Search

The partially overlapped ferroelastic/martensitic and para-ferromagnetic phase transitions of a Ni$_{50.53}$Mn${33.65}$In$_{15.82}$ metamagnetic shape memory alloy have been studied from calorimetric, magnetic and acoustic emission measurement. We have taken advantage of the existence of thermal hysteresis of the first order ferroelastic/martensitic phase transition ($\sim2.5$K) to discriminate the latent heat contribution $\Delta$Ht = 7.21(15) kJ/kg and the specific heat contribution $\Delta$Hc = 216(1) J/kg to the total excess enthalpy of the phase transition. The specific heat was found to follow a step-like behavior at this phase transition. The intermittent dynamics of the ferroelastic/martensitic transition has been characterized as a series of avalanches detected both from acoustic emission and calorimetric measurements. The energy distribution of these avalanche events was found to follow a power law with a characteristic energy exponent $\epsilon\sim2$ which is in agreement with the expected value for a system undergoing a symmetry change from cubic to monoclinic. Finally, the critical behavior of the para-ferromagnetic austenite phase transition that takes place at $\sim 311$K has been studied from the behavior of the specific heat. A critical exponent $\alpha\sim0.09$ has been obtained, which has been shown to be in agreement with previous values reported for Ni-Mn-Ga alloys but different from the critical divergence reported for pure Ni., Comment: 18 pages, 11 figures

The influence of the fabrication method on the magnetostructural properties of a Ni55Fe19Ga26 Heusler alloy, obtained both as a ribbon, by melt-spinning, and as a pellet, by arc-melting, has been analyzed. It has been found that, while the arc-melting technique leads to the precipitation of the gamma phase and to a non-modulated martensite structure, the alloy prepared by melt-spinning presents a fully 14M modulated martensitic structure at room temperature. The tetragonal non-modulated martensite in the arc-melted bulk sample transforms into the 14M structure after a long thermal treatment (at 1073 K for 24 h) and subsequent quenching. Characteristic temperatures of the martensitic transformation are higher for melt-spun ribbons than for bulk sample, due to the precipitation of the gamma phase and consequent different martensite composition. However, while the martensitic transformation temperature is practically constant in the case of the bulk sample, it changes by 150 K in the case of the ribbon sample submitted to the same thermal treatment applied to bulk samples. Finally, it was found that the martensitic transformation occurs in the paramagnetic regime of both types of samples.

In this study, the non-isothermal kinetics of the martensitic transition from 14M modulated martensite to austenite phase in Ni55Fe19Ga26 ribbons obtained by melt-spinning has been analyzed. The proximity of the martensitic transition to room temperature makes it very sensitive to pressure and subtle differences for different pieces of the ribbon (ascribed to stresses stored in the ribbon during its rapid solidification process). Despite the dispersion in the characteristic parameters of the transition, a general behavior is observed with a decreasing activation energy as the heating rate increases due to the nucleation driven character of the transition. It has been shown that a first-order autocatalysis can describe the temperature evolution of the austenite fraction using only two experimental temperatures. Predicted curves are in good agreement with experimental data

The presence of a distribution of transition temperatures (DTT) is ubiquitous in materials science. It is common to ascribe deviations from theoretical pure-phase behavior to this fact. To adapt the di erent pure phase models to systems with a DTT, the parameters of such distribution must be known or at least estimated. In this review, the di erent sources for the existence of such distributions and their e ects on magnetothermal properties are summarized. In addition, di erent models proposed to extract the parameters of the corresponding DTT are discussed and extended, starting from Weiss model, to account for other phenomenologies. Experimental results on amorphous Fe-Nb-B and intermetallic MnCo(Fe)Ge systems are also reported

Devitrification of mechanically alloyed amorphous Fe70Zr30 at. % compound consists on a two-step process: amorphous → amorphous + bcc Fe + Fe2Zr → Fe2Zr + Fe23Zr6. This sequence is inferred from the evolution of the Mossbauer spectra, the thermomagnetic experiments and the X-ray diffraction (XRD) patterns. Hyperfine parameters for both intermetallics have been obtained from Mossbauer spectroscopy in correlation with the phase identification from XRD results. The broadening of the stable compositional range of Fe2Zr intermetallic above 1000 K is responsible for a strong dependence of the phase fractions on heating and cooling rates. Despite the overlapping of the two processes involved in the devitrification, the individual Avrami exponents of each one have been estimated.

Ribbons of a Ni55Fe19Ga26 Heusler alloy were prepared by melt spinning technique. The effect of pressure on magnetostructural transitions was studied comparing as-spun ribbons to ribbons previously submitted to an axial pressure and to powder obtained from manually grinded as-spun ribbons. The martensitic transformation present in the as-spun ribbon almost vanishes after pulverization. The structural change driven by mechanical treatment is a stress-induced intermartensitic transformation from a 14 M modulated to a non-modulated tetragonal (NM) structure. There is a progressive change from the structure of as-spun samples to that of the pulverized one in samples submitted to axial pressures as it increases.

We propose a combination of the Kolmogorov–Johnson–Mehl–Avrami nucleation and growth theory and the Bean–Rodbell model to describe the field-induced transition in LaFe11.6Si1.4 alloys. The approach is applied to a set of bulk samples undergoing first-order transitions produced by different routes and including doping effects. The kinetic analysis of both magnetization and demagnetization processes reveals a nucleation and three-dimensional interface-controlled growth for these alloys. Introducing the kinetic process between the metastable and stable solutions of the Bean–Rodbell model, the field dependence of the magnetization/demagnetization processes, including magnetic hysteresis for different magnetic field sweeping rates, is better reproduced than with the pure model.

A systematic study of the magnetic and magnetocaloric properties around the ferromagnetic-paramagnetic phase transition temperature have been carried out on La0·7Ca0·3Mn1-xNixO3 (x = 0, 0.02, 0.07, 0.10) compounds synthesized by ball milling from oxides and pure Ni. The order of the transition, analyzed by the Banerjee's criterion and the field dependence of the magnetic entropy change, was found to be of second order for all the studied compositions. The existence of small traces of β -MnO2 phase and the distortions produced during the milling process may induce a competition character of the ferromagnetic and the antiferromagnetic interactions at low fields and motivate the existence of a distribution of Curie temperatures. Once the antiferromagnetic contributions are avoided, the parameters of the distribution, the average Curie temperature T C ¯ and the broadening of the distribution Δ T C , have been obtained from the analysis of the approach to saturation curves and the magnetocaloric effect.

Production of the intermetallic MnCo0.8Fe0.2Ge phase, interesting as magnetocaloric material, is obtained by using an almost fully amorphous mechanically alloyed precursor. Thermal treatment to develop the intermetallic phase has been reduced in time (from several hours or even days to few minutes), and temperature (from 1123 K to 723 K). Thermomagnetic measurements allow us to analyze the magnetocaloric effect and to compare the results with those obtained from conventional annealing treatments. Close relationship between magnetocaloric properties and crystal size provides further optimization of the system preserving the advantages of the production method proposed.

In this work, we propose a model for using the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) crystallization theory to analyze nanocrystallization processes as a set of multiple microprocesses. This model is based on the well-known microstructure observations of nanocrystalline systems for which, although the number of crystallites increases along the process, the growth of each crystallite is limited to a much shorter time than the needed one for completing the process. The very low values of the Avrami exponent, n∼1, are well reproduced assuming a set of multiple classical JMAK processes with constant nucleation rate and diffusion controlled growth, n = 2.5 for each i individual microprocess. It is shown that the values of Avrami exponent experimentally observed and lying out of the theoretical range can be assumed as effective values derived from a complex process consisting of multiple microprocesses, which can be individually treated as classical ones., This work was supported by MINECO and EU FEDER (project MAT2013-45165-P), and the PAI of the Regional Government of Andalucía. A.F. Manchón-Gordón acknowledges a research internship funded by the Spanish Ministry of Education.

The magnetic anisotropy field distribution is discussed for Hitperm alloys annealed under different field conditions leading to different induced magnetic anisotropies: zero (ZF), transversal (TF), and longitudinal (LF) field annealing and compared to that of as-quenched (AQ) melt-spun amorphous ribbon. In order to accurately use the present method, the demagnetizing factor has been obtained by analyzing the field dependence of the inverse of the field derivative of the magnetization. The coherence of the analysis is supported by testing the normalization of the complete distribution of anisotropy fields. Independently of the composition, two groups can be distinguished among the studied samples: those with mainly perpendicular anisotropy field contributions (ZF and TF samples) and those with mainly longitudinal anisotropy field contributions (LF and AQ samples). Behavior of TF samples is well reproduced using Stoner-Wohlfarth model and, in the case of as-quenched amorphous samples, the anisotropy field depends almost linearly on the thickness of the ribbon., Work supported by the Spanish MINECO and EU FEDER (Project MAT 2013-45165-P), the PAI of the Regional Government of Andalucía and the Slovak projects APVV-0492-11, VEGA 2/0192/13. The Slovak team acknowledges also the support of the Operational Program “Research and Development” financed through European Regional Development Fund through the projects ITMS 26220120019 and ITMS 26220220061.

Amorphous Fe75Nb10B15 and (Fe70Co30)75Nb10B15 alloys were prepared by mechanical alloying from the elemental constituents and hot compacted at different temperatures within the supercooled liquid region. After compaction, the microstructure studied by X-ray diffraction shows an increase in the crystalline fraction for both compositions. Magnetic properties and magnetic entropy change of compacted samples of the Co-free alloy were enhanced with respect to the powder sample, meanwhile, a deterioration in these properties was observed for the Co containing alloy after compaction. Both changes could be ascribed to an enrichment in boron in the amorphous phase.

Amorphous Fe-Zr and Fe-Nb alloys with 70 at.% Fe have been prepared by ball milling a mixture of elemental powders. The combination of Mössbauer spectroscopy, X-ray diffraction, scanning electron microscopy and differential scanning calorimetry techniques supplies detailed information about the composition as well as microstructural parameters of remaining crystalline and amorphous phases developed during milling. Detailed analysis of the Fe environments allows us to distinguish Fe atoms in paramagnetic sites between those incorporated to crystalline Zr or Nb rich phases and those Fe atoms in amorphous phase., This work was supported by the Spanish MINECO and EU FEDER (Project MAT 2013-45165-P), AEI/FEDER-UE (Project MAT-2016-77265-R) and the PAI of the Regional Government of Andalucía. A.F. Manchón-Gordón acknowledges a VPPI-US fellowship from University of Sevilla.

The magnetocaloric effect and its most straightforward application, magnetic refrigeration, are topics of current interest due to the potential improvement of energy efficiency of cooling and temperature control systems, in combination with other environmental benefits associated to a technology that does not rely on the compression/expansion of harmful gases. This review presents the fundamentals of the effect, the techniques for its measurement with consideration of possible artifacts found in the characterization of the samples, a comprehensive and comparative analysis of different magnetocaloric materials, as well as possible routes to improve their performance. An overview of the different magnetocaloric prototypes found in literature as well as alternative applications of the magnetocaloric effect for fundamental studies of phase transitions are also included., This work was supported by the Spanish MINECO (project MAT2013-45165-P), AEI/FEDER-UE (project MAT-2016-77265-R) and the PAI of the Regional Government of Andalucía. L.M. Moreno-Ramírez acknowledges a FPU fellowship from the Spanish MECD.

The field dependence of the magnetic entropy change (ΔS) after mechanical alloying of FeZr composition starting from high purity powders is studied. Samples with different shapes and different crystalline fraction X were analyzed. Although the results show that the proposed correction of the demagnetizing field has not a significant effect on ΔS(T) curves (∼5% underestimation), it is necessary in order to properly analyze the field dependence of this magnitude (|ΔS|=aH). This correction allows recovering the theoretically predicted field dependence as well as a deeper analysis on multiphase systems. In fact, the biphasic character of the studied system changes the value of the field exponent n: it decreases below two above Curie temperature (T), and it increases at T. We show that, assuming a non-interacting phase model, it is possible to obtain the value of the exponent n for the main phase from the intercept of n vs X/ΔS., This work was supported by the Spanish MINECO and EU FEDER (Project MAT-2013-45165-P), AEI/FEDER-UE (Project MAT-2016-77265-R) and the PAI of the Regional Government of Andalucía. A.F. Manchón-Gordón acknowledges a VPPI-US fellowship from the University of Sevilla. L.M Moreno-Ramírez acknowledges a FPU fellowship from the Spanish MECD. MINECO/ICTI2013-2016/MAT-2016-77265-R

In this work, the theoretical field dependence of the magnetic entropy change far away from the transition is used to analyze the field dependence of composite materials formed by fcc La(Fe,Si)13 and bcc α-Fe(Si) phases. A non-interacting phases approximation is followed in the analysis and results are in good agreement with microstructural data obtained from X-ray diffraction and Mossbauer spectroscopy. The range of validity of the approximation is estimated. It is concluded that the quadratic field dependence of magnetic entropy change is reached a few tens of kelvin above the transition temperature at 1.5 T. However, the linear dependence (characteristic of ferromagnets well below its Curie temperature) is only reached a few hundred kelvin below the transition. The results presented here can be used in the deconvolution of the contribution of impurities to the MCE signal in composites.

Since the discovery of the giant magnetocaloric effect (MCE) close to room temperature in FeRh and particularly in Gd5Si2Ge2 compounds, the study of this phenomenon has experienced an exponential growth. Among the different techniques used to produce magnetocaloric materials, ball milling has been shown as a very versatile one and presents several advantages over other preparation techniques (e.g., easy scale-up to industrial production). Although a general decrease of the peak value of the magnetic entropy change is observed for milled samples, it can be compensated by the large broadening of the MCE peak, leading to an increase of the refrigeration capacity. In this short review, several aspects inherent to powder samples affecting MCE will be discussed, such as the relevant effect of the demagnetizing field, the possible multiphase character, and the presence of Curie temperature distributions. In mechanically alloyed samples, the two latter factors are typically affected by the degree of integration of the different starting constituents.

The magnetocaloric effect (MCE) has been measured for an amorphous mechanically alloyed (MA) Co62Nb6Zr2B30. It is demonstrated that sample shape must be taken into account for a correct interpretation of the results, especially to correctly determine the field dependence of magnetic entropy change and refrigerant capacity. However, although correcting the influence of the demagnetizing factor allows us to recover the theoretically predicted behavior for these field dependencies, they differ from those obtained for a rapidly quenched (RQ) amorphous alloy of the same composition. This deviation is assigned to a broader Curie temperature distribution in MA alloys with respect to RQ alloys. Theoretical simulations based on the Arrott–Noakes equation of state and a Gaussian distribution of TC support this hypothesis.

A model is proposed to describe the evolution of the amorphization process driven by mechanical grinding. This model considers the interface controlled growth of the disordered regions at the boundaries between the crystallites (unlike the classical nucleation and growth theory, the growing particles are thus concave which affects the kinetic equation). The validity of this model has been tested for the amorphization process of mechanically alloyed Fe-Nb-B system at different frequencies. Predictions of the model are in agreement with the experimental results and values of the linear growth rate and the thickness of the starting disordered layer are obtained., This work was supported by MINECO and EU FEDER (projects MAT2013-45165-P and MAT2016-77265-R) and the PAI of the Regional Government of Andalucía. MINECO/ICTI2013-2016/MAT2016-77265-R

Gas-atomized MnAl particles constituted nominally of only ε- and γ-phases, i.e. no content of the ferromagnetic L1-type τ-phase, have been used to study the evolution of phases during short time of high-energy milling and subsequent annealing. Milling for 3 min is sufficient to begin formation of the τ-MnAl phase. A large coercivity of 4.9 kOe has been obtained in milled powder after annealing at 355 °C for 10 min. The large increase in coercivity, by comparison with the lower value of 1.8 kOe obtained for the starting material after the same annealing conditions, is attributed to the combined formation of the τ-MnAl and β-Mn phases and the creation of a very fine microstructure with grain sizes on the order of 20 nm. Correlation between morphology, microstructure and magnetic properties of the rapidly milled MnAl powders constitutes a technological advance to prepare highly coercive MnAl powders., The gas-atomized powders were provided by Prof. Ian Baker, Dartmouth College where they were produced via a grant from the U.S. Department of Energy, Advanced Research Projects Agency – Energy (ARPA-E) through REACT program contract no. DE-AR0000188. This research has been funded by MINECO (Spanish Ministry of Science and Innovation): ENMA-National project (MAT2014-56955-R) and Northeastern University, MINECO through M-era.Net Programme: NEXMAG project (PCIN-2015-126) and Regional Government (Comunidad de Madrid): NANOFRONTMAG (Ref. S2013/MIT-2850), MINECO (project MAT2013-45165-P) and the PAI of the Regional Government of Andalucía. N. López acknowledges MINECO for the “Promoción de Empleo Joven” (PEJ-2014) in the frame of Sistema Nacional de Garantía Juvenil and funding by FSE and IEJ.

The magnetocaloric effect of La(Fe,Si) samples deteriorates with the presence of secondary phases. However, it is highly challenging to produce single phase samples by conventional procedures and purification requires long annealing time. We propose grinding and particle size selection of non-optimal starting samples as procedure to enhance the magnetocaloric response. In this study a starting multiphase LaFeSi ingot was grinded and sieved to select three different particle size ranges. X-ray diffraction and Mössbauer spectrometry reveal that all samples mainly contain fcc-La(Fe,Si) and bcc-Fe(Si) phases. Microstructural and magnetic results show that the fcc-La(Fe,Si) phase fraction increases for samples with the smallest average particle size, increasing their magnetocaloric response in a factor larger than three with respect to that of the bulk sample., This work was supported by the Spanish MINECO and EU FEDER (projects MAT2013-45165-P and MAT2016-77265-R) and the PAI of the Regional Government of Andalucía. L.M. Moreno-Ramírez acknowledges a FPU fellowship from the Spanish MECD.

Covellite, CuS and chalcocite, Cu2S nanoparticles prepared in the explosive manner from elemental precursors were further ball-milled in order to observe additional changes caused by mechanical action. Three phases of chalcocite were interchanging during milling, monoclinic one being major at the equilibrium after 30 min. In the case of covellite synthesis, milling for 15 min brought about a significant diminishment in the content of digenite, Cu1.8S, impurity. Covellite powder exhibited finer character than chalcocite, as documented by crystallite size, grain size and specific surface area analysis. Finally, the effect of milling speed on the explosive character of the reaction and phase composition of chalcocite was investigated. The most drastic conditions favored the formation of the monoclinic phase with the lowest symmetry and the time and intensity of the explosion was found to depend on the milling speed. The whole process is mechanically driven. Slovak Research Development Agency APVV-14-0103 Slovak Grant Agency VEGA 2/0027/14 Ministerio de Ciencia e Innovación MAT-2013- 45165-P, MAT-2016-77265-R German Federal Ministry of Education and Research IB-COMSTRUC-010

Ball milling (BM) is a well-established technique for producing different materials in powder shape. Dynamical analysis of BM helps to optimize the process through simple but general relations (e.g., definition of an equivalent milling time). Concerning the field of study of magnetocaloric effect (MCE), BM is used in different ways: as a single step process (mechanical alloying), as an initial step to enhance mixing of the elements (e.g., to speed up the formation of the desired intermetallic phase) or as a final step (e.g., hydriding of La–Fe–Si). In this contribution, besides a simple description of the effects of some geometrical parameters on the power released during BM and a short review of the BM contribution to the research field of MCE, we will discuss the effect of the microstructure of the starting material and the granular shape inherent to BM on magnetic materials exhibiting MCE., Spanish MINECO and EU FEDER (Projects MAT 2013-45165-P), AEI/FEDER-UE (Project MAT-2016-77265-R) and the PAI of the Regional Government of Andalucía. L.M. Moreno-Ramírez acknowledges a FPU fellowship from the Spanish MECD. MINECO/ICTI2013-2016/MAT-2016-77265-R

Determining the composition at the nanoscale generally requires the use of experimental techniques such as 3D atom probe or nanoanalysis, which have limited availability, involve high economic cost and, moreover, imply aggressive sample preparations. However, the combination of Mossbauer spectrometry (MS), X-ray diffraction (XRD) and magnetization measurements can supply very detailed information on the average values of composition of tiny elements of the microstructure such as nanocrystals and boundary regions. Unlike nanoscale techniques, those techniques are widely accessible to most of the scientific community and do not require any special sample preparation, especially for powder samples. Two methods are proposed: the first method uses the ratio between the high field contributions to the MS spectra to extract the composition of the nanocrystals and allows us to follow its evolution; the second method uses average values of the hyperfine field and XRD data to study nanocrystalline samples. These procedures have been applied to two FeNb(B) powder samples obtained by mechanical alloying. The proposed procedures can be easily extended to systems containing other isotopes suitable for Mossbauer spectroscopy or to data from nuclear magnetic resonance experiments.

The structural relaxation of multicomponent Fe(Co)SiAlGaPCB amorphous alloys was investigated calorimetrically for annealed samples over a wide temperature range below the glass transition temperature. Upon heating, the annealed samples exhibit an endothermic reaction (enthalpy relaxation) starting around the annealing temperature and continuing over a temperature range of about 50-140 K, that it is followed by a broad exothermic reaction. Changes in the heat flow curves with annealing temperature and time were analyzed. Experimental values of the overall enthalpy change, ΔH, the peak temperature of the difference in heat flow between the annealed and the as-quenched samples, Tp, and Curie temperature, TC, were fitted by exponential functions including two relaxation times. Values of the two relaxation times are the same for different annealing temperatures regardless the considered property. Saturation values of these magnitudes show a linear dependence with the inverse of the annealing temperature. Tiny domains (2-3 nm in diameter) in the matrix observed by spherical aberration corrected high-resolution transmission electron microscopy could be attributed to some medium-range order in the atomic structure of these quenched alloys. Ministerio de Ciencia e Innovación MAT 2010-20537 European Commission (EC). Fondo Europeo de Desarrollo Regional (FEDER) MAT 2010-20537 Junta de Andalucía P10-FQM-6462 ESTEEM 312483 - ESTEEM2

The determination of the magnetocaloric magnitudes (specific magnetic entropy change, Δs , and adiabatic temperature change, ΔT ) from heat capacity (c ) measurements requires measurements performed at very low temperatures (∼0 K) or data extrapolation when the low temperature range is unavailable. In this work we analyze the influence on the calculated Δs and ΔT of the usually employed linear extrapolation of c from the initial measured temperature down to 0 K. Numerical simulations have been performed using the Brillouin equation of state, the Debye model and the Fermi electron statistics to reproduce the magnetic, lattice and electronic subsystems, respectively. It is demonstrated that it is not necessary to reach experimentally temperatures very close to 0 K due to the existence of certain starting temperatures of the experiments, the same for Δs and ΔT , that minimize the error of the results. A procedure is proposed to obtain the experimental magnitudes of Δs and ΔT with a minimum error from c data limited in temperature. It has been successfully applied to a GdZn alloy and results are compared to those derived from magnetization measurements., This work was supported by the Spanish MINECO and EU-FEDER (project MAT2013-45165-P and MAT2016-77265-R) and the PAI of the Regional Government of Andalucía. LM Moreno-Ramírez acknowledges a FPU fellowship from the Spanish MECD.

We propose a procedure for the determination of the parameters of the Curie temperature distribution (TCD) in a compositionally inhomogeneous ferromagnetic material. Assuming a Gaussian TCD and using a mean field approach based on the Brillouin function, we report that with respect to the average value of the distribution: a) both inflection point of magnetization, Tinf, and temperature at maximum magnetic entropy change curves, TpkMCE, shift to lower temperatures and b) temperature at maximum paramagnetic susceptibility, Tpkχ, shifts to higher temperatures. Using these evolutions as a function of the TCD broadening and fitting them to a second order polynomial function, a self-consistent procedure to determine the parameters of the distribution is supplied. These predictions have been experimentally tested for a ball milled Fe70Zr30 amorphous alloy using thermomagnetic and magnetocaloric measurements. The obtained parameters using the proposed procedure agree with those directly measured using Mössbauer spectrometry.

Specific heat at constant pressure, CP, was measured on amorphous, nanocrystalline and fully crystalline samples of Fe60Co18Nb6B16 alloy. Magnetic and calorimetric measurements agree, describing a continuously decreasing Curie temperature of the amorphous phase. A clear enhancement of CP over the Dulong-Pettit limit has been observed (from 14% to 25 %). Part of the enhancement is due to magnetic (mainly amorphous phase) and electronic contributions, although an excess volume can be inferred from the high value of the slope of CP versus temperature. © 2008 Elsevier B.V. All rights reserved.

Structural relaxation in FeSiB–CuNb alloys is studied from enthalpic and Curie temperature effects. Enthalpy release and Curie temperature changes can be fitted to an exponential behaviour as a function of the annealing time by using two different relaxation times. For ribbons melt-spun at different wheel speed, no significant differences in structural relaxation behaviour are found.

The broad compositional range in which transition metal (TM) based amorphous alloys can be obtained, yields an easily tunable magnetocaloric effect (MCE) in a wide temperature range. In some TM-based alloys, anomalous behaviors are reported, as a non-monotonous trend with magnetic moment (e.g. FeZrB alloys). Moreover, in certain Cr-containing Vitroperm alloys anomalously high values of the magnetic entropy change were published. In this work, a systematic study on MCE response of Cr-containing amorphous alloys of composition FeCrCuNbSiB (with x=2, 8, 10, 12, 13, 14 and 20) has been performed in a broad Curie temperature range from 100 K to 550 K. Curie temperature and magnetic entropy change peak of the amorphous alloys decrease with the increase of Cr content at rates of-25.6 K/at% Cr and-54 mJ kg K/at% Cr, respectively, following a linear trend with the magnetic moment in both cases. The presence of nanocrystalline phases has been considered as a possible cause in order to explain the anomalies. The samples were nanocrystallized in different stages, however, the magnetocaloric response decreases as crystallization progresses due to the large separation of the Curie temperatures of the two phases., This work was supported by the Spanish MINECO (Project MAT2013-45165-P) and the PAI of the Regional Government of Andalucía. L.M. Moreno-Ramírez acknowledges a FPU fellowship from the Spanish MECD.

Since the discovery of the giant magnetocaloric effect (MCE) close to room temperature in FeRh and particularly in Gd5Si2Ge2 compounds, the study of this phenomenon has experienced an exponential growth. Among the different techniques used to produce magnetocaloric materials, ball milling has been shown as a very versatile one and presents several advantages over other preparation techniques (e.g., easy scale-up to industrial production). Although a general decrease of the peak value of the magnetic entropy change is observed for milled samples, it can be compensated by the large broadening of the MCE peak, leading to an increase of the refrigeration capacity. In this short review, several aspects inherent to powder samples affecting MCE will be discussed, such as the relevant effect of the demagnetizing field, the possible multiphase character, and the presence of Curie temperature distributions. In mechanically alloyed samples, the two latter factors are typically affected by the degree of integration of the different starting constituents.

A new method is proposed for determining the Curie temperature from magnetocaloric measurements. It is based on the field dependence of the magnetic entropy change close to the Curie temperature. The main advantages over other methods are that the obtained temperature is field independent, and the process is noniterative and does not require a fitting procedure nor prior knowledge of the critical exponents of the transition. The reliability of the method is demonstrated using both simulated and experimental data for pure Ni and an Fe-based amorphous alloy.

Samples of FeCoNbBCu alloy were nanocrystallized under zero field annealing (ZF) and transverse field annealing (TF) conditions. A reduction in coercivity for TF samples with respect to ZF sample (16 and 45 A/m, respectively) is observed. Kerr microscopy images show a well-defined parallel domain structure, transversally oriented to the ribbon axis for the TF sample unlike for the ZF sample, for which a complex pattern is observed with large and small domains at the surface of the ribbon. Although Mössbauer spectra are clearly different for the two studied samples, Mössbauer studies confirm that there is no significant difference between the hyperfine field distributions of TF and ZF samples but only the relative intensity of the 2nd and 3rd lines A (related to the angle between the gamma radiation and the magnetic moments, α). However, for TF annealed samples α = 90 deg (A = 4), indicating that the magnetic moments lay on the plane of the ribbon in agreement with the well-defined domain structure observed by Kerr microscopy, ZF annealed samples show A = 1.8. This value is close to that of a random orientation (A = 2) but smaller, indicating a slight preference for out of plane orientations. Moreover, it is clearly smaller than that of the as-cast amorphous samples A = 2.8, with a preference to in-plane orientations. The application of the law of approach to saturation yields a larger effect of the inhomogeneities in ZF sample with respect to TF one., This work was supported by the Spanish Ministry of Science and Innovation and EU FEDER (Project MAT2013-45165-P), the PAI of the Regional Government of Andalucía, and the Slovak Projects APVV-0492-11, VEGA 2/0192/13. The Slovak team acknowledges also the support of the Operational Program “Research and Development” financed through European Regional Development Fund through the Projects ITMS 26220120019 and ITMS 26220220061.

Biphasic Gd + GdZn composites (GdZn; x = 0, 5, 15 and 25 at.%) were successfully obtained in a single fabrication step by induction melting. X-ray microdiffraction results reveal the homogeneity of all the prepared samples and indicate a combination of GdZn and Gd phases with different proportions. With this method, the main drawbacks of preparing composites have been avoided, providing the additional advantage of enhanced thermal conductivity between phases. The biphasic Gd + GdZn composite shows an enhanced refrigerant capacity in comparison to Gd (11%) as well as to single phase GdZn (45%). Heat capacity measurements provide an adiabatic temperature change of around 3.5 K for 20 kOe for the optimal composite., This work was supported by Spanish MINECO and EU-FEDER (Project MAT2013-45165-P) as well as the PAI of the Regional Government of Andalucía. L.M. Moreno-Ramírez acknowledges a FPU fellowship from Spanish MECD.

Different excitations, like temperature, magnetic field, or pressure, can drive a martensitic transition in Heusler alloys. Coupled phenomena in these materials lead to interesting magnetocaloric and barocaloric effects ascribed to this transition. In this work, we demonstrate that isothermal transformations induced by a magnetic field and isofield transformations induced by the temperature can be described using the same framework. By defining an effective temperature that relates field and temperature through the properties of the system (magnetic moment and entropy of the transition), both kinds of loops can be transformed into the other kind, therefore providing a more effective way of characterizing hysteretic samples. The validity of this effective temperature approach to describe the transition holds for martensite to austenite transformations as well as reversal ones, and thus, the hysteresis phenomena can be described using this single general excitation.

In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. This review is devoted to the main families of materials suitable for this application and to the procedures proposed to predict their response. Apart from the possible technological applications, we also discuss the use of magnetocaloric characterization to gain fundamental insight into the nature of the underlying phase transition. © Copyright ©2012 by Annual Reviews. All rights reserved.

Recent calculations have shown that the refrigerant capacity (RC) of magnetic refrigerants can be enhanced using multiphase materials or composites, which expand the temperature range over which a significant magnetic entropy change can be obtained. This work is a systematic experimental validation of the improvement of RC (RCI) using layered composites comprised of two Fe88−2yCoyNiyZr7B4Cu1 amorphous alloy constituents, with y=8.25 and y=11 compositions. RCI has a nonmonotonic dependence on the applied magnetic field H and the fraction x of the two constituent phases. In contrast to common assumptions, the composite has a smaller RCI than its constituent phases for small values of H and x, and there are critical values of each for which RCI is maximized. This work demonstrates the outstanding agreement between the experimental results and the continuous curves predicted by numerical calculations, indicating that this approach can be used to design magnetic refrigerant materials with enhanced magnetocaloric response for moderate magnetic fields.

After mechanical alloying Fe70Zr30 composition from pure starting powders, an amorphous phase with Curie temperature TC = 244 K and an intermetallic compound (that should be non-stoichiometric Zr-rich fcc Fe2Zr phase) with TC = 355 K are formed. Residual α-Fe crystallites are also found. The multiphase character of this system yields a non-monotonic dependence of the magnetocaloric effect (characterized by the refrigerant capacity, RC) on the fraction of phases. Among the samples studied in this work, RC is enhanced for samples with the highest fraction of intermetallic compound, although the maximum magnetic entropy change monotonically decreases with the increase of the fraction of this phase. This behaviour agrees with the predicted one for biphasic systems.

The milling time evolution of magnetic anisotropy of ball milled powders can be described considering two regimes. First, for short milling times, the main factor affecting the magnetic behavior of the alloy is the accumulation of internal stresses. Second, for long milling times, magnetic anisotropy can be explained using three contributions: long-range magnetoelastic, averaged short-range magnetoelastic and averaged magnetocrystalline anisotropies.

The influence of the Co content on the microstructure and magnetic behaviour of a series of amorphous and nanocrystalline (FeCo)79Mo8Cu1B12 alloys is reported. Changes in the magnetic properties provoked by the microstructural evolution upon different thermal treatments of as-cast samples are analyzed as well. Kinetics of nanocrystallization process can be described by an isokinetic approach. As the Co content in the alloy increases, the Curie temperature of the amorphous as-cast samples increases while the crystallization onset temperature decreases. The crystalline volume fraction as well as the mean grain size of the nanocrystals at the end of the nanocrystallization process are slightly higher for the lowest Co content alloy but smaller than in similar Hitperm Mo-free alloys. The average magnetic field and the average isomer shift of the as-cast samples show a linear increase with two slopes with the Co content of the alloy. The same tendency is found for the saturation magnetization. Combined TEM, XRD, and MS data indicate the presence of Mo atoms in the nanocrystals. At the end of the nanocrystallization process the softest magnetic behaviour corresponds to the lowest Co content alloy. The second crystallization process provokes a magnetic hardening of these alloys.

Instantaneous growth approximation is based on the fact that the time required by a new nucleus to reach its saturation size is negligible in comparison with the time required for the whole crystallization process. This approximation has been applied to analyze the kinetics of the nanocrystallization process of Co 60 Fe 18− x Mn x Nb 6 B 16 ( x = 0 and 4) alloys. Direct application of JMAK theory leads to divergences at very high transformed fractions. Although this divergence could be due to some artifacts, it can also be solved after a reinterpretation of the geometrical impingement effect on nanocrystalline systems. A modified Arrhenius relationship can be used for a general description of the double dependency of nucleation rate on the transformed fraction (only through the local activation energy) and the temperature.

The kinetics of amorphization during ball milling is generally analyzed using two different approaches: the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory and Delogu and Cocco’s model for which a region deterministically transforms after it reaches a certain number of collisions. The application of JMAK analysis to the latter model predicts Avrami exponents to be higher than the experimental ones (typically close to one). We develop simulations based on the probabilistic character of the nucleation phenomenon and concave growth of the amorphous phase in the core of a nanocrystal. The predictions of our simulations are in good agreement with the low Avrami exponents and with the size evolution of the remaining crystallites found experimentally. From these values, the parameters involved in the simulated model (growth rate and probability of nucleation) can be estimated.

The evolution of the amorphous fraction developed during the mechanical alloying of a mixture of pure 75 at.% Fe, 10 at.% Nb and 15 at.% B, X Am , has been followed by different techniques: X-ray diffraction, Mossbauer spectroscopy and magnetic permeability measurements; in order to compare their sensitivity in the detection of small fractions. The values obtained for the amorphous fraction from the three techniques show a roughly linear correlation above ∼20%. The most sensitive technique was Mossbauer spectroscopy ( X Am obtained from the low hyperfine field contributions) and the less sensitive technique was X-ray diffraction. The Curie temperature of the amorphous phase increases with the milling time due to a slow and progressive incorporation of boron into this phase and the nanocrystals from the boron inclusions.

Magnetic properties and magnetocaloric response of mechanically alloyed (Fe1−XCoX)75Nb10B15 (X = 0.15 and 0.30) powders have been studied as a function of crystalline fraction. From X-ray diffraction, it was observed that the amorphization process is delayed with the Co addition in the alloy. Crystallization temperature of the amorphous phase developed during milling decreases as Co content increases. Temperature dependence of magnetization curves shows that the Curie temperature of the amorphous phase increases as the Co content increases. The peak magnetic entropy change slowly increases with Co addition for low crystalline fractions and the refrigerant capacity decreases as Co increases in the alloy.