In the spin Seebeck effect (SSE), a temperature gradient generates a pure spin current in a ferromagnet. The magnetic insulator yttrium iron garnet Y3Fe5O12 (YIG) covered with a heavy metal (HM), such as Pt, is just a typical example. However, the mechanism of SSE still remains debatable because some of experiments exhibit the ferromagnetic characteristics in thin Pt films on the ferromagnetic insulator due to the magnetic proximity effects. In our previous preliminary PNR study (Xpress proposal performed in May 2016), the PNR data exhibits new route to investigate the origin of the SSE in HM/YIG. Here, we wish to use PNR to measure the magnetic properties of the interface of YIG/HM. The PNR data will allow us to develop a clear model to help explain the SSE in YIG/HM systems.