Spin valves and magnetic tunnel junctions, based on a structure of free ferromagnet (FM)/nonmagnetic metal or insulator/pinned FM, have been extensively used in magnetic field sensors. Recently, we demonstrated a new structure, FM/AFM/pinned FM, which showing a linear magnetic field response. Based on a preliminary polarized neutron reflectometry (PNR) measurement at ANSTO, we found a room temperature in-plane 90 degree interlayer coupling in the NiFe/FeMn/pinned-NiFe with 2-nm-thick FeMn layer. Indeed, further magnetic measurements suggest such 90 degree interlayer coupling may arise from the uncompensated layer at the FM/AF interface, which can be adjusted by the thickness of AF layer for the application of magnetic field sensor. Here, we propose the PNR study on the samples with different FeMn thickness to get a clear physics picture on the uncompensated layer at the FM/AF interface.