Abnormal Magnetoresistance on Non-superconducting NdFeAsO1-y

Abstract

The emergence of superconductivity in Fe-based oxypnictides has attracted much attention. To determination of the cause of the inhibition of superconductivity in the nonsuperconducting oxypnictides is crucial for further increasing superconducting transition temperature. Non-superconducting oxypnictides can also be useful for the future development of the Josephson device using these materials. Thus, the transport properties of non-superconducting oxypnictides should be studied. A typical phenomenon indicative of the temperature dependence of resistivity is the appearance of a peak at around 150K. The formation of a spin density wave (SDW) is strongly suggested at around this temperature. Below this temperature, the reported temperature dependence of resistivity varies. Some of reported ones show logarithmic divergence and others show no divergence. The common tendency is a large magnetoresistance below the temperature giving the peak of resistance. It remains to be determined which tendency reflects an intrinsic property. To answer this question, we prepare two types of samples with the same nominal composition and almost the same synthesis conditions. With these samples, we measured magnetoresistance and conclude that both tendencies reflect the true electronic property of non-superconducting oxypnictides. Polycrystalline samples were prepared by the highpressure synthesis method described in ref. 3. We selected two samples designated #080515 and #080520 for the transport study. The lattice parameters of #080515 and #080520 are a 1⁄4 3:967 A and c 1⁄4 8:570 A, a 1⁄4 3:967 A and c 1⁄4 8:576 A, respectively. The oxygen content of #080515 estimated by neutron diffraction analysis was NdFeAsO0:95, which is larger than the nominal value. The difference is probably due to the possible oxidation of the starting material (Nd, Fe, or As) or oxygen gain during the synthesis. Although the oxygen content of the sample #080520 was not estimated, judging from the lattice parameters, the difference in oxygen content between the two samples is small. For resistivity measurement, we adopted the four-terminal method. Using a Quantum Design Physical Property Measurement System (PPMS), we applied a magnetic field of up to 14 T perpendicular to the electric current, which is a 7.5Hz square wave excitation of 1mA. The voltage drop parallel to the excitation current was measured, and resistance was calculated. We show the X-ray diffraction (XRD) patterns of #080515 and #080520 in Fig. 1. These two samples are almost identical in their XRD patterns. Figure 2 shows the temperature dependence of resistivity. In contrast to that shown in the XRD pattern, the absolute values and temperature dependences of the resistivities of these two samples are markedly different. The difference results from the small difference in oxygen content between the samples. The resistivity of #080515 is 60% larger than that of #080520 at 300K. Although the resistivity of #080515 has a divergent tendency at a low temperature, that of #080520 show no clear upturn at a low temperature. In both samples, we show the peak of the resistivity at around 140K. Above this temperature, the temperature dependences of the resistivities of the two samples are similar. Below 140K, magnetoresistance becomes prominent, although the position of the peak itself is insensitive to magnetic field. We plot ðRH R0Þ=ð 0H R0Þ, where 0H is the magnetic field, RH is the resistance under the magnetic field, and R0 is the resistance under a zero magnetic field, in Fig. 1. ðRH R0Þ=ð 0H R0Þ is almost qualitatively equivalent to each other within an error of 10%. The general tendency of the magnetoresistance obtained by this measurement is consistent with those in other reports. There is a peak at around 150K and below it an abnormal magnetoresistance is observed. Two typical temperature dependences below 150K are reproduced. #080515 shows a logarithmic-like upturn at a low temperature but #080520 shows no such an upturn. Our major finding is the common temperature dependence of ðRH R0Þ=ð 0H R0Þ in these two samples. Because magnetoresistance sharply starts at around 150K near the peak position, it is easy to imagine that it relates to the origin of the 150K anomaly. This anomaly is usually interpreted as an indication of the emergence of SDW triggered by structural phase transition. Anti-ferromagnetic instability was also pointed out by band calculation. In te ns ity [ ar bi tr ar y un it]