Ferrimagnetic Dynamics Induced by Spin‐Orbit Torques

Abstract

Ferrimagnets are the magnetic materials with the fastest current‐induced dynamics reported so far. Among them, rare‐earth transition‐metal (RE‐TM) alloys offer a fertile playground for studying the behavior of multi‐sublattice systems with tunable composition and magnetic interactions. This review provides a survey of the magnetic dynamics excited by current‐induced spin‐orbit torques (SOTs) in RE‐TM ferrimagnets coupled to heavy‐metal layers. It summarizes the magnetic properties of RE‐TM alloys and discusses how interfacial SOTs result in efficient magnetization switching and fast domain‐wall motion close to the magnetization and angular momentum compensation points. Recent work shows that the switching is a multiphase process affected by significant stochastic fluctuations. However, strong SOTs results in fast and deterministic sub‐ns switching with minimal energy dissipation. In addition, the RE and TM magnetizations can respond asynchronously to SOTs during the reversal. This asynchronous dynamics pinpoints the different strength of the SOTs acting on the two sublattices and challenges the usual assumption of rigid inter‐sublattice antiferromagnetic coupling. Overall, the ability to tailor the timescale and reversal mode of RE‐TM alloys allows for optimizing the speed of ferrimagnetic spintronic devices and provides insight into the current‐induced transfer of angular momentum in systems with synergistic ferromagnetic and antiferromagnetic interactions.

Nonuniform Current-Driven Formation and Displacement of the Magnetic Compensation Point in Variable-Width Nanoscale Ferrimagnets

“…4 Thin-film structures based on ferrimagnets can combine the advantages of both, having high speed and efficiency of current-induced magnetization switching as in antiferromagnets combined with the ease of detecting magnetic states as in ferromagnets. 5,6 Such features are observed around compensation states characterized by zero values of magnetic and angular momenta. 7−9 Since the magnetic structure of a ferrimagnet is formed by two antiferromagnetically coupled sublattices of different elements, mutual compensation of parameters can be achieved by changing the concentration of these elements, 10 temperature, 11 or layer thickness.…”

“…Injection of spin-polarized current is a universal approach for local control of the magnetic structure of ferro-, ferri-, and antiferromagnets, which is a key method necessary for the development of spintronics. , This approach is more easily implemented in ferromagnetic structures, but it is more effective in antiferromagnets . Thin-film structures based on ferrimagnets can combine the advantages of both, having high speed and efficiency of current-induced magnetization switching as in antiferromagnets combined with the ease of detecting magnetic states as in ferromagnets. , Such features are observed around compensation states characterized by zero values of magnetic and angular momenta. – Since the magnetic structure of a ferrimagnet is formed by two antiferromagnetically coupled sublattices of different elements, mutual compensation of parameters can be achieved by changing the concentration of these elements, temperature, or layer thickness . A few approaches to local and reversible realizations of the compensation state were proposed.…”

Sperimagnetic states in amorphous Gd x (FeCo)1-x (0.2 ≤ x ≤ 0.3) alloys: insights from stochastic magnetic anisotropy analysis

“…Currently, amorphous GdFeCo films attract attention as a media for ultrafast magnetization reversal and also as a material for spintronic devices, which allows magnetization switching through spin-orbit torque [37][38][39][40]. Understanding of equilibrium properties of a material is essential for proper consideration of magnetization dynamics.…”

Aging and passivation of magnetic properties in Co/Gd bilayers

“…Ferrimagnetic spintronics is an active contemporary research field. 1,2 One major motivation for investigating these magnetic systems is to combine the easy characterization and manipulation of the magnetic order of ferromagnets with the fast exchange-driven dynamics and information robustness associated with antiferromagnetic coupling. One particular class of ferrimagnets that is of interest here is the so-called transition metal (TM) rare earth (RE) ferrimagnets, characterized by a composition consisting of one or more TMs (Co, Fe, and Ni) and one or more REs (e.g., Gd and Tb).…”

“…Moreover, the Pt/Co/Gd trilayer displays strong interfacial spintronic effects, such as perpendicular magnetic anisotropy, the spin-Hall effect, 25 and the interfacial Dzyaloshinskii-Moriya interaction, 26 which are essential ingredients for applications based on efficient domain wall motion or SOT-driven manipulation. 2,27 Nonetheless, one important challenge that yet needs to be addressed, before applications of spintronic devices based on synthetic RE-TM systems can be considered, is their long-term stability. The aging and sample structure of RE-TM alloys have been extensively studied in the past, where several factors gave rise to changing magnetic properties over time, like the capping layer, 28,29 compositional inhomogeneities at the scale of several nm, 30,31 and thermodynamically driven intermixing.…”