The Effect of Point Defects on the Compensation Temperature of Terbium Iron Garnet Thin Films

Recently, rare earth iron garnets (REIG) thin films with perpendicular magnetic anisotropy (PMA) have attracted a great deal of attention for spintronic applications. A subset of REIG materials exhibit magnetic compensation temperatures (T_comp), which may enable technologically interesting phenomena such as ultrafast domain wall velocities and ultrasmall skyrmions. For example, terbium iron garnet (TbIG) thin films have been grown by pulsed laser deposition with PMA and a T_comp of ~330 K, and their spin Hall magnetoresistance and spin orbit torque switching characteristics were investigated near compensation. Interestingly, however, the bulk T_comp­ of TbIG is closer to 250 K – about 80 K lower than the T_comp of the films. Hypotheses such as Tb antisites and/or iron vacancies have been suggested but have not been rigorously tested. Explaining this phenomenon could guide future efforts to engineer T_comp­.

In this study, we construct a site occupancy model for TbIG thin films. X-ray photoelectron spectroscopy scans reveal off-stoichiometry within the thin film (Tb:Fe =  0.86), implying the presence of iron vacancies. Hartree-Fock calculations are used to fit x-ray magnetic circular dichroism spectra at the Fe L3 edge to extract Fe site occupancies. This information is fed into molecular field simulations which in turn demonstrate that ~15% of the Fe vacancies must be filled by Tb ions to reproduce our observed T_comp. Our site occupancy model thereby explains the anomalous T_comp of TbIG thin films.