Frustrated pyrochlore oxides, ${\mathrm{Y}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$, ${\mathrm{Ho}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$, and ${\mathrm{Yb}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$: Bulk magnetism and magnetic microstructure

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 Å. Small angle neutron scattering data for all three materials are reported for the Q range ${10}^{\mathrm{-}2}$ A${\mathrm{̊}}^{\mathrm{-}1}$ to 2×${10}^{\mathrm{-}1}$ A${\mathrm{̊}}^{\mathrm{-}1}$ 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/${\xi }_1^2$)+B/(${\mathit{Q}}^2$+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 Å depending on the compound, while ${\xi }_2$ shows a very strong temperature dependence and reaches large values of ≳500 Å for Y and Ho and appears to saturate near 400 Å 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. © 1996 The American Physical Society.