Spin-lattice-coupled helical magnetic order in breathing pyrochlore magnets, CuAlCr$_{4}$S$_{8}$ and CuGaCr$_{4}$S$_{8}$

TL;DR

This study uncovers spin-lattice coupling and incommensurate cycloid magnetic order in breathing pyrochlore CuCrS compounds, revealing structural transitions and potential for magnetoelectric effects.

Contribution

It demonstrates the coexistence of structural and magnetic transitions with incommensurate cycloid order in CuCrS pyrochlores, highlighting the role of spin-lattice coupling and magnetic frustration.

Findings

Structural transition from cubic to orthorhombic symmetry at low temperatures.

Incommensurate magnetic modulation with specific wave vectors.

Evidence of strong spin-lattice coupling influencing ground state.

Abstract

We report low-temperature powder X-ray and neutron diffraction studies on breathing pyrochlore magnets Cu$M$Cr$_{4}$S$_{8}$ ($M$ = Al, Ga), which undergo a magnetic transition at $T_{\rm N} \approx$ 21 and 31 K for {$M$ = Al and Ga, respectively. X-ray diffraction reveals that the magnetic transition accompanies a structural transition from cubic $F{\overline 4}3m$ to polar orthorhombic $Imm2$ symmetry for both the compounds, with larger distortion observed for $M$ = Ga at low temperatures. Neutron diffraction reveals incommensurate magnetic modulation ${\mathbf Q} = (q_{\rm IC}, 0.5, 0)$ in the orthorhombic setting, where $q_{\rm IC} \approx$ 0.39 and 0.31 for $M$ = Al and Ga, respectively. Our magnetic-structure analysis suggests cycloid-type magnetic order but not proper-screw type for both the compounds. We find strong correlation between the local spin configuration and Cr-Cr bond lengths, indicating that the spin-lattice coupling as well as the magnetic frustration play an important role in determining the ground state. Cu$M$Cr$_{4}$S$_{8}$ potentially offers a platform to explore magnetoelectric effects arising from the helimagnet driven electric polarity.