Magnetic and Spintronic Properties of Rare-Earth Iron Garnets

Abstract

Rare earth iron garnets (REIG) can be grown as thin films with strain-induced perpendicular magnetic anisotropy (PMA), and their potential for spintronic device applications has been studied extensively in recent years. In particular, thulium iron garnet (TmIG) has excited great interest due to record-breaking spin orbit torque-driven domain wall velocities over 2km/s and the presence of the Dzyaloshinskii-Moriya interaction, which stabilizes chiral Néel domain walls. In order to optimize REIG for these applications, it is useful to develop methods for tuning their magnetic and spintronic properties. In this work, we accomplish this through varying the RE site occupancy.

We report the growth and characterization of fully-strained terbium iron garnet (TbIG) and europium iron garnet (EuIG) with PMA ranging in thickness from 10 to 80 nm. EuIG can be grown with PMA on (100) and (111) gadolinium gallium garnet (GGG) substrates, making it ideal for orientation-dependent studies. For instance, Pt/EuIG had similar (001) and (111) imaginary spin mixing conductances of 4.6-5.4×1012 Ω-1m-2 in contrast to similar studies on the Pt/CFO system. The (111) imaginary spin mixing conductance of Pt/TbIG (4.6×1012 Ω-1m-2) was similar to the Pt/EuIG system, and both Pt/TbIG and Pt/EuIG had comparable spin mixing conductances to Pt/TmIG.

The TbIG films had a low saturation magnetization (~30 emu/cc) at room temperature due to their easily accessible magnetic compensation point of 330K, and anomalous Hall effect measurements of Pt/TbIG showed a sign change at the compensation point. Through a combination of x-ray absorption measurements and molecular field simulations, we propose a model to explain this observation involving point defects such as iron vacancies and terbium antisite defects.

We also report the static and dynamic magnetic properties of yttrium substituted thulium iron garnet (Y:TmIG) thin films on GGG as a function of Y concentration. We report the tunability of the magnetic anisotropy energy, with full control achieved over the type of anisotropy (from perpendicular, to isotropic, to an in-plane easy axis) on the same substrate. In addition, we report a nonmonotonic composition-dependent anisotropy term, which we ascribe to growth-induced anisotropy similar to what has been reported in garnet thin films grown by liquid-phase epitaxy. Ferromagnetic resonance shows linear variation of the damping and the g-factor across the composition range, consistent with prior theoretical work. Domain imaging reveals differences in reversal modes, remanant states, and domain sizes in YxTm3-x iron-garnet thin films as a function of anisotropy.