Magnetism and superconductivity of Eu-based ‘122’ pnictides

With the discovery of high temperature superconductivity in Iron pnictides, investigation of transition metal pnictide compounds with layered tetragonal structure showing interesting physical properties has evolved as a topic of current interest in condensed matter physics. Among the ‘122’ pnictides, EuFe2As2 got particular attention as it shows interesting interplay between superconductivity and magnetism. The doping of iridium in EuFe2As2 induces superconductivity which coexists with Eu2+ magnetic order. The optimally doped system undergoes a superconducting transition below 22.2 K with double reentrant feature at lower temperature. The Eu2+ moments order magnetically at ~17K with an appreciable ferromagnetic (FM) component. The competition between the superconducting and the magnetic state leads to the reentrance of superconductivity. This presentation is further investigated using magnetic measurements, specific heat studies and Mössbauer spectroscopy measurements. In order to extend these studies we have synthesized a new compound EuCr2As2 in the ‘122’ pnictide family. The Eu-ions are in a stable divalent Eu2+ state in this compound with local magnetic moment of 7.0 µB. Magnetic susceptibility measurements on the EuCr2As2 crystals show isotropic ferromagnetic-like behavior below 21K but there is no hysteresis or remanent magnetization observed in the magnetization measurements. In addition, a large negative magnetoresistance is found at lower temperature. Magnetization studies and electronic structure calculations demonstrate that in addition to the local moment magnetism of Eu ions, Cr ions also possess itinerant moment which is expected to order antiferromagnetically above room temperature. This behavior is reminiscent of the parent compound of Fe based superconductor EuFe2As2 but with a slightly different magnetic structure.  We have further investigated the evolution of magnetism on Ge-doping at P-site in valence fluctuating EuNi2P2. While EuNi2P2 shows moderate heavy fermion behavior, EuNi2Ge2 exhibits antiferromagnetic order below TN = 30K. We have grown single crystals of EuNi2(P1-xGex)2 and investigated physical properties of these crystals. Valence fluctuation persists up to x = 0.3 and for     x = 0.5, Eu is in a divalent state and the Eu2+ moments order antiferromagnetically below 12 K.