Thermopower in the Ba1−δM2+xRu4−xO11<

Abstract

For comparison, the Ru metallic sample crystallizes in a 255 hexagonal structure, of the same space group of P6 3 /mmc as 256 the 124 ruthenates, and its elementary cell parameters are a = 257 b = 2.7092(1) Å and c = 4.2887(1) Å with a Ru-Ru distance 258 of about 2.67 Å. 259 B. 57 Fe Mössbauer experiments 260 For Sr 1−δ Fe 2.7 Ru 3.3 O 11 , Mössbauer spectrometry was used 261 to investigate in more detail the Fe distribution in the dif-262 ferent crystallographic sites and to investigate the Fe nature 263 in this oxide. The Mössbauer spectra of Sr 1−δ Fe 2.7 Ru 3.3 O 11 264 compound at 295 K (room temperature) and at 20 K (low 265 temperature) are shown in Fig. 4. The room-temperature 266 spectrum shape in the paramagnetic phase contains three 267 overlapping quadrupole doublets, each one associated with 268 a specific Fe site [Fig. 4(a)]. It is important to note that the 269 amount of metallic Fe impurity (0.02 wt.%) deduced from 270 the Rietveld refinement is too small to be detected by Möss-271 bauer spectroscopy, especially since the spectral area of such 272 The evolution of MR with T presented in Fig. 10(a) shows 455 that MR continuously decreases as a function of temperature, 456 except around T C where a maximum is observed for M = Co 457 and M = Mn. At low T, the high-field MR values follow the 458 magnetization behavior M, with MR the largest for M = Mn. 459 At 5 K and 5 T, the comparison with Fig. 7 shows that 460 these MR values follow the evolution of magnetization M 461 and moreover reflect the evolution of the M 2 behavior, with 462 MR ∼ M 2 , i.e., MR(M = Mn)/MR(M = Fe) = 3, similar to 463 the ratio M 2 (M = Mn)/M 2 (M = Fe), while these ratios reach 464 close values respectively, 2.5 and 3.5, for the comparison 465 between Fe and Co. This low-temperature M 2 behavior is 466 typically observed in the presence of spin-polarized tunnel-467 ing at grain boundaries as in manganites [42] or in SrRuO 3 468 [38] and most probably reflects the polycrystalline nature of 469 the samples. Consistently, the M(H) and MR(H) curves are 470 interrelated, as the MR curves are reversible for M = Co, 471 Mn, but display hysteresis below ∼25 kOe for M = Fe, in 472 good agreement with the large coercive field measured on the 473 M(H) loops (Fig. 7). It is interesting to note that three differ-474 ent behaviors are observed depending on the transition-metal 475 cation M, with a maximum of MR observed around T C for 476 M = Co, a maximum at 5 K observed for M = Mn together 477 with a secondary maximum around T C , and finally only very 478 small values for M = Fe and no peak around T C . The coupling 479 between magnetism and transport at T C is therefore maximum 480 for M = Co. 481 3. Thermopower 482 The temperature-dependent Seebeck coefficients S(T ) 483 of the three samples are presented in Fig. 11. In 484 A 1−δ Mn 2.4 Ru 3.6 O 11 , S(T ) is linear and positive from ∼100 to 485 750 K, reaching ∼22.5 μV K -1 . At T < 100 K, S is very close 486 Sr 2 RuO 4 [12] with Ru 4+ , which gave for the high-T limit 561 S = (k B ...