Relaxation of Strain in Terbium Iron Garnet Layers Detected by Magneto-optical Kerr Effect Spectroscopy
Abstract
Ferrimagnetic iron garnets are promising materials for various applications, such as magneto-optical (MO) isolators, spintronic and spin wave devices. Terbium iron garnet grows on (111) gadolinium gallium garnet substrate under compressive strain, that induces perpendicular magnetic anisotropy (PMA), which is a desired property for various applications.
The performance of particular device based on this material usually depends on the thickness of the layer. However, the desired PMA may not be retained in the whole layer above certain thicknesses.
To determine the state of the strain in the 264 nm thick TbIG layer, we measured spectral dependence of polar MO Kerr effect (MOKE) hysteresis loops in the range from 1.4 to 4.5 eV. The measurement showed that the hysteresis loops are composed of two contributions in the whole spectral range, which represent the PMA bottom layer, and top layer with relaxed strain.
Using the Yeh formalism, we obtained the spectral dependence of the off-diagonal elements of permitivity tensor from the saturated state at a magentic field of 1 T. Subsequently, using Yeh formalism again, we separately calculated the magneto-optical response of the strained and relaxed layers.
By matching the position of the spectral peaks, we estimated the thicknesses of the strained and relaxed layers to be 160 nm and 104 nm, respectively. The strain relaxation was further confirmed by the X-ray reciprocal space mapping (RSM) measurement.
The results show that the strain relaxation in thicker TbIG films may cause a magnetic anisotropy separation and field dependent MOKE spectroscopy is a robust and sensitive technique for probing the quality of the films.