Magnetic Circular Dichroism in Resonant Raman Scattering in the Perpendicular Geometry at theLedge of3dTransition Metal Systems

Abstract

We measured circular dichroism in resonant x-ray scattering 3d n ! 2p 5 3d n11 ! 3s 1 3d n11 with incidence perpendicular to the magnetization where the absorption dichroism vanishes. The advantages of photon scattering over other techniques make it possible to study a wide range of materials. The Ni L 3 dichroism in NiFe 2 O 4 is ͑28 6 5͒% in agreement with a localized model. In the metal Co the dichroism is reduced to ͑10.4 6 1͒% ͑L 3 ͒ and ͑6.8 6 1.5͒% (7.5 eV above L 3 ), indicating a large sensitivity to the nature of the valence states despite the fact that this spectroscopy is based on inner shell transitions. [S0031-9007(99)08431-8] PACS numbers: 78.70.En, 78.70.Ck, 78.70.Dm In recent years there has been a considerable effort in circular x-ray dichroism on magnetic materials [1], which provides important, and in many aspects unique, information especially by the use of the sum rules [2]. Much of this effort has been devoted to dichroism in x-ray absorption exploiting the difference in the number of the created core holes for parallel and antiparallel alignment of the magnetization and helicity vector of the incident light. The charge distribution of the excited core hole is not only characterized by its integral which is proportional to the absorption but also by its spatial distribution which is in general not spherical. Information on the deviation from the spherical symmetry, i.e., on the core hole polarization can be obtained from the angular dependence of the particles emitted by the decay of the core hole [3,4]. Since this deviation is directly connected to the ground state properties by the optical transition matrix elements of the absorption process, important information about the material properties is laying dormant in the angular emission when only absorption is measured. The dichroism in absorption vanishes when the incident light is perpendicular to the magnetization, i.e., in the socalled "forbidden" geometry. Let us consider the plane defined by the incident beam and by the emitted products detected in the experiment, with the magnetization in this plane (coplanar perpendicular geometry hereafter referred to as "perpendicular"). The detection breaks the mirror symmetry of the system, and one obtains information on the nonspherical part of the core hole distribution. So far this approach has been used only in experiments based on electron emission [3,5]. The possibility of detecting emitted photons has never been explored. The aim of the present Letter is not only to show that this is feasible but to present original results in this field. We concentrate on 3d TM systems, and we use the inner shell excitation of the type 3d n ! 2p 5 3d n11 ! 3s 1 3d n11 (i.e., final state with a 3s hole due to the filling of the intermediate 2p core hole state) which is an example of resonant Raman scattering (RRS). In Ni the L 3 excitation is around 853 eV and the Raman photons are around 742 eV; in the metal Co the energies are 778 and 677 eV, respectively. Perpendicular RRS (PRRS) has the potential t...

Angular dependence of resonant inelastic x-ray scattering: a spherical tensor expansion

“…Related results were obtained in specific experimental conditions [26,27] or in specific coordinate systems [28][29][30] but this general expression is new. Its coordinateless form enables descriptions of the general form of the resonant inelastic spectroscopy of isotropic samples.…”

StrongK-edge Magnetic Circular Dichroism Observed in Photon-in–Photon-out Spectroscopy

“…This results in sharp spectral features and often a rich multiplet structure that reveals electron-electron and spin-orbit interactions. Magnetic dichroism measurements have been combined with RIXS previously, but these studies were mainly performed at the L 2;3 absorption edges, probing low energy transfer excitations in 3d TM [20,21] or the quadrupole transitions (2p ! 4f) in lanthanides [22][23][24][25][26].…”

Angle resolved resonant inelastic X-ray scattering: intensity and dichroism from 3d transition metals in special geometries

“…A dichroic signal is also detected in resonant inelastic X-ray scattering (RIXS) experiments, even in the conditions where it is forbidden in absorption, i.e., when the incident radiation impinges orthogonally with respect to the magnetization axis (the so-called perpendicular geometry) [4]. In this geometry RIXS seems very promising in recovering the ionic multipole moments, even of higher order, exploiting a series of sum rules [5].…”

“…r Absorption of polarized X-rays at the 2p threshold of 3d metals is a test of the magnetic properties and the shape of absorption intensity and dichroism as a function of energy can be described in terms of transitions involving the ground state and excited manifolds [1]. The use of element selective sum rules can give information on the charge and magnetic multipole moments of the valence electron distribution [2,3].A dichroic signal is also detected in resonant inelastic X-ray scattering (RIXS) experiments, even in the conditions where it is forbidden in absorption, i.e., when the incident radiation impinges orthogonally with respect to the magnetization axis (the so-called perpendicular geometry) [4]. In this geometry RIXS seems very promising in recovering the ionic multipole moments, even of higher order, exploiting a series of sum rules [5].…”