Signatures of a magnetic superstructure phase induced by ultrahigh magnetic fields in a breathing pyrochlore antiferromagnet

Significance The search for exotic magnetic states, such as a quantum spin liquid and a magnetic superstructure, in geometrically frustrated magnets has been a central research topic in the recent 30 y. Theoretically, the mutual coupling of spin and lattice degrees of freedom has been proposed to induce rich magnetic phases in various spin models. Here, we observe an unconventional multistep magnetostructural transition in a breathing pyrochlore antiferromagnet in ultrahigh magnetic fields, signaling the emergence of a magnetic superstructure phase with a periodic array of collinear 3-up-1-down and canted antiferromagnetic states. This finding could be attributed to the interplay between the spin–lattice coupling and breathing anisotropy.