Coupling strength distribution in ferromagnetic/antiferromagnetic film systems

Abstract

Exchange coupling of a polycrystalline ferromagnetic(F)/antiferromagnetic(AF) film system is the superposed action of grains each with an individual coupling strength $j$ represented by a probability function $P(j)$. $P(j)$ governs the entire film coupling and the exchange bias field, and was not measurable until now. We propose a method to determine $P(j)$ from the high field rotational energy losses at low temperatures of a film system with low thickness of the AF layer. The method is verified by torquemetry in a rotating field after reversing its rotational sense. The transition to a new magnetic steady state after the reversal is analyzed within a Stoner-Wohlfarth model including thermal relaxation. This transition is completed earlier for strongly coupled grains than for grains with smaller $j$, which is reflected in the angular dependence of the hysteretic torque. We determined $P(j)$ for a sputtered $\mathrm{Ni}\mathrm{Fe}(16\phantom{\rule{0.3em}{0ex}}\mathrm{nm})∕\mathrm{Ir}\mathrm{Mn}(0.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ film at $T=10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the hysteretic range of coupling energies and found that $P$ decreases with increasing $j$.