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

Abstract

Advances in the synthesis and characterization of colloidal magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behavior, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic system, for example, achieved quantitative understanding of the superparamagnetic blocking temperature−particle diameter relationship. This hinged, however, on the critical assumption of a ferromagnetic NP volume lower than the chemical volume due to a nonferromagnetic dead shell indirectly deduced from magnetometry. Here, we determine both the chemical and magnetic average internal structures of Ni NP ensembles via unpolarized, half-polarized, and fully polarized small-angle neutron scattering (SANS) measurements and analyses coupled with X-ray diffraction and magnetometry. The postulated nanometric magnetic dead shell is not only detected but conclusively identified as a non-ferromagnetic Ni phosphide derived from the trioctylphosphine commonly used in hot-injection colloidal NP syntheses. The phosphide shell thickness is tunable via synthesis temperature, falling to as little as 0.5 nm at 170 °C. Temperature-and magnetic field-dependent polarized SANS measurements additionally reveal essentially bulk-like ferromagnetism in the Ni core and negligible interparticle magnetic interactions, quantitatively supporting prior modeling of superparamagnetism. These findings advance the understanding of synthesis−structure−property relationships in metallic magnetic NPs, point to a simple potential route to ligand-free stabilization, and highlight the power of the currently available suite of polarized SANS measurement and analysis capabilities for magnetic NP science and technology.

Biocompatible gelatin-coated ferrite nanoparticles: A magnetic approach to advanced drug delivery

“…In order to obtain focused and effective distribution, researchers have also looked at a variety of medications enclosed within gelatin-coated ferrite nanoparticles. With increased therapeutic effectiveness, Jain et al examined the creation and optimization of gelatin-coated cobalt ferrite nanoparticles for the controlled release of anticancer drugs ( Das et al, 2022 ). Similar research was conducted by Vandervoort et al, who investigated magnetically sensitive gelatin-coated manganese ferrite nanoparticles as prospective delivery systems for anticancer drugs ( Shende and Pathan, 2021 , Vandervoort and Ludwig, 2004 ).…”

Magnetic Coupling in Cobalt-Doped Iron Oxide Core–Shell Nanoparticles: Exchange Pinning through Epitaxial Alignment

“…Small-angle scattering using X-ray and neutron radiation (SAXS/SANS) allows the differentiation of the electron density and isotope-related neutron scattering length density profile within the nanoparticles, giving access to their local composition with spatial resolution from the core to the surface in the sub-nm range . Moreover, magnetic SANS is a powerful technique to explore the magnetic inhomogeneities on the nanoscale and address the nanoscale distribution of the collinear magnetization within magnetic nanoparticles. − A quantitative description of the individual core and shell magnetization in core–shell nanoparticles and their potential coupling, however, is so far missing.…”

Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

“…The structural and magnetic systems of each NP variant were explored using a variety of techniques including transmission electron microscopy (TEM), field- and temperature-dependent magnetometry, mesoscale magnetic simulations of NP ensembles, and also fully spin-polarized small-angle neutron scattering (SANS). The latter is a technique capable of providing both structural and magnetic details of the individual core–shell layers as well as interparticle spin correlations with differing length scales and degree of coherence. − The ensemble average of both the magnitude and direction of magnetic moments can be resolved allowing for differentiation between reduced moments and presence of tilted spins. Field- and temperature-dependent magnetometry were collected and compared to Langevin generated magnetization curves as such methods can also provide evidence of tilted spins in the NPs.…”