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In the spinel compounds AB2O4, the B sites form a pyrochlore lattice. GeCo2O4 and GeNi2O4 have been investigated using high magnetic field magnetization as well as neutron diffraction measurements. Both compounds become antiferromagnetic at low temperature saround 23 and 12 K, respectivelyd with the same propagation vector. The magnetization then presents two spin flop fields and one huge saturation field (above 50 T). The Curie-Weiss temperature is positive for the Co spinel and negative for the Ni one. A model taking into account the first neighbor interaction J1 together with two different third neighbor interactions J2 and J3 is able to account for the common observed magnetic behavior. J1 is ferromagnetic while J2 and J3 are antiferromagnetic, in agreement with Goodenough-Kanamori-Anderson rules. These competing interactions are responsible for the frustration in these compounds. [ABSTRACT FROM AUTHOR]

We report Raman spectroscopy measurements on ZnO crystals grown by the vapor transport method and annealed. Vacuum annealing is found to yield single crystals with ultra low density of defects. We focus on the optical E2 phonon linewidth temperature dependence in the 10–500 K range. The linewidth decrease at low temperature is analyzed and discussed in the light of anharmonic up- and down-conversion processes, unveiling strongly different behaviors for the two E2 phonons. [ABSTRACT FROM AUTHOR]

In the spinel compounds AB2O4, the B sites form a pyrochlore lattice. GeCo2O4 and GeNi2O4 have been investigated using high magnetic field magnetization as well as neutron diffraction measurements. Both compounds become antiferromagnetic at low temperature saround 23 and 12 K, respectivelyd with the same propagation vector. The magnetization then presents two spin flop fields and one huge saturation field (above 50 T). The Curie-Weiss temperature is positive for the Co spinel and negative for the Ni one. A model taking into account the first neighbor interaction J1 together with two different third neighbor interactions J2 and J3 is able to account for the common observed magnetic behavior. J1 is ferromagnetic while J2 and J3 are antiferromagnetic, in agreement with Goodenough-Kanamori-Anderson rules. These competing interactions are responsible for the frustration in these compounds. [ABSTRACT FROM AUTHOR]