Thermal effects on ESR signal evolution in nano and bulk CuO powder

In order to understand the effects of low dimension on the magnetic properties of CuO, a systematic electron spin resonance study is carried out on CuO nano and bulk powders. Sol–gel produced CuO nanopowder was calcined at temperatures from 200 to 1000 °C to produce nanoparticles of varying size. A broad electron spin resonance (ESR) signal was obtained for the CuO nanoparticles, representative of antiferromagnetic ordering. This ordering persists even at higher calcination temperatures. On the contrary, CuO bulk powder shows two separate ESR signals which merge into one at higher temperatures. These data indicate that the antiferromagnetic ordering is preserved up to 800 °C in CuO nanomaterial. In bulk CuO powder, the ESR signal breaks from antiferromagnetic ordering at lower temperatures. This is a result of the high latent heat of bulk material, which leads to early decomposition of the powder. With evidence from detailed thermal analysis, the observed differences in the ESR data for bulk and nano CuO as a function of calcination temperature are explained on the basis of particle size difference.