Magnetic properties, Mössbauer and specific-heat studies of R_1.5Ce_0.5Sr₂FeCu₂O₉ (R = Y, Eu, Gd and Dy) compounds

We have investigated R_1.5Ce_0.5Sr₂FeCu₂O₉ materials by complementary experimental techniques. All materials studied are not superconducting and the magnetic-susceptibility data reveal a magnetic anomaly for R = Y and Eu at T_N = 31 and 47 K, respectively. The anomaly is sensitive to oxygen concentration and shifts toward 260 K when oxygen is depleted. Mossbauer spectroscopic studies reveal two inequivalent iron sites, and that the Fe ions reside (60%) predominantly in the Cu(1) site. In both sites, the Fe does not order magnetically, and the low T_N obtained in this system, as compared to other isostructural materials, is due to frustration of the Cu moments by the presence of Fe in both sites. While the anomalies are clearly identified by magnetic measurements, no specific heat anomaly was observed at T_N. A strong two-dimensional quantum spin fluctuation is considered to be the cause of the unusual behavior at T_N of Cu.