Improving the Structural and Magnetic Properties of Yttrium Iron Garnet

Abstract

Both gadolinium gallium garnet (GGG) and yttrium aluminium garnet (YAG) substrates are used to grow thin YIG films. It is known that substrates of GGG are associated with reduced lattice strain (ε = 0.016%) relative to YAG substrates (ε = 1.12%) for YIG lattices. In addition, it was found that the structural and magnetic properties of thin YIG films are influenced by oxygen contents. For instance, we found in this work that the lattice constant for YIG/GGG films with 6.2% of the Oxygen content is similar to bulk value of YIG lattice constant ( 12.376 A) and the magnetic properties are similar to those of bulk YIG that have a coercive field (H c ) < 1 Oe and saturation magnetisation (M s= 143emu/cm 3 ).

The current study concerns all of the features of good quality nm-thin sputtered YIG films with the identification of interfacial diffusion and the impact it has in terms of suppressing magnetisation. The structural and magnetic properties of nm-thin YIG films deposited on GGG and YAG substrates by RF magnetron sputtering. A combination of X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM) were employed to assess the films’ morphology and structural characterisation. Meanwhile, both a VSM and SQUID magnetometer were employed to determine the associated magnetic qualities. The YIG films have a 111 crystalline orientation and a 1-3 A surface roughness. The YIG films have a coercive field of 0.30 ± 0.05 Oe, saturation magnetisation of 140 ± 6 emu/cc. Furthermore, the saturation magnetic moment’s thickness dependence has a dead layer. The dead layer thickness can be changed by changing the Oxygen contents during sputtering. The thinnest dead layer was 2.35nm in YIG film with 6.2% of Oxygen and the thickest dead layer was 6 nm in YIG film with 5% of Oxygen.

When a normal metal (NM) makes contact with a magnetic material, this can result in spin Hall magnetoresistance (SHMR). Insulating magnetic materials help to avoid magnetic effects because they are able to effectively restrict electrons from leaving the NM. To help analyse this, an innovative means of producing YIG has been devised that utilises sputtering methods, thereby enabling applications to be created in less time and at lower cost relative to the liquid phase epitaxy (LPE) approach. The magnetic qualities of the YIG material have been specified at varying thicknesses. For thin samples, it is known that magnetisation is lower than the bulk value and this is understood to be attributable to the substrate interface having a layer that is interdiffused. Owing to the fact that the SHMR is an interface effect, it is necessary to employ AFM and XRR to examine how rough the films are.

The first measurements of SHMR were made using platinum and the results achieved broadly matched those reported in the empirical literature. A cryostat is used to measure angular dependence with four probe resistance measures and a split pair magnet which enables the sample to be positioned in all conceivable directions relative to the applied field. This concurs with the established theory and research was conducted across various temperatures from 2K to 300K. A range of models were then applied to fit the data for the spin diffusion length and it was found that the temperature dependence is best explained by the Elliot-Yafet mechanism (EY). The research is conducted into magneto-transport with bilayers of platinum and YIG using different interfaces (2 nm of Pt on 40 nm of YIG with different Oxygen contents) . Measuring resistivity as a function of temperature, conventional magnetoresistance and angle-dependent magnetoresistance reveals that there is considerable reliance on the interface. Consequently, it is necessary to develop the methods for preparing interfaces so as to observe a pure SHMR from enhanced spin mixing conductance.