One impurity effect of Tm in YSe. Kondo effect and Van Vleck behavior

— Low temperature magnetization and resistivity measurements in the dilute Tm^Y! _JSe system show both Tm 2 + and Tm 3 + like behavior. However, since the results are nearly linear with concentration, they apparently reflect the properties of each thulium site. This paper presents a new approach to the TmSe mixed valence problem [1]. The idea is to build up the rather complex properties of the concentrated system starting from the substitution of Tm ions in non magnetic YSe, which has a lattice constant equal to that of stoichiometric TmSe (5.71 A). We report here on the first step of this study, namely the low concentration single impurity limit. The inverse susceptibilities of two samples of TmxY1_xSe with x = 0.01 and 0.052 are shown in figure 1 between 4.2 and 300 K. The temperatureindependent contribution of the matrix which is responsible for the curvatures can be deduced from a %T vs T plot of the less concentrated sample. Correcting our data for this contribution (X0 = 6.52 x 10 uem/moleYSe) results in Curie-Weiss law from which a Curie constant of 3.70 uem/mole Tm is obtained in both samples. It is remarkable that this value is intermediate between those of Tm (2.57 uem) andTm (7.13 uem) ions, indicating that the situation in these dilute alloys is very similar to that already encountered in concentrated TmSe ; the proportion of Tm is about 75 % in the present case. The low temperature inverse susceptibilities (T < 4.2 K) shown in figure 2 can be interpreted in the same model, by assuming both Tm + and Tm to be in their crystal field ground states (Tm + : Van Vleck, non-magnetic ; Tm + : magnetic). We have fitted our results below 4.2 K to the law :