Unusual thermal dependence of saturation magnetization in zinc oxide nanoparticles doped with transition metals obtained by sol gel method

Abstract We present a study about the thermal dependence of saturation magnetization in ZnO nanoparticles doped with Co and Cr, obtained by the sol gel method. The X-ray diffraction (DRX), confirmed a single crystalline phase corresponding to the wurtzite structure of ZnO with hexagonal structure. The lattice parameter, a exhibits values from 3.2456 A to 3.2446 A (for samples doped with Cr) and 3.2466–3.2460 A (for samples doped with Co) lower than that for undoped sample (a = 3.2512 A). The defects and the presence of oxygen vacancy (Vo) have been estimated through Raman and FTIR spectroscopic studies. All samples exhibit a ferrimagnetic behavior with changes in the saturation magnetization and coercive field for different dopant concentration. The ferromagnetic ordering in these samples may be induced via surface defect like oxygen vacancy and inclusion of magnetic ions. Adjustments by means of the extended Bloch law show that the exponent takes values different from those expected for ferromagnets with a Heisenberg type Hamiltonian (9/2, 2, 3, 3/2 and 5/2). The Bloch exponent value for the pure ZnO sample is 1.58. For samples doped with Co, this value varies between 1.06 and 2.20, while for samples doped with Cr they oscillate between 1.25 and 2.61.