Coexistence of ferromagnetism and spin glass behavior in antiferromagnetic Y2BaCuO5

We report the synthesis of a series of Y2BaCuO5 samples by varying the annealing temperature with a citrate pyrolysis technique. X-ray diffraction patterns, scanning electron microscopy and transmission electron microscopy observation show the formation of a columnar Y2BaCuO5 phase and these samples are composed of many irregular particles with different particle size. Magnetic measurements show that these samples exhibit room temperature ferromagnetism and the saturation magnetization decreases with increasing sintering temperature. Post-heating treatment under argon atmosphere can enhance the ferromagnetism greatly, suggesting that the magnetism is attributed to the surface oxygen defects. By measuring magnetization versus temperature curves after zero field cooling with various applied magnetic fields, two magnetic phase transitions located at about 11 and 110 K are revealed. The position of the peak at about 11 K is independent of the magnetic field; the other peak, however, becomes rounder and shifts to lower temperatures with increasing the magnetic field, showing a strong field dependence. In addition, the virgin magnetization curves with the measured temperature below 110 K display an S-type. These features are suggestive of an antiferromagnetic phase transition at about 11 K and a spin glass transition at about 110 K.