Ferromagnetism out of charge fluctuation of strongly correlated electrons in $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br

TL;DR

This study reveals that charge fluctuations in a strongly correlated organic material induce ferromagnetism, with experimental evidence showing emergent weak ferromagnetic moments arising from a complex interplay of charge and spin dynamics.

Contribution

It introduces a model where charge fluctuations in a dipole-liquid state generate ferromagnetic interactions in a strongly correlated electron system.

Findings

Magnetic susceptibility follows Curie-Weiss law below transition.

Emergent ferromagnetic moments involve about 1/6 of localized spins.

Charge fluctuations induce weak ferromagnetism in the material.

Abstract

We perform magnetic susceptibility and magnetic torque measurements on the organic $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br, which is recently suggested to host an exotic quantum dipole-liquid in its low-temperature insulating phase. Below the metal-insulator transition temperature, the magnetic susceptibility follows a Curie-Weiss law with a positive Curie-Weiss temperature, and a particular $M\propto \sqrt{H}$ curve is observed. The emergent ferromagnetically interacting spins amount to about 1/6 of the full spin moment of localized charges. Taking account of the possible inhomogeneous quasi-charge-order that forms a dipole-liquid, we construct a model of antiferromagnetically interacting spin chains in two adjacent charge-ordered domains, which are coupled via fluctuating charges on a Mott-dimer at the boundary. We find that the charge fluctuations can draw a weak ferromagnetic moment out of the spin singlet domains.