Ferrimagnetic rare earth iron garnets (X3Fe5O12, where X denotes a rare earth element) are promising materials for various applications, such as magneto-optical (MO) isolators, spintronic and spin wave devices. The three types of cation sites (dodecahedral, octahedral and tetrahedral) can be occupied by a wide range of ions resulting in a large variation of the physical properties of the garnet. This can be utilized to tailor its MO response or magnetic properties.

To determine the contribution of each sublattice to the MO response and therefore to characterize sublattice magnetism, an analysis of spectrally dependent MO hysteresis loops can be used 1. For this purpose, we measured field dependent polar MO Kerr effect (MOKE) spectra in the range from 1.4 eV to 4.5 eV of various ferrimagnetic garnet thin films, such as terbium iron garnet (TbIG) or yttrium iron garnet (YIG), prepared by pulsed laser deposition on GGG substrates. This allowed us to extract MOKE hysteresis loops with very high spectral resolution. The resulting hysteresis loops exhibited spectrally dependent shapes and were modelled as a sum of two hysteresis loops, shown for 300 nm thick epitaxial TbIG in Fig. 1. Moreover, their contributions to the MOKE spectra were separated as shown in Fig. 2. These results suggest that rather than sublattice contributions this analysis led to the identification of components with two different magnetic anisotropies within this particular film. The TbIG has a strain-induced perpendicular magnetic anisotropy when grown on GGG. This anisotropy, however, may not be retained for thicker samples 2 due to strain relaxation leading to an evolution of magnetic anisotropy across the layer. These results can be compared with those on YIG films, where the spectral response of the octahedral and tetrahedral Fe sublattices can be separated. This work demonstrates field dependent MOKE spectra as an effective tool to separate not only sublattice contributions but also an evolution of magnetic anisotropy across the film.
References:
1 M Deb, E Popova, A Fouchet and N Keller, J. Phys. D: Appl. Phys. 45 455001 (2012)
2 V. H. Oritz, M. Aldosary, J. Li, et al. APL Materials 6, 121113 (2018)

Fig. 1 Separation of MOKE hysteresis loop of 300 nm thick TbIG film on GGG

Fig. 2 Individual loop contributions to the MOKE spectra of 300 nm TbIG on GGG