Size-dependent multiple magnetic phases and exchange bias effect in hole-doped double perovskite La1.6Sr0.4NiMnO6

Abstract

La1.6Sr0.4NiMnO6 nanoparticles of different sizes (18–150 nm) have been prepared by a sol–gel method, and the size effects on their magnetic properties are investigated. It is found that there is a ferromagnetic (FM) transition at TC ∼ 245 K for the sample with particle size D ∼ 150 nm. As the particle size decreases, a spin glass (SG) transition appears at TG ∼ 60 K and becomes more obvious. Meanwhile, the TC shifts to a lower temperature and the FM transition becomes indistinct. The size dependent saturation magnetization MS, which is in the range 0.78 ∼ 1.27 μB/f.u., shows a non-monotonic variation with a maximum of 1.27 μB/f.u. at the particle size DC ∼ 42 nm. The exchange bias (EB) effect is clearly observed for D > 32 nm and becomes indistinct as D ⩽ 32 nm. As the particle size decreasing, the EB field HE decreases and becomes near nil at DC. Meanwhile, the coercive field HC first increases to a maximum at DC and then decreases. All the results indicate the coexistence of AFM APBs related to the anti-site disorders, FM domains, and SG phase in La1.6Sr0.4NiMnO6 nanoparticles. The magnetic properties of the nanoparticles are understood by a complex core-shell model, i.e. both AFM APBs and FM domains coexist in the core, and the SG phase mainly resides on the surface (shell) of each particle, which reveals that the size effects on the FM phase, AFM APBs and surface SG phase in grain are different, even are opposite. The nanometer size effect is a crucial factor to influence the magnetic properties of La1.6Sr0.4NiMnO6 nanoparticles.