Polarized neutron reflectometry study of thin Fe films prepared on V (100)

Abstract

The present work emphasizes on the polarized neutron reflectometry (PNR) method as a powerful tool to study details of the magnetization of an ultrathin film by combining in-situ and ex-situ PNR measurements as function of the film thickness for the first time. The aim is to separate and estimate the magnetization contributions from the interface, the inner part and the free surface of the film by investigating capped and uncapped films.A series of samples of V / t Fe Fe / V(100) and t Fe Fe / V(100) all grown on MgO (100) substrates with different Fe thickness t Fe were prepared by molecular beam epitaxy (MBE).The critical part of the preparation of the Fe/V samples was the preparation of a chemically clean Vanadium substrate layer with a good crystalline quality. Instead of using a V single crystal substrate, V was evaporated on a MgO(100) substrate together with an additional ultrathin V toplayer by MBE with various heat treatments. This procedure was successful leading to a smooth and clean V(100) surface with good crystalline quality. Concerning the samples for the ex-situ measurements it was established with the help of x-ray reflectometry (XRR) that V 2 O 3 was formed on top of the V capping layer after the samples were exposed to air. The oxidation, however, did not penetrate as deep as to reach the Fe/V interface which would have influenced the Fe magnetization.The samples were studied using ex-situ PNR, ex-situ XRR, in-situ PNR, and in-situ magneto-optical Kerr effect (MOKE). The ex-situ measurements were done on V-capped samples, while uncapped Fe films were used in the in-situ measurements. According to the results obtained the product of the magnetic moment per atom and the Fe thickness of ultrathin Fe(100) films was plotted as a function of the Fe thickness. Analyzing these plots the different contributions to the magnetization were estimated. The slope of this kind of plot is correlated with the magnetization from the inner part of the Fe layer. On the other hand, the intersection at the abscissa may help to separate the contributions from the interface and free surface.All the PNR data are consistent with the concept of a linear superposition of a volume and a surface term for describing the magnetization of a thin film, where the volume term grows linearly with the thickness. The experiments clearly indicate that the capping of the thin Fe film with V changes the total magnetization, whereas it does not affect the magnetic moment of the inner part of the Fe layer (which is that of bulk Fe). Combining both series of PNR experiments demonstrates the existence of a reduced magnetic moment at the Fe/V interface, estimated to be a reduction of -0.75(±0.05) µ B /atom which corresponds to about 35% of the Fe bulk value. In contrast, an enhanced magnetic moment at the free Fe surface is derived (estimated to be an enhancement of +0.3(±0.9) µ B /atom). (100) durch MBE-Epitaxie aufgedampft und unter verschiedenen Behandlungen schließlich mit einer ultradünnen V-Schicht abgedeckt. Diese Prozedur w...