Zinc substituted nickel ferrite (Zn x Ni 1 − x Fe 2 O 4 ) is investigated under density functional theory (DFT) within the DFT + U approximation for x ≤ 0.50, with particular interest in understanding the effect of Zn on the net magnetization. Using as a reference ZnFe 2 O 4 , the localization of the Zn d band is proved to have a large impact on the preference for Zn to occupy either tetrahedral (Zn A ) or octahedral (Zn B ) coordination sites, which in ZnFe 2 O 4 is equivalent to the relative stability of the direct and inverse spinel forms. This affects the lattice volume, with Zn A favoring larger lattice expansions. Additional important consequences emerge on the magnetism of the system, as Zn A and Zn B alter the balance of atoms at the magnetic sublattices in a different way: while Zn A enhances the global magnetization by reducing the minority spin contribution, the opposite occurs for Zn B . On the other hand, the dominant magnetic exchange interactions are not significantly altered by Zn independently of its distribution, while the magnetic anisotropy of soft NiFe 2 O 4 is further weakened. Our simulations support the presence of a significant ratio of Zn atoms at octahedral positions at Zn x Ni 1 − x Fe 2 O 4 , mainly as the Zn concentration increases, putting limits to the ability to increase the magnetization of NiFe 2 O 4 by Zn substitution.

Abstract The magnetic properties of $${\text{SrFe}}_{12}{\text{O}}_{19}$$ SrFe 12 O 19 , a paradigmatic hexaferrite for permanent magnet applications, have been addressed in detail combining density functional theory including spin–orbit coupling and a Hubbard U term with Monte Carlo simulations. This multiscale approach allows to estimate the Néel temperature of the material from ab initio exchange constants, and to determine the influence of different computational conditions on the magnetic properties by direct comparison versus available experimental data. It is found that the dominant influence arises from the choice of the Hubbard U term, with a value in the 2–3 eV range as the most adequate to quantitatively reproduce the two most relevant magnetic properties of this material, namely: its large perpendicular magnetocrystalline anisotropy and its elevated Néel temperature.

We address the detailed description of the magnetic properties of the (001) and (111) interfaces between the hard CoFe 2 O 4 (CFO) and soft NiFe 2 O 4 spinel ferrites, used in bicomponent systems for permanent magnet applications. The similarity between the electronic properties, magnetic order and lattice structure of both oxides allows us to isolate the effect of ideal interface formation on the magnetism of the combined system based on density functional theory. While the magnetic moments and long-range magnetic order preserve bulk-like values, the magnetic anisotropy is reduced by the presence of the interface, and depends on the specific termination. Partial inversion of CFO is also relevant, as it enhances the net magnetization and alters both the magnitude and spatial dependence of the magnetic anisotropy.

A beyond mean-field study of the Tavis-Cummings (TC) model is developed. This is the simplest model for describing the interaction of a radiation field with a system composed by an array of two-level atoms. The first correction to the mean-field ground state energy and the energy gap between the ground and the first excited states are computed. For the ground-state energy our result improves the mean-field calculation, as expected. For the gap, that cannot be calculated at mean-field level, a drop down to zero is obtained at the critical point where the system undergoes a second-order quantum phase transition.

Platelets of strontium hexaferrite (SrFe 12 O 19 , SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulations.