Antiferro to superparamagnetic transition on Mn doping in NiO

We report the structural and magnetic properties of Ni1−xMnxO (x=0x=0 to 0.05) prepared by chemical method. Within the XRD resolution, no impurity phases could be detected up to 5 at.% Mn doping in NiO. The fcc structure and the lattice parameter of host NiO matrix is not altered on Mn doping. The average crystallite size was found to remain almost constant (28 nm) up to 3 at.% Mn doping, beyond which it decreases to 21 nm for 5 at.% Mn doping in NiO. The magnetic properties on the other hand showed a drastic change with Mn doping. While 0 and 1 at.% Mn doped NiO showed antiferromagnetic behaviour down to 10 K, 3 and 5 at.% Mn doped NiO were superparamagnetic at 300 K with a blocking temperature of 186 and 171 K respectively. Clear hysteresis loops were thus observed for these samples at 10 K. The distribution of blocking temperature of the Mn doped NiO particles matches well with the distribution of particle size as obtained from TEM. The observed antiferro to superparamagnetic transition on Mn doping in NiO is understood on the basis of Mn occupying Ni site and breaking the translational symmetry of the parent antiferromagnetic correlation.