Electric quadrupole interaction in cubic BCC alpha-Fe

TL;DR

This study measures the electric field gradient in cubic alpha-Fe using Moessbauer spectroscopy, revealing the EFG's dependence on local electronic structure and spin-orbit interactions, with implications for understanding hyperfine interactions in ferromagnetic materials.

Contribution

The paper provides the first detailed measurement of the electric field gradient in cubic alpha-Fe, highlighting the role of local electronic wave functions and spin-orbit coupling in EFG origin.

Findings

EFG principal component Vzz measured as +1.61(4) x 10^19 V/m^2 in single crystal

EFG principal component Vzz measured as +1.92(4) x 10^19 V/m^2 in polycrystalline foil

EFG origin primarily due to local electronic wave function distortion from spin-orbit interaction

Abstract

Moessbauer transmission spectra for the 14.41-keV resonant line in 57Fe have been collected at room temperature by using 57Co(Rh) commercial source and alpha-Fe strain-free single crystal as an absorber. The absorber was magnetized to saturation in the absorber plane perpendicular to the gamma-ray beam axis applying small external magnetic field. Spectra were collected for various orientations of the magnetizing field, the latter lying close to the [110] crystal plane. A positive electric quadrupole coupling constant was found practically independent on the field orientation. One obtains the following value Vzz=+1.61(4)x10^19 V/m^2 for the (average) principal component of the electric field gradient (EFG) tensor under assumption that the EFG tensor is axially symmetric and the principal axis is aligned with the magnetic hyperfine field acting on the 57Fe nucleus. The nuclear spectroscopic electric quadrupole moment for the first excited state of the 57Fe nucleus was adopted as +0.17 b. Similar measurement was performed at room temperature using as-rolled polycrystalline alpha-Fe foil of high purity in the zero external field. Corresponding value for the principal component of the EFG was found as Vzz=+1.92(4)x10^19 V/m^2. Hence, it seems that the origin of the EFG is primarily due to the local (atomic) electronic wave function distortion caused by the spin-orbit interaction between effective electronic spin S and incompletely quenched electronic angular momentum L. It seems as well that the lowest order term proportional to the product L.LAMBDA.S dominates, as no direction dependence of the EFG principal component is seen. The lowest order term is isotropic for a cubic symmetry as one has LAMBDA=lambda.1 for cubic systems with the symbol 1 denoting unit operator and lambda being the coupling parameter.