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.