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 S Aydil, Chris Leighton

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


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 non-ferromagnetic 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.

Original languageEnglish (US)
Pages (from-to)33491-33504
Number of pages14
JournalACS Applied Materials and Interfaces
Issue number29
StatePublished - Jul 17 2022

Bibliographical note

Funding Information:
This work was supported primarily by the US Department of Energy through the University of Minnesota Center for Quantum Materials under DE-SC-0016371. Parts of this work were performed in the Characterization Facility, UMN, which receives partial support from NSF through the MRSEC program. Access to polarization capabilities on the NG7 SANS instrument was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under agreement no. DMR-2010792. We are grateful to Jeff Krzywon for technical assistance on NG7 and to Shannon Watson and Taufique Hassan for assistance with the He polarization analyzer. Any mention of commercial products is for information only; it does not imply recommendation or endorsement by NIST. 3

Publisher Copyright:
© 2022 American Chemical Society.


  • colloidal synthesis
  • dead shell effects
  • magnetic nanoparticles
  • polarized neutron scattering
  • small-angle neutron scattering

MRSEC Support

  • Shared

PubMed: MeSH publication types

  • Journal Article


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