63,65Cu Nuclear Resonance Study of the Coupled Spin Dimers and Chains Compound Cu2Fe2Ge4O13

TL;DR

This study uses NMR and NQR techniques to investigate the magnetic properties of Cu2Fe2Ge4O13, revealing that Fe chains do not alter the Cu spin gap and identifying the orbital state of Cu ions in the compound.

Contribution

It provides the first detailed comparison of spin dynamics and internal magnetic fields in Cu2Fe2Ge4O13 and its isostructural counterpart, highlighting the role of Fe chains.

Findings

Fe chains do not affect the Cu spin gap energy.

A large internal magnetic field of 3.39 T was observed at Cu sites.

The Cu 3d hole is mainly in the d(x2-y2) orbital state.

Abstract

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) of Cu have been measured in a coupled spin dimers and chains compound Cu2Fe2Ge4O13. Cu NQR has also been measured in an isostructural material Cu2Sc2Ge4O13 including only spin dimers. Comparison of the temperature dependence of the 63Cu nuclear spin-lattice relaxation rate between the two compounds reveals that the Fe chains in Cu2Fe2Ge4O13 do not change a spin gap energy of the Cu dimers from that in Cu2Sc2Ge4O13, contributing additionally to the relaxation rate at the Cu site. A modestly large internal field of 3.39 T was observed at the Cu site in the antiferromagnetic state of Cu2Fe2Ge4O13 at 4.2 K, which is partly because of quantum reduction of the ordered moment of a Cu atom. The internal field and the ordered moment of Cu are noncollinear due to large anisotropy of the hyperfine interaction at the Cu site. A model analysis of the internal field based on the fourfold planar coordination of Cu suggests that a 3d hole of the Cu2+ ion is mainly in the d(x2-y2) orbital state.