Magnetic Switching In Permalloy Ellipses By Field Pulse*

The switching behavior of patterned Ni$_{81}$Fe$_{19 } $(permalloy) particles induced by a magnetic field pulse has been studied using magnetic force microscopy (MFM). Arrays of permalloy elliptical elements, 340 nm$\times $130 nm with a thickness of 40 nm, were patterned directly onto a copper microstrip line. The magnetization of the ellipses was first saturated in one direction and then a pulsed magnetic field was applied along their hard axis with a constant bias field (348 Oe), which was lower than their switching field $H_{s}$ ($\sim $ 0.85$H_{s})$, along their easy axis. The switching probabilities $P(t)$ of the elements were measured repeating the process 100 times at room temperature with various durations and amplitudes of the field pulse. The ellipses showed quite different behaviors in $P(t)$ even though their static switching fields had a similar angular dependence. The data will be discussed in terms of a model for the thermally activated switching and the Stoner-Wohlfarth model.