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    Item type:Doctoral Dissertation, Access status: Open Access ,
    Ciepło właściwe zwiazków $RMn_{2}H_{x}$
    (2004-01-19) (Data obrony: 2008) Kolwicz-Chodak, Liliana
    Wydział Fizyki i Informatyki Stosowanej
    The $RMn_{2} (R=Gd, Tb, Er, Nd, Ho)$ can easily absorb a large amount of hydrogen up to 4.5 at.H/f.u. at low pressures and temperatures. In this compounds, the $Mn-Mn$ distance plays a crucial role in inducing a $Mn$ magnetic moment. An increase of the atomic distance by increasing hydrogen content in the $RMn_{2}H_{x}$ implies a very strong influence on their physical properties. A new method for the sample preparation for specific heat (SH) measurements was developed. It is based on mixing the powder of $RMn_{2}H_{x}$ with $Cu$ or $Ag$ and then compressing. SH measurements of $RMn_{2}H_{x}$ have been performed in two different set-ups. In the temperature range of 50 K -f 320 K by a quasi-adiabatic technique and in the temperature range from 2 K to 350 K in the magnetic field of 0 and 9 T in the PPMS by a relaxation method. The specific heat results of $RMn_{2}H_{x}$ show that: The hydrogenation implies a large addition to the specific heat in the whole temperature range of 50 K-300 K For the pure sample the phase transition was revealed by a single SH peak at the $T_{N}$, whereas a multiple SH peak were observed for the hydrides The increase of hydrogen concentration causes a large increase o the $T_{N}$ and was confirmed by magnetic measurements. Usually the highest temperature peak of SH is related to the antiferromagnetic phase transition what was confirming by magnetic measurements and by neutron scattering for $Tb$ and $Er$ samples $(x=2)$ Peaks near TN are considered to be connected with the hydrogen ordering In some cases an additional anomaly is attributed to inhomogeneous distribution of hydrogen in the sample.