Frustrated pyrochlore oxides, Y₂Mn₂O₇, Ho₂Mn₂O₇, and Yb₂Mn₂O₇: Bulk magnetism and magnetic microstructure

The bulk magnetic properties, including de and ac susceptibilities and heat capacity, of the pyrochlore oxides Ho2Mn2O7 and Yb2Mn2O7 are reported and compared with those of the previously studied Y2Mn2O7. In the latter case the magnetic Mn4+ ions occupy the 16c sites in Fd3m which define a potentially frustrated three-dimensional array of corner sharing tetrahedra. For Ho2Mn2O7 and Yb2Mn2O7 magnetic rare earth ions occupy the 16d sites, as shown by powder neutron diffraction, which are topologically equivalent to the 16c sites but displaced by a vector (1/2 1/2 1/2). Ho2Mn2O7 and Yb2Mn2O7 display sharp increases in both dc and ac susceptibilities near 38 K. In addition field-cooled-zero-field-cooled irreversibilities appear, also at 38 K, followed by broad maxima centered near 30 K. The ac data are similar with frequency variability setting below 38 K and broad, frequency dependent maxima at somewhat lower temperatures. Heat capacity data show only broad maxima centered near 30-35 K with a high temperature tail extending up to 80 K. The bulk behavior of Ho2Mn2O7 and Yb2Mn2O7 just described parallels the spin-glass-like behavior of Y2Mn2O7 but with a doubling of the temperature scale. Surprisingly, neutron diffraction data for both Ho2Mn2O7 and Yb2Mn2O7 show resolution limited reflections of magnetic origin in contrast to the heat capacity results. The resolution of the neutron diffraction data places a minimum on the correlation length of 100 Angstrom. Small angle neutron scattering data for all three materials are reported for the Q range 10(-2) Angstrom(-1) to 2x10(-1) Angstrom(-1) and the temperature range 6-100 K. Data for the full Q range can be fitted for all three materials to a model consisting of a Lorentzian and a Lorentzian-squared term, i.e., I(Q)=A/(Q(2)+1/xi(1)(2))+B/(Q(2)+1/xi(2)(2))(2), a cross section commonly found in spin-glass-like materials. A surprising result is that the correlation lengths xi(1) and xi(2) are unequal and in general xi(2)>xi(1). xi(1) remains finite reaching maximum values which range from 10 to 20 Angstrom depending on the compound, while xi(2) shows a very strong temperature dependence and reaches large values of >500 Angstrom for Y and Ho and appears to saturate near 400 Angstrom for Yb. The temperature dependence of the product B xi(2) is order parameter like. The above behavior is compared to that of reentrant spin glasses but with ferromagnetic and spin glass temperatures nearly coincident.