Structural and magnetic properties of YIG thin films and interfacial origin of magnetisation suppression

This work covers the complete study of the properties of high quality nm-thick sputtered Yttrium iron garnet (Y3Fe5O12) films, with the discovery of interfacial diffusion and its effect on the magnetisation suppression. Here we report the structural and magnetic properties of YIG nano films deposited on Gadolinium gallium garnet (GGG) substrate by RF magnetron sputtering. The structural characterisation and morphology of the films were analysed using X-ray reflectivity (XRR), X-ray diffraction (XRD) and atomic force microscopy (AFM). The magnetic properties were investigated using VSM and SQUID magnetometer. The films in the 10 - 60 nm thickness range have surface roughness of 1-3 Å, and (111) crystalline orientation. The saturation magnetisation, coercive field and the Curie temperature observed in our YIG films are 144 ± 6 emu/cc, 0.30 ± 0.05 Oe and 559 K, respectively. The thickness dependence of the saturation magnetic moment shows the existence of a 6 nm dead layer. The temperature dependence of the magnetization M(T) in YIG reveals a reduction in magnetization at low temperature, below ~ 100 K. Through an extensive analysis using STEM, we discovered an interdiffusion zone of 4 - 6 nm at the YIG/GGG interface where Gd from the GGG and Y from
the YIG diffuse. Analysis of XRR data also confirms the presence of Gd-rich diffused layer of 5 - 6 nm thick at the interface. This Gd-rich YIG layer having compensation temperature at 100 K corresponds to 40% Gd diffusion,
that aligns antiparallel to the net moment of YIG, resulting in the magnetisation suppression in YIG at low temperature. Our polarised neutron reflectivity results also revealed the magnetization downturn in 80 nm YIG film. FMR results showed narrow FMR linewidth and a small
Gilbert damping, for e.g. (2.6 ± 0.3) x 10−4 in 38 nm thick YIG. The temperature dependence of the Gilbert damping factor in YIG and YIG/Pt showed a linewidth broadening and increased damping below 50 K. Our
current induced FMR results demonstrate the dominating role of Oersted field torque in driving the magnetization dynamics in YIG/Pt bilayer films. Our investigation on the effect of C60 molecules on the damping of YIG in
YIG/C60 hybrid structures shows an increase in damping in thin YIG films, but it decreases between 80 - 160 nm. Our findings widen the applications of sputtered nm-thick epitaxial YIG films and YIG-based multilayers in magnonics, spin caloritronics and insulator-based spintronics devices.