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