Magnetotransport in Tb2Fe17 single crystals

We have performed measurements of the Hall effect, electrical resistivity, and magnetization on Tb2Fe17 single crystals in the 5 to 300 K temperature range, and in magnetic fields of up to 5 T. We find that the anomalous Hall effect of this ferromagnet depends strongly on the magnetization direction relative to the crystal axes. The AHE resistivity, measured with an applied magnetic field H perpendicular to the c-axis, is very large and varies linearly with the longitudinal resistivity ρ, whereas the AHE resistivity for H along the hard magnetization direction is much smaller and increases as ρ2. For the latter configuration, the electrical resistivity shows a sharp decrease at a field-induced first-order magnetization process (FOMP) which is observed in H ~ 2.7 T up to a temperature of 250 K.This work was supported in part by grant MAT2008/03074 from the Spanish Ministerio de Ciencia. Additional support from Diputación General de Aragón (DGA-IMANA) is also acknowledged.Peer Reviewe

Effect of electric current annealing in phase transition of Mn-Al alloy

The electronic structure of any material can be modified when it is exposed to high density electric currents or high strength electric fields, caused by the increased electronic/ionic mobility. Electromigration effects can have desirable uses (1), but also be a problem, for example in IC circuit design (2). However, the increased electronic/ionic mobility can be used to tailor the material properties by modifying e.g. phase formation, phase stability, density of defects etc. Our goal is to understand, by theoretical (DFT calculation) and experimental approaches, and utilize these effects in the processing of hard magnetic materials and to quantify the influence of the electric current on microstructure and magnetic properties. Please click Additional Files below to see the full abstract

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

Analysis of the magnetocaloric effect in Heusler alloys: study of Ni50CoMn36Sn13 by calorimetric techniques

This is an open access article distributed under the Creative Commons Attribution License.Direct determinations of the isothermal entropy increment, −ΔST, in the Heusler alloy Ni50CoMn36Sn13 on demagnetization gave positive values, corresponding to a normal magnetocaloric effect. These values contradict the results derived from heat-capacity measurements and also previous results obtained from magnetization measurements, which indicated an inverse magnetocaloric effect, but showing different values depending on the technique employed. The puzzle is solved, and the apparent incompatibilities are quantitatively explained considering the hysteresis, the width of the martensitic transition and the detailed protocol followed to obtain each datum. The results show that these factors should be analyzed in detail when dealing with Heusler alloys.Financial support from Projects MAT2011-23791, MAT2013-44063-R and MAT2014-53921-R from the Spanish MEC, DGA Consolidated Groups E100 and E34, RFBR 12-07-00676-a, RF President MD-770.2014.2, RSF 14-12-00570 and from the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of MISiS are acknowledged.We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).Peer Reviewe

Analysis of the magnetocaloric effect in Heusler alloys: Study of Ni50CoMn36Sn13 by calorimetric techniques

Direct determinations of the isothermal entropy increment, -¿ST , in the Heusler alloy Ni50CoMn36Sn13 on demagnetization gave positive values, corresponding to a normal magnetocaloric effect. These values contradict the results derived from heat-capacity measurements and also previous results obtained from magnetization measurements, which indicated an inverse magnetocaloric effect, but showing different values depending on the technique employed. The puzzle is solved, and the apparent incompatibilities are quantitatively explained considering the hysteresis, the width of the martensitic transition and the detailed protocol followed to obtain each datum. The results show that these factors should be analyzed in detail when dealing with Heusler alloys

Magnetic properties of Nd6Fe13Cu single crystals

The understanding of coercivity mechanism in high performance Nd-Fe-B permanent magnets relies on the analysis of the magnetic properties of all phases present in the magnets. By adding Cu in such compounds, a new Nd6Fe13Cu grain boundary phase is formed, however, the magnetic properties of this phase and its role in the magnetic decoupling of the matrix Nd2Fe14B grains are still insufficiently studied. In this work, we have grown Nd6Fe13Cu single crystals by the reactive flux method and studied their magnetic properties in detail. It is observed that below the N\'eel temperature (TN = 410 K), the Nd6Fe13Cu is antiferromagnetic in zero magnetic field; whereas when a magnetic field is applied along the a-axis, a spin-flop transition occurs at approx. 6 T, indicating a strong competition between antiferromagnetic and ferromagnetic interactions in two Nd layers below and above the Cu layers. Our atomistic spin dynamics simulation confirms that an increase in temperature and/or magnetic field can significantly change the antiferromagnetic coupling between the two Nd layers below and above the Cu layers, which, in turn, is the reason for the observed spin-flop transition. These results suggest that the role of antiferromagnetic Nd6Fe13Cu grain boundary phase in the coercivity enhancement of Nd-Fe-B-Cu magnets is more complex than previously thought, mainly due to the competition between its antiferro- and ferro-magnetic exchange interactions.Comment: 15 pages, 4 figure

Reversibility of minor hysteresis loops in magnetocaloric Heusler alloys

The unavoidable existence of thermal hysteresis in magnetocaloric materials with a first-order phase transition is one of the central problems limiting their implementation in cooling devices. Using minor loops, however, allows achieving significant cyclic effects even in materials with relatively large hysteresis. Here, we compare thermometric measurements of the adiabatic temperature change Delta T-ad and calorimetric measurements of the isothermal entropy change Delta S-T when moving in minor hysteresis loops driven by magnetic fields. Under cycling in 2 T, the Ni-Mn-In-Co Heusler material provides a reversible magnetocaloric effect of Delta S-T(rev) = 10.5 J kg(-1) K-1 and Delta T-ad(rev) = 3.0 K. Even though the thermodynamic conditions and time scales are very different in adiabatic and isothermal minor loops, it turns out that after a suitable scaling, a self-consistent reversibility region in the entropy diagram is found. This region is larger than expected from basic thermodynamic considerations based on isofield measurements alone, which opens new opportunities in application. Published by AIP Publishing

Tuning magnetocrystalline anisotropy of Fe3_{3}Sn by alloying

The electronic structure, magnetic properties and phase formation of hexagonal ferromagnetic Fe$_{3}$Sn-based alloys have been studied from first principles and by experiment. The pristine Fe$_{3}$Sn compound is known to fulfill all the requirements for a good permanent magnet, except for the magnetocrystalline anisotropy energy (MAE). The latter is large, but planar, i.e. the easy magnetization axis is not along the hexagonal c direction, whereas a good permanent magnet requires the MAE to be uniaxial. Here we consider Fe$_{3}$Sn$_{0.75}$M$_{0.25}$, where M= Si, P, Ga, Ge, As, Se, In, Sb, Te and Bi, and show how different dopants on the Sn sublattice affect the MAE and can alter it from planar to uniaxial. The stability of the doped Fe$_{3}$Sn phases is elucidated theoretically via the calculations of their formation enthalpies. A micromagnetic model is developed in order to estimate the energy density product (BH)max and coercive field $\mu_{0}$H$_{c}$ of a potential magnet made of Fe$_{3}$Sn$_{0.75}$Sb$_{0.25}$, the most promising candidate from theoretical studies. The phase stability and magnetic properties of the Fe$_{3}$Sn compound doped with Sb and Mn has been checked experimentally on the samples synthesised using the reactive crucible melting technique as well as by solid state reaction. The Fe$_{3}$Sn-Sb compound is found to be stable when alloyed with Mn. It is shown that even small structural changes, such as a change of the c/a ratio or volume, that can be induced by, e.g., alloying with Mn, can influence anisotropy and reverse it from planar to uniaxial and back