Field-induced staggered magnetic moment in the quasi-two-dimensional organic Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl

TL;DR

This study reveals that in a quasi-two-dimensional organic Mott insulator, an applied magnetic field induces a staggered magnetic moment and transforms the antiferromagnetic transition into a crossover, emphasizing the role of Dzyaloshinsky-Moriya interactions.

Contribution

It demonstrates the field-induced staggered moments and crossover behavior in a Mott insulator, supported by magnetization, NMR measurements, and molecular-field calculations including DM interactions.

Findings

Field induces local staggered moments perpendicular to the applied field.

Antiferromagnetic transition becomes a crossover under finite magnetic field.

Molecular-field calculations qualitatively reproduce the experimental observations.

Abstract

We investigated the magnetism under a magnetic field in the quasi-two-dimensional organic Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl through magnetization and $^{13}$C-NMR measurements. We found that in the nominally paramagnetic phase (i.e., above N\'eel temperature) the field-induced local moments have a staggered component perpendicular to the applied field. As a result, the antiferromagnetic transition well defined at a zero field becomes crossover under a finite field. This unconventional behavior is qualitatively reproduced by the molecular-field calculation for Hamiltonian including the exchange, Dzyaloshinsky-Moriya (DM), and Zeeman interactions. This calculation also explains other unconventional magnetic features in $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl reported in the literature. The present results highlight the importance of the DM interaction in field-induced magnetism in a nominally paramagnetic phase, especially in low-dimensional spin systems.