Magnetic Ordering in V-Layers of the Superconducting System of Sr₂VFeAsO₃

Abstract

Results of transport, magnetic, thermal, and 75 As-NMR measurements are presented for superconducting Sr2VFeAsO3 with an alternating stack of FeAs and perovskite-like block layers. Although apparent anomalies in magnetic and thermal properties have been observed at ∼150 K, no anomaly in transport behaviors has been observed at around the same temperature. These results indicate that V ions in the Sr2VO3-block layers have localized magnetic moments and that V-electrons do not contribute to the Fermi surface. The electronic characteristics of Sr2VFeAsO3 are considered to be common to those of other superconducting systems with Fe-pnictogen layers. 2−4) This implies that Sr 2 VFeAsO 3 may give us an opportunity to examine whether Fermi-surface nesting is important for the occurrence of superconductivity of Fe pnictides. On this point, Mazin has reported that, because the FSs constructed by only the Fe orbitals of Sr 2 VFeAsO 3 are similar to those expected in other Fe pnictides, the nesting condition is also satisfied in the present system. 5) However, it seems important to experimentally ensure whether or not the electrons of V ions are itinerant and really contributing to FSs, before studying the relation between Fermi-surface nesting and superconductivity. KEYWORDSWe have carried out transport, magnetic, thermal, and 75 As-NMR measurements, and found that although apparent anomalies in the temperature (T ) dependences of the magnetic and thermal properties of Sr 2 VFeAsO 3 exist at ∼150 K, no anomalies in the transport properties have been observed. On the basis of these results, we argue the electronic state of Sr 2 VFeAsO 3 and answer the question regarding the contribution of V ions to FSs.Polycrystalline samples of Sr 2 VFeAsO 3 were prepared as described in refs. 1 and 6. SrAs powder was first obtained by annealing mixtures of Sr and As in an evacuated quartz tube at 850• C. Mixtures with proper ratios of SrAs, FeAs, SrO, Fe, and V 2 O 3 were pressed into pellets, sealed in an evacuated quartz tube with Ti powder (Ti/Fe = 1/4), which probably acts as the reducing agent, and then fired for 10 h at 900• C and for 30 h 1050• C, successively. The X-ray powder diffraction pattern of one of the * corresponding author(i45323a@cc.nagoya-u.ac.jp) obtained pellets with CuKα radiation at a step of 0.01• of the scattering angle 2θ is shown in Fig.1 (a), where the reflection indices are attached to the corresponding peaks of Sr 2 VFeAsO 3 , and the asterisks indicate the peaks from impurity phases of Sr 2 VO 4 or Sr 3 V 2 O 7 .7,8) The lattice parameters a and c were estimated to be 3.9329(1) and 15.6703(18)Å, respectively.The magnetic moments M were measured at magnetic fields H of 10 Oe and 1 T using a Quantum Design MPMS. The electrical resistivity ρ was measured by the four-terminal method with increasing temperature T at H = 0. The thermoelectric power S was measured by a dc method, where the typical temperature range between two ends of the sample was 0.2−2 K, depending on the temperature region. Details of the me...

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