Contrasting magnetic behaviors of Ni3Al nanoparticles with and without NiO shell

In the nanocrystalline Ni3Al sample with the average crystallite size d=50 nm but without NiO shell, magnetization does not saturate even at magnetic fields as high as H=90 kOe, no long-range ferromagnetic order exists even at T=2.5 K and the exchange bias (EB) effect is completely absent. By contrast, in the nanocrystalline sample with ferromagnetic Ni3Al core (diameter 5 nm) and antiferromagnetic NiO shell (thickness ≃1nm), saturation in magnetization is reached at fields as low as 1 kOe, saturation magnetization is ≃5 times higher, super-spin glass state coexists with inhomogeneous ferromagnetic ordering and a novel type of EB effect is observed.