Application of polarization modulation spectroscopy to the study of magnetic materials (abstract) (invited)

Abstract

Over the last decade, x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic scattering (XRMS) have become important experimental techniques in the study of magnetic materials. In order to reduce systematic errors in these measurements, typical XMCD and XRMS experiments are carried out using magnetization switching, although there are many situations where polarization switching is clearly desirable. Recently, fast polarization modulation techniques have been developed using either polarization conversion optical elements, such as phase plate, or special insertion devices. At the National Synchrotron Light Source (NSLS), an elliptically polarized wiggler (EPW), jointly developed by NSLS, Advanced Photon Source, and BINP of Novosibirsk, for this purpose. The EPW consists of a permanent magnet vertical wiggler and an electromagnet horizontal wiggler. The polarization of the device can be switched up to 100 Hz by switching the electromagnet. To take advantage of the fast switching capability, a phase sensitive detection system was also implemented. The sensitivity of using fast polarization modulation is demonstrated by measuring the element specific hysteresis loop and MCD spectrum of Cu induced moment at the interface of a Co/Cu multilayer. By comparing with results obtained using conventional measurements from similar samples, it clearly shows the advantages of using polarization modulation for small MCD effects. The sensitivity of this technique and the possibility of performing magnetic field dependent measurements of using polarization modulation have been applied to a number of magnetic systems. First, the spatial distribution of Cr induced moment in an ideal exchange-biased Fe/Cr multilayer was measured using soft-x-ray XRMS. Specular reflectivity was measured as a function of both angle and energy near Cr and Fe L3 edges. The Cr induced moment was clearly observed. Moreover, the spatial distribution of the induced Cr moment was determined by combining the measurements with detailed simulation of the energy and angle dependent specular reflectivity. Second, the magnetic behavior and spatial distribution of the interfacial Ni spins in an exchange-biased Co/NiO bilayer were studied using angle and energy dependent specular reflectivity as well as total electron yield. These results will be discussed in relation to several existing models of interface spin structures for exchange bias. Other applications of polarization modulation will also be discussed.