Chemically-induced Magnetic Dead Shells in Superparamagnetic Ni Nanoparticles from Polarized Small-Angle Neutron Scattering

Abstract

Advances in synthesis and characterization of magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behaviour, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic (FM) system achieved quantitative understanding of the superparamagnetic blocking temperature-particle diameter relationship1. This hinged, however, on the assumption of an FM NP volume lower than the chemical volume due to a non-FM surface dead shell, indirectly deduced from magnetometry. Here, we completely determine both the chemical and magnetic internal structure of such Ni NPs, via unpolarized, half-polarized, and fully-polarized small-angle neutron scattering (SANS) measurements and analyses, coupled with X-ray diffraction and magnetometry. The postulated magnetic dead shell is not only detected but conclusively identified as a Ni phosphide, derived from the trioctylphosphine commonly used in hot-injection syntheses. The shell thickness (t*) can be controlled by synthesis temperature (Ts), falling to as little as 0.5 nm at 170 °C (Fig. 1). Polarized SANS further reveals bulk-like ferromagnetism in the Ni core, the magnetic scattering length density (SLD) being in ideal agreement with magnetometry (Fig. 2), quantitatively supporting prior modeling of superparamagnetism. These findings advance the understanding of synthesis-structure-property relationships in metallic magnetic NPs, point to a route to ligand-free stabilization, and highlight the power of polarized SANS measurement and analysis capabilities for magnetic NP science and technology.Work supported by the UMN DOE Center for Quantum Materials.  Fig. 2. Ni core magnetic SLD vs. T (circles) from half-polarized SANS for the NPs at Ts = 170 °C, superimposed on magnetometry field-cooled (FC) (solid line) and zero field-cooled (ZFC) (dashed line) data measured at 50 Oe.  Fig. 2. Ni core magnetic SLD vs. T (circles) from half-polarized SANS for the NPs at Ts = 170 °C, superimposed on magnetometry field-cooled (FC) (solid line) and zero field-cooled (ZFC) (dashed line) data measured at 50 Oe.