We report on the crystal structure, physical properties, and electronic structure calculations for the ternary pnictide compound EuCr2As2. X-ray diffraction studies confirmed that EuCr2As2 crystalizes in the ThCr2Si2-type tetragonal structure (space
group I4/mmm). The Eu ions are in a stable divalent state in this compound. Eu moments in EuCr2As2 order magnetically below Tm = 21 K. A sharp increase in the magnetic susceptibility below Tm and the positive value of the paramagnetic Curie temperature obtained from the Curie-Weiss fit suggest dominant ferromagnetic interactions. The heat capacity exhibits a sharp {lambda}-shape anomaly at Tm, confirming the bulk nature of the magnetic transition. The extracted magnetic entropy at the magnetic transition temperature is consistent with the theoretical value Rln(2S+1) for S = 7/2 of the Eu2+ ion. The temperature dependence of the electrical resistivity r{ho}(T) shows metallic behavior along with an anomaly at 21 K. In addition, we observe a reasonably large negative magneto-resistance (~ -24%) at lower temperature. Electronic structure calculations for EuCr2As2 reveal a moderately high density of states of Cr-3d orbitals at the Fermi energy, indicating that the nonmagnetic state of Cr is unstable against magnetic order. Our density functional calculations for EuCr2As2 predict a G-type AFM order in the Cr sublattice. The electronic structure calculations suggest a weak interlayer coupling of the Eu moments.
The J1-J2 model on a square lattice exhibits a rich variety of different forms of magnetic order that depend sensitively on the ratio of exchange constants J2/J1. We use bulk magnetometry and polarized neutron scattering to determine J1 and J2 unambi
guously for two materials in a new family of vanadium phosphates, Pb2VO(PO4)2 and SrZnVO(PO4)2, and we find that they have ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground state is reduced, and the diffuse magnetic scattering is enhanced, as the predicted bond-nematic region of the phase diagram is approached.
In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination, magnetic susceptibility, electrical resistivity, and specific heat are reported. The observati
on of bulk superconductivity in all thermodynamic properties -- despite strong disorder in the Fe-As layer -- favors an itinerant picture in contrast to the cuprates and renders a p- or d-wave scenario unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3) leads to the suppression of the magnetic ordering present in SrFe_2As_2 due to a rigid down-shift of the Fe-3d_(x^2-y^2) related band edge in the density of states.
