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



Bhaskar Das, Joseph Batley, Kathryn L. Krycka, Julie A. Borchers, Patrick Quarterman, Caroline Korostynski, My Nguyen, Ishita Kamboj, Eray Aydil, Chris Leighton


Advances in 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 achieved quantitative understanding of the superparamagnetic blocking temperature-particle diameter relationship, for example. This hinged, however, on the critical assumption of a ferromagnetic NP volume lower than the chemical volume due to a non-ferromagnetic dead shell, as indirectly deduced from magnetometry. Here, we determine both the chemical and magnetic average internal structure 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 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 inter-particle magnetic interactions, quantitatively supporting prior modelling 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.
ACS Applied Materials & Interfaces


Magnetic nanoparticles, Dead layers, SANS, Blocking temperature


Das, B. , Batley, J. , Krycka, K. , Borchers, J. , Quarterman, P. , Korostynski, C. , Nguyen, M. , Kamboj, I. , Aydil, E. and Leighton, C. (2022), Chemically Induced Magnetic Dead Shells in Superparamagnetic Ni Nanoparticles Deduced from Polarized Small-Angle Neutron Scattering, ACS Applied Materials & Interfaces, [online],, (Accessed April 23, 2024)
Created July 17, 2022, Updated March 22, 2024