Increased Gilbert damping in yttrium iron garnet by low temperature vacuum annealing

The effect of thermal surface cleaning on the Gilbert damping (α) of yttrium iron garnet (YIG), before capping with a metallic layer, has been investigated. Our results show that α is strongly affected by relatively mild annealing conditions (T = 300 °C) when performed in a vacuum. This increase needs to be taken into account when obtaining the spin-mixing conductance from spin pumping measurements. We measure an increase in α by a factor of ×8 when the YIG is vacuum annealed at 300 °C. No such changes in α are observed when annealed at the same temperature in 1 × 10−1 mbar of oxygen. We suggest that the main driver for the increase in α is the reduction of Fe3+ to Fe2+, as demonstrated by soft x-ray magnetic spectroscopy.