Speaker
Description
The beamline for advanced dichroism experiments (BLADE) is a soft x-ray beamline at the UK synchrotron facility Diamond light source. It delivers soft x-ray beam in the energy range from $0.4$ to $1.6$ keV. This energy range is optimized for the dipolar transitions of 3$d$ transition metals ($L_2,_3$ edges) and rare earth elements ($M_4,_5$ edges). So the absorption effect benefits from a strong resonance and directly probes the electronic states responsible for magnetism.
The dichroic effect is measured as difference between circular polarization with opposite handiness in absorption. The absorption is detected by measuring the total electron yield (drain current) and fluorescence signal (photodiode facing the sample) simultaneously.
The absorption branch of BLADE is equipped with superconducting split pair coil that can deliver the magnetic field up to $14$T. The sample can be cooled by a variable temperature insert down to $3$K. The set up is complemented with an electromagnet providing the magnetic field up to $1.5$T. This end station is using a Janis cryostat with the base temperature of $20$K.
In three examples, we demonstrate the capabilities of XMCD and how it can be applied to different magnetic systems. As x-ray magnetic circular dichroism (XMCD) is a resonant effect, the technique is element selective. We recorded the element specific hysteresis loops in exchange biased IrMn/CoFeB interface. The results reveal that Fe spins align antiparallelly with interfacial uncompensated Mn spins, whose strength increases with the decrease in temperature. The uncompensated Mn spins are composed of rotatable and pinned spins. Furthermore, the pinned spins can be switched by annealing under different fields.
Spinel ferrites nanoparticles show significant difference in the surface structure compared to the bulk. This modification also manifests in the redistribution of Fe cations in tetrahedral and octahedral sites. XMCD in combination with the semi-empirical quantum many-body program QUANTY has been used to determine the degree of inversion.
Series of transition metal dopants in single-crystal linearly coordinated molecules are studied by angular dependent XAS and XMCD measurements. $L_2,_3$ edge XAS is developed to be a probe of $3d$–$4s$ mixing via the quantification of ligand-field splitting. Analysis of XAS with support from ab-initio calculations determines the presence of significant $3d$–$4s$ mixing across the transition metal series with maximal mixing for Cu and Ni that then decreases along the series to Mn. XMCD is applied to decompose spin and orbital contributions to easy-plane (Mn and Co) versus easy-axis (Fe and Ni) magnetic behavior.
On the scattering branch of BLADE, soft x-ray diffractometer RASOR (Reflectivity and Advanced Scattering from Ordered Regimes) is equipped with a polarization analyzer, small magnetic field and sample cryostat. It can be used to study the structure and dynamics of the topological objects such as skyrmions.
