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We performed temperature-dependent optical pump -THz emission measurements in Y3Fe5O12 (YIG)|Pt from 5 K to room temperature in the presence of an externally applied magnetic field. We study the temperature dependence of the spin Seebeck effect and observe a continuous increase as temperature is decreased, opposite to what is observed in electrical measurements where the spin Seebeck effect is suppressed as 0 K is approached. By quantitatively analysing the different contributions we isolate the temperature dependence of the spin-mixing conductance and observe features that are correlated to the bands of magnon spectrum in YIG.The longitudinal spin Seebeck effect (LSSE) 1 describes the transfer of a spin current from a magnetic insulator driven by a temperature gradient. An adjacent heavy metal (HM) layer with large spin orbit coupling is typically used to convert the spin current into an electrical signal via the inverse spin Hall effect (ISHE). 2,3 The LSSE has been measured in a variety of different materials such as ferromagnets 1,4,5 , anti-ferromagnets 6,7 and paramagnets. 8 Magnetic insulators (MI) such as Y3Fe5O12 (Yttrium Iron Garnet -YIG) are particularly interesting for studies on the LSSE since the absence of electron charge transport allows the roles of magnons and phonons to be identified in the spin transfer. 1,3,9,10 Temperature, thickness and magnetic field dependence studies have contributed to a phenomenological picture of magnon-driven spin current. [11][12][13][14][15] A temperature gradient across the magnetic insulator thickness leads to the diffusion of thermal magnons that accumulate at the interface with the HM. 16,17 The temperature dependence of the magnon propagation length m results in a characteristic peak in the SSE signal at low temperature when the thickness of the MI is comparable to m . 12 Low frequency magnons play a dominant role due to their large population and

Yttrium iron garnet is a complex ferrimagnetic insulator with 20 magnon modes which is used extensively in fundamental experimental studies of magnetisation dynamics. As a transition metal oxide with moderate gap (2.8 eV), yttrium iron garnet requires a careful treatment of electronic correlation. We have applied quasiparticle self-consistent GW to provide a fully ab initio description of the electronic structure and resulting magnetic properties, including the parameterisation of a Heisenberg model for magnetic exchange interactions. Subsequent spin dynamical modelling with quantum statistics extends our description to the magnon spectrum and thermodynamic properties such as the Curie temperature, finding favourable agreement with experimental measurements. This work provides a snapshot of the state-of-the art in modelling of complex magnetic insulators.

5d metals are used in electronics because of their high spin-orbit coupling (SOC) leading to efficient spin-electric conversion. When C60 is grown on a metal, the electronic structure is altered due to hybridisation and charge transfer. In this work we measure the spin Hall magnetoresistance for Pt/C60 and Ta/C60, finding they are up to a factor 6 higher than for the pristine metals, indicating an increase in spin Hall angle of 20-60%. At low fields of 1-30 mT the presence of the C60 increased the anisotropic magnetoresistance by up to 700%. Our measurements are supported by non-collinear Density Functional Theory calculations, which predict a significant SOC enhancement by C60 that penetrates through the Pt layer, concomitant with trends in the magnetic moment of transport electrons acquired via SOC and symmetry breaking. The charge transfer and hybridisation between the metal and the C60 can be controlled by gating, so our results indicate the possibility of dynamically modifying the SOC of thin metals using molecular layers. This could be exploited in spin transfer torque memories and pure spin current circuits.

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