| Autor: |
Das B; Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States., Batley JT; Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States., Krycka KL; NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States., Borchers JA; NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States., Quarterman P; NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States., Korostynski C; Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States., Nguyen M; Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States., Kamboj I; Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States., Aydil ES; Department of Chemical and Biomolecular Engineering, New York University Tandon School of Engineering, Brooklyn, New York 11201, United States., Leighton C; Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States. |
| Abstrakt: |
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. |
