The consequences of nonmagnetic-ion dilution for the pyrochlore family Y$_{ 2}$($M_{ 1-x}N_{ x}$)$_{ 2}$O$_{ 7}$ ($M$ = magnetic ion, $N$ = nonmagnetic ion) have been investigated. As a first step, we experimentally examine the magnetic properties of Y$_{ 2}$CrSbO$_{ 7}$ ($x$ = 0.5), in which the magnetic sites (Cr$^{ 3+}$) are percolative. Although the effective Cr-Cr spin exchange is ferromagnetic, as evidenced by a positive Curie-Weiss temperature, $\Theta_\mathrm{ { CW}}$ = 20.1(6) K, our high-resolution neutron powder diffraction measurements detect no sign of magnetic long range order down to 2 K. In order to understand our observations, we performed numerical simulations to study the bond-disorder introduced by the ionic size mismatch between $M$ and $N$. Based on these simulations, bond-disorder ($x_{ b}$ $\simeq$ 0.23) percolates well ahead of site-disorder ($x_{ s}$ $\simeq$ 0.61). This model successfully reproduces the critical region (0.2 < $x$ < 0.25) for the N\'eel to spin glass phase transition in Zn(Cr$_{ 1-x}$Ga$_{ x}$)$_{ 2}$O$_{ 4}$, where the Cr/Ga-sublattice forms the same corner-sharing tetrahedral network as the $M/N$-sublattice in Y$_{ 2}$($M_{ 1-x}N_{ x}$)$_{ 2}$O$_{ 7}$, and the rapid drop in magnetically ordered moment in the N\'eel phase [Lee $et$ $al$, Phys. Rev. B 77, 014405 (2008)]. Our study stresses the nonnegligible role of bond-disorder on magnetic frustration, even in ferromagnets.
Categories: cond-mat.str-el cond-mat.dis-nn