Neutron scattering studies of magnetic pyrochlores

The pyrochlore antiferromagnets in which spins interact in a network of corner-sharing tetrahedra have macroscopic ground state degeneracy, that leads to exotic low temperature properties. Spinels AB$_2$O$_4$ realize the pyrochlore lattice if the B ions couple antiferromagnetically. This talk will start with a quick review of several novel properties found in spinels, such as the spin liquid state in ZnCr$_2$O$_4$, the 3D spin-Peierls transition in ZnCr$_2$O$_4$, the spin-orbital coupling in ZnV$_2$O $_4$, and the heavy fermionic behaviors in LiV$_2$O$_4$. A discussion will follow on our recent neutron and X-ray scattering works on ACr$_2$O$_4$ (A=Cd, Hg). We will show that the 3D spin-Peierls transition in CdCr$_2$O$_4$ is different from that observed in ZnCr$_2$O$_4$, and that the magnetic field-induced half-magnetization plateau state in HgCr$_2$O$_4$ has the P4$_3$32 symmetry. Our results provide direct tests of theoretical models proposed to understand the complex behaviors of the Heisenberg pyrochlore antiferromagnets. A quantum spin pyrochlore system will also be discussed.