Phase transformations and thermodynamic properties of nanocrystalline FePt powders

The solid state reactions and structural evolution in nanocrystalline FePt powders during mechanical ball milling at 77 K and subsequent annealing have been investigated. Above 310 °C the formation of L10 FePt is observed. As the milling time increases, the enthalpy evolved during the transformations is reduced, whereas the corresponding activation energy increases. For accelerating the ordering process a fine lamellar structure of Fe and Pt is favourable

Dynamics of the first-order metamagnetic transition in magnetocaloric La(Fe,Si)(13): Reducing hysteresis

The influence of dynamics and sample shape on the magnetic hysteresis in first‐order magnetocaloric metamagnetic LaFe13–xSix with x = 1.4 is studied. In solid‐state magnetic cooling, reducing magnetic and thermal hysteresis is critical for refrigeration cycle efficiency. From magnetization measurements, it is found that the fast field‐rate dependence of the hysteresis can be attributed to extrinsic heating directly related to the thickness of the sample and the thermal contact with the bath. If the field is paused partway through the transition, the subsequent relaxation is strongly dependent on shape due to both demagnetizing fields and thermal equilibration; magnetic coupling between adjacent sample fragments can also be significant. Judicious shaping of the sample can both increase the onset field of the ferromagnetic–paramagnetic (FM–PM) transition but have little effect on the PM–FM onset, suggesting a route to engineer the hysteresis width by appropriate design. In the field‐paused state, the relaxation from one phase to the other slows with increasing temperature, implying that the process is neither thermally activated or athermal; comparison with the temperature dependence of the latent heat strongly suggests that the dynamics reflect the intrinsic free energy difference between the two phases

Magneto-caloric effect in the pseudo-binary intermetallic YPrFe17 compound

We have synthesized the intermetallic YPrFe17 compound by arc-melting. X-ray and neutron powder diffraction show that the crystal structure is rhombohedral with View the MathML source space group (Th2Zn17-type). The investigated compound exhibits a broad isothermal magnetic entropy change {\Delta}SM(T) associated with the ferro-to-paramagnetic phase transition (TC \approx 290 K). The |{\Delta}SM| (\approx 2.3 J kg-1 K-1) and the relative cooling power (\approx 100 J kg-1) have been calculated for applied magnetic field changes up to 1.5 T. A single master curve for {\Delta}SM 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.Comment: Preprint, 5 pages (postprint), 4 figures, regular pape

Overview of the Characteristic Features of the Magnetic Phase Transition with Regards to the Magnetocaloric Effect: the Hidden Relationship Between Hysteresis and Latent Heat

This article was published in the journal, Metallurgical and Materials Transactions E [Springer / © The Minerals, Metals & Materials Society and ASM International]. The final publication is available at Springer via http://dx.doi.org/10.1007/s40553-014-0015-8The magnetocaloric effect has seen a resurgence in interest over the last 20 years as a means towards an alternative energy efficient cooling method. This has resulted in a concerted effort to develop the so-called “giant” magnetocaloric materials with large entropy changes that often come at the expense of hysteretic behavior. But do the gains offset the disadvantages? In this paper, we review the relationship between the latent heat of several giant magnetocaloric systems and the associated magnetic field hysteresis. We quantify this relationship by the parameter Δμ 0 H/ΔS L, which describes the linear relationship between field hysteresis, Δμ 0 H, and entropy change due to latent heat, ΔS L. The general trends observed in these systems suggest that itinerant magnets appear to consistently show large ΔS L accompanied by small Δμ 0 H (Δμ 0 H/ΔS L = 0.02 ± 0.01 T/(J K−1 kg−1)), compared to local moment systems, which show significantly larger Δμ 0 H as ΔS L increases (Δμ 0 H/ΔS L = 0.14 ± 0.06 T/(J K−1 kg−1))

Magnetic Properties and Specific Heat of Laves Phase Tb1−xScxNi2Tb_{1-x}Sc_{x}Ni_2 (x = 0.1, 0.2) Solid Solutions

Magnetic and specific heat measurements have been performed on polycrystalline $TbNi_2,$ $ScNi_2$ and their solid solutions $Tb_{1-x}Sc_{x}Ni_2$ (x = 0.1, 0.2). These compounds were synthesized using high-purity rare-earth metals. It has been found that the magnetic susceptibility of the nonmagnetic ScNi_2 compound exhibits a very weak temperature dependence characteristic of the Pauli paramagnets. $TbNi_2,$ $Tb_{0.9}Sc_{0.1}Ni_2$ and $Tb_{0.8}Sc_{0.2}Ni_2$ are typical Curie-Weiss paramagnets and are ferromagnetically ordered below 36 K. As revealed by room-temperature X-ray powder diffraction all the $Tb_{1-x}Sc_{x}Ni_2$ solid solutions have the cubic Laves C15-type superstructure. The Debye temperature, phonon and conduction electron contributions as well as the magnetic part of heat capacity were determined. The magnetocaloric effect has been studied by means of specific heat measurements in magnetic fields of 0.42 and 1 T. The effect of rare-earth substitution in $ScNi_2$ on the magnetic and magnetocaloric properties will be discussed

Thermopower of LaFe

We measured the thermopower of LaFe11.6Si1.4 and its hydride LaFe11.6Si1.4Hy to investigate changes in the electronic structure induced by hydriding. Using a model based on a density-of-states (DOS) function we can accurately describe a non-linear temperature dependence of the thermopower over a wide temperature range. The fit of the model to experimental data yields a significantly broader maximum in the DOS function near the Fermi energy of the hydride as compared to LaFe11.6Si1.4. Additionally, a new scattering mechanism leading to a decreased thermopower is observed in LaFe11.6Si1.4Hy at low temperatures which is attributed to scattering of electrons on magnetic excitation