Spin canting across core/shell Fe3O4/MnxFe3-xO4 nanoparticles

Abstract

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/Mn_xFe_3-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.

Figure 1

(a) Schematic showing chemical and physical composition of core/shell nanoparticles. (b) Representative figure showing coherent spin canting across nanoparticles under application of applied magnetic field. Due to interparticle correlation of canted spins, the domain has a net canted magnetization that can be measured in terms of parallel and perpendicular components. Particles are physically separated from one another due to a stabilizing, surfactant coating (oleic acid) on their surface.

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. (c) integrated map of the Fe L_2,3 EELS signal, where red intensity shows the Fe distribution. (d) Composite image showing Fe-rich core and Mn-rich shell in the nanoparticles.

Figure 3

Mössbauer spectrum of core/shell nanoparticles measured at 10 K. Subspectra (I − IV) describe the simulated Fe-sites within the core and shell as described in the text and Supplementary Table S2.

Figure 4

Calculations of (a) XAS and (b) XMCD of Fe²⁺ B-, Fe³⁺ A-, and Fe³⁺ B sites, along with the experimental total electron yield (TEY) data at 10 K with ±1 T. The experimental spectra are shown in comparison with a weighted sum of calculated spectra (26% Fe³⁺ A-, 31% Fe²⁺ B-, and 43% Fe³⁺ B-site).

Figure 5

Calculations of (a) XAS and (b) XMCD of Mn²⁺ A-, Mn²⁺ B-, and Mn³⁺ B-sites. Also shown is a comparison of the XAS measured at 75 K with the sum of the calculated spectra of (i) 62% Mn²⁺ A-, 16% Mn²⁺ B-, and 22% Mn³⁺ B-sites, and (ii) 80% Mn²⁺ A-, 20% Mn²⁺ B-sites. (b) XMCD of Mn²⁺ A-, Mn²⁺ B-, and Mn³⁺ B-sites and a comparison of the XMCD measured at 10 K in ±1 T with the sum of (i) 62% Mn²⁺ A-, 16% Mn²⁺ B-, and 22% Mn³⁺ B-sites, and (ii) 80% Mn²⁺ A-, 20% Mn²⁺ B-sites.

Figure 6

Magnetization and magnetic correlations in the core/shell NPs. (a) Magnetization curve at 200 K for a dilute dispersion of the nanoparticles, compared with both the standard and a modified Langevin function, using the independently determined value of μ.

Figure 7

(a) Schematic of PASANS measurement process and (b) PASANS intensity data on dense, crystallized assemblies of the core/shell nanoparticles as a function of scattering vector Q for 200 K and a variety of applied magnetic field (H) conditions. Note that the magnitude of |Q| = 4πsin(ϕ/λ) where 2ϕ is the scattering angle between the source and detector and λ is the neutron wavelength. Data are normalized against the unchanging peak of the structural scattering (N²) with the N², M²_PERP, and M²_PAR components extracted as described in the Supplementary Data Note S5, using sector averages of the 2D data, with θ defined as the angle from Q in the detector plane to the field (or X) axis.

Figure 8

(a) Visualization of the simulated spin configuration of a Fe₃O₄ core and Mn(Fe_1−xMn_x)₂O₄ shell including Dzyaloshinskii-Moriya interactions on Mn B sites. The simulation temperature is set at 0 K in a 0.1 T externally applied field along the [001] crystal direction. The shell shows significant local disorder due to the strong effect of the DMI on the octahedral Mn sites with D = J_ij and particularly at the surface. The frustration leads to canting of the net magnetization of the NP from the field direction of around 12 degrees, similar to an effective anisotropy. The surface frustration also gives a small intersublattice canting of the octahedral Fe sites in the shell of around 1.5 degrees compared to the core. In a NP assembly this could lead to coherent canting after field cooling, and in large fields the particle canting is significantly reduced. (b) Enlargement of a region of Fig. 8a, with dashed line to show the boundary between core and shell. Color bar indicates direction of spin magnetization (blue, +1 or red, −1) on the spin sites.