UC Davis

Permanent magnets composed primarily of rare earth elements are critical components in electric vehicle motors. With the recent surge in electric vehicle sales, demand for rare earth elements has dramatically increased, elevating costs. Thus, there is strong interest in identifying rare-earth-free alternatives for permanent magnet applications. The multi- principal element alloy (MPEA) family has emerged as a promising candidate. MPEAs contain three or more principal constituent elements, some of which may be magnetic such as Fe, Co, Cr, Mn, and Ni. In this thesis, I will first discuss our investigation of the magnetic properties of FeCoCrMnSi-based MPEAs. Multiple magnetic phase transitions were observed via zero-field-cooled/field-cooled magnetization measurements, indicating a rich magnetic phase diagram. Spectroscopic measurements revealed ferromagnetic ordering of Fe, Co and Cr, while Mn exhibited no long-range magnetization. Our results illustrate the rich and complex magnetic properties of MPEAs.

In addition to permanent magnet applications, controlling mesoscopic magnetic textures may enable next-generation energy-efficient magnetic memory and data storage. The second portion of my talk will focus on ultrafast manipulation of magnetic textures. Domain wall motion in ferromagnets driven by magnetic fields, electrical currents, or spin waves is typically quite slow, below 100 m/s. Furthermore, the Walker breakdown phenomenon limits domain wall velocities at higher driving fields or currents as precession of spins approaches the ferromagnetic resonance frequency. However, far-from-equilibrium dynamics induced through ultrafast optical excitation may allow overcoming these limitations and accessing nonequilibrium material behavior. In fact, recent theoretical work predicted superdiffusive spin current-driven domain wall velocities up to 14 km/s in optically pumped ferromagnets. We experimentally tested this prediction in a magnetically textured CoFe/Ni multilayer film using time-resolved extreme ultraviolet magnetic scattering with 50 fs resolution. For the highest optical pump fluences, we observed domain wall velocities up to 66 km/s, approaching the theoretical limit set by magnon group velocity. These findings demonstrate that far-from-equilibrium optical excitation can dramatically accelerate mesoscale magnetic textures. Our studies open the possibility of manipulating the ground state to achieve far-from-equilibrium effects at mesoscopic length scales. The implications of such nonequilibrium spin kinetics likely extend to understanding and harnessing ultrafast phenomena in quantum materials.