Recent Progress of the X-ray Magnetic Circular Dichroism Technique for Element-Specific Magnetic Analysis

Abstract

Recent trend of the X-ray magnetic circular dichroism (XMCD) spectroscopy-based technique and the related scientific topics at SPring-8 are reviewed. At the third-generation synchrotron radiation facility, the key developments in the XMCD technique include i) significant improvements in the accuracy and sensitivity of XMCD measurements with a quick helicity switching of circularly polarized X-rays, ii) introduction of extreme conditions (strong pulsed magnetic field and high pressures), and iii) new experimental setup using the high-brilliance X-ray source (microfocusing, reflection geometry, and high-pressure cells). These achievements have effectively extended the capability of the XMCD technique, a unique magnetic probe with an element-specificity, providing fundamental scientific insights in spintronics and magnetic recording devices, magnetic nanoparticles, and magnetism under high-magnetic field and high pressures.KEYWORDS: X-ray magnetic circular dichroism, element specific magnetic hysteresis, synchrotron radiation X-rays Historical ReviewElement-specific magnetic measurements are widely recognized as a powerful experimental technique for the study of magnetic materials and have previously been exploited mainly by Mössbauer spectroscopy. For the last 30 years, an energy-tunable and polarized X-ray source at synchrotron radiation facilities has given rather expansive possibilities of element-specific magnetic measurements using X-ray spectroscopy. Indeed, X-ray magnetic circular dichroism (XMCD) spectroscopy 1,2) has become one of the leading techniques for element-specific magnetic measurement using a resonant effect in the core-shell absorption with circularly polarized X-ray photons.The XMCD effect was first observed at the Fe K edge in the hard-X-ray region in 1987 by Schütz et al.3) In their experiment, an XMCD spectrum of pure iron was obtained by reversing the direction of magnetic fields applied to a sample, and the circularly polarized X-rays were provided by the off-orbital radiation from a bending magnet (off-plane method).3) The recorded XMCD signal was quite small, of the order of ÁI=I $ 5 Â 10 À3 . Subsequently, XMCD spectra at the L 2;3 edges of rare-earth elements [4][5][6] and at the L 2;3 edges of 5d metals in some alloys with 3d transition metals were also reported. 7) XMCD measurements over a wide energy range that is similar to the extended X-ray absorption fine structure (EXAFS), called magnetic EXAFS, in Fe and Gd 3 Fe 5 O 12 showed a capability of analyses of local magnetic structures using dichroic effects. 8,9) In the soft-X-ray region, the first XMCD spectrum at the Ni L 2;3 edges in a Ni single crystal was reported by Chen et al. in 1990. 10) The circularly polarized soft X-rays were obtained by the off-plain method, similar to the previous XMCD measurement at the Fe K edge.3) The intensity of the observed XMCD signal was about 8% with respect to the resonant absorption, the so called white line, and was much larger than that recorded at the K edge. The strong XMCD value at the...

Bayesian spectroscopy of synthesized soft X-ray absorption spectra showing magnetic circular dichroism at the Ni-L3, -L2 edges

“…Such evaluation of the posterior probability distributions is meaningful when measurements with a high signal-to-noise (S/N) ratio are difficult. Although XMCD microscopic measurements [28], which capture magnetic domain in magnetic materials, become available with recent developments [29] in synchrotron radiation measurement techniques, it is still difficult to obtain a high S/N ratio for in situ measurements that capture the dynamics of magnetic domain formation. Of course, although the estimation accuracy will degrade with bad S/N ratios, the evaluation of posterior probability distributions by Bayesian spectroscopy becomes more important in such situations.…”

X-ray Diffraction and Absorption Spectroscopy in Pulsed High Magnetic Fields

“…The development of techniques for the soft X-ray spectroscopy in pulsed high magnetic fields as a recent breakthrough of the high- magnetic-field X-ray experiments is presented in an article of this special edition by Suzuki and Nakamura. 72) X-ray absorption spectroscopy is expected to be a microscopic probe for the investigation of electronic states in matter at high magnetic fields. The L 2;3 absorption edge of rare-earth elements is a means of determining valence states.…”

Applications of Field-reversal and Angle-dependent XMCD Techniques to Mn-based Diluted Magnetic Materials

“…A variety of switching and modulation techniques (such as currentreversal, mechanically reversal the superconducting coils, fast helicity reversal by kicker system or phase retarder, moving the magnet array of undulator, high-speed chopper, etc.) have been reported with the development of advanced insertion devices and third-generation light source [10]. These modulation techniques can be divided into two kinds of equivalent categories: the magnetic-field switching method and the photon-helicity switching method.…”