The mutual coupling of spin and lattice degrees of freedom is ubiquitous in magnetic materials and potentially creates exotic magnetic states in response to the external magnetic field. Here, we investigate phase transitions of a geometrically frustrated Heisenberg antiferromagnet LiGaCr 4 O 8 , comprised of a breathing pyrochlore network of spin-3/2 Cr 3+ ions, up to 600 T by means of magnetization and magnetostriction measurements. We observe a two-step magnetostructural transition between 150 T and 200 T prior to a half-magnetization plateau. Considering a microscopic magnetoelastic theory, the intermediate-field phase can be assigned to a magnetic superstructure with a three-dimensional periodic array of 3-up-1-down spin molecules and canted 2-up-2-down ones. We attribute the emergence of the magnetic superstructure to a unique combination of the strong spinlattice coupling and large breathing anisotropy.

We investigate the crystallographic and magnetic properties of a chromium-based thiospinel CuGaCr 4 S 8 . From a synchrotron x-ray diffraction experiment and structural refinement, Cu and Ga atoms are found to occupy the tetrahedral A-sites in an alternate way, yielding breathing pyrochlore Cr network. CuGaCr 4 S 8 undergoes a magnetic transition associated with a structural distortion at 31 K in zero magnetic field, indicating that the spin-lattice coupling is responsible for relieving the geometrical frustration. When applying a pulsed high magnetic field, a sharp metamagnetic transition takes place at 40 T, followed by a 1/2-magnetization plateau up to 103 T. These phase transitions accompany dielectric anomalies, suggesting the presence of helical spin correlations in low-field phases. The density-functional-theory calculation reveals that CuGaCr 4 S 8 is dominated by antiferromagnetic and ferromagnetic exchange couplings within small and large tetrahedra, respectively, in analogy with CuInCr 4 S 8 . We argue that A-site-ordered Cr thiospinels serve as an excellent platform to explore diverse magnetic phases along with pronounced magnetoelastic and magnetodielectric responses.