The Ho2MnRuO7 pyrochlore oxide: Magnetic structure versus magnetic frustration

A pyrochlore-like phase of composition Ho(2)MnRuO(7) has been synthesized by a soft-chemistry procedure followed by thermal treatments at moderate temperatures up to 900 degrees C for 12 h in air. It has been characterized by x-ray diffraction and neutron powder diffraction (NPD), as well as dc and ac susceptibilities. As in the parent Ho(2)Mn(2)O(7) oxide, the magnetic Mn and Ru ions statistically occupy the 16c sites in a cubic unit cell with space group Fd (3) over barm, which define a potentially frustrated three-dimensional array of corner sharing (Mn, Ru)(4) tetrahedra. The dc and ac magnetic susceptibilities of Ho(2)MnRuO(7) display a sharp increase near 60 K. In addition, the field-cooled and zero-field-cooled curves diverge below 30 K. The ac data present frequency variability below 60 K and broad frequency dependent maxima at lower temperatures (similar to 30 K), suggesting a spin-glass like behavior similar to Ho(2)Mn(2)O(7). However, a low-temperature NPD study of the magnetic structure unveils an antiferromagnetic coupling of two subsets of Mn(4+)/Ru(4+) spins, indicating that the magnetic frustration is partially relieved by the random distribution of Mn and Ru over the 16c sites; at lower temperatures there is a polarization of the Ho(3+) magnetic moments, which also participate in the magnetic structure. Under an external magnetic field the Ho moments become totally polarized, giving a saturation magnetization at 2 K of 10.8 mu(B)/f.u. at 5 T. (C) 2010 American Institute of Physics. [doi:10.1063/1.3393994]