MRSEC at the University of Utah; “Optical Detection of Transverse Spin-Seebeck Effect in Permalloy Films using Sagnac Interferometer Microscopy” MRSEC at the University of Utah; DMR 1121252 2017 R. McLauglin, D. Sun, and Z.V. Vardeny; University of Utah The field of spintronics, which involves the design of novel electronic devices that utilize the spin of electrons, requires researchers to develop a robust source of spin-polarized currents to fuel future technology. One promising avenue for a reliable spin-current source is the spin Seebeck effect, where a thermal gradient is used to drive a spin-current in a ferromagnet. We used the magneto-optical Kerr effect (the rotation of polarized light by a magnetic medium) to gain an unprecedented look into the microscopic physics of the spin-Seebeck effect in a permalloy ferromagnet film. The breakthrough required the ultra-high sensitivity of our homebuilt fiber optic Sagnac interferometer, which we adapted into a diffraction-limited confocal microscope in order to spatially map the magnetization change of magnetic thin films with 20 nano-radian angular resolution. A thermal gradient is applied to the permalloy (Ni81Fe19) ferromagnetic thin film, while the Sagnac interferometer microscope watches for subtle changes in magnetization due to spin currents in the film. This non-contact optical technique allows the magnetization and/or thermal gradient to be in-plane or out-of-plane for studying the underlying spin-Seebeck physics. Macroscopic spatial spin distribution caused by the application of an in-plane thermal gradient in a conducting ferromagnetic film, known as transverse spin-Seebeck effect (TSSE), is in many cases overshadowed by thermoelectric and magnetothermoelectric effects when using the conventional electrical detection via the inverse spin Hall effect. The figure schematically shows an optical technique for the detection and characterization of TSSE response in permalloy films using magneto-optical Kerr effect with an ultrasensitive fiber-optic Sagnac interferometer microscope that is free of magnetothermoelectric artifacts, which also allows measurements with field direction parallel and perpendicular to the film surface. We found a substantial anisotropy in the permalloy TSSE coefficient, where the “in-plane magnetization” coefficient is much larger than that in the “out-of-plane magnetization.” R. McLaughlin, D. Sun, C. Zhang, M. Groesbeck, and Z. V. Vardeny, Phys. Rev. B  95, 180401 (2017).