MAGNETOCALORIC EFFECT AND PHASE TRANSFORMATION IN Ho_(1-x)Gd_xCo_2 AND Ho_(1-x)Tb_xCo_2 COMPOUNDS

摘要

The rare - earth transition metal intermetallics RCo_2 have attracted much attention due torntheir interesting magnetic properties related to the magnetic instability of the cobalt 3d electrons.rnThis instability induces a first order magnetic transition (FOMT) leading to large magneticrnentropy changes in some RCo_2 compounds such as ErCo_2, HoCo_2 and DyCo_2. Here, we presentrnrecent results obtained with Ho_(1-x)Tb_xCo_2 and Ho_(1-x)Gd_xCo_2 solid solutions. Structural, magneticrnphase transition and magnetocaloric properties of these materials (0 ≤ x ≤ 1) were investigatedrnby X – ray diffraction and magnetic measurements and then analyzed in terms of Landau theory.rnThe Ho_(1-x)Gd_xCo_2 and Ho_(1-x)Tb_xCo_2 alloys present the C15 cubic MgCu_2 – type structure. The cellrnparameter a and the Curie temperature T_C both increase, increasing the Tb and Gdrnconcentrations. TC shifts from 81 to 233 K (x = 0 to 1) for Ho_(1-x)Tb_xCo_2 and from 81 to 300 K (xrn= 0 to 0.6) in the case of Ho_(1-x)Gd_xCo_2. The change of isothermal entropy ΔS was calculatedrnaccording to magnetic measurements, using the thermodynamic Maxwell's relation. Besides, thernincrease of TC is accompanied by a decrease of the entropy change. The reduction of thernmagnetocaloric effect (MCE) for Ho_(1-x)(Gd, Tb)_xCo_2 compounds could be explained by the factrnthat substitution of Tb and Gd to Ho weakens and supersedes the field–induced magneticrntransition existing close to T_C and accordingly transforms the transition from first to second orderrntype.