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 rep orted and compared with those of the previously studied Y2Mn2O7. In th e latter case the magnetic Mn4+ ions occupy the 16c sites in Fd3m whic h define a potentially frustrated three-dimensional array of corner sh aring tetrahedra. For Ho2Mn2O7 and Yb2Mn2O7 magnetic rare earth ions o ccupy 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-fiel d-cooled irreversibilities appear, also at 38 K, followed by broad max ima centered near 30 K. The ac data are similar with frequency variabi lity setting below 38 K and broad, frequency dependent maxima at somew hat lower temperatures. Heat capacity data show only broad maxima cent ered near 30-35 K with a high temperature tail extending up to 80 K. T he bulk behavior of Ho2Mn2O7 and Yb2Mn2O7 just described parallels the spin-glass-like behavior of Y2Mn2O7 but with a doubling of the temper ature 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 Ang strom. 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 f itted for all three materials to a model consisting of a Lorentzian an d 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 materi als. 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 reachi ng maximum values which range from 10 to 20 Angstrom depending on the compound, while xi(2) shows a very strong temperature dependence and r eaches large values of >500 Angstrom for Y and Ho and appears to satur ate near 400 Angstrom for Yb. The temperature dependence of the produc t B xi(2) is order parameter like. The above behavior is compared to t hat of reentrant spin glasses but with ferromagnetic and spin glass te mperatures nearly coincident.