Engineering of spin canting across core/shell Fe3O4/MnxFe3−xO4 nanoparticle

The past decade has seen major advances in terms of sub-nanometer probes of materials and fabrication techniques, which have allowed exploration of magnetic effects due to a non-uniform spin configuration within a nanoparticle. Core/shell nanoparticles with hard/soft magnetic layers can be synthesized with high structural precision for engineering the magnetic interactions between the layers. Controlled magnetic interactions between core/shell layers have been used to tailor the magnetic response of cubic nanoparticles and for enhanced heat generation for magnetic hyperthermia. Particular attention has been paid to Fe3O4/Mn-ferrite core/shell structures with strong exchange coupling in nanoparticle systems. Spin canting at high fields was first proposed to explain a reduced moment in Mn ferrite found by both neutron diffraction and Mossbauer spectroscopy, and later supported by high field differential susceptibility and thermos magnetization measurements. Since then numerous studies of ferrite NPs have identified a separate component in Mossbauer spectra attributed to canted spins, or to disordered surface spins. Small angle neutron scattering is used to demonstrate the spin canting that was coherent within a surface shell for Fe3O4 NPs7 ; this has been confirmed by a combination of high resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculations.

In the recently published paper in Scientific Reports, Sara A. Majetich and their colleagues have demonstrated the engineering of spin canting across a Magnetic nanoparticles (MNP) via the Dzyaloshinskii-Moriya interaction (DMI). In this paper, they have shown that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results have illuminated how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.

They have studied ~7.4nm diameter, core/shell Fe3O4/MnxFe3−xO4 MNPs with a 0.5nm Mn-ferrite shell. They have used mossbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism to determine chemical structure of core and shell. Polarized small angle neutron scattering have shown parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. By atomistic simulations, they have revealed the underlying mechanism of the observed spin canting. The results show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment.

In summary, Sara's group have shown the nanoparticles with a magnetite core and manganese ferrite shell with canted spins in moderate fields. They have shown the multiparticle correlations both parallel and perpendicular to the applied field in ordered assemblies. By atomistic simulation, they have revealled the magnetic frustration in the shell, which may originate from DM interactions of Mn B site ions, leads to a modest amount of surface canting, which can act as a source of anisotropy. Strong exchange coupling between the core and shell causes the core spins to cant, as well. In dense assemblies, magnetostatic interactions among the particles favor canting of the particle moments in the same direction. This coherent canting results in a canted superferromagnet or canted supermagnet, a nanoparticle composite that collectively shows canted ferromagnetic behavior. While these core/shell nanoparticles are highly complex, a similar spin canting mechanism may be responsible for variations in performance seen in nanoparticles used for magnetic hyperthermia. Surface frustration due to either DMI or local strains could affect the response of entire particles to AC magnetic fields, which would impact the heat generation. Specifically, tailoring the magnetic response and magnetization reversal in the particle should be possible by precisely tuning the DM interaction between the core and shell.

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