EPR measurements of Eu{+2} spins in metallic EuCo{2-y}As2 single crystals

TL;DR

This study uses electron-paramagnetic-resonance (EPR) to investigate Eu{+2} spins in EuCo{2-y}As2, revealing temperature-dependent g-factor enhancements linked to magnetic correlations in a metallic compound.

Contribution

It applies a modified Bloch equation approach to analyze EPR data, providing new insights into the microscopic magnetic properties and temperature-dependent g-factor behavior of Eu spins in a metallic environment.

Findings

Eu spins exhibit a g-factor increasing from 2.00 to 2.16 as temperature decreases.

EPR analysis reveals local ferromagnetic correlations between Eu spins and Co 3d electrons.

The modified susceptibility model predicts different absorption behavior compared to traditional methods.

Abstract

The Eu{+2} spins S = 7/2 in the metallic compound EuCo{2-y}As2 order into an antiferromagnetic helical structure below a Neel temperature TN = 40 to 45 K. The effective magnetic moment mu_eff of the Eu spins in the paramagnetic state from 100 to 300 K is found from static magnetic susceptibility measurements to be enhanced by about 7% compared to the value expected for spectroscopic splitting factor g = 2, and the saturation moment at high applied fields H and low temperatures T is also sometimes enhanced. Here electron-paramagnetic-resonance (CW EPR) measurements versus applied magnetic field H at fixed X-band rf (microwave) angular frequency omega were carried out using a linearly-polarized rf magnetic field oriented perpendicular to H to study the microscopic magnetic properties of the Eu spins. In order to analyze the data, the complex magnetic susceptibility chi(omega) at fixed H was used that was derived for linearly-polarized rf fields from the modified Bloch equations [M. A. Garstens and J. I. Kaplan, Phys. Rev. 99, 459 (1955)] (GK). It is shown that their formulation when applied to calculate the Dysonian absorptive susceptibility chi_D''(H) of local magnetic moments in metals yields a prediction that can be very different from the traditionally-used form of chi_D''(H). By fitting the derivative of the field-swept CW EPR data for EuCo{2-y}As2 by chi_D''(H) at fixed omega derived from the GK chi_D''(omega) at fixed H, the Eu spin spectroscopic splitting factor (g-factor) is found to be approximately 2.00 from 300 to ~125 K, and then to continuously increase to approximately 2.16 on further cooling to 50 K. We speculate that the enhancement of the Eu g-factor on cooling from 125 to 50 K arises from continuously-increasing local short-range ferromagnetic correlations between the Co 3d-band electrons and the Eu spins.