Size -dependence of the exchange bias e ff ect observed in Nd 1-x Dyx CrO3 ( x=0.05 and 0.20) compounds

The particle-size effects on the exchange bias phenomenon have been investigated in perovskite Nd (1)-(x) Dy-x CrO3 (x=0.05 and 0.20) compounds, which are obtained via co-precipitation method. The analysis of the X-ray diffraction data through Rietveld refinement show that, independently of calcination temperatures and nominal compositions, samples exhibit a single phase with orthorhombic structure and Pnma space group. The calculated particle average diameters via Scherrer formula range from 50 to 90nm. The magnetic-field and T-dependencies of magnetization using the field-cooled-cooling protocol are performed. For all samples, the MvsT curves present a first magnetic transition at T approximate to 210 K likely associated with the antiferromagnetic order of Cr3+ ions and a second one at T approximate to 40 K assigned to their spin reorientation. Magnetic hysteresis loops show the presence of a negative exchange bias phenomenon for all samples, whose values of exchange bias field, , decrease with increasing particle size. We argue that this behavior is mainly associated to the uncompensated surface spins, which are dependent on the particle size.