Source of anomalies in the temperature dependence of ferromagnetic resonance and susceptibility data in the FeAg heterosystem

Abstract

Nanometer size Fe particles in an Ag matrix were prepared by pulsed laser deposition and were characterized from 4 to 300 K using ferromagnetic resonance (FMR) at 9.46 GHz and superconducting quantum interference device magnetometry. The magnetic behavior changes systematically from superparamagnetic to ferromagnetic with increasing Fe concentration, increasing deposition or postdeposition annealing temperature, or decreasing measurement temperature; FMR linewidths and anisotropy fields also increase. At low temperatures a dramatic further increase in linewidth occurs, accompanied by a strong downward shift in the resonance field for all orientations, and zero field cooled susceptibility decreases sharply. These phenomena indicate the development of much larger anisotropy than in bulk Fe. For samples annealed in H gas above 600 K, this anisotropy is largely absent, indicating that the anomalous behavior is induced by a surface oxide layer on the Fe, as in the exchange bias effect, despite the absence of oxygen in the nominal composition.