Magnetism, Volume 2, Issue 1 (March 2022) – 6 articles

Maxwell’s equations and the Lorentz force equation form the foundation of classical electromagnetic theory and their discovery led to the development of special relativity. Despite this achievement, their universal compatibility with the conservation of momentum and relativistic energy transformations is still debated. Incorporating effects of hidden momentum with the Lorentz force equation or using the Einstein–Laub formula are two common approaches to address some of these concerns. Which method to use, or if a change to classical electromagnetism is even required, remains controversial. A new theoretical approach is presented in this paper to address this using relativistic electromagnetic energy inertial frame transformations. These transformations identify a situation where an apparent violation of conservation laws could occur and how to consolidate this with electromagnetic theory. An explanation regarding the elementary nature of magnetism and the relationship between inertia and electromagnetic energy is also commented on. Full article

The rare earth-free Mn₅Ge₃ compound shows magnetocaloric properties similar to those of pure Gd; therefore, it is a good candidate for magnetic refrigeration technology. In this work, we investigate the influence of chemical substitution on the crystal structure and the magnetic, thermodynamic, and magnetocaloric properties of a polycrystalline Mn₅Ge₃ compound prepared by induction melting. For this purpose, we replaced 5% of the Mn with Cr or Co and 5% of the Ge with B or Al. The additional chemical elements were shown not to change the crystal structure of the parent compound (space group P6₃/mcm, No. 193). In the case of the magnetic properties, all samples remained ferromagnetic with the ordering temperature (T_C) lower than for the original compound (T_C = 295(1) K). The exception was the sample with B, where we observed an increase in T_C by 3 K. The maximum value of the magnetic entropy change, |∆S_m|^MAX (for a magnetic field change of 5 T), decreased from 7.1(1) for Mn₅Ge₃ to 6.2(1), 6.8(1), 4.8(1), and 5.8(1) J kg⁻¹ K⁻¹ for the alloys with B, Al, Cr, and Co, respectively. The adiabatic temperature change (∆T_ad) (for a magnetic field change of 1 T) was determined from the specific heat measurements and was equal to 1.1(1), 1.2(1), 1.2(1), 0.8(1), and 0.8(1) K for Mn₅Ge₃, Mn₅Ge_2.85B_0.15, Mn₅Ge_2.85Al_0.15, Mn_4.75Cr_0.25Ge₃, and Mn_4.75Co_0.25Ge₃, respectively. The obtained data were compared with those from the literature. It was found that the substitution allowed for tuning of the ordering temperature in a wide temperature range. At the same time, the reduction in the magnetocaloric parameters’ values was relatively small. Therefore, the produced Mn₅Ge₃-based alloys allow for the expansion of the operation temperature range of the parent compound as a magnetocaloric material. Full article

Stoichiometric compositions of NiO were prepared by the standard chemical co-precipitation method to inspect the effect of the calcination temperature on structures, morphology, and physical properties. The samples were calcined at three different temperatures viz. 350 °C, 550 °C, and 650 °C for 5 h. X-ray diffraction analysis confirmed the cubic (Fm-3m) structure of the prepared samples. The average crystalline size increases from 41 nm to above 100 nm as the calcination temperature increases in the same time period. In Fourier transform infrared spectra, the spectral absorption bands were observed at ~413, 434, and 444 cm^–1. The bandgap energy of NiO particles is decreased from 3.6 eV to 3.41 eV as the calcination temperature increases. The magnetic analysis confirms that the magnetization value of NiO is invariably decreased with a rise in the calcination temperature. Full article

We report magnetism of tetragonal β-Fe₃Se₄ nanoplates controllably synthesized by thermal decomposition at 603 K of inorganic–organic (β-Fe₂Se₃)₄[Fe(tepa)] hybrid nanoplates (tepa = tetraethylenepentamine). (β-Fe₂Se₃)₄[Fe(tepa)] hybrid precursor and β-Fe₃Se₄ nanoplates are in single crystal features as characterized by selected area electron diffraction. Rietveld refinements reveal that ordered inorganic–organic (β-Fe₂Se₃)₄[Fe(tepa)] hybrid nanoplates are in a tetragonal layered crystal structure with a space group of I4cm (108) and room-temperature lattice parameters are a = 8.642(0) Å and c = 19.40(3) Å, while the as-synthetic tetragonal β-Fe₃Se₄ nanoplates have a layered crystal structure with the P4/nmm space group, and room-temperature lattice parameters are a = 3.775(8) Å and c = 5.514(5) Å. Magnetic measurements show the weak ferrimagnetism for (β-Fe₂Se₃)₄[Fe(tepa)] hybrid nanoplates at room temperature, while the as-synthetic β-Fe₃Se₄ nanoplates are antiferromagnetic in a temperature range between 120 and 420 K but in a ferrimagnetic feature below ~120 K. The as-synthetic β-Fe₃Se₄ nanoplates are thermally instable, which are transformed to ferrimagnetic β-Fe₃Se₄ nanoplates by annealing at 623 K (a little higher than the synthetic temperature). There is an irreversible change from antiferromagnetism of the as-synthetic β-Fe₃Se₄ phase to the ferrimagnetism of the as-annealed β-Fe₃Se₄ phase in a temperature between 420 and 470 K. Above 470 K, the tetragonal β-Fe₃Se₄ phase transforms to monoclinic Fe₃Se₄ phase with a Curie temperature (T_C) of ~330 K. This discovery highlights that crystal structure and magnetism of Fe-Se binary compounds are highly dependent on both their phase compositions and synthesis procedures. Full article

An analytical approach for computing the coefficient of refrigeration performance (CRP) was described for materials that exhibited a giant inverse magnetocaloric effect (MCE), and their governing thermodynamics were reviewed. The approach defines the magnetic work input using thermodynamic relationships rather than isothermal magnetization data discretized from the literature. The CRP was computed for only cyclically reversible temperature and entropy changes in materials that exhibited thermal hysteresis by placing a limit on their operating temperature in a thermodynamic cycle. The analytical CRP serves to link meaningful material properties in first-order MCE refrigerants to their potential work and efficiency and can be employed as a metric to compare the behaviors of dissimilar alloy compositions or for materials design. We found that an optimum in the CRP may exist that depends on the applied field level and Clausius–Clapeyron (CC) slope. Moreover, through a large literature review of NiMn-based materials, we note that NiMn(In/Sn) alloys offer the most promising materials properties for applications within the bounds of the developed framework. Full article

Thin films of magnetic topological insulators (TIs) are expected to exhibit a quantized anomalous Hall effect when the magnetizations on the top and bottom surfaces are parallel and a quantized topological magnetoelectric effect when the magnetizations have opposite orientations. Progress in the observation of these quantum effects was achieved earlier in the films with modulated magnetic doping. On the other hand, the molecular-beam-epitaxy technique allowing the growth of stoichiometric magnetic van der Waals blocks in combination with blocks of topological insulator was developed. This approach should allow the construction of modulated heterostructures with the desired architecture. In the present paper, based on the first-principles calculations, we study the electronic structure of symmetric thin film heterostructures composed of magnetic MnBi${}_2$Se${}_4$ blocks (septuple layers, SLs) and blocks of Bi${}_2$Se${}_3$ TI (quintuple layers, QLs) in dependence on the depth of the magnetic SLs relative to the film surface and the TI spacer between them. Among considered heterostructures we have revealed those characterized by nontrivial band topology. Full article