Noncollinear magnetization between surface and bulk Y3Fe5O12

Yttrium iron garnet (YIG, Y3Fe5O12) is a magnetic insulator that has been widely used to generate spin-wave spin current via the longitudinal spin Seebeck effect. Spin current can be converted to charge current by the inverse spin Hall effect (ISHE) in an attached metal layer with a spin direction given by the magnetization of the YIG. However, both the ISHE voltage from the thermal transport measurement and the magnetoresistance (MR) from the electrical transport measurement of the metal/YIG structure show a clear plateau behavior in the low-field range, which is inconsistent with the magnetization reversal behavior of the YIG slab. In this work, we provide direct evidences by using the highly sensitive micro-magneto-optic Kerr effect (micro-MOKE) measurement to demonstrate that the plateau behavior in the thermal and electrical transport measurement of metal/YIG is due to the noncollinear magnetization configuration between the bulk and surface of YIG. In addition, keeping the measured surface of YIG unaltered, we show that its surface magnetization can be systematically controlled by varying the thickness. We further demonstrate that the magnetic coupling between the surface magnetization of YIG and an attached ferromagnetic layer exhibits long-range interaction due to the magnetic dipole-dipole interaction.