D. Qu

Intrinsic spin Seebeck effect in Au/YIG.

The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin polarized density functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG.

Absence of anomalous Nernst effect in spin Seebeck effect of Pt/YIG

The Pt/YIG structure has been widely used to study spin Seebeck effect (SSE), inverse spin Hall effect, and other pure spin current phenomena. However, the magnetic proximity effect in Pt when in contact with YIG, and the potential anomalous Nernst effect (ANE) may compromise the spin current phenomena in Pt/YIG. By inserting a Cu layer of various thicknesses between Pt and YIG, we have separated the signals from the SSE and that of the ANE. It is demonstrated that the thermal voltage in Pt/YIG mainly comes from spin current due to the longitudinal SSE with negligible contribution from the ANE.

Separation of spin Seebeck effect and anomalous Nernst effect in Co/Cu/YIG

The spin Seebeck effect (SSE) and Anomalous Nernst effect (ANE) have been observed in Co/Cu/YIG (yttrium iron garnet) multi-layer structure, where the ferromagnetic insulator YIG acts as the pure spin injector and the ferromagnetic metal Co layer acts as the spin current detector. With the insertion of 5u2009nm Cu layer, the two ferromagnetic layers are decoupled, thus allowing unambiguous separation of the SSE and ANE contributions under the same experimental conditions in the same sample.

Thermal spin transport and applications

Spin caloritronics, exploiting the interaction between spin with heat currents, offers a promising path to further reduction in both the size and power consumption of solid state devices. Despite recent observations of spin dependent thermal transport by several groups, the underlying physical mechanism remains unsettled.1-3 Our study has demonstrated the profound effect of substrate on the spin-dependent thermal transport by patterned ferromagnetic thin films.4 This unexpected behavior is due to an out-of-plane temperature gradient imposed by the thermal conduction through the substrate, resulting in a mixture of anomalous Nernst effects (ANE) and spin Seebeck effect (SSE)1-3. Only with substrate-free sample have we determined the intrinsic spin-dependent thermal transport with characteristics and field sensitivity similar to those of anisotropic magnetoresistance (AMR) effect and planer Hall effect (PHE).4 These effects are sensitive to magnetic fields, encouraging for future applications such as spin thermoelectric coating, and sensors.

Chapter Three - Charge, Spin, and Heat Transport in the Proximity of Metal/Ferromagnet Interface

The spin Seebeck effect (SSE) is one of the few routes for exploiting the spin degree of electronic transport. The observation of SSE has been reported using transverse and longitudinal geometries with in-plane (∇xT) and out-of-plane temperature (∇zT) gradient, respectively. However, it is hard to create purely in-plane temperature gradient for thin magnetic films on thick substrates. The transverse geometry where temperature gradient is applied along in-plane direction is always accompanied by an unintended out-of-plane temperature gradient due to substrates, which gives rise to the anomalous Nernst effect (ANE). The voltages of transverse SSE with ∇xT and ANE with ∇zT have the same field dependence and angular symmetry and thus are entangled. In the longitudinal SSE applicable for ferromagnetic insulator (e.g., yttrium iron garnet, YIG), the temperature gradient is exclusively out of plane. However, one encounters a different issue of magnetic proximity effects (MPEs) when the spin current detector, Pt, is in contact with YIG where unusual transport phenomena, including magnetoresistance and anomalous Hall effects, exist. These complications present difficulties for the unequivocal establishment of SSE in Pt/YIG. Very recently, the intrinsic SSE using the longitudinal geometry in Au/YIG was reported without appreciable MPE.