Size and voltage dependence of effective anisotropy in sub-100-nm perpendicular magnetic tunnel junctions

Magnetic tunnel junctions with perpendicular magnetic anisotropy are investigated using a conductive atomic force microscope. The 1.23-nm Co40Fe40B20 recording layer coercivity exhibits a size dependence which suggests single-domain behavior for diameters <= 100 nm. Focusing on devices with diameters smaller than 100 nm, we determine the effect of voltage and size on the effective device anisotropy K-eff using two different techniques. K-eff is extracted both from distributions of the switching fields of the recording and reference layers and from measurement of thermal fluctuations of the recording layer magnetization when a field close to the switching field is applied. The results from both sets of measurements reveal that K-eff increases monotonically with decreasing junction diameter, consistent with the size dependence of the demagnetization energy density. We demonstrate that K-eff can be controlled with a voltage down to the smallest size measured, 64 nm.