We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca3 Co Nb2O9, in which the effective spin of Co2+is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant θCW ^-55 K and the low Neel temperature TN~ 1.45 K give a frustration factor f =| θCW/TN|≈ 38, suggesting that Ca3 Co Nb2O9resides in strong frustration regime. Slightly below TN, deviation between the susceptibility data under zero-field cooling(ZFC)and field cooling(FC) is observed. A new magnetic state with 1/3 of the saturate magnetization Ms is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca3 Co Nb2O9is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.
We report 75As NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1-xPrxFe2As2 . In both cases of x = 0.075, 0.15, a large increase of νq upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structure transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/75T1 is enhanced and forms a peak at T ≈ 25 K, which may be caused by the magnetic ordering of the Pr3+ moments or spin dynamics of mobile domain walls.
The newly discovered iron-based superconductors have triggered renewed enormous research interest in the condensed matter physics community. Nuclear magnetic resonance (NMR) is a low-energy local probe for studying strongly correlated electrons, and particularly important for high-T C superconductors. In this paper, we review NMR studies on the structural transition, antiferromagnetic order, spin fluctuations, and superconducting properties of several iron-based high-T C superconductors, including LaFeAsO1-xF x , LaFeAsO1-x , BaFe2As2 , Ba1-xKxFe2As2 , Ca0.23Na0.67Fe2As2 , BaFe2 (As1-xPx )2 , Ba(Fe1-xRux )2As2 , Ba(Fe1-x Cox )2 As2 , Li1+xFeAs, LiFe1-xCoxAs, NaFeAs, NaFe1-xCoxAs, KyFe2-xSe2 , and (Tl,Rb)yFe2-x Se2.
