Thermal spin excitations in epitaxial Fe nanostructures on GaAs(001)

Abstract

Thermal spin excitations in confined ferromagnetic structures become increasingly important, e.g., because they reduce tunnel magnetoresistance in highly integrated magnetic memories and the stability of stored information. Here, the effect of lateral confinement on the temperature dependence of magnetization in ultrathin films was studied. Epitaxial Fe films were grown on GaAs(001) by molecular beam epitaxy. Patterning into dot arrays with several million dots of well defined circular shape was accomplished by electron beam lithography, lift-off, and ion beam etching. The magnetic properties of the samples were investigated by superconducting quantum interference device magnetometry between 10 and 350 K. All films—in addition to the fourfold magnetocrystalline anisotropy—have an in-plane uniaxial magnetic anisotropy with the easy axis along [110], which is fully conserved during patterning. The temperature dependence of the spontaneous magnetization for T<0.5TC can be well described by Bloch’s law, MS(T)=M(0)(1−BT3/2), for all samples. For a dot diameter of 500 nm the spin wave parameter B is significantly increased compared to the extended 14 ML film, which in turn shows about twice the bulk value of BFe=5×10−6 K−3/2. The enhancement of spin wave excitations with decreasing film thickness and lateral dimension is discussed in comparison to existing theories and model simulations.