The design, construction, and testing of a highly sensitive magnetometer with a wide temperature range for the study of magnetic phase transitions and the effect of thermal processing on heterostructured magnetic films is reported. The magnetometer is based on the magneto-optic Kerr effect which produces a change in polarization of light reflected from a magnetic surface. A laser with a wavelength of 632 nm is used, and the magnetometer measures the magnetic orientation of layers within the skin depth of the incident radiation. For the systems studied in this work, the probed magnetization is assumed to follow the magnetic behavior of the topmost magnetic layer. The apparatus has a temperature range from 65 K to 760 K, enabling the study of a wide range of heterostructured thin films. Alloys of Gd with transition metals have a strong temperature dependence since pure Gd has a ferromagnetic Curie temperature around 300 K. Single-layers of Co, Gd and Co-Gd alloy were used as prototype magnetic system for commissioning the apparatus. The phase transition behaviors as determined by the magnetometer are reported. For testing the thermal processing, bilayers incorporating antiferromagnetic NiMn and NiMn-Gd with the above ferromagnetic layers were studied. These bilayers exhibited magnetic behaviors that were dependent on the thermal processing history, as annealing at 573K and cooling in a static magnetic field produced a change in the exchange bias, coercivity, and shape of the magnetic hysteresis loops.