Superparamagnetic States and Perpendicular Magnetic Anisotropy in Ultrathin MgO/CoFeB/Ta Structures

High perpendicular magnetic anisotropy (PMA) has been observed in MgO/CoFeB/Ta thin films if the thickness of CoFeB is in the range between 1.1 and 1.5 nm. However, both the coercivity and remanence vanish when the thickness of CoFeB is less than 1.1 nm, indicating a characteristic of superparamagnetic state. The magnetization versus external field of these thinner films shows a fit with a Langevin model. In addition, a temperature dependent study identifies a blocking temperature of 160 K by field cool and zero field cool measurements. The particle size of ~ 23 nm in diameter is estimated for a pancake shape cluster and the small particles are probably formed at the CoFeB/MgO interface at the beginning of the growth. Moreover, even the thicker films, which show ferromagnetic response at room temperature, also exhibit the temperature-dependent characteristic of superparamagnetic state due to the pinning at the edge of the small particles.