Magnetic properties of mechanically milled nanosized cupric oxide

Abstract

Detailed magnetic measurements have been carried out in order to understand the effects of ball-mill-induced defects on the magnetic properties of cupric oxide nanoparticles. A progressive decrease in the crystallite size and a concomitant increase in the induced strain have been observed with the milling times. The field-cooled (FC) and zero-field-cooled (ZFC) measurements indicate a presence of an exchange bias field, whose value increases with the increase in the milling time, reaching a maximum value of 750 Oe at the 50 h milling time. However, the ZFC coercivity first increases up to the 30 h milling time and then decreases upon prolonged milling. The temperature-dependent magnetization measurements indicate the presence of a hysteresis loop even at room temperature. The presence of an exchange bias field is indicative of the presence of an exchange interaction between the ferromagnetic surface, arising from uncompensated moments generated by mechanical strain, and the antiferromagnetic (AFM) core. The exchange bias field reduces to zero at about the Néel temperature of the AFM core of the nanoparticles. The Néel temperature of the AFM core is milling time dependent; for example, the 50 h milled sample shows a higher Néel temperature ( T N∼100 K) than the 30 h ( T N∼30 K) milled sample. The presence of weak ferromagnetism and an exchange bias field is attributed to the uncompensated surface spins resulting from induced defects via mechanical milling.