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College Park, Maryland      June 6 - 10 , 2004

TP64: Magnetic and Structural Studies of the Spinels GeNi2O4 and GeCo2O4

M.K. Crawford, R.L. Harlow, R. Flippen (DuPont CR&D, Wilmington, DE), R.W. Stevens, B.F. Woodfield, J. Boerio-Goates (Brigham Young University, Provo, UT), P.L. Lee, Y. Zhang (Advanced Photon Source, Argonne National Laboratory, Argonne, IL), J. Hormadaly (Ben Gurion University, Beer Sheeva, Israel), R.A. Fisher (Lawrence Berkeley National Laboratory), Q. Huang, J. W. Lynn, Y. Qiu, J.R.D. Copley (NIST Center for Neutron Research)

Transition metal oxide spinels that have magnetic ions located only on the B-sublattice exhibit interesting phenomena, including Néel ordering accompanied by structural phase transitions. Neutron powder diffraction measurements show that GeNi2O4 (spin-1) and GeCo2O4 (spin-3/2) become antiferromagnetic at TN = 12 K and 21 K, respectively. However, GeNi2O4 remains cubic, while GeCo2O4 becomes tetragonal, below TN. Thus the behavior of GeCo2O4 is similar to that of ZnCr2O4 (spin-3/2), although the Co2+ ion has unquenched orbital angular momentum and the Cr3+ ion does not. The Néel transition in GeNi2O4 occurs in two discrete steps, while the other materials have single Néel transitions. Neutron inelastic scattering measurements demonstrate that both GeNi2O4 and GeCo2O4 have energy gaps in their Néel states, consistent with the results of heat capacity measurements. The Néel transition and magnetic structure of GeCo2O4 are very sensitive to an applied magnetic field, whereas the Néel transitions in GeNi2O4 are only weakly affected by a field. In this presentation we will describe the results of magnetic and structural studies of these materials, and discuss their different behaviors.

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