Численное моделирование циклов магнитного охлаждения Брайтона

Была разработана компьютерная модель работы магнитного холодильника, работающего по циклу Брайтона при температурах вблизи комнатных. Данная модель использовалась для расчета теоретического предела рабочей разности температур и мощности охлаждения. Мощность охлаждения была рассчитана для цикла Брайтона с одним и двумя рабочими телами, в которых в качестве рабочих тел использовался редкоземельный металл гадолиний. Полученные результаты наглядно демонстрируют функциональные диапазоны магнитных холодильников, работающих по циклу Брайтона

Phase transformations and thermodynamic properties of nanocrystalline FePt powders

The solid state reactions and structural evolution in nanocrystalline FePt powders during mechanical ball milling at 77 K and subsequent annealing have been investigated. Above 310 °C the formation of L10 FePt is observed. As the milling time increases, the enthalpy evolved during the transformations is reduced, whereas the corresponding activation energy increases. For accelerating the ordering process a fine lamellar structure of Fe and Pt is favourable

The influence of magnetocrystalline anisotropy on the magnetocaloric effect: A case study on Co 2B

The influence of magnetocrystalline anisotropy on the magnetocaloric effect (MCE) was studied on single crystals of CoB and compared to measurements on polycrystalline samples. Large differences in adiabatic temperature change Δ T a d and magnetic entropy change Δ S M were found along the different crystallographic directions. The magnetocaloric effect differs by 40% in the case of Δ T a d in a field change of 1.9 T when applying the field along the hard axis and easy plane of magnetization. In the case of Δ S M, the values differ 50% and 35% from each other in field changes of 1 and 1.9 T, respectively. It was found that this anisotropy effect does not saturate in fields up to 4 T, which is higher than the anisotropy field of CoB ( ≈2 T). A simple model was developed to illustrate the possible effect on magnetocrystalline anisotropy, showing large differences especially in application relevant fields of about 1 T. The results strongly suggest that the MCE could be maximized when orienting single crystalline powders in an easy axis parallel to the applied field in active magnetocaloric regenerator structures, and therefore the overall device efficiency could be increased.Unión Europea FP7/2007-2013DRREAM No. 310748DAAD A/13/09434MINECO EU-FEDER MAT2013-45165-P MAT2016-77265-RNUST MISiS No. K4-2015-01

Magnetic properties of (Fe1−x_{1-x}Cox_x)2_2B alloys and the effect of doping by 5dd elements

We have explored, computationally and experimentally, the magnetic properties of \fecob{} alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co$_2$B, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between $x=0.1$ and $x=0.5$. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated non-monotonous temperature behaviour is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutional doping the transition metal sublattice by the whole range of 5$d$ transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5~at.\% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50\% compared to its parent (Fe$_{0.7}$Co$_{0.3}$)$_2$B compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.Comment: 15 pages 17 figure

Evolution of magnetic and microstructural properties of thick sputtered NdFeB films with processing temperature

Ta (100 nm) / NdFeB (5 $\mu$m) / Ta (100 nm) films have been deposited onto Si substrates using triode sputtering (deposition rate ~ 18 $\mu$m/h). A 2-step procedure was used : deposition at temperatures up to 400 C followed by ex-situ annealing at higher temperatures. Post-deposition annealing temperatures above 650 C are needed to develop high values of coercivity. The duration of the annealing time is more critical in anisotropic samples deposited onto heated substrates than in isotropic samples deposited at lower temperatures. For a given set of annealing conditions (750 C/ 10'), high heating rates (≥ 2000 C / h) favour high coercivity in both isotropic and anisotropic films. The shape and size of Nd2Fe14B grains depend strongly on the heating rate