Investigation of exchange bias in thin films by ferromagnetic resonance spectroscopy

Abstract

In this work the ferromagnetic resonance spectroscopy experimental technique is employed to investigate the mechanisms of magnetic anisotropy in ferromagnet/antiferromagnet thin films, as well as the influence of antimony interlayers in such systems. Antimony is a diamagnetic semimetal known to possess topological surface states and unique quantum properties. Vibrating sample spectroscopy was also used to confirm the existence of exchange bias by looking at their magnetization curves. Regarding the first experimental section, the effect of magnetic annealing on the crystal lattice and magnetic anisotropy of YIG/IrMn bilayers was evaluated in order to establish procedures that give certain desired properties, with special attention to the induction of unidirectional anisotropy. It was found that cubic and uniaxial anisotropies became neater after annealing, indicating improvement of the crystallographic structure. Furthermore, an expressive sevenfold increase of the exchange bias effective field could be accomplished for one of the samples. In the second part, evidence of exchange bias in FM/Sb/AFM trilayers was gathered in two different FM (YIG and Py), for several antimony thicknesses in the order of a few dozens of nanometers. I hereby present the hypothesis that a coupling between the spins from antimony’s spin-momentum locked electrons and the FM electrons might be occurring. Spin-momentum locking stands for spins’ orientations being locked perpendicular to the electrons’ linear momentum due to Rashba spin-orbit splitting, thereby generating spin polarization at the interface. Assuming antimony to be hosting surface states, the spin pumped current from Sb would be topologically locked, which could result in coupling with the FM spins by means of the exchange interaction. If confirmed, the induction of exchange bias mediated by spin-momentum locking could open new avenues in spintronics, such as the control of exchange bias through the surface currents in antimony.