CDW’s AND SDW’s IN Ta0.95Fe0.05S3 WITH MAGNETIC FIELDS AT T = 4.2 K

Quasi-one-dimensional Ta0.96Fe0.0SS3 has been studied in magnetic fields applied at T = 4.2 K by Miissbauer spectrcxcopy. The amplitudes of the CDW's are increased. Also a phase transition is induced; SDW's then coexist with CDW's. When an external magnetic field is applied to charge-density-wave (CDW) conductors at low temperature new phenomena are sometimes observed. For example, Parilla et al. [I] find an anomalous magneto-resistance in monoclinic NbSe3; it seems the ratio of condensed (CDW's) to normal electrons is increased. When a field of 3.5 kOe is applied to the twodimensional material 2H-TaS at T = 1.5 K evidence for the coexistence of a spin-density wave (SDW) with a CDW was obtained [2]. A Mossbauer study of CDW's in the quasi-onedimensional family Tal-,Fe,S3 has been reported [3]. We now present a study of T~.~sFeo.osS3 when an external magnetic field is applied at T = 4.2 K. The sample was made by employing high temperature and high pressure [3]. The crystal structure is monoclinic, the same as for monoclinic TaS3 and similar to that for NbSes. Six sulfur ions form the corners of a trigonal prism with a Ta ion at the center. Neighboring prisms share triangular faces and are stacked along the b axis to form lengthy chains. Three types of prism, designated I, I1 and 111, have different S-S bond lengths in the basal plane perpendicular to the b axis, namely 0.2068,0.2105 and 0.2835 nm, respectively [4]. For the similar compound NbSe3, NMR measurements show that chains I and II are in the CDW state and chain I11 is an insulator [5]. Our earlier Mossbauer data show that the probability of an iron atom in chains I, I1 and I11 is 3 : 3 : 4, respectively [3]. The absorber was in the form of a disk of randomly oriented particles immobilized in benzophenone. The external magnetic field was perpendicular to the plane of the disk. The Mossbauer source was Co in a Rh host, a triangular drive was used and the data folded. Since the S-S bonds of chains I and I1 are nearly equal, the assumption was made that these two chains are in the same CDW state. Also, the iron atoms were assumed to have the same probability to lie in the electron excess region (the peak of the CDW's or P-branch) and the deficient region (the valley of the CDW's, or Vbranch) of chains I and 11. The number of iron atoms on chain I11 was fixed at 40 % . It was then found that three patterns provided an adequate fit to each Mossbauer spectrum. VELOCITY (mrn s-' ) Fig. 1. Mossbauer spectra (dots) at T = 4.2 K in various external magnetic fields. For H = 0 and 0.5 kOe 3 doublets are fitted, for higher fields 3 sextets (solid lines). The Mossbauer spectra obtained as a function of the external magnetic field are shown in figure 1. Clearly, a marked change in the spectra occurs at H = 10 kOe, that is, at a threshold field somewhat less than 10 kOe a phase change occurs. Three doublets are fitted to the spectra at H = 0 and 0.5 kOe. For H 2 10 kOe some and perhaps all of the subpatterns are hyperfine-field split. However, because the magnetic and quadrupole interactions are comparable in magnitude and because each spectrum is rather featureless, an unambiguous fitting is difficult. A fit with two sextets and one doublet was Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988864 C8 156 JOURNAL DE PHYSIQUE not obtained. A satisfactory fit was provided by three sextets; some simplifying assumptions were needed. The assignments of two subpatterns to the P-branch and V-branch of the I and I1 chains and the third to chain 111 are indicated by the bar diagrams in figure 1. It is unexpected that chain I11 has a hyperhe field; its origin is presumably the interactions with chains I and 11. The hyperfine parameters deduced from the data, vie, the isomer shifts, 6, the quadrupole shifts, E , and the hyperfine fields, Hhf, are plotted in figure 2. The isomer shift of the P-branch decreases and that of the V-branch increases as the external field increases; a small discontinuity occurs at the phase transition. Since the charge. density at the Fe nuclei is inversely proportional to 6, it follows that the amplitude of the CDW's increases with H. The implication is that the electronic states near the Fermi level are altered so that the ratio of condensed (CDW) to normal electrons is increased. The isomer shift for chain I11 is almost unchanged. The quadrupole splitting does increase with H for all patterns; it seems that the lattice distortion increases as the field is increased. The increase for chain I11 may occur via the long-range Coulomb forces with chains I and 11. When a hyperfine field is present it usually follows that magnetic ordering has occurred. The magnetieation when H = 30 kOe as a function of temperature is shown in figure 3; the data suggest the ordering is antiferromagnetic. Hence, SDW's appear to be induced and coexist with the CDW's. Acknowledgment. This research was partially funded by the Natural Sciences and Engineering Research Council of Canada. [I] Parilla, P., Hundley, M. F. and Zettl, A., Phys. Rev. Lett. 57 (1986) 617. [2] Butz, T., Ebeling, K.-H., Hagn, E., Saibene, S., Zech, E. and Lerf, A., Phys. Rev. Lett. 56 (1986) 639. [3] Dai, S., Zhou, X. Z., Morrish, A. H. and Shan, Z., Phys. Rev. B 36 (1987) 1. [4] Meerschant, A., Guemas, L. and Rowel, J., C.R. Acad. Sci. Ser. C 290 (1980) 215. [5] Devreux, F., J. Phys. France 43 (1982) 1489. EXTERNAL FIELD (kOe) Fig. 2. Hyperfine parameters as a function of external magnetic field applied. Fig. 3. The magnetization as a function of temperature. , TEMPERATURE (K) 0.20 P E, 0.15 g N 0.10 z 8 I 1 I Tao.95 Fe0.05 S3 7 Applied Magnetic Field : 30 kOe -