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

Abstract

The bulk magnetic properties, including dc and ac susceptibilities and heat capacity, of the pyrochlore oxides ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ are reported and compared with those of the previously studied ${\mathrm{Y}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$. In the latter case the magnetic ${\mathrm{Mn}}^{4+}$ ions occupy the 16c sites in Fd3m which define a potentially frustrated three-dimensional array of corner sharing tetrahedra. For ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ 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). ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ 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 ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ just described parallels the spin-glass-like behavior of ${\mathrm{Y}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ but with a doubling of the temperature scale. Surprisingly, neutron diffraction data for both ${\mathrm{Ho}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Yb}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ 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 \AA{}. Small angle neutron scattering data for all three materials are reported for the Q range ${10}^{\mathrm{\ensuremath{-}}2}$ A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ to 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}1}$ A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ 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/(${\mathit{Q}}^{2}$+1/${\ensuremath{\xi}}_{1}^{2}$)+B/(${\mathit{Q}}^{2}$+1/${\ensuremath{\xi}}_{2}^{2}$${)}^{2}$, a cross section commonly found in spin-glass-like materials. A surprising result is that the correlation lengths ${\ensuremath{\xi}}_{1}$ and ${\ensuremath{\xi}}_{2}$ are unequal and in general ${\ensuremath{\xi}}_{2}$\ensuremath{\gtrsim}${\ensuremath{\xi}}_{1}$. ${\ensuremath{\xi}}_{1}$ remains finite reaching maximum values which range from 10 to 20 \AA{} depending on the compound, while ${\ensuremath{\xi}}_{2}$ shows a very strong temperature dependence and reaches large values of \ensuremath{\gtrsim}500 \AA{} for Y and Ho and appears to saturate near 400 \AA{} for Yb. The temperature dependence of the product B${\ensuremath{\xi}}_{2}$ is order parameter like. The above behavior is compared to that of reentrant spin glasses but with ferromagnetic and spin glass temperatures nearly coincident. \textcopyright{} 1996 The American Physical Society.