The Néel-Brown model for magnetization reversal over an energy barrier due to thermal excitation is a widely accepted mechanism for magnetization reversal, and has been experimentally verified to hold for many systems. Calculations of switching field distributions based on this model predict that the mean switching field 〈H〉 should decrease and the switching field distribution width ΔH should increase as the temperature is increased. However, for some systems like amorphous microwires of soft ferromagnets like Fe-Co-Si-B, ΔH has been reported to follow the opposite trend, which was attributed to magnetostriction effects in the wires.

In this article, we report the anomalous temperature dependence of ΔH in a system with little or no magnetostriction: permalloy thin film hall bars on Si(100) through a measurement of its switching field distribution by planar Hall effect. We perform calculations based on the N-B model as well as an extension of it which considers the possibility of having multiple reversal modes in the same system, described by a Gaussian distribution of anisotropy fields. Our calculations show that the simple N-B model cannot reproduce the temperature dependence of ΔH under any conditions. However, under the assumption of very low relaxation times (~10⁻⁴⁰ s and less), the extended N-B model can qualitatively reproduce the ‘anomalous’ temperature dependence of ΔH, even though quantitative agreement with our experiment is not obtained.