Condensed Matter Seminars

Frustrated magnets show interesting phenomena originating from the macroscopic ground state degeneracy. Exotic states can be chosen as the ground state from many possible candidates. Usually, geometrically frustrated systems consist of edge- and corner-shared triangles (triangular and kagomé lattices, respectively) and tetrahedra (pyrochlore lattice). However, even unfrustrated lattice systems, such as square or honeycomb lattice systems, can have frustrating interactions in the presence of antiferromagnetic further-neighbor interactions. We have recently studied two interesting frustrated systems using neutron scattering technique. One is the Cr-based spinel with pyrochlore lattice and another is the honeycomb lattice system with competing interactions. Cr-based spinel compounds ACr2O4 (A=Mg, Zn, Cd, and Hg) are so far the best model systems for a network of corner-sharing tetrahedrons with isotropic nearest-neighbor antiferromagnetic interactions. The systems exhibit novel spin-Peierls phase transitions from cubic spin liquid to non-cubic Néel states at low temperatures. They also show the magnetic field-induced half-magnetization plateau states that are stable over a wide range of field. Using an elastic neutron scattering technique under magnetic field, we determined the magnetic structure in the half-magnetization plateau phase in the spinel HgCr2O4 and CdCr2O4. The magnetic structure has a common cubic P4332 symmetry. This suggests that there is a universal field induced spin-lattice coupling mechanism at work in the Cr-based spinels. This is consistent with the theoretical prediction based on the simplest Hamiltonian for the spin-lattice coupling with the nearest-neighbor exchange interaction and an elastic energy term. Bi3Mn4O12(NO3), in which the Mn4+ ions carry S=3/2, is the first honeycomb lattice system that shows no long-range magnetic order. Using neutron scattering technique, we determined that a short-range antiferromagnetic correlation develops at low temperatures. Applying magnetic field, an interesting field-induced magnetic transition occurs, in which the short-range order abruptly expands into a long-range order.