Structure and magnetism of BCC-Fe/Co multilayers studied by means of perturbed angular correlation spectroscopy

Abstract

We have used perturbed angular correlation spectroscopy to study Fe/Co superlattices grown on (1 – 10)-GaAs with a Co thickness around 20 Å and Fe thicknesses between 10 and 40 Å. We found that all hyperfine fields in these layers are along the [1 1 0]-axis in the plane of the multilayers. Measurements of the temperature dependence of the Cd hyperfine field at the center of the (1 – 10)-Co layers were interpreted in terms of gradually changing Co-structure. Below the growth temperature, thermal lattice expansion accounts for the anomalous temperature dependence of the hyperfine field. At higher temperature the monoclinic deformation of the Co lattice gradually relaxes until a nearly undeformed BCC-structure is achieved around 570 K. Further increasing the temperature causes a phase transition of the Co lattice that results in interdiffusion of Fe and Co. These results in accordance with theoretical calculations suggest that the BCC-like Co structure is stabilized by an interfacial energy contribution, particularly by lattice strain and that BCC-Co is not a thermodynamically metastable phase. We resolved satellite hyperfine fields that are unambiguously attributed to probe atoms in plateaus near a sharp interface. In addition, broad frequency distributions associated with a diffuse interface were obvious in the data. Therefore, we proposed a growth model for (1 – 10)-Fe/Co where interfaces of Co on top of Fe are sharp and those of Fe on top of Co are diffuse. Within the extended Stearns hyperfine-field model, the hyperfine-field data were interpreted in terms of magnetic moments. The moment profile derived for (1 – 10)-Fe/Co reveals a Friedel oscillation of both the Fe and Co near interface moments. The spatial variation of the moments is best described by an amplitude, decaying exponentially with the distance to the interface and modulated by a sine function of the same distance. Between 90 and 570 K, the amplitude, the decay constant and the wave vector of the modulation are constant within the values determined. The Friedel oscillation of the near interface moments is understood as due to spin waves traveling across the interface and interfering with those of the penetrated lattice. The experimental results suggest that the nodes of the spin waves should be pinned to the interface. The resulting oscillation is superimposed on the bulk moment. This picture evidently explains why the experimental moment profile at (1 – 10)-Fe/Co is symmetric about the interface.