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We have studied the correlation between the broadening of the isothermal magnetic entropy change and the Curie temperature (TC) distribution in nanostructured Pr2Fe17 and Nd2Fe17 alloys produced by high-energy ball-milling after milling times of 10, 20, and 40 h. The changes in the microstructure affect the Fe local environments and as a consequence the magnetic interactions, giving rise to TC distributions centered around 285K and 330K for the Pr2Fe17 and Nd2Fe17 alloys, respectively. The width of the distributions enlarges (up to 60K) as the milling-time increases, and consequently, the isothermal magnetic entropy change curves show an extended full width at half maximum. [ABSTRACT FROM AUTHOR]

Abstract: We have synthesized the Dy2Fe17 alloy with an unconventional crystal structure ( space group). Neutron powder thermo-diffraction and X-ray powder diffraction under high pressure (up to 15GPa) show that the rhombohedral crystal structure is stable. Likewise, the alloy with the usual hexagonal crystal structure (P63/mmc), the rhombohedral variant of Dy2Fe17 compound, exhibits a collinear ferrimagnetic order below the Curie temperature (T C ≈363K), with antiparallel mutual alignment of the Dy and Fe sublattices. Additionally, we have observed two distinctive issues related to magneto-volume anomalies: (i) the pressure dependence of the cell volume at room temperature shows the existence of a critical pressure at which the compound is no longer ferrimagnetic; and (ii) the cell volume shows invar-like behaviour in a wide temperature range (2–290K), with a minimum at T ≈380K. The spontaneous volume magnetostriction reaches ωS =1.6×10−2 at 2K that decreases to zero at T ≈500K (≈1.4 T C), which is associated with the existence of short-range magnetic correlations. [Copyright &y& Elsevier]

Abstract: We have synthesized the intermetallic YPrFe17 compound by arc-melting. X-ray and neutron powder diffraction show that the crystal structure is rhombohedral with space group (Th2Zn17-type). The investigated compound exhibits a broad isothermal magnetic entropy change ΔS M (T) associated with the ferro-to-paramagnetic phase transition (T C ≈290K). The |ΔS M | (≈2.3Jkg−1 K−1) and the relative cooling power (≈100Jkg−1) have been calculated for applied magnetic field changes up to 1.5T. A single master curve for ΔS M under different values of the magnetic field change can be obtained by a rescaling of the temperature axis. The results are compared and discussed in terms of the magneto-caloric effect in the isostructural R2Fe17 (R=Y, Pr and Nd) binary intermetallic alloys. [Copyright &y& Elsevier]

Abstract: We have synthesized three pseudo-binary (Ce,R)2Fe17 intermetallic alloys, where cerium has been partially substituted by R = Y, Pr and Dy, with the aim of tuning the Curie temperature (between 253 and 273 K) for obtaining the maximum magneto-caloric response of the alloys just below room temperature. The analysis of the x-ray powder diffraction patterns show that the three samples crystallize in the rhombohedral Th2Zn17-type crystal structure (space group ). We report the temperature dependence of the isothermal magnetic entropy change, |ΔSM |(T), the magnetic field dependence of its maximum value, |ΔSM ma x|(H), and the relative cooling power around the second-order magnetic transition for magnetic field changes μ 0ΔHmax  = 5 T. The collapse of the normalized ΔSM (θ)/ΔSM max vs. temperature (where θ is a rescaled temperature) into a single master curve allows the extrapolation of |ΔSM |(T) curves for higher magnetic field changes, and/or the estimation of the temperature dependence of the magnetic entropy for other 2:17 pseudo-binary compounds. The lower values of |ΔSM ma x| compared with those of the R2Fe17 compounds with R = Pr or Nd are explained in terms of the decrease of the saturation magnetization. The magnetic field dependence of the |ΔSM ma x| indicates that only Pr1.5Ce0.5Fe17 alloy roughly follows a mean-field behaviour. [Copyright &y& Elsevier]

Abstract: Using high-energy ball milling, nanostructured Pr2Fe17 powders can be obtained from their arc-melted bulk alloys. High-resolution X-ray and neutron powder diffraction experiments reveal that the Th2Zn17-type rhombohedral crystal structure is maintained, after milling for 10h, with almost unchanged values for both crystalline lattice parameters (Δa; Δc <0.05%) and vanishing mechanically induced microstrain (<0.1%). Although the mean crystalline size decreases down to 20±3nm, magnetovolume anomalies observed in pure Pr2Fe17 are still present with a significant volume decrease on heating from 5 up to 320K. After the milling, a significant increase in the magnetic anisotropy, due to the drastic reduction in crystalline size, is observed, while the value of the magnetic moment seems to be increased slightly (5%). In addition, the magnetocaloric effect of bulk and nanostructured Pr2Fe17 has been investigated. The magnetic entropy change, |ΔSM |, decreases from 6.3 to 4.5J kg−1 K−1 under an applied magnetic field μ 0 H =5T after the milling process. However, the width of the |ΔSM |(T) curve is substantially enlarged and hence the refrigerant capacity is enhanced. These findings make the iron-based nanostructured Pr2Fe17 powders interesting for applications in magnetic refrigeration at around room temperature. [Copyright &y& Elsevier]

Recent investigations in R2Fe17 intermetallic compounds have evidenced that these materials present a moderate magnetocaloric effect (MCE) near room temperature. A series of accurate magnetization measurements was carried out to show that the value of the demagnetizing factor has a significant influence on the absolute MCE value of Er2Fe17. In addition, the critical exponents determined from heat capacity and magnetization measurements allow us to describe the field dependence of the observed MCE around the Curie temperature.

Nanocrystalline Nd(2)Fe(17) powders have been obtained by means of high-energy ball milling from nearly single-phase bulk alloys produced by arc melting and high temperature homogenization annealing. The rhombohedral Th(2)Zn(17)-type crystal structure of the bulk alloy remains unaltered after the milling process, with almost unchanged values for the cell parameters. However, the severe mechanical processing induces drastic microstructural changes. A decrease of the mean crystalline size down to around 10 nm is observed, giving rise to a considerable augmentation of the disordered inter-grain boundaries. This modification of the microstructure affects the magnetic behaviour of the milled powders, although the magnetic structure remains collinear ferromagnetic. While a unique ferro-to-paramagnetic transition temperature, T(C) = 339 ± 2 K, is observed in the bulk alloy, the nanocrystalline samples exhibit a more likely distribution of T(C) values. The latter seems to be responsible for the significant broadening of the temperature range in which magneto-caloric effect is observed, and the lowering of the maximum value of the magnetic entropy change.