Anomalous magnetic exchange in a dimerized quantum-magnet composed of unlike spin species

TL;DR

This study investigates the magnetic properties of a dimerized quantum magnet composed of two different spin species, revealing an unusual exchange mechanism mediated by bridging oxygen atoms and identifying the critical magnetic field for singlet-triplet gap closure.

Contribution

It provides the first detailed experimental and theoretical analysis of a mixed-spin dimer system with unusual exchange interactions mediated by bridging oxygen.

Findings

Magnetic field of 13.1 T closes the singlet-triplet gap.

Intradimer exchange energy J0 is approximately 21 K.

DFT predicts antiferromagnetic coupling with spins of different species antiparallel.

Abstract

We present here a study of the magnetic properties of the antiferromagnetic dimer material CuVOF$_4$(H$_2$O)$_6\cdot$H$_2$O, in which the dimer unit is composed of two different $S = 1/2$ species, Cu(II) and V(IV). An applied magnetic field of $\mu_0H_{\rm c1} = 13.1(1)~\rm T$ is found to close the singlet-triplet energy gap, the magnitude of which is governed by the antiferromagnetic intradimer, $J_0 \approx 21~\rm K$, and interdimer, $J' \approx 1~\rm K$, exchange energies, determined from magnetometry and electron-spin resonance measurements. The results of density functional theory (DFT) calculations are consistent with the experimental results and predicts antiferromagnetic coupling along all nearest-neighbor bonds, with the magnetic ground state comprising spins of different species aligning antiparallel to one another, while spins of the same species are aligned parallel. The magnetism in this system cannot be accurately described by the overlap between localized V orbitals and magnetic Cu orbitals lying in the Jahn-Teller (JT) plane, with a tight-binding model based on such a set of orbitals incorrectly predicting that interdimer exchange should be dominant. DFT calculations indicate significant spin density on the bridging oxide, suggesting instead an unusual mechanism in which intradimer exchange is mediated through the O atom on the Cu(II) JT axis.