pulsed laser deposition. We utilize x-ray photoemission spectroscopy depth profiling to independently confirm data from cold neutron depth profiling which reveal thermally activated B-accumulation at the B:Cr₂O₃/ vacuum interface within a layer of about 50 nm thickness. We attributed the stable B-enrichment to surface segregation.   

Electrical manipulation of spin-splitting antiferromagnet

The recently discovered spin-splitting antiferromagnets (AFMs) inherit advantages of both ferromagnets and AFMs, which provide unprecedented opportunities for the long-desired non-volatile, high-density, and ultra-fast memories with opposite Néel vectors as binary "0" and "1". However, as crucial components, the electrical detection and electrical 180^o switching of the Néel vector as well as the corresponding spin-splitting, are very challenging and still missing. Here, we demonstrate that in spin-splitting AFM Mn₅Si₃, the unique anomalous Hall effect (AHE) can be adopted for electrical readout of opposite Néel vectors, which cannot be distinguished by conventional methods such as magnetoresistance in spin-degenerate AFMs. Moreover, we proposed a new mechanism for the electrical 180^o switching of the Néel vector via damping-like spin-orbit torques by designing asymmetric switching barriers and experimentally achieved it. Based on our novel readout and manipulation methods, we fabricated a prototype memory device that can accomplish robust write and read cycles. All of the experimental results are well supported by our first-principles calculations and atomic spin simulations. Our work lays the foundation for practical AFM spintronics and fundamental studies based on the AFM spin-splitting band structure.   

Landau Theory of Altermagnetism

We formulate a Landau theory for altermagnets, a class of colinear compensated magnets with spin-split bands. Starting from the non-relativistic limit, this Landau theory goes beyond a conventional analysis by including spin-space symmetries, providing a simple framework for understanding the key features of this family of materials. We find a set of multipolar secondary order parameters connecting existing ideas about the spin symmetries of these systems, their order parameters and the effect of non-zero spin-orbit coupling. We account for several features of canonical altermagnets such as RuO2, MnTe and CuF2 that go beyond symmetry alone, relating the order parameter to key observables such as magnetization, anomalous Hall conductivity and magneto-elastic and magneto-optical probes. Finally, we comment on generalizations of our framework to a wider family of exotic magnetic systems deriving from the zero spin-orbit coupled limit.   

Topological Phase Transition in One-Dimensional Magnonic Crystals Induced by Field Orientation

Recently, it has been reported that the magnonic crystals satisfying certain conditions such as the filling fraction and the magnetic properties may have topologically non-trivial properties. Nevertheless, such parameters are not favorable candidates to realize the topological phase transition as they are not changeable in already fabricated magnonic devices. Instead, we propose orientation of the external field as a new variable to achieve the topological phase transition in the semi-infinite one-dimensional magnonic crystals. We analyzed an YIG based one-dimensional magnonic crystal polarized to an arbitrary direction by solving the Landau-Lifshitz-Gilbert equation. We compared magnon band structures and the Zak phases of magnon bands at various orientation of the bias field. For the finite structure analysis, we employed a micromagnetic modeling software to simulate the dispersion relations and the boundary modes. The result shows that the topological phase transition of the magnonic crystals can be induced by the orientation of the magnetic field and suggests new magnonic devices based on the topological properties.