Superparamagnetic effects in the ferromagnetic resonance of silica supported nickel particles

Abstract

A silica supported Ni catalyst has been investigated by electron magnetic resonance between 293 and 4.2 K. A relatively narrow absorption signal with linewidth ΔH p p ?400 Oe is observed at temperatures ?150 K as a result of motional averaging of the magnetocrystalline anistropy field H a of the Ni particles resulting from superparamagnetism. Broadening of the resonance occurs at lower temperatures because of a reduction of the influence of thermal fluctuations. A line shape analysis of the observed resonance signals is carried out employing the theory of ferromagnetic resonance in polycrystallinematerial. The calculated anisotropy field is reduced in the particles as compared to bulk Ni. The Landau–Lifshitz relaxation frequency obtained from the calculated ’’single crystal’’ linewidths are found to be larger than those of bulk Ni single crystals. Concurrently with line broadening, the resonance shifts to lower magnetic fields. The resonance shifts are approximately H a /2. The appearance of magnetoelastic effects in the resonance below 120 K indicates the presence of small but significant strain in the particles resulting from particle‐support interaction.