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

Abstract

Magnetic tunnel junctions with perpendicular magnetic anisotropy are investigated using a conductive atomic force microscope. The $1.23\ensuremath{-}\mathrm{nm}$ ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ recording layer coercivity exhibits a size dependence which suggests single-domain behavior for diameters $\ensuremath{\le}\phantom{\rule{0.16em}{0ex}}100$ nm. Focusing on devices with diameters smaller than 100 nm, we determine the effect of voltage and size on the effective device anisotropy ${K}_{\text{eff}}$ using two different techniques. ${K}_{\text{eff}}$ 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 ${K}_{\text{eff}}$ increases monotonically with decreasing junction diameter, consistent with the size dependence of the demagnetization energy density. We demonstrate that ${K}_{\text{eff}}$ can be controlled with a voltage down to the smallest size measured, 64 nm.