Session B50: Antiferromagnetic Spintronics and Magnonics

Effects of spin injection on the anisotropic magnetoresistance in antiferromagnetic Sr2IrO4

The emerging field of antiferromagnetic (AFM) spintronics aims at the active control and manipulation of AFM moments to be used in future spintronic devices. Crucial to its success is a rigorous method to detect and monitor the orientation of AFM moments. Very large anisotropic magnetoresistance (MR) observed in antiferromagnetic iridates [1] is touted to provide just that. Here we study the effects of spin injection on the magnetoresistance in AFM Mott insulator Sr₂IrO₄. To generate and inject a pure spin current into a single-crystal Sr₂IrO₄ we exploit the spin Hall effect in a thin Pt layer deposited directly onto the crystal. The magnetoresistance effect in Sr₂IrO₄ was measured with and without the spin injection from Pt. We observed a significant increase of MR with the spin current. The increase may be tentatively associated with a spin-transfer torque generated by the current on local magnetic moments in Sr₂IrO₄. [1] C. Wang et al., Phys. Rev. X 3, 041034 (2014).   

Intrinsic nonlinear Hall effect in antiferromagnetic tetragonal CuMnAs

Detecting the orientation of the N\'eel vector is a major research topic in antiferromagnetic spintronics. Here we recognize the intrinsic nonlinear Hall effect, which is independent of the relaxation time, as a prominent contribution to the time-reversal-odd second order conductivity and can be used to detect the flipping of the Neel vector. In contrast, the Berry-curvature-dipole-induced nonlinear Hall effect depends linear on relaxation time and is time-reversal-even. We study the intrinsic nonlinear Hall effect in an antiferromagnetic metal: tetragonal CuMnAs, and show that its nonlinear Hall conductivity can reach the order of mA/V^2. The dependence on the chemical potential of such nonlinear Hall conductivity can be qualitatively explained by a tilted massive Dirac model. Moreover, we demonstrate its strong temperature dependence and briefly discuss its competition with the second order Drude conductivity. Finally, a complete survey of magnetic point groups are presented, providing guidelines for finding more antiferromagnetic materials with the intrinsic nonlinear Hall effect.   

Direct detection of spin Hall effect in an antiferromagnetic material

The spin Hall effect is one of the key phenomena in spintronics since it enables efficient electrical manipulation of magnetization. The figure of merit of this effect is the spin Hall angle given by the ratio of spin to charge currents. Antiferromagnetic materials have already demonstrated high spin Hall angles as detected using indirect electrical measurements [1,2]. Here, we report the direct measurement of the interfacial spin accumulation induced by the spin Hall effect in antiferromagnetic PtMn thin films using magnetic circular dichroism (XMCD)- photoemission electron microscopy (PEEM). We show that the XMCD has opposite sign at the L₃ edge of Mn for opposite charge current directions and scales linearly with current density. We quantitatively determine the current-induced spin accumulation at the PtMn interface as 8.8×10⁻¹² μ_BA⁻¹cm² per atom averaged over the probing depth which translates into a positive spin Hall angle of 0.25 (±0.1). Our results show the direct, spatially resolved, interface free and element-selective measurement of the SHE in a CuAu-I-type antiferromagnetic material by means of X-ray microscopy.

[1] W. Zhang, M. B. Jungfleisch, W. Jiang et al., Phys. Rev. Lett. 113 196602 (2014)

[2] Y. Ou, S. Shi, D. C. Ralph et al., Phys. Rev. B 93 220405(R) (2016)   

Controlling antiferromagnetic magnon polarization by interfacial exchange interaction

Antiferromagnetic (AFM) spintronics holds the great promise of ultrafast spin operation and immunity against magnetic field perturbations. However, manipulating the spin degree of freedom in AFM materials requires magnetic fields orders of magnitude stronger than in ferromagnets. Here we realize a highly efficient control of the magnon spins by the interfacial exchange interaction in an insulating type of thin film heterostructures of ferrimagnetic yttrium iron garnet (YIG) and AFM Cr₂O₃. At low temperatures, the exchange interaction lifts the degeneracy between the two AFM magnon modes, resulting in a net spin polarization from the more populated left-handed magnons even at zero applied magnetic field. This effect manifests as a sign change of the spin Seebeck effect (SSE) signal in YIG/Cr₂O₃(t)/Pt (t=0.7 nm and 12 nm) as temperature is varied. In addition, the SSE signal polarity flips when a magnetic field switches the YIG magnetization but not sufficiently strong to induce the spin-flop transition in Cr₂O₃, demonstrating the ability to control the magnon quantum states in AFM materials via interfacial exchange coupling. Our findings pave the way towards insulating spintronics where magnons function with two quantum spin states in analogy to electrons in metallic spintronics.   

Spin-flip switching of a nanoscale synthetic antiferromagnet by excitation of individual spin-wave modes

Synthetic antiferromagnets (SAF) offer increased functionalization of spintronic devices, thus stimulating fundamental and application-driven research. Understanding and controlling critical phenomena of the SAF spin dynamics, such as magnetic switching, plays a central role. Here, we investigate magnetic switching of a nanoscale SAF with perpendicular order parameter, incorporated in a magnetic tunnel junction (MTJ) structure [1,2]. We drive the MTJ with microwave pulses and observe spin-flip switching of the SAF by resonant excitation of an individual magnon mode of the SAF. The magnon spectrum of the nanoscale SAF is discrete. Evaluation of the switching probability distributions shows that various modes from this spectrum can facilitate the switching. Moreover, it suggests a substantial contribution of the thermal spin-torque. The results point the way toward energy-efficient SAF-based applications and open avenues for functionalizing SAFs in nanoscale heterostructures.

[1] A. Etesamirad et al., ACS Appl. Mater. Interfaces 13, 20288 (2021)

[2] I. Barsukov et al., Sci. Adv. 5, eaav6943 (2019)   

Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: A first-principles Floquet-nonequilibrium Green function study

We compute and compare pumped spin currents from sub-THz-microwave-driven uniaxial antiferromagnetic insulator (AFI) MnF2 into heavy metal (HM) Pt with strong spin-orbit (SO) coupling. In contrast to using the concept of interfacial spin mixing conductance (SMC) which fails in the presence of SO coupling at the interface, we use noncollinear DFT Hamiltonian with Floquet-nonequilibrium Green functions to take into account SO-proximitized AFI due to adjacent HM; SO coupling at interfaces, and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region conducting. The DC component of pumped spin current vs. precession cone angle (theta) of the Neel vector of AFI does not follow putative relation proportional to sin^2 theta, except for very small angles theta less than 10 deg for which we define an effective SMC from the prefactor and find that it doubles from MnF2/Cu to MnF2/Pt interface. The angular dependence spin current is twice as large as SMC for the right-handed as for the left-handed chirality of the precession modes of localized magnetic moments within AFI at resonance.   

Superluminal propagation of antiferromagnetic magnons in nanometer-scale NiO

Magnons, the quasiparticles of spin waves, are the elementary low-energy collective excitations in magnetic materials. Antiferromagnetic insulators (AFMIs) can host terahertz (THz)-frequency magnons to carry angular momentums without moving charges, enabling high-speed and low-dissipation operation of spin-devices. In addition, compact AFMI devices at nanoscales are desirable due to the absence of stray field. However, the magnon propagation speed, which is a key parameter to determine data operation time, remains elusive in AFMIs particularly at nanometer distances due to the lack of sufficiently fast probes. Here, we report the direct time-domain measurement of the velocity of antiferromagnetic magnons in NiO with optical-driven THz emission [1]. We find the magnons propagate in nonmagnetic Bi₂Te₃/antiferromagnetic insulator NiO/ferromagnetic Co trilayers at a superluminal velocity (up to 650 km/s) at nanoscales in NiO (≤ 50 nm), which exceeds far beyond the limiting magnon group velocity (~40 km/s) obtained by dispersion relation using inelastic neutron scattering. We attribute this finding to the fact that a finite damping makes the dispersion anomalous at small wavenumbers and yields the superluminal magnon propagation. Our observation suggests the prospects of energy-efficient nanodevices using AFMIs considering finite dissipation in real materials.   

Harmonic Analysis of Spin Torques in Antiferromagnet/Heavy Metal Heterostructures

Harmonic analysis is a powerful technique to quantify current-induced torques acting on magnetic materials, but so far it has never been applied to antiferromagnets. We formulate an analytical theory of the first and second harmonic responses of an antiferromagnet/heavy metal (HM) heterostructure driven by current-induced torques and other concomitant effects, which can well be confirmed by numerical simulations. By rotating a magnetic field of variable strength in the xy, yz, and xz planes, we are able to clearly distinguish the contributions of the field-like torque, the damping-like torque, and the thermal effects through analyzing the second harmonic signals. Using Fe₂O₃/HM and NiO/HM heterostructures, we demonstrate the validity of our theory. Our theory provides direct and general guidance to current and future experiments.   

Quantifying Spin-Orbit Torques in AFM/HM Heterostructures by Harmonic Hall Measurements

Modifying the magnetic order of insulating antiferromagnets (AFMs) with spin currents is of fundamental interest and can enable new applications of AFMs. Toward this goal, characterizing the type and amplitude of the spin-orbit torques (SOT) associated with AFM/Heavy metal interfaces is important. In this study, we report the full angular dependence of harmonic Hall voltage measurements in a predominantly easy-plane AFM, epitaxial c-axis oriented α-Fe₂O₃, with an interface to Pt. Hall measurements are conducted while rotating an applied field in 3-orthogonal planes. By modeling the harmonic resistance signals together with the α-Fe₂O₃ magnetic parameters, such as its exchange field, magnetic anisotropy and DMI, we determine the amplitudes of field-like and damping-like SOTs. Out-of-plane field scans are shown to be essential to determining the damping-like torques. Our results indicate that the amplitude of the field-like torques are significantly larger than the damping-like torques and also consistent with a small angle tilt of the α-Fe₂O₃ hard axis with respect to the c-axis.   

Robust Spin Injection at Antiferromagnet/Ferromagnet Interface

With ultrafast spin dynamics and robustness against perturbation to magnetic fields, antiferromagnets are promising material candidates for information technologies [1,2]. Spin injection and detection in antiferromagnet-based systems is a prerequisite for the development of next-generation spintronic applications. Here, we demonstrate spin injection from a ferromagnetic metal, Permalloy (Py), into a thin film of an insulating antiferromagnet, Cr2O3. Using temperature gradient as a thermal drive [3], we observe spin pumping by magnons with different sign across the spin-flop transition of Cr2O3. In heterostructures with nonmagnetic metals, Cr2O3/Pt, the spin current rapidly falls with increasing temperature. However, we find the spin current injection from the ferromagnetic metal to rise with increasing temperature and to persist up to the characteristic collapse at the Néel temperature. The results point the way for robust spintronic applications that take advantage of both antiferromagnetic spin dynamics and spin-orbitronic phenomena inherent to ferromagnetic metals.

[1] Baltz et al., Rev. Mod. Phys. 90, 015005 (2018); [2] Etesamirad et al., ACS Appl. Mater. Interfaces 13, 20288 (2021); [3] Arkook et al., arXiv:1909.12445   

Spin Polarization of Noncollinear Antiferromagnetic Antiperovskites

Spin-polarized currents play a key role in spintronics. Recently, it has been found that antiferromagnets with a non-spin-degenerate band structure can efficiently spin-polarize electric currents, even though their net magnetization is zero. Among the antiferromagnetic metals with magnetic space group symmetry supporting this functionality, the noncollinear antiferromagnetic antiperovskites ANMn₃ (A = Ga, Ni, Sn, and Pt) are especially promising. This is due to their high Néel temperatures and a good lattice match to perovskite oxide substrates, offering possibilities of high structural quality heterostructures based on these materials. We investigate the spin polarization of antiferromagnetic ANMn₃ metals using first-principles calculations. We find that the spin polarization of the longitudinal currents in these materials is comparable to that in widely used ferromagnetic metals, and thus can be exploited in magnetic tunnel junctions and spin transfer torque devices. Moreover, for certain film growth directions, the out-of-plane transverse spin currents with a giant charge-to-spin conversion efficiency can be achieved, implying that the ANMn₃ antiperovskites can be used as efficient spin sources. These properties make ANMn₃ compounds promising for application in spintronics.   

Large Hall Signal due to Electrical Switching of an Antiferromagnetic Weyl Semimetal State

The electrical manipulation of a topological state is crucial for utilizing the robust properties of topological materials in electronic devices ^[1,2]. Recently, such manipulation is realized in the antiferromagnetic Weyl semimetal Mn₃Sn through the readout signal of anomalous Hall effect in the Mn₃Sn/heavy metal (Pt, W) heterostructures ^[3]. Here, we reported that the switching of Hall signal of Mn₃Sn/heavy metal multilayer can be significantly enhanced by: (i) changing the crystal orientation of Mn₃Sn by removing Ru buffer layer, and (ii) changing the interfacial condition by annealing at the interface between Mn₃Sn and the heavy metal. Compared to the reported switching Hall signal in Ru/Mn₃Sn/Pt multilayers, the switching Hall resistance becomes one order larger, ~0.35 Ω, in the Mn₃Sn/W devices. The readout voltage can be increased to mV order by increasing the read current. Moreover, by investigating the thickness dependence of Mn₃Sn layer, we found that the effective switching thickness in Mn₃Sn layer could go up to 40nm, which is much thicker than ferromagnetic materials.

[1] S. Nakatsuji, N. Kiyohara, T. Higo, Nature 527, 212-215 (2015).

[2] T. Higo, et al. APL 113, 202402 (2018).

[3] H. Tsai, T. Higo, et al. Nature 580, 608–613 (2020).     

Antiferromagnetic and ferromagnetic spintronics: spin transport in the two-dimensional ferromagnet and the role of in-chain and inter-chain interactions

The spintronics demands the  spin transport study, where the spin current plays a central role in order of spintronics phenomena to occur  in magnetic materials and also in various other materials including semiconductors and oxides. [1,2]

Spin-transport and current-induced torques in ferromagnet heterostructures given by a ferromagnetic domain wall are investigated.[3] Furthermore, the continuum spin conductivity is studied in a frustrated spin system given by the Heisenberg model with ferromagnetic in-chain interaction J₁ < 0 between nearest neighbors and antiferromagnetic next-nearest-neighbor in-chain interaction J₂ > 0  with aim to investigate the effect of the phase diagram of the critical ion single anisotropy D_c as a function of J₂ on conductivity. We consider the model with the moderate strength of the frustrating parameter such that in-chain spin-spin correlations that are predominantly ferromagnetic. In addition, we consider two inter-chain couplings J_perp,y and J_perp,z corresponding to the two axes perpendicular to chain where ferromagnetic as well as antiferromagnetic interactions are taken into account.