Successive magnetic phase transitions in $\bm{\alpha}$-RuCl$_3$: XY-like frustrated magnet on the honeycomb lattice

TL;DR

This study reveals that $ ext{α}$-RuCl$_3$ exhibits multiple magnetic phase transitions due to XY-like frustrated interactions on a honeycomb lattice, with strong anisotropy and complex magnetic behavior.

Contribution

The paper provides detailed experimental evidence that $ ext{α}$-RuCl$_3$ is a three-dimensionally coupled XY-like frustrated magnet with multiple magnetic phases.

Findings

Multiple magnetic phase transitions at zero field.

Strong anisotropic $g$-factors indicating XY-like interactions.

Magnetic phase diagram for $H \,{\parallel}\,ab$ obtained.

Abstract

The layered compound $\alpha$-RuCl$_3$ is composed of a honeycomb lattice of magnetic Ru$^{3+}$ ions with the $4d^5$ electronic state. We have investigated the magnetic properties of $\alpha$-RuCl$_3$ via magnetization and specific heat measurements using single crystals. It was observed that $\alpha$-RuCl$_3$ undergoes a structural phase transition at $T_{t}\,{\simeq}\,150$ K accompanied by fairly large hysteresis. This structural phase transition is expected to be similar to that observed in closely related CrCl$_3$. The magnetizations and magnetic susceptibilities are strongly anisotropic, which mainly arise from the anisotropic $g$-factors, i.e., $g_{ab}\,{\simeq}\,2.5$ and $g_c\,{\simeq}\,0.4$ for magnetic fields parallel and perpendicular to the $ab$ plane, respectively. These $g$-factors and the obtained entropy indicate that the effective spin of Ru$^{3+}$ is one-half, which results from the low-spin state. Specific heat data show that magnetic ordering occurs in four steps at zero magnetic field. The successive magnetic phase transitions should be ascribed to the competition among exchange interactions. The magnetic phase diagram for $H\,{\parallel}\,ab$ is obtained. We discuss the strongly anisotropic $g$-factors in $\alpha$-RuCl$_3$ and deduce that the exchange interaction is strongly XY-like. $\alpha$-RuCl$_3$ is magnetically described as a three-dimensionally coupled XY-like frustrated magnet on a honeycomb lattice.