Ferromagnetic correlations in the highly frustrated, spin-glass-like, pyrochlore oxide Y2Mn2O7, a neutron scattering study

Previous studies have shown that Y2Mn2O7 is spin-glass-like with no long range order to 2 K in spite of bulk magnetic susceptibility data which suggest ferromagnetic order below 20 K. This compound has been studied by small angle neutron scattering (SANS) over the Q-range 0.02 Å−1 to 0.15 Å and the temperature range 8 K to 100 K. Direct evidence for the presence of finite-size ferromagnetic clusters is seen in the form of strong scattering at low Q below 20 K. The SANS line shape is Lorentzian, allowing determination of the correlation length ξ. This quantity shows a power law temperature dependence from 20 K to 13.5 K where ξ = 104(10) Å. Below this temperature ξ continues to increase but cannot be measured with precision due to limitations of instrumental resolution. The temperature dependence of the O-integrated SANS intensity shows a change in slope below 15 K which corresponds to a prominent anomaly in the a.c. susceptibility. Thus, in Y2Mn2O7 fairly large, 100 Å, but subcritical ferromagnetic regions co-exist with very short range, antiferromagnetic regions with correlations extending only to a few nearest neighbors.