Spin-lattice instability to a fractional magnetization state in the spinel HgCr ₂ O ₄

Magnetic systems are fertile ground for the emergence of exotic states when the magnetic interactions cannot be satisfied simultaneously owing to the topology of the latticea situation known as geometrical frustration. Spinels, AB ₂ O ₄ , can realize the most highly frustrated network of corner-sharing tetrahedra. Several novel states have been discovered in spinels, such as composite spin clusters and novel charge-ordered states. Here, we use neutron and synchrotron X-ray scattering to characterize the fractional magnetization state of HgCr ₂ O ₄ under an external magnetic field, H. When the field is applied in its Néel ground state, a phase transition occurs at H10 T at which each tetrahedron changes from a canted Néel state to a fractional spin state with the total spin, Stet, of S/2 and the lattice undergoes an orthorhombic to cubic symmetry change. Our results provide the microscopic one-to-one correspondence between the spin state and the lattice distortion.