Magnetocaloric effect and phase transformation in Ho 1-x Gd x Co 2 AND Ho 1-x Tb x Co 2 compounds

International audienceThe rare-earth transition metal intermetallics RCo 2 have attracted much attention due to their interesting magnetic properties related to the magnetic instability of the cobalt 3d electrons. This instability induces a first order magnetic transition (FOMT) leading to large magnetic entropy changes in some RCo 2 compounds such as ErCo 2 , HoCo 2 and DyCo 2. Here, we present recent results obtained with Ho 1-x Tb x Co 2 and Ho 1-x Gd x Co 2 solid solutions. Structural, magnetic phase transition and magnetocaloric properties of these materials (0 ≤ x ≤ 1) were investigated by X – ray diffraction and magnetic measurements and then analyzed in terms of Landau theory. The Ho 1-x Gd x Co 2 and Ho 1-x Tb x Co 2 alloys present the C15 cubic MgCu 2 – type structure. The cell parameter a and the Curie temperature T C both increase, increasing the Tb and Gd concentrations. T C shifts from 81 to 233 K (x = 0 to 1) for Ho 1-x Tb x Co 2 and from 81 to 300 K (x = 0 to 0.6) in the case of Ho 1-x Gd x Co 2. The change of isothermal entropy S Δ was calculated according to magnetic measurements, using the thermodynamic Maxwell's relation. Besides, the increase of T C is accompanied by a decrease of the entropy change. The reduction of the magnetocaloric effect (MCE) for Ho 1-x (Gd, Tb) x Co 2 compounds could be explained by the fact that substitution of Tb and Gd to Ho weakens and supersedes the field–induced magnetic transition existing close to T C and accordingly transforms the transition from first to second order type

Negative magnetocaloric effect in Fe 1-x Rh x compounds

International audienceOn increasing temperature, the Fe1-xRhx alloys present a transition from an antiferromagnetic to a ferromagnetic state, which induces a negative magnetocaloric effect (MCE). The magnetocaloric effect, in particular of the Fe0.49Rh0.51 alloy, was studied by direct measurements and accordingto specific heat measurements. Here, we report the recent results obtained on the annealed Fe0.48Rh0.52 compound we prepared by arc melting. The isothermal entropy change ΔS allowing an estimation of the MCE was determined from magnetic measurements. The ΔS experimentally found is larger than that reported for the annealed Fe0.49Rh0.51 sample. However a significant difference occurs between transition temperature of the annealed sample Fe0.48Rh0.52 and that reported in literature. This result shows that magnetic and magnetocaloric properties in this kind of materials are very sensitive to the technique of samples preparation. Besides, the magnetocaloric effect close the ferromagnetic – paramagnetic transition in Fe0.48Rh0.52 is also discussed

Analysis and modeling of magnetocaloric effect near magnetic phase transition temperature

International audienceMagnetocaloric behavior of gadolinium near room temperature can be correctly described by the Weiss molecular field theory especially in the paramagnetic state. In this paper, this approach is generalized for binary rare earth alloys which present as Gd a second order phase transition. The magnetic entropy variation can be calculated as a function of the temperature and the applied field. This model was tested on a laboratory synthesized samples of Gd-Tb. The agreement between calculations and experiments shows that this model can be easily used for these alloys in order to optimize their composition and adjust their Curie temperatures. For first order transition materials, the observed magnetocaloric effect enhancement can be explained by magnetoelastic effects which are due to the spontaneous crystal deformation and the structure transformation. A model based on the phenomenological approach of Bean Rodbell is developed to describe such a behavior. It highlights the link between the nature of magnetic transition and the magnetocaloric effect. It can be identified by only two parameters: T 0 the Curie temperature without deformation and η an order parameter which characterizes the transition nature. In this paper we apply this model to describe the giant magnetocaloric effect exhibited by the new Mn 1-x (Ti 0.5 V 0.5) x As materials

Giant magnetocaloric effect in Mn 1-t (Ti 0.5 V 0.5 ) t as compounds near room temperature

International audienceMn 1-t (Ti 0.5 V 0.5) t As compounds with t varying from 0 to 0.20 were synthesised by arc melting and subsequently annealed. The X-ray diffraction analysis reveals pure and fairly crystallised samples. Magnetisation measurements show that the Curie temperature decreases to room temperature with t the substitution rate. The sharp and abrupt character of the 1 st order transition of MnAs-type turns to a less marked variation of the magnetic entropy in the vicinity of the transition temperature, profit made to a wider temperature range of MCE efficiency

Contribution à l'étude des propriétés magnétiques des composés de basse symétrie entre les terres rares et les métaux de transition

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Matériaux Magnétocaloriques

Ce dossier fait le point sur l'état actuel des recherches sur les matériaux magnétocaloriques les plus prometteurs pour la réfrigération. Après une première partie consacrée au contexte dans lequel se situent ces recherches, une description plus détaillée de l'effet magnétocalorique et sa détermination expérimentale sont traitées dans une deuxième partie. La troisième partie, la plus importante, présente les propriétés et les performances des différents composés auxquels on s'intéresse en raison de leurs performances élevées. La disponibilité et la mise en œuvre sont rapidement abordées dans la quatrième partie. Enfin, les systèmes de réfrigération existants sont à peine esquissés dans la cinquième partie

Magnetic behaviour and experimental study of the magnetocaloric effect in the pseudobinary Laves phase Er1−xDyxCo2

International audienceWe have investigated the crystal structure, the bulk magnetization characteristics and the magnetocaloric properties of Er1−xDyxCo2 compounds. X-ray diffraction (XRD) analyses confirm that all these Lavesphase type compounds crystallize in the cubic MgCu2-type structure. First, the magnetization behaviour and the magnetic transition are analyzed in terms of Landau theory. Then, a direct correlation was pointed out between the character of the magnetic transition and the behaviour of the cell parameter versus x. Substitution of Dy to Er enhances markedly the Curie temperature TC from 35 to 142 K, while ΔS the corresponding change of isothermal entropy decreases significantly. The refrigerant capacity of the Er1−xDyxCo2 compounds is discussed and our experimental data are compared with the corresponding theoretical results reported in the literature [de Oliveira, von Ranke, J. Magn. Magn. Mater. 264 (2003) 55]

Magnetism: Materials and Applications

This book treats permanent magnet (hard) materials, magnetically soft materials for low-frequency applications and for high-frequency electronics, magnetostrictive materials, superconductors, magnetic-thin films and multilayers, and ferrofluids. Chapters are dedicated to magnetic recording, the role of magnetism in magnetic resonance imaging (MRI), and instrumentation for magnetic measurements.  

Effect of interstitial nitrogen on magnetism and entropy change of LaFe11.7Si1.3 compound

International audienceCrystal structure, magnetism and magnetocaloric properties of LaFe11.7Si1.3Ny (y = 0, 1.3) compounds have been studied by X-ray diffraction and magnetic measurements. The LaFe11.7Si1.3Ny compounds present a cubic NaZn13-type structure. Insertion of 1.3 nitrogen atoms per LaFe11.7Si1.3 formula increases the lattice parameter and Curie temperature from 11.467 to 11.733 A and from 190 to ∼230K, respectively. Besides, the absorption of nitrogen drives drastically the magnetic transition from first to second order and accordingly strongly decreases the magnetocaloric effect compared to the parent alloy. Under an external field change of 5T, the value of isothermal entropy change -AS is about 28 and 3.5J/kgK for LaFe11.7Si1.3 and LaFe11.7Si1.3N1.3, respectively, close to their Curie temperature. However, the relative cooling power RCP(S) of the nitride is about half that of the parent alloy