Antiferromagnetic and Orbital Ordering on a Diamond Lattice Near Quantum Criticality

Abstract

We present neutron scattering measurements on powder samples of the spinel FeSc_{2}S_{4} that reveal a previously unobserved magnetic ordering transition occurring at 11.8(2) K. Magnetic ordering occurs subsequent to a subtle cubic-to-tetragonal structural transition that distorts Fe coordinating sulfur tetrahedra and lifts the orbital degeneracy. The orbital ordering is not truly long ranged, but occurs over finite-sized domains that limit magnetic correlation lengths. The application of 1 GPa hydrostatic pressure appears to destabilize this Néel state, reducing the transition temperature to 8.6(8) K and redistributing magnetic spectral weight to higher energies. The relative magnitudes of ordered ⟨m⟩^{2}=3.1(2) μ_{B}^{2} and fluctuating moments ⟨δm⟩=13(1) μ_{B}^{2} show that the magnetically ordered state of FeSc_{2}S_{4} is drastically renormalized and close to criticality.