Spin-glass behavior in the (formula presented) fcc ordered perovskite (formula presented)

The ordered perovskite (formula presented) of monoclinic symmetry [space group (formula presented) at (formula presented)] has been synthesized using standard solid-state chemistry techniques. The difference in the size and charge of the cations induces an ordering of the B site (formula presented) and (formula presented) ions which leads to a distorted fcc lattice of spin-(formula presented) (formula presented) (formula presented) moments. dc magnetic susceptibility measurements indicate a maximum at (formula presented) and an irreversibility in the field-cooled and zero-field-cooled data at 22 K that is believed to be caused by the geometric frustration inherent in the fcc structure. Neutron-scattering measurements confirm the absence of magnetic long-range order, and muon spin relaxation experiments indicate the presence of an abrupt spin freezing at (formula presented) Specific heat measurements reveal a broad anomaly typical of spin glasses and no sharp feature. 65% of the spin entropy is released at low temperatures. The low-temperature data do not show the expected linear temperature dependence, but rather a (formula presented) relationship, as is observed, typically, for antiferromagnetic spin waves. The material is characterized as an unconventional, essentially disorder-free, spin glass.