Transport Properties And Magnetic Torque Study Of ThCr₂Si₂-Type Iron-based Superconductors And Related Magnetic Materials

There are many different kinds of materials which have a ThCr₂Si₂-type layered structure, people mainly focused on their magnetic properties. Recently, the discovery of 122-type iron-based superconductors（FeSCs） spurs people’s interest to this kind of materials again. It may help us to discover some new superconductors and understand the mechanism of high temperature superconductivity by studying and comparing the physical properties in different kinds of ThCr₂Si₂-type materials. In this thesis, we report the studies of superconductivity in electron-doped BaFe2-x NixAs2 and magnetism in Ca_1-xPr_xCo₂As₂ system, respectively, by the measurements of transport properties and magnetic torque. The main contents are shown as following:（1） Some basic physical properties in FeSCs BaFe2-x NixAs2 are introduced in this part, including structure phase transition, spin density wave（SDW） transition, upper critical fields Hc2, anisotropy and quantum criticality et al. Then, a systematically investigation of magneto-resistivity in BaFe（2-x）Ni_x As2 revealed the existence of filamentary superconductivity（FLSC） in the underdoped region. Besides, compared with bulk SC, we found that FLSC exhibits little doping dependence and it will vanish abruptly in the overdoped region where the long-range antiferromagnetic（AFM） order is absent. Our results suggest a close relation between FLSC and magnetic order.（2） Angular dependent of out-of-plane magnetic torque have been performed on a series of high quality BaFe_2-xNi_xAs₂ single crystal. Firstly, SDW transition temperature TSDW and anisotropy parameter γ are obtained from the reversible magnetic torque, as the corresponding results obtained from transport measurements. Then, our results in underdoped samples show that the irreversible torque signal survives up to a temperature Tirr well above Tc, suggesting the existence of superconducting vortex above Tc. However, this irreversibility vanishes above Tc in optimally-doped and overdoped compounds. Compared with some relevant results in high-Tc copper oxides, we speculate that the existence of irreversibility above Tc in underdoped samples may be due to the strong superconducting fluctuations in this region.（3） A series of Ca_1-xPr_xCo₂As₂ single crystals are grown by self-flux method. Firstly, the magnetic properties of CaCo₂As2 single crystal are systematically studied by using dc magnetization and magnetic torque mesurements. An A-type antiferromagnetic（AFM） ground state with TN = 74 K and with easy axis aligned along the c axis are revealed in CaCo₂As2. For H || c, the external magnetic field will induce spin-flop transition and finally the magnetization tends to saturate with increasing magnetic field. The possible magnetic structure and detailed magnetic phase diagram are also given by the combination of magnetization and magnetic torque measurements. Then, we report the magnetic properties in Ca_1-xPr_xCo₂As₂ single crystals. We find that the ground state changed from AFM in CaCo₂As2 to ferromagnetic（FM） in Pr-doped CaCo₂As2. Besides, for the Pr content x≥0.25, the phenomenon of magnetic pole reversal are observed at low temperature, suggesting the antiferromagnetic coupling between the spins of Pr 4f electrons and Co 3d electrons.