Spin glass transitions in the absence of chemical disorder for the pyrochlores A₂Sb₂O₇ (A=Mn, Co, Ni)

The pyrochlores in the series A₂Sb₂O₇ have been synthesized and characterized as exhibiting spin glass transitions at T_SG=41, 4.5, and 2.6 K (for A=Mn²⁺S = frac(5, 2), Co²⁺S = frac(3, 2) and Ni²⁺ S=1, respectively) despite the lack of chemical disorder. Since the Curie-Weiss temperature remains essentially constant for all members in the series (θ ∼ - 40 K), the frustration index for these materials increases significantly as the moment size is reduced from f = | θ | / T_SG = 1.1 (Mn²⁺), to 9.3 (Co²⁺) to 14.6 (Ni²⁺). There is also a corresponding change in the spin dynamics measured by the shift in the AC susceptibility signal as a function of frequency. These new materials provide an avenue to investigate the effect of quantum fluctuations on the Heisenberg pyrochlore lattice in the low spin limit, and show there is a dramatic change in the spin dynamics as the quantum regime is approached.