Spin-Orbit Torque and Magnetization Dynamics

When ferromagnets and antiferromagnets are driven away from equilibrium, their magnetization undergoes precessional dynamics at GHz to THz frequencies. These dynamics are closely intertwined with various interesting phenomena including magnon propagation, magnetic resonance, and spin torque which are important for energy-efficient magnetic memory and logic applications.

One of the most efficient mechanisms for exciting magnetization dynamics and magnetic switching of a nanomagnet is the spin-orbit torque. Here, a charge current through a heavy metal with large spin-orbit coupling such as platinum generates a transverse spin current through the spin Hall effect. The resulting spin current enters the adjacent magnetic layer and creates a torque on the magnetization as a result of angular momentum transfer. This pushes the magnetization away from equilibrium and induces magnetization dynamics and even magnetization switching if a sufficient current is applied.

Spin torque phenomena have traditionally been studied in metallic multilayers and magnetic tunnel junctions. However, it was found that using topological insulators for the spin-orbit layer leads to much stronger spin-orbit torques. An interesting question is whether other quantum materials can be used to further improve the spin-orbit torque efficiency. We are therefore investigating various quantum materials for spin-orbit torque including 2D magnets, topological insulators, magnetic topological insulators, kagome magnets, Dirac and Weyl semimetals, and 2D semimetals.

As the spin torque phenomena are closely intertwined with magnetization dynamics, one of our methods for investigating spin-orbit torque is through ultrafast optical probes of magnetization dynamics as discussed below. These methods directly measure the magnetization of the magnetic layer and could probe precession frequencies up to ~1 THz due to the temporal resolution of ~150 femtoseconds. This is could be beneficial for investigating antiferromagnetic dynamics in the THz range, which is very difficult using traditional magnetic resonance methods.


Spin−Orbit Torque in Bilayers of Kagome Ferromagnet Fe3Sn2 and Pt

Spin−orbit torque phenomena enable efficient manipulation of the magnetization in ferromagnet/heavy metal bilayer systems for prospective magnetic memory and logic applications. Kagome magnets are of particular interest for spin−orbit torque due to the interplay of magnetic order and the nontrivial band topology. In this study, we demonstrated spin−orbit torque in a bilayer system of topological kagome ferromagnet Fe3Sn2 and platinum. We used two different techniques, one based on the quasistatic magneto-optic Kerr effect (MOKE) and another based on time-resolved MOKE, to quantify spin−orbit torque efficiency.

The time-resolved MOKE method is illustrated to the left. Using a femtosecond pulsed laser to generate pump and probe pulses, we use Kerr rotation of the probe pulse to track the magnetization dynamics after a thermal pump pulse. From a simplified viewpoint, the presence of damping-like spin-orbit torque will change the overall damping and hence modify the spin lifetime. Similarly, the presence of field-like spin-orbit torque will change the precession frequency. The figure (panel c) shows three traces with three different values for the charge current. In the probe delay scan, it can be seen that the precessional spin lifetime varies with the charge current. Detailed analysis of the spin lifetime as a function of the charge current (panels d and e) yields an effective spin Hall angle of ~6% in the platinum. We plan to use this technique to investigate a variety of quantum materials for spin-orbit torque. For further details, read Lyalin et al, “Spin−Orbit Torque in Bilayers of Kagome Ferromagnet Fe3Sn2 and Pt” Nano Letters 21, 6975 (2021).

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