Journal of Magnetism and Magnetic Materials 272–276 (2004) 2391–2392 Influence of hydrogen on the magnetic behaviour of Gd 5 Ge 2 Si 2 H x ,0:1pxp2:5 Cleber S. Alves a, *, C! esar C. Colucci a , S. Gama b , Alexandre M.C. Carvalho b , Adelino A. Coelho b a Universidade Estadual de Maring ! a, CTM, R. Cantor Raul Seixas, s/n, 87053-240, Maring ! a PR Brazil b Unicamp, IFGW, DFA, GPCM, CP 6165, Campinas, SP, 13083-970, Brazil Abstract The influence of hydrogen absorption on T c and M s of the Gd 5 Ge 2 Si 2 H x alloys, with 0:1pxp2:5 was studied by thermomagnetic analysis from 2 to 370 K. The M(T) curves show the first order magnetic transition at 260 K only for the sample with x ¼ 0:1: All the samples presented the second-order magnetic transition between 296 ðx ¼ 0:1Þ and 212K ðx ¼ 2:5Þ: For the sample with xX1:2 is also observed a new antiferromagnetic transition at E45 K. The MðHÞ curves revealed that the M s tends to increase with the hydrogen amount into the alloy. r 2004 Published by Elsevier B.V. PACS: 75.30.Sg; 75.30.Kz; 51.60.+a Keywords: Magnetocaloric effect; Magnetic-phase transition; Magnetic properties The magnetocaloric effect (MCE) is the ability of some magnetic materials to heat up when they are magnetized, and cool down when removed from the magnetic field in an adiabatic process. Recently a giant MCE (GMCE) was discovered for the Gd 5 (Si 2 Ge 2 ) alloy, making it a potential candidate for magnetic refrigeration at B276K [1]. The GMCE in these materials was associated with a simultaneous first- order structural and magnetic-phase transition, that occurs near its ordering temperature of 276 K. A solid magnetic refrigerant material is an essential part of the magnetic refrigerator, making them more efficient and more ecologically clean than the gas- compression refrigerations. Also, the cooling power in the magnetocaloric refrigerators can be varied by scaling from milliwatts to a few hundred watts as the cooling efficiency of magnetic refrigerators working with Gd reach values of 60% of the theoretical limit, comparated to only about 40% in the best gas-compression refrigerators [2]. The Gd 5 Ge 2 Si 2 alloys were prepared by arc melting of the pure constituintes (Gd = 99.8 at%, Si and Ge both >99.99 at%) in an argon atmosphere under normal pressure. Each ingot had the total weight of B5g and was arc melted four times with the button being turned over each time to ensure sample homogeneity. The heat treatment of all alloys was carried out in an induction furnace in an argon atmosphere under pressure of 1 atm for 48h at 1650  C. The annealed sample was hand- ground and sieved prior the hydrogenation. The particles had diameter in the 20–32 mm range. After- wards, the hydrogenation of the powder was carried out in a Sieverts apparatus. The reactor was heated at 400  C and the temperature was continously monitored by a type K themocouple, placed inside the reactor. The high purity hydrogen gas was admitted to the reactor chamber and the pressure change was registered until the final concentration was reached. Magnetic measurements for the Gd 5 Ge 2 Si 2 H x alloys, with 0:1pxp2:5 were carried out using a Quantum Design dc magnetometer, model MPMS XL. ARTICLE IN PRESS *Corresponding author. Tel.: +55-44-226-7471; fax: +55- 44-263-4334. E-mail address: cleber@deq.uem.br (C.S. Alves). 0304-8853/$-see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.01.054