Quantitative size-structure-magnetic property relationships in thoroughly-characterized metallic Ni nanoparticle assemblies*

Recent advances in colloidal synthesis and other approaches have realized nanoparticles(NPs) with controlled structure, size, functionalization, and self-assembly. In magnetism, the focus on hard ferromagnetic(FM) NPs and oxides has left relatively little research on metallic soft FM NPs. Here, Ni NPs have been synthesized via injection of a Ni-oleylamine(OAm) complex into 200°C trioctylphosphine(TOP). Size control was obtained by varying TOP/OAm ratio, reaction time, and by differential centrifugation. Characterization was performed with X-ray diffraction, transmission electron microscopy, Raman and Fourier-transform infrared spectroscopy, and SQUID magnetometry. Polycrystalline FCC NPs are obtained, with mean diameter, <D>, tunable from 4—22 nm, and dispersion, σ/<D> = 10-30%. Superparamagnetic blocking can thus be tuned between 15 and >300 K. Size distributions and inter-particle interactions are explicitly accounted for, quantitatively reconciling differences between the true mean blocking temperature, <T_B>, and the zero field cooled magnetization peak. The resulting <T_B>vs.<D> yields an effective anisotropy surprisingly close to that of bulk Ni. Reduced saturation magnetization is observed, but is consistent with a Ni_xP_y or spin-canted shell ~1 nm thick.