Abstract
The crystallographic and physical properties of polycrystalline and single-crystal samples of and K-doped are investigated by x-ray and neutron powder diffraction, magnetic susceptibility , magnetization, heat capacity nuclear magnetic resonance (NMR), and electrical resistivity measurements versus temperature . The crystals were grown using both Sn flux and CoAs self-flux, where the Sn-grown crystals contain 1.6–2.0 mol% Sn. All samples crystallize in the tetragonal -type structure (space group . For , powder neutron diffraction data show that the -axis lattice parameter exhibits anomalous negative thermal expansion from 10 K to 300 K, whereas the axis lattice parameter and the unit cell volume show normal positive thermal expansion over this range. No transitions in were found in this range from any of the measurements. Below 40–50 K, we find , indicating a Fermi liquid ground state. A large density of states at the Fermi energy states/(eV f.u.) for both spin directions is found from low- measurements, whereas the band-structure calculations give states/(eV f.u.). The enhancement of the former value above the latter is inferred to arise from electron-electron correlations and the electron-phonon interaction. The derived intrinsic monotonically increases with decreasing , with anisotropy . The shift data versus have the same dependence as the derived data, demonstrating that the derived data are intrinsic. The observed nuclear spin dynamics rule out the presence of Néel-type antiferromagnetic electronic spin fluctuations, but are consistent with the presence of ferromagnetic and/or stripe-type antiferromagnetic spin fluctuations. The crystals of were grown in Sn flux and show properties very similar to those of undoped . On the other hand, the crystals from two batches of grown in CoAs self-flux show evidence of weak ferromagnetism at K with small ordered moments at 1.8 K of and per formula unit, respectively.
20 More- Received 27 June 2014
- Revised 15 August 2014
DOI:https://doi.org/10.1103/PhysRevB.90.064517
©2014 American Physical Society