|
(1) Vidyadhar Singh, (2) V. Srinivas and (3) S. Ram
(1), (2) Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur - 721302, India (3) Materials Science Center, Indian Institute of Technology, Kharagpur 721302, India Borohydride reduction of metal ions is currently used extensively for the production of nanoscale metal, metal boride and metal borate particles. Further, this method has also been used to synthesize core shell structured nanoparticles such as Ni/B2O3. Since the shell has the excellent ability to block the oxygen influx at higher temperature, such stable magnetic nanocomposites are useful for high temperature application. Ferromagnetic (FM) core and Antiferromagnetic (AFM) shell nanoparticles are useful in several ways. Superparamagnetism (SPM) blocking temperature can be modified by introducing a AFM shell. In the present analysis, we investigate nickel oxide (NiO) nanoparticles core and AFM Nickel borate (Ni3B2O6) shell structures. The samples are synthesized by simple and versatile borohydride reduction of NiCl2 aqueous solution. Subsequently, structural and magnetic study of these nanoparticles is determined by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), and SQUID measurements. The as prepared samples appear to have two-phase mixer and on annealing in air at 7000C transforms to NiO/Ni3B2O6 nanocomposites. The XRD patterns show that Ni3B2O6 crystallizes in a kotoite-type structure with the space group Pnnm and NiO has cubic structure. The magnetic measurements show on these samples show a strong anomaly at T=48 K, in the magnetization (M) vs temperature (T) and M vs field (H) curves and identified as the ordering temperature of the NiO/ Ni3B2O6 nanocomposites. The peak at T=120 K in the ZFC curve of this sample has been attributed to the blocking of the SPM NiO shell of the particles. The AFM state of Ni3B2O6 is characterized by weak ferromagnetism due to spin canting of fcc Ni. Above the Neel temperature, the fits to Curie-Weiss law resulting in negative sign of the asymptotic θp which indicates dominant AFM interactions. A comparative study of nano NiO and NiO/Ni3B2O6 confirms the SPM/AFM nature of NiO/Ni3B2O6 nanocomposites. |
|