Tunable magnetocaloric effect around room temperature by Fe doping in Mn0.98Cr(0.02-x)FexAs compound

In this work, we present an investigation of the magnetic and magnetocaloric properties of Mn0.98Cr(0.02-x)FexAs compounds with x = 0.002, 0.005 and 0.010. Our findings show that as Fe content increases the unit cell volume decreases, which indicates that Fe doping emulates the pressure effect on the crystalline structure. The transition temperature TC decreases as x increases and it can be set at approximate value of room temperature by changing the doping level. In addition, the magnetic entropy change ΔSM was determined using a discontinuous measurement protocol, and realistic values from the magnetocaloric effect presented by MnAs-type compounds under pressure (emulated pressure) could be obtained. The values of ΔSMMAX are very large, around −11 Jkg−1K−1 with ΔH = 15 kOe, which is higher than that observed for most compounds with TC around room temperature. However, ΔSM is confined to a narrow temperature range of 11 K. To overcome this drawback, the composition of a theoretical composite formed by our samples was calculated in order to obtain a table-shaped ΔSM curve. The simulated composite showed a high value of full width at half maximum δTFWHM of 33 K, which is much higher than that of single sample.Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro - FAPERJ E-26/110.393/2014, E-16/210.263/2014Coordenação de aperfeiçoamento de pessoal de nivel superior - CAPES 485200/2013-9Fundación de Apoyo a la Investigación del Estado de São Paulo - FAPESP 2012/03480-0Ministerio de Economía y Competitividad MAT2013-45165-

Near Room Temperature Magnetocaloric Response of an (FeNi)ZrB Alloy

Mechanical alloying of (Fe70Ni30)89Zr7B4 powders leads to the formation of a γ-FeNi phase. In order to obtain single γ-phase, a powdered sample was solution annealed in the γ-phase field and water quenched. The Curie temperature of this powder was slightly higher than room temperature. The refrigerant capacity calculated for this alloy, RCFWHM ~ 330 J·kg-1 for 5 T, is comparable to other prominent room temperature magnetocaloric materials, making this alloy a good candidate for magnetic refrigeration near room temperature with additional benefits that is non-rare earth containing and less expensive than many alternatives.</p

Mechanical amorphization and recrystallization of Mn-Co(Fe)-Ge(Si) compositions

Mechanical alloying using a planetary ball mill allowed us to obtain two homogeneous systems formed by units with nanometer size and MnCo0.8Fe0.2Ge1-xSix stoichiometry (x = 0 and 0.5). The phase evolution of the systems with the milling time was analyzed using X-ray diffraction. Thermal stability of the final products was studied using differential scanning calorimetry. Room temperature 57Fe Mössbauer spectroscopy was used to follow the changes in the Fe environments. A paramagnetic Co-based amorphous phase developed in both alloys as milling progressed. However, while the presence of Si stabilized the Mn-type phase, mechanical recrystallization was observed in a Si-free composition leading to the formation of a MnCo(Fe)Ge intermetallic (Pnma space group) with a crystal size of 7 ± 1 nm. Mössbauer results indicate that Fe atoms migrate from the initial bcc phase to the amorphous and intermetallic phases

Mechanical Amorphization and Recrystallization of Mn-Co(Fe)-Ge(Si) Compositions

Mechanical alloying using a planetary ball mill allowed us to obtain two homogeneous systems formed by units with nanometer size and MnCo0.8Fe0.2Ge1&#8722;xSix stoichiometry (x = 0 and 0.5). The phase evolution of the systems with the milling time was analyzed using X-ray diffraction. Thermal stability of the final products was studied using differential scanning calorimetry. Room temperature 57Fe M&#246;ssbauer spectroscopy was used to follow the changes in the Fe environments. A paramagnetic Co-based amorphous phase developed in both alloys as milling progressed. However, while the presence of Si stabilized the Mn-type phase, mechanical recrystallization was observed in a Si-free composition leading to the formation of a MnCo(Fe)Ge intermetallic (Pnma space group) with a crystal size of 7 &#177; 1 nm. M&#246;ssbauer results indicate that Fe atoms migrate from the initial bcc phase to the amorphous and intermetallic phases

A procedure to obtain the parameters of Curie temperature distribution from thermomagnetic and magnetocaloric data

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.Peer reviewe

Study of the kinetics and products of the devitrification process of mechanically amorphized Fe70Zr30 alloy

Devitrification of mechanically alloyed amorphous FeZr at. % compound consists on a two-step process: amorphous → amorphous + bcc Fe + FeZr → FeZr + FeZr. This sequence is inferred from the evolution of the Mössbauer spectra, the thermomagnetic experiments and the X-ray diffraction (XRD) patterns. Hyperfine parameters for both intermetallics have been obtained from Mössbauer spectroscopy in correlation with the phase identification from XRD results. The broadening of the stable compositional range of FeZr 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

Kinetic Analysis of the Transformation from 14M Martensite to L21 Austenite in Ni-Fe-Ga Melt Spun Ribbons

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 dataPeer reviewe

Comparative study of structural and magnetic properties of ribbon and bulk Ni55Fe19Ga26 Heusler alloy

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.Peer reviewe

Tunable magnetocaloric effect around room temperature by Fe doping in Mn0.98Cr(0.02-x)FexAs compound

In this work, we present an investigation of the magnetic and magnetocaloric properties of MnCrFeAs compounds with x = 0.002, 0.005 and 0.010. Our findings show that as Fe content increases the unit cell volume decreases, which indicates that Fe doping emulates the pressure effect on the crystalline structure. The transition temperature T decreases as x increases and it can be set at approximate value of room temperature by changing the doping level. In addition, the magnetic entropy change ΔS was determined using a discontinuous measurement protocol, and realistic values from the magnetocaloric effect presented by MnAs-type compounds under pressure (emulated pressure) could be obtained. The values of ΔS are very large, around −11 JkgK with ΔH = 15 kOe, which is higher than that observed for most compounds with T around room temperature. However, ΔS is confined to a narrow temperature range of 11 K. To overcome this drawback, the composition of a theoretical composite formed by our samples was calculated in order to obtain a table-shaped ΔS curve. The simulated composite showed a high value of full width at half maximum δT of 33 K, which is much higher than that of single sample.We acknowledge FAPERJ (Grant Nos. E-26/110.393/2014 and E-16/210.263/2014), CAPES, CNPq (Grant No. 485200/2013-9), FAPESP (Grant No. 2012/03480-0), PROPPI-UFF, MINECO and EU FEDER (project MAT2013-45165-P) and the PAI of the Regional Government of Andalucía for financial support