Session A41: Spin Transport and Excitations in Antiferromagnets

Spin Torque Switching of Antiferromagnet

Antiferromagnets with zero net magnetic moment, strong anti-interference and ultrafast switching speed have potential competitiveness in high-density data storage. Electrical switching of antiferromagnets is at the heart of their device application [1,2]. The antidamping torque-induced switching of Néel order is attained in a biaxial antiferromagnetic insulator NiO, which is manifested electrically via spin Hall magnetoresistance in NiO (100)/Pt bilayers [3]. The antiferromagnetic moments are switched towards the current direction, different from the vertical configuration in the fieldlike torque scenario (e.g., CuMnAs and Mn₂Au) [4,5]. On the other hand, electric field is used to switch the magnetic moment of Mn₂Au films grown on piezoelectric Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMN-PT) (011) substrates. When the electric field is swept, the easy axis of Mn₂Au is switched between [100] and [01-1] directions of PMN-PT (011) at room temperature, exhibiting a butterfly-like swithing feature. This feature indicates that the underlying mechanism is the electric field-induced ferroelastic strain. Such a transition of the easy axis leads to the change of threshold current for the field-like torque switching of Mn₂Au [6]. Electrical switching of antiferromagnetic moments pave the way for all-electrical writing and readout in antiferromagnetic spintronics.
[1] C. Song, et al. Nanotechnology 29, 112001 (2018)
[2] C. Song, et al. Prog. Mater. Sci. 87, 33 (2017)
[3] X. Z. Chen, et al. Phys. Rev. Lett. 120, 207204 (2018)
[4] P. Wadley, et al. Science 351, 587 (2016).
[5] X. F. Zhou et al. Phys. Rev. Appl. 9, 054028 (2018)
[6] X. Z. Chen, et al. To be submitted.
  

Spin Dissipation Independent of Antiferromagnetic Order in IrMn

We investigate the interaction of spin current with antiferromagnetic order in polycrystalline IrMn by resonant spin pumping. We use spin-valve-like multilayers (NiFe/Cu/IrMn/CoFe), in which no direct magnon coupling is present between the NiFe spin source and the exchange-biased IrMn/CoFe spin sink; the pumped pure spin current is mediated solely by electrons through the Cu spacer layer. We observe no anisotropic spin dissipation with respect to the exchange bias direction, nor do we observe any difference in spin dissipation for samples with and without pinned antiferromagnetic order. Moreover, although there is a pronounced resonance linewidth increase in NiFe that coincides with the switching of IrMn/CoFe, we show that this is not indicative of anisotropic spin dissipation in IrMn. Our results demonstrate that the dissipation of electron-mediated spin current is remarkably insensitive to the magnetization state of the antiferromagnetic IrMn spin sink.   

Two-Magnon Scattering Enhanced by Randomly-Distributed Antiferromagnetic Exchange Field

We report a quantitative study of two-magnon scattering in Ni₈₁Fe₁₉/NiO bilayers with various NiO thicknesses. The magnetic damping of the Ni₈₁Fe₁₉/NiO bilayer was found to have strong dependence on NiO thickness. The amplitude of the two-magnon scattering is enhanced with increasing the thickness of the antiferromagnetic layer, which was evaluated from the out-of-plane-angular-dependent spectral linewidth of ferromagnetic resonance. The origin of this enhancement in the Ni₈₁Fe₁₉/NiO bilayer is the increase of randomly-distributed antiferromagnetic exchange fields. We have calculated the spin-mixing conductance by eliminating the effect of the two-magnon scattering, and found that the value is at 8.1 nm^-2 for the Ni₈₁Fe₁₉/NiO interface. Skipping this process leads to overestimation of spin-mixing conductance in ferromagnet/antiferromagnet bilayer structures. Our result gives further insight on the role of the two-magnon scattering in manipulating magnetic damping, which is crucial for generation and transmission of spin currents in widely-studied ferromagnet/antiferromagnet systems.   

A Dirac nodal line metal for topological antiferromagnetic spintronics

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which exploits the Néel vector to control the topological electronic states and the associated spin-dependent transport properties. A recently discovered Néel spin-orbit torque has been proposed to electrically manipulate Dirac band crossings in antiferromagnets; however, a reliable AFM material to realize these properties in practice is missing. Here, we predict that room temperature AFM metal MnPd₂ allows the electrical control of the Dirac nodal line by the Néel spin-orbit torque. Based on first-principles density functional theory calculations, we show that reorientation of the Néel vector leads to switching between the symmetry-protected degenerate state and the gapped state associated with the dispersive Dirac nodal line at the Fermi energy. The calculated spin Hall conductivity strongly depends on the Néel vector orientation and can be used to experimentally detect the predicted effect using a proposed spin-orbit torque device. Our results indicate that AFM Dirac nodal line metal MnPd₂ represents a promising material for topological AFM spintronics.

[1] Ding-Fu Shao, Gautam Gurung, Shu-Hui Zhang, and Evgeny Y. Tsymbal, arXiv:1810.09033.   

Antiferromagnetic Mn3NiN thin films supporting giant piezomagnetism

Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite Mn₃NiN, facilitated by its geometrically frustrated antiferromagnetism opening the possibility of new memory device concepts. Films of Mn₃NiN with intrinsic biaxial strains of 0.25% result in Néel transition shifts up to 60K and magnetisation changes consistent with theory [1]. Films grown on BaTiO₃ display a striking magnetisation jump in response to uniaxial strain from the intrinsic BaTiO₃ structural transition, with an inferred 44% strain coupling efficiency and a magnetoelectric coefficient approximately a 1000-fold increase over Cr₂O₃ as predicted previously by theory. Overall our observations pave the way for further research into the broader family of Mn-based antiperovskites where yet larger piezomagnetic effects are predicted to occur at room temperature [2]. In this talk we will review progress towards application of thin film piezomagnetism in Mn3NiN.
[1] J. Zemen et al., Phys Rev B 96, 024451 (2017) [2] D. Boldrin et al., ASC Appl. Mater. Interfaces 10 18863 (2018)   

Spin Hall Effects in Antiferromagnets

Recent experiments demonstrate that antiferromagnets exhibit a spin Hall effect. Calculations also indicate that the intrinsic contribution is important in determining the magnitude of the spin Hall angle. However, we do not know how the mean free path, exchange interaction, and spin-orbit coupling govern these results and how these factors might influence our understanding of experiments. To address these questions, we consider a minimal model of an antiferromagnet. We numerically compute the spin Hall conductance as a function of impurity concentration, exchange energy, and spin-orbit coupling. We find that the spin Hall conductance is considerably larger in antiferromagnetic systems compared to normal metals. This opens yet another avenue of using antiferromagnets in spintronics devices.   

Spin Orbit Torque Switching Mediated by Antiferromagnetic Insulators

Spin transport and magnetic dynamics in antiferromagnetic (AF) insulators have attracted wide research interests recently. In contrary to the popular belief of AF being an inactive element for spin transport, recent experiments based on spin pumping, nonlocal spin transport and spin Seebeck effect suggest efficient spin current transmission can be realized in various antiferromagnetic systems, via the mediation of antiferromagnetic magnons. In this work, we show initial experimental evidences towards this direction, where by utilizing the current induced spin orbit torque in Pt layer, we achieved magnetic switching in Co_xTb_1-x free layer across a thin AF insulator NiO. In the ultrathin spacer thickness regime (1~2nm), we even observed an enhancement of the spin orbital torque efficiency compared with the Pt/Co_xTb_1-x bilayer film. The realization of magnetic switching in Pt/NiO/Co_xTb_1-x heterostructures provides an existence proof on AF insulator mediated spin orbit torque, enabling promising material platform and device structures for energy favorable spin manipulation.   

Nonlocal Coupling between Antiferromagnets and Ferromagnets in Cavities

Microwaves couple to magnetic moments in both ferromagnets and antiferromagnets. Although the magnons in ferromagnets and antiferromagnets radically differ, they can become hybridized via strong coupling to the same microwave mode in a cavity. The equilibrium configuration of the magnetic moments crucially governs the coupling between the different magnons, because the antiferromagnetic and ferromagnetic magnons have opposite spins when their dispersion relations cross. We derive analytical expressions for the coupling strengths and find that the coupling between antiferromagnets and ferromagnets is comparable to the coupling between two ferromagnets. Our findings reveal a robust link between cavity spintronics with ferromagnets and antiferromagnets.   

Gigahertz frequency antiferromagnetic resonance and strong magnon-magnon coupling in the layered crystal CrCl3

Antiferromagnetic spintronics is an emerging field with the potential to realize logic and memory devices with high speed and bit density. Compared with well-studied ferromagnetic materials, the understanding on antiferromagnetic dynamics remains very limited, partly due to the ultrahigh instrinsic frequency, which often requires specialized terahertz techniques beyond the reach of facile device integration. Here we report broadband microwave magnetic resonance spectroscopy of the layered antiferromagnet CrCl₃. We observe a rich structure of resonances arising from quasi-two-dimensional antiferromagnetic dynamics. Due to the weak interlayer magnetic coupling in this material, we are able to observe both optical and acoustic branches of antiferromagnetic resonance in GHz frequency range. By breaking the rotational symmetry of the crystal, we further show that strong magnon-magnon coupling with large tunable gaps in orders of GHz can be induced between the two resonant modes. Our results therefore provide a versatile system for microwave control of antiferromagnetic dynamics.   

Electrical Signal Generation at Magnetic Resonance in Insulating Antiferromagnets

Antiferromagnetic materials, particularly antiferromagnetic insulators provide an alternative to present ferromagnetic spin-transfer torque based devices which suffer from limitations in terms of density and speed owing to their magnetic anisotropy dominated spin dynamics. In antiferromagnets spin dynamics are governed by the interatomic exchange interaction energies which are orders of magnitude larger than the magnetic anisotropy energy, leading to the potential for ultrafast information processing and communication in the THz frequency range. We will present studies of spin pumping at Manganese Difluoride(MnF₂) / Platinum (Pt) interfaces at temperatures below the MnF₂ Néel temperature (T_N = 67.34K). In particular, measurements of the inverse spin Hall Effect (ISHE) voltage arising from the interconversion of the dynamically injected spin currents into Pt will be reported. We observe clear electrostatic potential signals coinciding with the MnF₂ magnetic resonance positions including the spin-flop transition (H_SF = 9T). The signals reverse by switching the polarity of the magnetic field, and display a marked dependence on the power of the microwave stimuli, as expected from the ISHE.   

Spin Nernst Effect in the Paramagnetic Regime of An Antiferromagnetic Insulator

We theoretically investigate a pure spin Hall current driven by a longitudinal temperature gradient, i.e., the spin Nernst effect (SNE), in a paramagnetic state of a collinear antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction. The SNE in a magnetic ordered state in such an insulator was proposed by Cheng et al. [R. Cheng, S. Okamoto, and D. Xiao, Phys. Rev. Lett. 117, 217202 (2016)]. Here we show that the Dzyaloshinskii-Moriya interaction can generate a pure spin Hall current even without magnetic ordering. By using a Schwinger boson mean-field theory, we calculate the temperature dependence of SNE in a disordered phase. We also discuss the implication of our results to experimental realizations.   

Spin-torque control of the anomalous Hall effect in non-collinear antiferromagnets

Non-collinear antiferromagnets have recently aroused significant interest due to the non-vanishing Berry curvature and the associated anomalous Hall effect sensitive to the magnetic ordering. In this work, we explore series of antiperovskite compounds Ga_1-xNi_xNMn₃ with competing Γ_4g and Γ_5g phases, where the magnetic moments of Mn atoms are aligned in the (111) plane. An electric current flowing and polarized in the [111] direction produces a spin-transfer torque rotating the magnetic moments in the (111) plane. Based on the Landau-Lifshitz-Gilbert-Slonczewski equation, we explore spin dynamics in these non-collinear antiferromagnets and obtain conditions at which a pulse of the spin-polarized current can reverse the Néel vector and thus change the sign of the anomalous Hall conductivity. We find that the strength of the applied spin-polarized current density largely depends on the magnetocrystalline anisotropy energy controlling the stability of the Γ_4g and Γ_5g phases. Using density functional theory methods, we calculate the magnetocrystalline anisotropy energy in these antiperovskite compounds and find the optimum conditions for the current-induced reversal of the Néel vector.