Ordering Phenomena in Transition Metal Oxides

Frustrated diamond antiferromagnetism in A-site magnetic spinels   

We study antiferromagnetism in the diamond sublattice of A-site magnetic spinels.� These can exhibit "bond frustration" due to the competing effects of first and second neighbor interactions.� A classical Heisenberg model exhibits a remarkable ground state degeneracy of coplanar spirals, in which the wavevector of the spiral can lie anywhere on a "spiral surface" in momentum space.� Thermal fluctuations lead to a broad spin liquid regime, with unique properties.� At low temperatures, a variety of effects can split the degeneracy. �We discuss the resulting ordered states in clean materials, the nature of magnetic anisotropy, improper ferroelectricity, and commensurability effects with the lattice.� We also discuss the variable sensitivity of the magnetism to disorder, and how this is related to the frustration-induced degeneracy.