Signature of antiphase boundaries in iron oxide nanoparticles

Abstract

Iron oxide nanoparticles find a wide variety of applications, including targeted drug delivery and hyperthermia in advanced cancer treatment methods. An important property of these particles is their maximum net magnetization, which has been repeatedly reported to be drastically lower than the bulk reference value. Previous studies have shown that planar lattice defects known as antiphase boundaries (APBs) have an important influence on the particle magnetization. The influence of APBs on the atomic spin structure of nanoparticles with the γ-Fe2O3 composition is examined via Monte Carlo simulations, explicitly considering dipole–dipole interactions between the magnetic moments that have previously only been approximated. For a single APB passing through the particle centre, a reduction in the magnetization of 3.9% (for 9 nm particles) to 7.9% (for 5 nm particles) is found in saturation fields of 1.5 T compared with a particle without this defect. Additionally, on the basis of Debye scattering equation simulations, the influence of APBs on X-ray powder diffraction patterns is shown. The Fourier transform of the APB peak profile is developed to be used in a whole powder pattern modelling approach to determine the presence of APBs and quantify them by fits to powder diffraction patterns. This is demonstrated on experimental data, where it could be shown that the number of APBs is related to the observed reduction in magnetization.

Magnetic neutron scattering from spherical nanoparticles with Néel surface anisotropy: atomistic simulations

“…The magnetodipolar interaction has been ignored in our simulations. This is motivated by the numerical complexity of this energy term, in particular for atomistic simulations (here, for a 10 nm diameter particle the number of spins is N = 11 633), and by the expectation that it is of minor relevance for smaller-sized nanomagnets (Ko ¨hler et al, 2021;Pathak & Hertel, 2021).…”

“…Scattering techniques, in particular employing X-rays and neutrons, have proved to be very powerful in this endeavour, since they provide statistically averaged information on a large number of scattering particles. For instance, using Monte Carlo simulations of a discrete atomistic spin model, Ko ¨hler et al (2021) have numerically studied the influence of antiphase boundaries in iron oxide nanoparticles on their spin structure. These authors used the Debye scattering equation to relate the internal spin disorder to the broadening of certain X-ray Bragg peaks.…”

Magnetic neutron scattering from spherical nanoparticles with Néel surface anisotropy: analytical treatment

“…In (2), the two surface integrals take into account the boundary conditions for the magnetization on the surface (@V) of the NM of volume V, which result from the exchange interaction and the Ne ´el term. The magnetodipolar energy has been ignored in the calculations because of its mathematical complexity and since it is expected to be of minor relevance for smaller-sized NMs [see the recent atomistic simulations by Ko ¨hler et al (2021b)].…”

On the angular anisotropy of the randomly-averaged magnetic neutron scattering cross section of nanoparticles

“…Nonuniformities in the magnetization distribution of nanoparticles are e.g. caused by surface anisotropy, vacancies, or antiphase boundaries [7,[10][11][12][13][14][15][16][17]. Micromagnetic simulations that take into account the relevant interactions such as isotropic exchange, antisymmetric exchange, magnetic anisotropy, Zeeman energy, and the magnetodipolar interaction are an important tool for advancing the understanding of magnetic SANS of nanomagnets [1].…”