Engineering topics

Souza-Neto, N. M.

Publications and source records attributed to Souza-Neto, N. M..

5 records

DOE OSTI2022

Pressure-tuning of the electronic and magnetic properties of EuPt 2 Si 2

Using the element and orbital selectivity of x-ray absorption spectroscopy at Eu and Pt L 3 edges we investigate the effects of the lattice contraction, induced by temperature and external pressure, on the magnetic and electronic properties of the EuPt 2 Si 2 system. Our findings point to a clear relationship between the volume and the Eu valence in this material. From XANES experiments as a function of pressure we show that the Europium valence tends to stabilize at 3+ for pressures up to 27 GPa. The XMCD results demonstrate that pressure induced valence change of the Europium ion leads to a suppression of the magnetic ordering of the material. Finally, altogether our results provide direct evidence that Eu-4f /5d electronic hybridization effects underlie the mechanism that regulates the valence and magnetic ordering in this material.

36 MATERIALS SCIENCE↗

DOE OSTI2020

A versatile X-ray phase retarder for lock-in XMCD measurements

X-ray magnetic circular dichroism (XMCD) is a technique commonly used to probe magnetic properties of materials with element and orbital selectivity, which requires the use of circularly polarized (CP) X-rays. It is possible to accomplish XMCD experiments with fixed CP and alternating the magnetic field orientation, but most reliable data are obtained when alternating the magnetization orientation and the polarization between right and left helicities. A versatile strategy has been developed to perform XMCD experiments using a hard X-ray quarter-wave plate, at both polychromatic dispersive and conventional monochromatic optics, in combination with synchronous data acquisition. The switching frequency waveform is fed into a lock-in amplifier to detect and amplify the XMCD signal. The results on a reference sample demonstrate an improvement in data quality and acquisition time. The instrumentation successfully generated 98% of CP X-rays switching the beam helicity at 13 Hz, with the possibility of faster helicity switching once it is installed at the new Brazilian fourth-generation source, SIRIUS.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2018

Tuning magnetic coercivity with external pressure in iron-rhenium based ferrimagnetic double perovskites

We studied the effect of physical pressure on the electronic and magnetic properties of ferrimagnetic double perovskites A 2 FeReO 6 (A = Ca, Ba) using Re L 2,3 edge x-ray absorption spectroscopy and powder diffraction measurements. Volume compression is shown to dramatically increase the magnetic coercivity (H c ) in polycrystalline samples of both compounds with ΔH c /ΔV similar to 150-200 Oe/Å 3 . A nearly eight-fold increase in H c , from 0.2 to 1.55 T, is obtained in Ba 2 FeReO 6 at P = 29 GPa. While no signs of structural phase transitions are seen in either sample to similar to 30 GPa, the structural data points to a pressure-driven increase in tetragonal distortion of ReO 6 octahedra. A sizable but pressure-independent Re orbital-to-spin magnetic moment ratio is observed, pointing to the critical role of spin-orbit interactions at Re sites. We present a J(eff) description of the electronic structure that combines effects of crystal field and spin-orbit coupling on the Re 5d 2 orbitals and use this description to provide insight into the pressure-induced enhancement of magnetic anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Oscillatory interlayer coupling in spin Hall systems

Many spintronics applications consist of ultrathin magnetic and nonmagnetic multilayers and require an in-depth understanding of interfacial magnetism and spin transport. Here, we study permalloy/copper/platinum multilayer systems. We find that magnetic damping, perpendicular anisotropy, and proximity magnetization exhibit correlated oscillations as a function of the copper thickness. We ascribe these observations to an oscillatory interlayer coupling between permalloy and platinum. Such interlayer coupling may have a significant impact on the performance of spintronics applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2017

Unraveling 5f-6d hybridization in uranium compounds via spin-resolved L-edge spectroscopy

Here, the multifaceted character of 5f electrons in actinide materials, from localized to itinerant and in between, together with their complex interactions with 6d and other conduction electron states, has thwarted efforts for fully understanding this class of compounds. While theoretical efforts abound, direct experimental probes of relevant electronic states and their hybridization are limited. Here we exploit the presence of sizable quadrupolar and dipolar contributions in the uranium L 3 -edge X-ray absorption cross section to provide unique information on the extent of spin-polarized hybridization between 5f and 6d electronic states by means of X-ray magnetic circular dichroism. As a result, we show how this 5f-6d hybridization regulates the magnetism of each sublattice in UCu 2 Si 2 and UMn 2 Si 2 compounds, demonstrating the potentiality of this methodology to investigate a plethora of magnetic actinide compounds.

74 ATOMIC AND MOLECULAR PHYSICS↗