Doping effect on the local structure of metamagnetic Co doped Ni/NiO:GO core-shell nanoparticles using X-ray absorption spectroscopy and the pair distribution function.

Core-shell nanoparticles of Co (0%, 3%, and 5%) doped Ni/NiO and incorporated (5 mg) graphene oxide (GO) sheets were synthesized by a sol-gel auto-combustion method. X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and pair distribution function measurements were employed for the determination of the local structure and structural properties of the samples. Due to the effect of Co doping the bond lengths of all coordinate atoms were varied. The microstructural features in the core-shell structured particles were evaluated with high-resolution transmission electron microscopy (HRTEM). Magnetic properties of the samples revealed that both the interface of Ni/Co and NiO crystal lattices and the weight fraction of Ni have significant impact on their magnetic properties at 5 K to 300 K. Experimental results show that Co doping and GO incorporation into Ni/NiO suppress the antiferromagnetic charge ordering and lead to a spin-flop metamagnetic behavior at 5 K to 300 K temperatures. Above 5 K, the step-like transitions transform into broad ones. This step-like feature is correlated with the collapse of the balance between the magnetic energy and elastic energy at the core-shell interface. It is confirmed from M-T measurements that the blocking temperature of Ni/NiO was reduced with Co content.