Magnetism, Volume 4, Issue 3 (September 2024) – 9 articles

This work describes an automated pilot plant for the extraction of rare-earth (RE) permanent magnets from computer hard-disk drives (HDDs), demonstrating a commercially viable way to exploit these abundant sources of end-of-life (EOL) magnets. A mobile approach is provided for the on-site destruction of the HDDs in server farms, in compliance with the European Data Protection Regulation (GDPR), enabling both separation of the magnets and automated shredding of the data carrier. This fully automated process identifies (both optically and magnetically) the location of the rare-earth magnets and cuts off the corner of the hard drive containing the rare-earth material in the voice coil motor. This allows for a significant reduction in magnet extraction time (6 s per HDD) compared to previously reported semi-automated (2 min) and manual (5 min) dismantling times. This work will also help to transfer the experience gained in the mobile pilot plant to other future sources of EOL materials such as drive motors and mixed electronic scrap. Full article

We investigate the emergence of magnetic skyrmion crystals with swirling topological spin textures in itinerant magnets with an emphasis on momentum-resolved multi-spin interactions. By performing the simulated annealing for the effective spin model with the two-spin and four-spin interactions on a two-dimensional triangular lattice, we show that various types of four-spin interactions become the microscopic origin of the magnetic skyrmion crystal with the skyrmion numbers of one and two. We find that the four-spin interactions between the different wave vectors lead to the skyrmion crystal with the skyrmion number of one, whereas those at the same wave vectors lead to the skyrmion crystals with the skyrmion number of one and two. Our results indicate that the multi-spin interactions arising from the itinerant nature of electrons provide rich topological spin textures in magnetic metals. Full article

NiO nanoparticles have garnered significant interest due to their diverse applications and unique properties, which differ markedly from their bulk counterparts. NiO nanoparticles are p-type semiconductors with a wide bandgap, high discharge capacity, and high carrier density, making them ideal for use in batteries, sensors, and catalysts. Their ability to generate reactive oxygen species also imparts disinfectant and antibiotic properties. Additionally, the higher Néel temperature of NiO compared with other antiferromagnetic materials makes it suitable for high-temperature applications in spintronic devices and industrial settings. This review focuses on the critical role of structure and composition in determining the magnetic properties of NiO nanoparticles. It examines how finite-size surface effects, morphology, crystallinity, and nickel distribution influence these properties. Fundamental physical properties and characterization techniques are discussed first. Various synthesis methods and their impact on NiO nanoparticle properties are then explored. Their magnetic phenomenology is examined in detail, highlighting the effects of finite size, particle composition and surface, and crystal quality. The review concludes with a summary of key insights and future research directions for optimizing NiO nanoparticles in technological applications. Full article

This work presents an investigation of the degradation of paracetamol via heterogeneous photocatalysis, aiming to magnetically immobilize the catalyst in a continuous process. Catalyst immobilization was conducted on aggregated flower-like structures. The CoFe₂O₄@Nb₅O₂ catalyst was characterized using a Vibrating Sample Magnetometer (VSM). The effects of the magnetic immobilization of the catalyst, flow, residence time, adsorption, and photolysis were evaluated. Additionally, catalyst reuse cycles were analyzed. The results indicated that a longer residence time favors the degradation of paracetamol due to the increase in the contact time of the effluent catalyst. At a flow rate of 20 mL·min⁻¹, a degradation of 27% was obtained. Photolysis and adsorption tests indicated that residence time was not an important factor for paracetamol degradation. For the photolysis test, in the first cycle, the values obtained were in the range of 6.0–8.5%. The adsorption results indicated ~10% removal. Full article

In this article, a comprehensive review is presented of recent technological advancements utilizing electromagnetic sensors in the microwave range for detecting human vital signs and lung water levels. With the main objective of improving detection accuracy and system robustness, numerous advancements in front-end architecture, detection techniques, and system-level integration have been reported. The benefits of non-contact vital sign detection have garnered significant interest across a range of applications, including healthcare monitoring and search and rescue operations. Moreover, some integrated circuits and portable systems have lately been shown off. A comparative examination of various system architectures, baseband signal processing methods, system-level integration strategies, and possible applications are included in this article. Going forward, researchers will continue to focus on integrating radar chips to achieve compact form factors and employ advanced signal processing methods to further enhance detection accuracy. Full article

In 1971, Chua defined an ideal memristor that links magnetic flux φ and electric charge q. In a magnetic lump with a current-carrying conductor, we found that the direct interaction between physical magnetic flux φ and physical electric charge q is memristive by nature in terms of a time-invariant φ-q curve being nonlinear, continuously differentiable and strictly monotonically increasing. Although we succeeded in demonstrating that the “ideal/real/perfect/… memristor” needs magnetism, the structure still suffers from two serious limitations: 1. a parasitic “inductor” effect and 2. bistability and dynamic sweep of a continuous resistance range. Then, we discussed how to overcome these two limitations to make a fully functioning ideal memristor with multiple or an infinite number of stable states and no parasitic inductance. We then gave a number of innovations to the current memristor structure, such as an “open” structure, nanoscale size, magnetic materials with cubic anisotropy (or even isotropy) and sequential switching of the magnetic domains. Contrary to the conjecture that “an ideal memristor may not exist or may be a purely mathematical concept”, we remain optimistic that an ideal memristor will be discovered in nature or will be made in the laboratory. Our finding of the memristive flux–charge interaction may advance the development and application of the memristor technology. Full article

Spin models like the Heisenberg Hamiltonian effectively describe the interactions of open-shell transition-metal ions on a lattice and can account for various properties of magnetic solids and molecules. Numerical methods are usually required to find exact or approximate eigenstates, but for small clusters with spatial symmetry, analytical solutions exist, and a few Heisenberg systems have been solved in closed form. This paper presents a simple, generally applicable approach to analytically solve isotropic spin clusters, based on adapting the basis to both total spin and point group symmetry to factor the Hamiltonian matrix into sufficiently small blocks. We demonstrate applications to small rings and polyhedra, some of which are straightforward to solve by successive spin-coupling for Heisenberg terms only; additional interactions, such as biquadratic exchange or multi-center terms necessitate symmetry adaptation. Full article

The exact formulae for calculating the demagnetizing factors of a general ellipsoid along the three main axes a ≥ b ≥ c have been long known. According to these formulae, the demagnetizing factors depend only on the axial ratios b/a and c/a. Although the calculation of the demagnetizing factors is a straightforward task, the calculation itself is not a simple one. Therefore, tabular and graphical representations of these demagnetizing factor data have also been presented which can then be used for approximating the demagnetizing factors of a rectangular ferromagnetic slab with the same axial ratios. It turned out in our recent study, however, that, in some ranges of axial ratios (e.g., for very small c/a values), the available tables and graphs do not provide sufficient resolution for obtaining the demagnetizing factors with reasonable accuracy. It was decided to calculate these missing values, and they are presented here in both tabular and graphical form by giving instructions for how to obtain conveniently further interpolated data. In addition, the previous and current demagnetizing factor data have been replotted and fitted to a polynomial function with high accuracy. The functional form of these fitting polynomials is presented in a table for the whole range of the axial ratios b/a and c/a. By graphically displaying these functions, one can obtain, in a relatively simple manner, the demagnetizing factors of a general ellipsoid with known axial ratios without the need to directly calculate through the exact formulae. This may be helpful in obtaining a quick estimate for the demagnetizing factors of any rectangular ferromagnetic slab of interest. Full article

Multipolar bonded magnets based on a thermoset matrix provide the opportunity to expand the applications of bonded magnets, especially within the drive technology industry, in terms of the high thermal and chemical resistance, along with a higher utilisation of the magnetic potential. To realize the application of polymer bonded magnets based on thermosets within the drive technology industry, general design parameters in terms of the material, the process parameters, and the tool concept are needed. These allow for a fundamental realization of multipolar bonded magnets with complex geometries in drive technologies, based on thermosets as the matrix material. This paper investigates the impact of the material (matrix material and filler grade), the process conditions (holding pressure (p_h) and heating time (t_h)), and the tool concept (gating position and system, sleeve material, pole division, and sample thickness) on the magnetic properties in terms of the remanence (B_R) and the deviation (Δs) of the pole division, as well as the orientation of the fillers in the middle of the pole and at the pole pitch. For each parameter, an optimised value is derived. In the majority of the cases, this value is equal in terms of the magnetic properties and the orientation. In terms of the sleeve material and the sample thickness, the ideal value differs between the two criteria. Therefore, an optimised value for each criterion, as well as an overall value, is defined. In terms of the material, PF, along with a high filler grade; in terms of the process conditions, a high holding pressure (p_h) and a low heating time (t_h); and in terms of the tool concept, a two-pinpoint gating system, located in the middle of the pole, a Ferro-Titanit-Cromoni sleeve material, a high pole division, as well as small sample thickness, should be selected to improve the properties of polymer bonded magnets based on thermosets. Full article