CoFeB/MgO heterostructures are a promising candidate for an integral component of spintronic devices due to their high magnetic anisotropy, low Gilbert damping, and excellent magnetoresistive properties. Here, we present experimental measurements and atomistic simulations of the temperature and CoFeB thickness dependence of spontaneous magnetization and magnetic anisotropy in CoFeB/MgO ultrathin films. We find that the thermal fluctuations are different between the bulk and interface magnetizations, and that the interfacial anisotropy originates from a two-site anisotropic exchange interaction. These effects lead to a complex temperature and thickness dependence of the magnetic properties critical to device operation and stability at elevated temperatures.
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