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 TSG=41, 4.5, and 2.6K (for A=Mn2+ S=52, Co2+ S=32 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∼−40K), the frustration index for these materials increases significantly as the moment size is reduced from f=|θ|/TSG=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.