Tuning the magnetic anisotropy and phase transition in superparamagnetic cobalt oxide nanostructures synthesized through novel solution-growth technique

Abstract

In the present work, we show that the in situ growth of oriented nanometric aggregates of cobalt oxide can be explored to alter and tune magnetocrystaline anisotropy. As such, the impact of interparticle interaction on magnetic properties of the aggregates is studied. Mono-dispersed cobalt oxide (CoO) nanostructures were prepared through decomposition of metal-organic compound of cobalt (II) in an alcoholic solution under controlled microwave irradiation, below 150 ⁰C. CoO nanostructures were effectively synthesized without the use of any surfactant. The structures were found to be inert to oxidation in ambient air for several months. We attribute the stability of CoO nanospheres to their dense packing, the driving force being, minimization of surface area and surface energy [1]. X-ray diffractometry (XRD) measurements indicate the formation of well crystallized CoO nanoparticles, without the need for post-synthesis annealing. Deconvolution of the active modes in Raman spectra obtained at room temperature reveals the Oh symmetry in rock-salt CoO, induced by microwave-assisted synthesis route [2]. Its effect on magnetic characteristics of CoO is analyzed here. Field-dependent magnetization (MH) and temperature-dependent magnetization (MT) measurements show a phase transition from paramagnetic to superparamagnetic interactions in CoO nanostructures at around 10 K, at which the average particle size was estimated to be 9 nm. The nanostructures however do not exhibit a distinct antiferromagnetic transition around 300 K as the bulk sample [3], but instead show a hysteresis below a blocking temperature of ~10 K (Figure 1). This finding reveals the magnetic behavior of CoO nanostructures that opens new opportunities for future applications as anisotropy source for magnetic recording.