Abstract
We present results of transport and magnetic properties and heat capacity measurements on polycrystalline CeFeAsO, PrFeAsO and NdFeAsO. These materials undergo structural phase transitions, spin density wave-like magnetic ordering of small moments on iron and antiferromagnetic ordering of rare-earth moments. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient and magnetoresistance are reported. The magnetic behavior of the materials have been investigated using Mössbauer spectroscopy and magnetization measurements. Transport and magnetic properties are affected strongly by the structural and magnetic transitions, suggesting significant changes in the band structure and/or carrier mobilities occur, and phonon–phonon scattering is reduced upon transformation to the low-temperature structure. Results are compared with recent reports for LaFeAsO, and systematic variations in properties as the identity of Ln is changed are observed and discussed. As Ln progresses across the rare-earth series from La to Nd, an increase in the hole contributions to the Seebeck coefficient and increases in magnetoresistance and the Hall coefficient are observed in the low-temperature phase. Analysis of hyperfine fields at the iron nuclei determined from Mössbauer spectra indicates that the moment on Fe in the orthorhombic phase is nearly independent of the identity of Ln, in apparent contrast to reports of powder neutron diffraction refinements.
Highlights
Soon after the discovery of superconductivity in doped LnFeAsO, studies of the undoped materials revealed interesting structural and magnetic properties
The structural phase transition is reported to occur near 158 K in CeFeAsO [21], 153 K in PrFeAsO [22] and 150 K in NdFeAsO [23]
Stripe-like magnetic order of small moments on the Fe sites is observed by neutron diffraction, suggesting the formation of a commensurate spin density wave (SDW)
Summary
NdFeAsO was made from FeAs, Nd2O3 and Nd powders. The rare-earth oxide was used as the oxygen source because we have found this route to give the highest purity samples of LaFeAsO [20]. Single-phase samples of CeFeAsO were made from CeAs, Fe2O3 and Fe powders. Rare-earth arsenides and FeAs starting materials were prepared as reported previously [13, 20]. Stoichiometric mixtures of the starting materials (1–4 g total masses) were ground and mixed thoroughly in a helium-filled glovebox, pressed into one half-inch diameter pellets and sealed in silica tubes under about 0.2–0.3 atm ultra-high-purity argon. Powder x-ray diffraction showed the products to be single-phase LnFeAsO (limiting impurity concentrations to less than a few percent) with the ZrCuSiAs structure type [figure 1(c)]
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