Complex interactions between spin, charge, and heat currents have set the stage for some of the most exciting physics experiments of the early 21 st century. The desire to understand these interactions has inspired physicists, engineers, and material scientists, particularly in the fields of spin caloritronics and magnonics, both of which are focused on answering questions involving spin and heat interactions and the transport of pure spin current.

To provide a means for understanding both the physical phenomena and technological applications associated with spin transport, the opto-thermal measurement has been developed. This experimental technique serves as a way to probe the generation, transport, and detection of spin currents by utilizing the spin Seebeck effect (SSE). The measurement involves using a laser to induce a highly localized thermal gradient, which results in the generation of a spin current in a magnetic insulator (FM). This spin current is then detected in an adjacent normal metal (NM) via the inverse spin Hall effect. The opto-thermal technique is used to study the generation of spin current in the presence of photo excited charge carriers and also to study the SSE on an ultrafast time scale. It is demonstrated that the opto-thermal technique is highly efficient for studying the myriad of different FM/NM systems relevant to the SSE.

The nonlocal opto-thermal measurement is used to study the spin diffusion length in Yttrium Iron Garnet (YIG). It is found that thermally excited magnon spins travel over 120 mum and have a spin diffusion length of 47 mum at 23 K. A temperature dependence study of the spin diffusion length using the nonlocal opto-thermal measurement reveals a long range spin diffusion length of > 80 mum at low temperature. 3D finite element modeling is used to show that the spin current in YIG can be broken into two components. One of the components is proportional to ∇mum, the gradient of the spin chemical potential, while the other is proportional to ∇ Tm, the gradient of the magnon temperature. It is shown that ∇Tm, is the driving force responsible for the long range diffusion.