Antiferromagnetic Spin-torque Oscillators: Ultrafast Dynamics and Possible Applications

Abstract

Antiferromagnets are prospective materials for spintronic applications due to their internal ultrafast dynamics, robustness with respect to the external magnetic fields, and high sensitivity to spin-torques of different nature. Moreover, in contrast to ferromagnets, spin-torques can induce steady precession of the Néel vector at THz frequencies whose values can be controlled by the current. Hence, antiferromagnets can be considered as tunable current controlled auto-oscillators, which is the main component of phase-controlling devices. An auto-oscillation regime is supported solely by antidamping torques that compensate the internal energy losses. However, the frequency of the precession, threshold value, the type of dynamics etc can be effectively manipulated by applying the field-like torques. In this presentation we review recent developments in the field of antiferromagnetic spin-torque oscillators and discuss new functionalities stemming from the combining effect of the field-like and antidamping-like torques. To demonstrate the effects of different torques we start from the dynamics of ultrafast switching and discuss various mechanisms including the Néel spin-orbit torques [1], thermomagnetoelastic mechanism [2], and optically-induced spin-transfer torques [3]. We illustrate possible applications of nontrivial antiferromagnetic dynamics for the detection and emission of teraherz radiation in altermagnets CuMnAs and Mn2Au in which electrical current can induce simultaneously the ac Néel spin-orbit torque and dc spin-orbit torque [1]. Using NiO antiferromagnet as an example, we discuss and compare various scenarios of ultrafast dynamics, such as: i) reorientation of the Néel vector due to the field-like torques of magnetoelastic nature and spin-orbit torques due to spin-pumping [4]; ii) fast switching and stable precession induced by the ac spin-transfer torques whose frequency coinsides with the frequency of antiferromagnetic resonance. We also analyse the chaotic regimes of an antiferromagnetic oscillator that appear at certain combination of field-like and damping-like torques (see Fig.1) and and discuss possible applications for neuromorphic computations [5]. As an outline of future development, we discuss influence of the magnetoelastic and thermal effects and combination of different stimulus for the effective control and manipulation of the antiferromagnet-based devices.  Current-induced transition to chaos traced by variation of the largest of three Lyapunov's exponents. Red lines on the spheres show trajectories of the Neel vector. From [5].