We investigated the unstable interface between vortices and anti-vortices in single crystals of iron-based superconductors (Ba0.67Rb0.33)Fe2As2 (Tc ∼ 37.6 K). These interfaces, also known as the Meissner hole, were captured with magneto-optical imaging in both pristine crystal and one with artificial point defects. Local excess current were observed flowing along the interface. In addition, we confirmed that the creep mode of such an interface follows the prediction of the critical state model, accompanied with a finite relaxation rate of magnetization.

Novel nematic states as well as remarkable superconducting properties were reported in iron-based superconductors. We synthesized high-quality single crystals of (Ba1-x Rb x )Fe2As2 close to the optimal composition of x ≈ 0.4, and have characterized normal state and superconducting properties for pristine and proton irradiated samples. The highest Tc was found to be ∼36.6 K for pristine samples. A maximum critical current density of 12 MA/cm2 was achieved in a proton irradiated sample, which is comparable to that in irradiated (Ba0.6K0.4) Fe2As2. Twin boundaries were observed optically in both non-doped and under-doped samples. We also attempted local magnetic field measurements induced by transport current in a sample with twin boundaries for future exploration of nematic states in different regions of doping.

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(Fe,Ni)2(P,Si) compounds were synthesized and characterized. Ni substitution in Fe1.95-xNixP0.7Si0.3 is found to favor the formation of Fe2P-type hexagonal structure. The samples appear nearly single phase. Powder oriented in the magnetic field shows a pronounced uniaxial magnetic anisotropy with c axis as the easy axis. Magnetization measurements carried out along and perpendicular to the c crystal axis demonstrate a significant magnetic anisotropy, making these materials potential candidates for permanent magnet applications. We found that (Fe,Ni)2(P,Si) compound has no remanent magnetic field and coercivity, but it has a large magnetocrystalline anisotropy at room temperature. Therefore, doping Fe2P type compounds with a small amount of Ni and Si may be a promising way to create new materials with large magnetocrystalline anisotropy at room temperature, and thus rare-earth free permanent magnet.