In this study, magneto-optical Kerr effect (MOKE) is measured on magnetic thin-film samples composed of permalloy (Py) and samarium-cobalt (SmCo) on a silicon substrate with 102 A of Py, but varying thicknesses of SmCo (344 to 1146 A ). This is done to see the effects of SmCo layer thickness on the magnetic switching of top Py layer. A new optical element, a photoelastic modulator (PEM), is introduced to the existing MOKE system and allows for the measurement of the Kerr ellipticity of the samples. Longitudinal and polar components of magnetization were analyzed from Kerr ellipticity hysteresis loop. It is shown that the coercivity values of the samples obtained from Kerr signal in this research stayed similar. However, the magnetic switching behavior shows a correlation to sample thickness. These results show that the magnetic switching behavior of the top Py layer is directly affected by varying SmCo layer thickness and this clearly shows up in Kerr rotation and Kerr ellipticity hysteresis loops.