MAGNETORESISTANCE OF Sm1-xLaxS, Sm1-xYx S AND SmS1-xAsx ALLOYS

The e l e c t r o n i c t r a n s i t i o n in SmS involv ing a change in the valence s t a t e of Sm from 2+ towards 3+ , normally induced under p r e s sure / l / can a l so be induced a t atmospheric p re s su re e i t h e r by r ep lac ing Sm by a t r i v a l e n t r a r e e a r t h ion / 1 3 / or by a l l o y i n g SmS with SmP / 4 , 5 / SmAs / 6 / or SmSb /I/ . In t h i s paper , we w i l l focus our a t t e n t i o n on three systems: S m i x L a x S ' S m l x Y x S a n d SmS. As . The e l e c t r i c a l r e s i s t i v i t i e s p 1-x x and Hall c o e f f i c i e n t R„ of these a l l o y s a re very d i f f e r e n t from one system t o another and depend on the Sm valence s t a t e . Sm, La S forms a continuous s e r i e s of 1-x x s o l i d s o l u t i o n s , the valence being always in t e rmed ia t e between 2 and 3 / 2 / . All the a l l o y s of t h i s system e x h i b i t a m e t a l l i c c h a r a c t e r , with R„ nega t ive and approximat e l y independent of T / 8 / . T he c a r r i e r s a r e in t roduced e i t h e r by the La, o the r t r i v a l e n t r a r e e a r t h s or de fec t s which are a l so p r e sent in pure SmS . For low La concen t r a t i ons (x < 0 .05) the r e s i s t i v i t y passes through a Or minimum near 100 K and below this increases as Log I .This has been interpreted as a 34Kondo effect due to Sm with localized magnetic moments acting as impurities /8/. 3+The concentration of Sm , deduced from magnetization measurements, was between 1% and 3 .5% . The systems Sm. Y S and SmS, As J 1-x x 1-x x exhibit a large volume decrease and color change from black to gold, with increasing x. In the Sm Y S system, stable black samples are obtained for 0 < x < 0 .15 and gold in° Present address : University of California Dept of ?hys. Riverside.. CA 92521, USA. termediate valence (IV) samples for x ̂ 0.15 /9/ . All are metallic at room temperature {RT) /10/ . However, only the gold samples with x > 0 .4 and the black samples show the normal metallic variation of p and negative R„ between RT and low temperatures . For 0 .15 < x < 0 .4 p and R„ display more complicated variations /10/ because cooling below room temperature converts the samples from gold to black /9/. These properties have been interpreted by a simple density of states model /ll/ . In the SmS, As system, the alloys are semiconductors for x < 0 .05 and metallic at RT for higher.x /6/. They are IV forO.C7< x: < 0 .5 and purely Sm for x > 0.5. There is a concentration range (0 .07 < x < 0 .15) where the alloys transform from metallic to semiconducting character on cooling below RT. The resistivity thus shows a simple metallic variation for x :> 0 .15 (although R„ is temperature independent only for x> 0 .5) /6/ . Hence, it seemed interesting to compare the effect of a magnetic field on the resistivity of these different alloys . We have measured the magnetoresistance of typical alloys of each system in fields up to 180 kOe at 4 .2 K and 1-5K. However, we have eliminated from this comparison the alloys of the Sm, Y S and SmS. As systems which trans1-x x 1-x x J form on cooling . T he resistivities at RT, 4 .2 K and 1 .5 K, of the samples studied are shown in table I . The measurements where made by the Van der Pauw method, with the magnetic field H perpendicular to the sample . Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980531