Thermal stability of the magnetization of 150 nm×230 nm Ni19Fe81 elements

The time- and temperature-dependencies of the switching fields of arrays of 230 nm×150 nm Ni19Fe81 rectangular elements have been investigated. The elements have thicknesses of 3 nm, 5 nm, and 10 nm. For temperatures from 100 K to 300 K, all three arrays show a linear dependence of switching field on temperature, with similar slopes, but below 100 K, a deviation from linearity occurs, indicating a change in the switching process. The energy barrier for reversal at zero field has been estimated as 70kT300, 125kT300, and 280kT300 for the 3-nm, 5-nm, and 10-nm thick elements, respectively, which suggests that the energy barrier scales approximately linearly with film thickness. For uncoupled permalloy elements with these lateral dimensions, superparamagnetism is expected at room temperature for thicknesses below about 1 nm.