Magnetostructural Transitions in a Frustrated Magnet at High Fields

Ultrasound and magnetization studies of bond-frustrated ZnCr₂S₄ spinel are performed in static magnetic fields up to 18 T and in pulsed fields up to 62 T. At temperatures below the antiferromagnetic transition at T_N1 ≈ 4 K, the sound velocity as a function of the magnetic field reveals a sequence of steps followed by plateaus indicating a succession of crystallographic structures with constant stiffness. At the same time, the magnetization evolves continuously with a field up to full magnetic polarization without any plateaus in contrast to geometrically frustrated chromium oxide spinels. The observed high-field magnetostructural states are discussed within a H-T phase diagram taking into account the field and temperature evolution of three coexisting spin structures and subsequent lattice transformation induced by the magnetic field.