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 angstrom-1 to 0.15 angstrom 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-xi. This quantity shows a power law temperature dependence from 20 K to 13.5 K where xi = 104(10) A. Below this temperature xi 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 angstrom, but subcritical ferromagnetic regions co-exist with very short range, antiferromagnetic regions with correlations extending only to a few nearest neighbors.