Abstract
Ni nanoparticles (Ni-NPs), with diameter (D) ranging 5–30 nm, were synthesized by reducing nickel chloride with NaBH4 in the presence of polymer molecules of poly-vinyl alcohol (PVA) in cold water. Nickel chloride was dispersed in the PVA molecules which stabilized the resulting Ni-NPs. Experiments were carried out with and without PVA to elucidate the effect of PVA molecules on the structural and magnetic properties of Ni-NPs. It was found that both uncoated (uc) and PVA-coated (pc) Ni-NPs exhibit a tetragonal (t) crystal structure, i.e. different from the cubic (fcc) structure of bulk nickel. pc Ni-NPs (paramagnetic in nature) converted to fcc Ni (spherical shape, D ∼ 12 nm) on annealing at 573 K in air, exhibiting a saturation magnetization M s = 20.5 emu/g, squareness ratio M r /M s = 0.48 and coercivity H c = 248 Oe, which is higher than the bulk Ni (0.7 Oe). uc Ni-NPs showed little improvement in M s and H c on air annealing. The core–shell structure resulted in a high H c value in stable pc Ni-NPs in air. Electron magnetic resonance revealed exchange interaction between the core and shell, which changes on annealing in air.
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