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Submitted on 1 Jan 1978
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SUPERCONDUCTIVITY IN TRANSITION METAL
SUBSTITUTED A-15 SYSTEMS
F. Cadieu, N. Chencinski
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 8, Tome 39, aozit 1978, page C6-391
F.J. Cadieu and M. Chencinski
Department of Physisc, w e n s College of Cuny, Flushing, New York 11367, U.S.A.
R6sumb.- Diverses phases 1 haut Tc dgrivant de NbsGe par substitution d'blbments de transition voi- sins, Mo, Ti, Zr ont 6tb synth6tis6e.s par "sputtering" sur substrat chauffb. Le changement de Tc est relie 1 la variation du nombre moyen d'blectrons d et du rayon atomique. Quelques mesures de dHc2/dT ont 6tB effectuees jusque dans des champs magnstiques de 10 T.
Abstract.- We have been synthesizing high Tc systems based on NbsGe in which another transition metal atom, either Mo, Ti or Zr, is partially substituted for Nb. Systems have been prepared by selectively thermalized sputtering onto heated substrate. The systems investigated allow changes in Tc to be ob- served due to a variation in the electron per atom ratio as well as due to atom size effects. Some measurements of dHc2/dT at fields up to 10 T have been made.
For the Nb-Ge system by a process of selective thermalization in sputtering onto heated substrates we have been synthesizing samples in which another
transition metal atom is partially substituted for the Nb atom to optimize the effective transition me- tal atom in this system. Samples have been synthesi- zed of the form (Nbl-yTy)l-xGex in which T = Mo,Ti or Zr which are neighboring transition metal atoms to Nb in the periodic table. Some work has been re- ported earlier/l,Z/. We have now added more samples and some magnetic field measurements of dH /dT near c2 the superconducting transition temperature, Tc, for fields up to 10 T. For the case of Mo versus Zr sub- stitutions. Mo adds one additional outer electron per Mo atom over that of a Nb atom while Zr adds one less per Zr atom. This allows a systematic method of varying the electron population in the system. For the case of Ti and Zr substitutions, these two metals occur in the same column next to Nb in the periodic table and hense should contribute the same number of outer electrons to the crystal bonding. One of the major differences in the effect of substi- tuting these two atoms is expected to arise from the difference in the A-15 radii for Ti and Zr. Since the Ti atom is smaller than the Zr atom it is expec- ted that Ti could be more easily incorporated into the structure than Zr. Such effects are observed in that higher Tcls for the same amount of Ti versus Zr substitution have been observed. Some movement of the A-15 phase field to higher Ge concentrations has also been observed for the Ti case.
Samples have been synthesized onto heated po- + Supported by PSC-BHE Award Program and NSF DMR 74-18138
lycrystalline alumina substrates by trisputtering under conditions of selective thermalization/3/. The superconducting transition temperatures were measu- red inductively so that only bulk Tc's were recor- ded. In the cases reported complete inductive tran- sitions have been observed. To determine the actual atomic composition along a substrate of varying com- position we have employed quantitative X-ray fluo- rescence spectroscopy. The relative X-ray intensity spectrum for Ge to Nb was then converted to composi- tion by a fairly elaborate standardization procedu- re which resulted in f 1 at% absolute error.
In figure 1 we present some data which show the variation of T versus measured Ge fraction for Nbl-xGex' (Nb0.99Zr~.~l)l-xGex,
and for (Nbo,96Tio.os)l-xGex. These systems show a maximum in T versus Ge content at about 21-22 at % for the Nb-Ge system and at about 21 at % for the Nb-Ti-Ge and Nb-Zr-Ge systems. The Nb-Ge system for these deposition conditions exhibits a maximum T of 21.1 K and a lattice parameter of 5.152 for the single phase A-15 deposit at a Ge concentration of 21 at %. The deposit is single phase A-15 material from 15-21 at % Ge. The relative changes in Tc pro- duced upon substituting small amounts of Ti are less than the changes produced by adding the same frac- tion Zr. For 4 at % of the Nb substituted with Zr the T is less than 6 K while for.the same Ti sub- stitution the T is still above 12 K. For
(Nbo,99Tio,o~)o,~9Geo.~~ a maximum in T of 21.0 K with a width of 1 K is observed. The resistance ra-
tio R/R is 2.2 at this composition point and then rapidly drops toward one to either side of this point. The inductive measured transition shows no
References
~ i 1 ~: .(Top) : the Tc versus Ge atomic fraction for (Nb-Ge)Ti. (Bottom) the Tc versus Ge atomic frac- tion for Nb-Ge and for (Nb-0.01Zr)Ge.
structure and drops to zero indicating that there is no amorphous component in the film.For (Nbo,ggZro,o~) 0,81Ge0,19 a maximum Tc of 17.6 K has been achieved. The exact Tc value is sensitive to the sputt'ering conditions employed. Crystal phase analysis shows that for 1 at % Zr substitution the single phase A-15 boundary has been pushed down to 19 at % Ge while 4 at % Ti substitution the single phase A-15 boundary is still about 21 at % Ge. These changes are as expected for atomic size considerations. We are presently working on adjustments of the sput- tering conditions.
For the (Nbo,g~Zro,ol)l-xGex system measure- ments of dH /dT for fields up to 10T for the resis-
c2
tive Tc onset point went from 3.6k0.4 T/K to2.6k0.3 T/K as x varied from 0.24 to 0.19. At x = 0.23 the resistive T onset was 18.9 K (H = 0). Extrapolation /4/ to 0 K for no Pauli paramagnetic limiting gives H (T = 0 K) = 47k5 T.
c2
The assistance in the Z-ray fluorescence ana- lysis of L. Bergel is gratefully acknowledged.
/1/ Cadieu,F.J., Chencinski,N. and Rosen,C.Z., J. Appl. Phys.
48
(1977) 686121 Cadieu,F.J., Chencinski,N., Proc. Phys. Transi- tion Metals, Toronto, 1977
/3/ Cadieu,F.J., Chencinski,N., IEEE Trans. Magn. MAG-11 (1975) 227
