Spin-lattice-coupled helical magnetic order in breathing pyrochlore magnets, CuAlCr₄S₈ and CuGaCr₄S₈

We report low-temperature powder X-ray diffraction and neutron scattering studies on breathing pyrochlore magnets CuMCr₄S₈ (M = Al, Ga), which undergo a magnetic transition at T_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 F43m to polar orthorhombic Imm2 symmetry for both compounds, with larger distortion observed for M = Ga at low temperatures. Neutron scattering reveals incommensurate magnetic modulation Q = (q_IC, 0.5, 0) in the orthorhombic setting, where q_IC approx. = 0.39 and 0.31 for M = Al and Ga, respectively. We propose that a cycloid-type order, rather than a proper screw-type one, is one of the candidate magnetic structures for both compounds. The spin-lattice coupling as well as magnetic frustration should play an important role in determining the ground state, as suggested by the correlation between the local spin configuration and the Cr-Cr bond lengths. CuMCr₄S₈ potentially offers a platform to explore magnetoelectric effects arising from the helimagnet driven electric polarity. (author)