Pure spin currents, i.e. the net flow of spin angular momentum without an accompanying charge current, represent a new paradigm for spin transport and spintronics. Different schemes for the generation and the detection of pure spin currents have been heavily investigated both from experimental and theoretical sides in the last years. In these experiments either the magnetization is driven out of equilibrium by thermal (spin Seebeck effect) or microwave radiation (spin pumping) or the combined action of spin Hall and inverse spin Hall effect is exploited (spin Hall magnetoresistance). In previous pure spin current studies the experimental data are usually described within a simple, single band, low-energy magnon (spin wave) picture. Neither the magnonic band structure, nor higher energy magnon modes are considered in this approach. In our recent experiments we investigated a more complex class of electrically insulating ferrimagnets, namely ferrimagnets with a magnetization compensation point. We have fabricated gadolinium iron garnet/platinum (GIG/Pt) thin film heterostructures, and measured the spin Seebeck, spin pumping and spin hall magnetoresistance effect in these samples as a function of temperature.For the spin Seebeck effect (SSE), we observe two consecutive sign changes in the SSE signal. The first sign change occurs around the GIG magnetic compensation temperature, and can be straightforwardly understood in terms of the reorientation of the iron sublattice magnetizations at this temperature. The second, more gradual SSE sign change takes place at a temperature below the compensation point. Our analysis reveals that the thermally generated net spin current and SSE signal reflect a complex interplay between two magnon branches . These findings are further confirmed by addition spin pumping experiments, where we selectively can excite only one magnon mode and don’t observe a sign change in the spin pumping signal as a function of temperature. For the spin Hall magnetoresistance (SMR), we observe a sign change in the SMR signal around the compensation temperature. This sign change in SMR can be understood in terms of the spin canting phase of the three magnetic sublattices and their individual contribution to the SMR in the compensated garnet system .
Last but not least, we studied the local and non-local magnetoresistance of thin Pt strips deposited onto yttrium iron garnet . We compare the local magnetoresistance response of a Pt strip under current bias and the non-local voltage drop along a second, electrically isolated Pt strip, which is separated from the current carrying one by a few 100 nm. The non-local magnetoresistance exhibits the symmetry expected for a magnon spin accumulation-driven process . Our experiments as a function of temperature and applied field orientation show that the magnon mediated magnetoresistance (MMR) is qualitatively different from the SMR. Especially, the MMR vanishes at temperatures below 10 K while the SMR prevails even at low temperatures.
Financial support by the DFG via SPP 1538 (project no. GO 944/4) and the Nanoinitiative Munich (NIM) is gratefully acknowledged.
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