Abstract:Recently, an easily scalable process for the production of small (3−7 nm) monodisperse superparamagnetic ferrite nanoparticles MeFe 2 O 4 (Me = Zn, Mn, Co) from iron metal and octanoic acid has been reported (Salih et al., Chem. Mater. 25 1430-1435 2013). Here we present a Mössbauer spectroscopic study of these ferrite nanoparticles in external magnetic fields of up to B = 5 T at liquid helium temperatures. Our analysis shows that all three systems show a comparable inversion degree and the cationic distributi… Show more
“…When the temperature is raised to reflux at 300 °C, magnetite cores nucleate and grow first, followed by growth of the Mn ferrite shells. In some cases core/shell NPs have been prepared by design with a two-stage process 8,13,27 , while in other some cases the differential decomposition may have unintentionally generated core/shell NPs 20,28,29 .Figure 2( a ) HAADF-STEM images of nanoparticles showing spinel structure in magnetite cores. ( b ) integrated map of the Mn L 2,3 EELS signal, where green intensity shows the Mn distribution.…”
Section: Resultsmentioning
confidence: 99%
“…Since then numerous studies of ferrite NPs have identified a separate component in Mössbauer spectra attributed to canted spins 17–20 , or to disordered surface spins 21 . Small angle neutron scattering demonstrated spin canting that was coherent within a surface shell for Fe 3 O 4 NPs 7 ; this has been confirmed by a combination of high resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculations 9 .…”
Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe3O4/MnxFe3−xO4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.
“…When the temperature is raised to reflux at 300 °C, magnetite cores nucleate and grow first, followed by growth of the Mn ferrite shells. In some cases core/shell NPs have been prepared by design with a two-stage process 8,13,27 , while in other some cases the differential decomposition may have unintentionally generated core/shell NPs 20,28,29 .Figure 2( a ) HAADF-STEM images of nanoparticles showing spinel structure in magnetite cores. ( b ) integrated map of the Mn L 2,3 EELS signal, where green intensity shows the Mn distribution.…”
Section: Resultsmentioning
confidence: 99%
“…Since then numerous studies of ferrite NPs have identified a separate component in Mössbauer spectra attributed to canted spins 17–20 , or to disordered surface spins 21 . Small angle neutron scattering demonstrated spin canting that was coherent within a surface shell for Fe 3 O 4 NPs 7 ; this has been confirmed by a combination of high resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculations 9 .…”
Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe3O4/MnxFe3−xO4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.
“…Surface spin disorder as a result of symmetry breaking in the crystal structure at the NP surface and altered exchange interactions lead to canted spins and a reduction in magnetic moment of the NP . In general, surface spins are more susceptible to spin canting than interior moments, and such canting has been observed experimentally in many ferrites and iron oxides NPs. − The degree of canting is influenced by anisotropy, intraparticle effects due to Dzyaloshinskii–Moriya interactions and NP size allowing for possible ways to tailor spin disorder in core–shell NPs through careful selection of constituent materials and synthesis parameters.…”
We investigate the spatial distribution of spin orientation
in
magnetic nanoparticles consisting of hard and soft magnetic layers.
The nanoparticles are synthesized in a core–shell spherical
morphology where the target stoichiometry of the magnetically hard,
high anisotropy layer is CoFe2O4 (CFO), while
the synthesis protocol of the lower anisotropy material is known to
produce Fe3O4. The nanoparticles have a mean
diameter of ∼9.2–9.6 nm and are synthesized as two variants:
a conventional hard/soft core–shell structure with a CFO core/FO
shell (CFO@FO) and the inverted structure FO core/CFO shell (FO@CFO).
High-resolution electron microscopy confirms the coherent spinel structure
across the core–shell boundary in both variants, while magnetometry
indicates the nanoparticles are superparamagnetic at 300 K and develop
a considerable anisotropy at reduced temperatures. Low-temperature M vs H loops suggest a multistep reversal process. Small
angle neutron scattering (SANS) with full polarization analysis reveals
a considerable alignment of the spins perpendicular to the field even
at fields approaching saturation. The perpendicular magnetization
is surprisingly correlated from one nanoparticle to the next, though
the interaction is of limited range. More significantly, the SANS
data reveal a pronounced difference in the reversal process of the
magnetization parallel to the field for the two nanoparticle variants.
For the CFO@FO nanoparticles, the core and shell magnetizations appear
to track each other through the coercive region, while in the FO@CFO
variant, the softer Fe3O4 core reverses before
the higher anisotropy CoFe2O4 shell, consistent
with expectations from mesoscale magnetic modeling. These results
highlight the interplay between interfacial exchange coupling and
anisotropy as a means to tune the composite properties of the nanoparticles
for tailored applications including biomedical/theranostic uses.
“…Surface spin disorder as a result of symmetry breaking in the crystal structure at the NP surface and altered exchange interactions lead to canted spins and a reduction in magnetic moment of the NP [2]. In general, surface spins are more susceptible to spin canting than interior moments and such canting has been observed experimentally in many ferrites and iron oxides NPs [15][16][17][18][19][20][21]. The degree of canting is influenced by anistropy [22], intraparticle effects due to Dzyaloshinskii-Moriya interactions and NP size [23] allowing for possible ways to tailor spin disorder in core/shell NPs through careful selection of constituent materials and synthesis parameters.…”
We investigate the spatial distribution of spin orientation in magnetic nanoparticles consisting of hard and soft magnetic layers. The nanoparticles are synthesized in a core / shell spherical morphology where the magnetically hard, high anisotropy layer is CoFe 2 O 4 (CFO) while the lower anisotropy material is Fe 3 O 4 (FO). The nanoparticles have a mean diameter of ∼9.2 -9.6 nm and are synthesized as two variants: a conventional hard / soft core / shell structure with a CFO core / FO shell (CFO@FO) and the inverted structure FO core / CFO shell (FO@CFO). High resolution electron microscopy confirms the coherent spinel structure across the core / shell boundary in both variants while magnetometry indicates the nanoparticles are superparamagnetic at 300 K and develop a considerable anisotropy at reduced temperatures. Low temperature M vs. H loops suggest a multi-step reversal process. Temperature dependent small angle neutron scattering (SANS) with full polarization analysis reveals a strong perpendicular plane alignment of the spins near zero field, indicative of spin canting, but the perpendicular alignment quickly disappears upon application of a weak field and little spin ordering parallel to the field until the coercive field is reached. Above the coercive field of the sample, spins orient predominantly along the field direction. At both zero field and near saturation, the parallel magnetic SANS peak coincides with the structural peak, indicating the magnetization is uniform throughout the nanoparticle volume, while near the coercive field the parallel scattering peak shifts to higher momentum transfer (Q), suggesting that the coherent scattering volume is smaller and likely originates in the softer Fe 3 O 4 portion of the nanoparticle.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
