Abstract
Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE.
Highlights
Magnons are the elementary excitations of a magnetically ordered system
It is widely accepted that the thermopower signals measured in ferromagnetic insulator/ normal metal (FMI/NM) bilayers in the longitudinal spin Seebeck effect (SSE) geometry are a consequence of magnonic spin currents generated by a temperature gradient[2,5,6,7,8,15]
The detection of the magnonic spin currents via the inverse spin Hall effect (ISHE) in the adjacent normal metal layer has been analysed based on an effective spin current combined with an effective spin mixing conductance that governs the spin current transport across the interface
Summary
Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. For Gd3Fe5O12/Pt (GdIG/Pt) bilayers as a test bed, we demonstrate that the SSE in complex ferrimagnets results from the balance of thermal spin pumping between multiple magnon modes where distinct magnetic moments might contribute differently to the spin mixing conductance. This leads to different efficiencies for the spin transport across the GdIG/Pt interface. Our theoretical analysis reveals that the thermally generated net spin current giving the SSE signal reflects the complex interplay of two magnon branches and possibly the exchange coupling at the interface This shows that the description of the SSE needs to take into account the magnon dispersion relation including the polarization of spin waves as well as interface effects that have previously been neglected
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