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An amorphous GdNiAl sample was prepared by mechanical grinding performed on a crystallised intermetallic. The treatment changes greatly the ferromagnetic behaviour of this compound; its Curie temperature decreases from 57 K (unmilled sample) to 29 K (milled sample). Specific heat and magnetisation measurements reveal that amorphous GdNiAl exhibits an interesting magnetocaloric effect; for an applied magnetic field of 5 T a change of ≅8.9 J/kg K is observed at 36 K for the isothermal magnetic entropy.

Hydrogenation of CeNiSn provokes a change of the ground state of cerium from Kondo insulator to antiferromagnetic ( CeNiSnH 1.0 ) or ferromagnetic ( CeNiSnH 1.8 ) behaviour. We report heat capacity measurements in CeNiSnH 1.0 performed in magnetic fields up to 90 kOe. The calculated magnetocaloric effect has a maximum at temperatures close to T N = 4.5 K . The values of the isothermal magnetic entropy ( Δ S m ) and the adiabatic temperature change ( Δ T ad ) are modest, and similar to those found in other Ce compounds. The field dependence of both, heat capacity and magnetocaloric effect clearly indicate the nature of the magnetic transition.

The ternary compounds CeCoSi and CeCoGe exhibit an antiferromagnetic ordering at 8.8 and 5.0 K, respectively. We present magnetization and specific heat measurements on these compounds and their hydrides, CeCoSiH and CeCoGeH. The hydrogen absorption reveals an original transition from antiferromagnetic to spin fluctuation behavior. CeCoSiH and CeCoGeH present a spin fluctuation temperature T sf around 130 and 15 K, respectively.

We have studied both the crystal chemistry and magnetic, transport, and thermal properties of the hydride $\mathrm{Ce}\mathrm{Ni}\mathrm{Ga}{\mathrm{H}}_{1.1(1)}$. This compound crystallizes in the hexagonal $\mathrm{Al}{\mathrm{B}}_{2}$-type structure with a random distribution of nickel and gallium atoms on the B site, which has an important influence upon the macroscopic properties. Its thermoelectric power versus temperature indicates that cerium is in a trivalent state. The electrical resistivity displays two minima, which could be expected for the Kondo-type interactions in the presence of crystal field effects. Specific heat measurements up to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ allow us to determine the splitting energies ${\ensuremath{\Delta}}_{1}=100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${\ensuremath{\Delta}}_{2}=159\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A broad maximum is observed around $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the specific heat in the low temperature region. This maximum and its evolution with the applied magnetic field, are discussed in the framework of the existing theories, which point toward the existence of short-range magnetic correlations and spin glasslike freezing below $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This study reveals: (i) that the hydrogenation of the intermediate valence gallide CeNiGa induces a valence transition for cerium which is purely trivalent in the hydride and (ii) the absence above $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of long-range magnetic ordering resulting from structural disorder around Ce atoms.

The hydrogenation process of the intermediate valence compounds CeNiAl, CeNiGa, CeNiIn, CeNiSi and CeNiGe and of the Kondo semiconductor stannide CeNiSn has been investigated. Magnetization and electrical resistivity measurements reveal interesting physical transition induced by the insertion of hydrogen: (i) the Kondo temperature decreases from 600(20) to 220(10) K in the sequence CeNiGe → CeNiGeH1.6(1); (ii) the hydrides CeNiAlH1.93(5) and CeNiGaH1.1(1) contain cerium in trivalent state and the former shows the presence of magnetic correlations around 3.6(2) K; (iii) in contrast, CeNiInH1.8(2) presents a ferromagnetic transition at TC=6.8(2) K and a magnetic transition Kondo semiconductor → antiferromagnet (TN=4.5(2) K) → ferromagnet (TC=7.0(2) K) is evidenced in the CeNiSnHY system. These results are explained by a modification of the competition between RKKY magnetic interaction and Kondo interaction induced by the hydrogenation of these intermetallics.

The new hydrides CeNiSnH 1.0(2) and CeNiSnH 1.8(2) have been investigated by X-ray powder diffraction, magnetization, electrical resistivity and specific heat measurements. The hydrogen insertion into CeNiSn reveals an interesting magnetic transition: Kondo insulator (CeNiSn)→antiferromagnet (CeNiSnH 1.0(2) )→ferromagnet (CeNiSnH 1.8(2) ). The physical properties of these compounds are influenced by the Kondo effect.

The Laves phases GdMn 2 has been recently prepared in amorphous structure. We present heat capacity measurements under magnetic fields up to 9 T in both polycrystalline and amorphous states. No distinguishable anomaly at the magnetic order temperature is observed in the amorphous sample. In both cases a very low field dependence of the heat capacity is observed. The calculated magnetocaloric effects give rise to a maximum centred at 37 K in the crystalline form and a low, but broad maximum centred at 130 K in the amorphous sample.

In the intermediate valence (IV) CeNiIn, the Ce ions evolve towards a trivalent state with a ferromagnetic transition at low temperature when hydrogen is interstitially inserted. In this work, we present heat capacity measurements on CeNiInH 1.8 carried out under magnetic fields up to H = 90 kOe . The observed λ -anomaly and its evolution with magnetic field confirm the existence of the ferromagnetic transition. The total entropy shifts to higher temperatures with increasing field, giving rise to the appearance of magnetocaloric effect. We have found that both the isothermal magnetic entropy ( Δ S m ) and the adiabatic temperature change ( Δ T ad ) display significant peaks around 6.5 K, despite the small magnetic moment of the Ce ions.

The isostructural Co 2− x Cu x (OH)PO 4 ( x =0, 1 and 2) phases have been prepared from hydrothermal synthesis and characterized from powder X-ray diffraction. The structure consists of a three-dimensional framework in which M(1)O 5 -trigonal bipyramid dimers and M(2)O 6 -octahedral chains are simultaneously present. Magnetization measurements of Co 2 (OH)(PO 4 ) show the existence of two maxima attributed to a three-dimensional antiferromagnetic ordering at 70 K and a spin-glass-like state at 12 K. When Co 2+ is substituted by Cu 2+ ions, the spin-glass behavior disappears and the magnetic order is decreased.