Search

International audience; The previously accepted notion that the spontaneous magnetization of Sm2Fe17 lies in the basal plane of the crystal is true only approximately, and then only around room temperature. At low temperatures the magnetization, whose orientation is not fixed by the symmetry, is found to deviate from the basal plane by as much as 10 degrees. The threefold symmetry axis is a hard direction; to magnetize the crystal in this direction, a magnetic field of about 9 T is required. The hard-axis magnetization arrives at saturation discontinuously, by way of a first-order phase transition. The behavior is a general one for trigonal ferromagnets where K-1 < 0 and the leading trigonal anisotropy constant is nonzero, K-2' not equal 0. Although of universal occurrence, the first-order transition is only visible at low temperatures, where it is accompanied by a magnetization anomaly of sufficient size.

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., 15 pages 17 figures

We study the dependence of the magnetocaloric effect on the magnetic field-change-rate the first order magnetostructural transition in Mn3GaC by measuring the adiabatic temperature change ΔT at three different time scales: 11 mTs-1, 700 mTs-1, and ~1000 Ts-1. We find that the Maximum adiabatic temperature-change of about 5 K is reached in the 11 mTs-1 and 700 mTs-1 rates, whereas for the ~1000 Ts-1 the transition lags the change in the magnetic field so that the maximum adiabatic temperature-change is not attained.

We have explored, computationally and experimentally, the magnetic properties of (Fe1-xCox)(2)B alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co2B, 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 nonmonotonic behavior 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 substitutionally doping the transition metal sublattice by the whole range of 5d 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 (Fe0.7Co0.3)(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.

Fully hydrogenated Mn containing LaFeSi alloys provide long time stability of attractive magnetocaloric properties; issues of previously reported induced phase co-existence can be avoided. In this paper, two series LaFe11.8 xSi1.2Mnx and LaFe11.6 xSi1.4Mnx (x = 0, 0.1, 0.2, 0.3, 0.4), before and after hydrogenation are presented. In the non-hydrogenated parent samples, microstructural evolution and phase formation with increasing Mn content are studied. It is found that materials combination within the sample series can provide constant MCE over a large working temperature range. As a peculiarity, the hydrogenated compounds show a different dependence of the transition temperature as a function of Si content than the parent compounds. This behaviour is explained by comparing the amount of hydrogen incorporated in samples with different Si content. Moreover, an estimation for the maximum adiabatic temperature change and optimal magnetic field is given exemplarily for a non-hydrogenated and a hydrogenated compound.

es hat kein Abstract vorgelegen

We calculated magnetic exchange constants and magnetocaloric properties of Ni2Mn1−xCuxGa Heusler alloys by ab initio and Monte Carlo methods. The ab initio study of the influence of the Cu excess x on the strength of magnetic interactions revealed that Cu weakens Mn-Ni interaction and has a complex impact on the Mn-Mn interactions. Theoretically calculated magnetic phase diagram of Ni2Mn1−xCuxGa is in a good agreement with available experimental data. Calculated by the Monte Carlo method the isothermal magnetic entropy change DSmag in a Ni2Mn0.75Cu0.25Ga alloy is significantly smaller around the coupled magnetostructural phase transition temperature than the reported earlier experimental DSmag. This discrepancy is ascribed to an overestimation of the experimental DSmag at the magnetostructural phase transition. Theoretically determined adiabatic temperature change DTad in Ni2Mn0.75Cu0.25Ga agrees well with DTad measured experimentally by a direct method.

Strips of cold-rolled gadolinium have been produced starting from an ingot of 99.9%-pure metal. The rolling has been carried out in a particularly severe regime, the thickness has been reduced 550 times. The rolled strips feature significantly (several times) diminished DTDT and DSDS effects, as well as a very large induced magnetic anisotropy, estimated to be between 10 and 15 MJ/m3.

The magnetization of a Ho2Fe17 single crystal has been measured along the principal crystallographic directions in pulsed magnetic fields up to 60 T. Stepwise discontinuities in the magnetization occur at 45 and 55 T along the [120] and [100] directions, respectively. The data allowed us to deduce the molecular field at the Ho site. As a cross check, the molecular field was determined as well from a magnetization measurement when the Ho2Fe17 single crystal was let rotate freely. Both values are in good agreement with each other.

Исследована кинетика роста новых фаз в ходе индуцированного водородом прямого фазового превращения в сплаве Sm2Fe17. Показано, что развитие прямого фазового превращения замедляется с понижением температуры превращения. Анализ кинетики роста новых фаз в ходе индуцированных водородом прямого фазового превращения в сплаве Sm2Fe17 позволил установить, что процесс роста новых фаз в исследованном интервале температур контролируется диффузией больших атомов (Fe, Sm) компонентов сплава. There kinetics of growth of new phases during hydrogen-induced direct phase transformation in Sm2Fe17 alloy was analysed. It is shown that evolution of direct phase transformation decreases with temperature transformation decreasing. Analysis of kinetics of growth of new phases during hydrogeninduced direct phase transformations in Sm2Fe17 alloy allowed to establishe that the process of new phases growth in analysed temperature interval is controlled by diffusion of big atoms of alloy’s components (Fe, Sm). Рыбалка Сергей Борисович – кандидат физико-математических наук, доцент, кафедра физики, физико-металлургический факультет, Донецкий национальный технический университет. e-mail: rybalka@rambler.ru. Додонова Елена Викторовна – магистр, аспирант, кафедра физики, физико-металлургический факультет, Донецкий национальный технический университет. Скоков Константин Петрович – кандидат физико-математических наук, доцент, кафедра магнетизма, физико-технический факультет, Тверской государственный университет. Rybalka Sergey Borisovich is Cand. Sc. (Physics and Mathematics), Associate Professor, Physics Department, Donetsk National Technical University. e-mail: rybalka@rambler.ru. Dodonova Elena Victorovna is a master student, Post-graduate Student, Physics Department, Physics and Metallurgic Faculty, Donetsk National Technical University. Skokov Konstantin Petrovich is Cand. Sc. (Physics and Mathematics), Associate Professor, Magnetism Department, Physics and Technical Faculty, Tver State University.

Most of the modern permanent magnet materials are based on the combination of a 3d transition metal (T) and a rare earth (R). The inter-sublattice exchange field binds the high magnetic moment of the former with the high anisotropy of the latter, either ferri- or ferromagnetically. Applying high external magnetic field one can break down the ferrimagnetic ground state and drive the system towards ferromagnetic order via a non-collinear intermediate phases. This manifests itself in peculiarities of the magnetization curve. In some cases the value of the exchange field can be extracted form those experiments [1, 2]. Here we report on a magnetization measurement on single crystals of Ho2Fe17 in the fields up to 60 Tesla. Magnetization measured in the basal plane shows jumps. The highest one, measured along [100] is observed at about 55 Tesla . Considering that both R and T moments rotation is constrained within the basal plane we describe observed curve shape.

The critical parameters of a first-order magnetization process, Hcr and mcr, are used to determine the anisotropy constants K1 and K2 of Tb2Fe17 in the temperature range between 5 and 300 K. Both anisotropy constants are proportional to the critical field Hcr (times spontaneous magnetization), whereas their ratio is independent of Hcr and depends solely on the dimensionless reduced critical magnetization mcr. The employed method of determining K1 and K2 is complementary to the classical Sucksmith–Thompson technique, which is inapplicable in the case of a first-order magnetization process.

If the easy magnetization direction of a 3d-4f ferrimagnet is perpendicular to a high-symmetry axis, a magnetic field applied in the easy direction may induce a number of first-order transitions, the first (lowestfield ) one of which carries the information sufficient for an unambiguous determination of the intersublattice molecular field. This idea has been used to find the molecular field in Er2Fe17. To this end, magnetization curves have been measured in pulsed magnetic fields of up to 50 T applied along [100] or [001]. In order to obtain a reference value of the molecular field by a conventional method, high-field measurements have also been performed on crystals free to rotate. The molecular fields determined by both techniques are in good agreement with each other as well as with the values deduced from literature data.