Experimentelle Bestimmung der Supraleitungsparameter einiger Verbindungen vom β-Wolframtyp aus Magnetisierungsmessungen

A complete analysis of magnetization curves is given for bulk sintered samples of Nb₃Sn with low hysteresis and for finely powdered sintered and melted samples of Nb₃Sn, Nb₃Al, V₃Si and V₃Ga. The Ginzburg-Landau parameterκ and the penetration depthλ of weak magnetic fields is calculated from the temperature dependence of the thermodynamic and upper critical fieldsH _c andH _c2. The slope of theH _c ?T curve is about 360 Oe/°K and approximately the same for all measured samples. There is an extremely high increase of the lower critical fieldH _c1 if the grain size approachesλ. The occurence of field expulsion and partially reversible magnetization curves of sintered Nb₃Sn samples with high sintering temperatures and high pore volumes is explained by means of connections between single grains which have lower transition temperatures.