Measurements of the heat capacity (2 to 30 K) and the magnetic susceptibility (2 to 300 K) are reported for nearly homogeneous YbFe,Mo,S, compounds of the Chevrel phase, with x = 0, 0.02, and 0.04. For x = 0 and x = 0.02 the samples show a superconducting transition a t T, z = 9 K and T, = 3 K, respectively, whereas for x = 0.04 the superconductivity is completely suppressed by the iron content. All samples show a sharp L-type anomaly in nearly the same form within the specific heat a t the same temperature TI. = 2.6 K. The magnetic susceptibility for the YbFeo.ojMo,S, compound reveals in low magnetic fields a t the same temperature an anomaly which is typical for an antiferromagnetic ordering. Therefore i t is concluded that the I-anomaly can be caused by a "soft" antiferromagnetic long-range ordering of the Yb3+ magnetic moments arranged in the superconducting matrix. Es werden die Warmekapazitat (2 bis 30 K) und die magnetische Suszept,ibilitat (2 bis 300 I<) fur nahezu homogene YbFe,Mo,S,-Verbindungen der Chevrel-Phase mit x = 0; 0,02 und 0,04 gemessen. Fur x = 0 und x = 0,02 zeigen die Proben einen Supraleitungsiibergang bei T , = 9 K bzw. T , = 3 K, wahrend fur z = 0,04 die Supraleitung durch den Eisengehalt vollstandig unterdruckt wird. Alle Proben zeigen eine scharfe I-Anomalie von nahezu gleicher Form im Verlauf der spezifischen Warme bei der gleichen Temperatur T, = 2,6 I(. Die magnetische Snszeptibilitiit fur die YbFeo,ojMo,S,-Verbindung zeigt bei kleinen Magnetfeldern bei der gleichen Temperstur eine Anomalie, die fur eine antiferromagnetische Ordnung typisch ist. Deshalb wird angenommen, daB die I-Anomalie durch eine ,,weiche'' antiferromagnetische Fernordnung der magnetischen Yb3+-I\lomente, die in der supraleitenden Matrix angeordnet sind, verursacht sein konntc.Recently the interest turned upon compounds, in which magnetic order phenomena appeared in the superconducting state [l to 51. I n all these compounds the magnetic effects observed are related to RE-ions which are present in them. I n some of them thc occurrence of the magnetic order (predominantly ferromagnetism) is related to the destruction of the superconducting state. Such compounds seem to confirm the theoretical predictions of the Mueller-Hartmann-Zittartz theory [6].According to other authors the occurrence of the magnetic order (predominantly antiferromagnetism) in the superconducting state represents a coexistence of both effects. The precondition for the appearance of the above-mentioned effects is that the exchange interactions between the magnetic moments of the RE-ions and the conduction electrons must be weak. On the other hand, a concentration of magnetic ions too small for a magnetic long-range order can already lead t o a considerable decrease of the critical temperature T, necessary for the transition into the superconducting state.The results obtained by investigating the heat capacity of YbMo,S, [7] in the temperature range from 2 to 20 K showed the appearance of the superconductivity l) 53-529 Wroclaw, Poland. 2) Prese...
