Study On Magnetic Properties Of Charge Ordering State In Na_xCoO₂ System And Iron-based Superconductors

In conventional theory of condensed matter physics,scientists have not paid enough attention on electron-electron interaction and this was always ignored in many situations.However,with more and more discovery of new materials,scientists found that the electron-electron interaction would play an improtant role in these new materials than their conventional counterparts,such as high-Tc cuprates,manganites with colossal magnetoresistance,heavy fermion materials etc.How to understand these materials became a research frontier in condensed matter physics-strong correlated electron system.In this dissertatiori,we have studied two kinds of correlated electron system.One is Na_xCoO₂ system with triangle lattice.The other is the iron-based high-Tc superconductor.Transport,magnetic properties,Raman spectra and electron spin resonance spectra have been measured to study novel properties in these materials.In last chapter,we have also studied the anisotropic magnetoresistance in lightly doped Nd_2-xCe_xCuO₄.The whole dissertation is constructed as following:1.Brief overview of research on Na_xCoO₂ system and iron-based superconductor In this chapter,the author has reviewed the basic physical properties and some hot topics in Na_xCoO₂ system and iron-based superconductors.For Na_xCoO₂ system,we have reviewed the crystalline structure and phase diagram,and also discuss the two hot issues on charge ordering and Na⁺ ion ordering.For iron-based superconductors, we have introduced the crystalline structure and phase diagram.The electronic structure and theoretical models for iron-based superconductor have been also reviewed.2.Magnetic-field-induced spin-flop transition in Na_xCoO₂(0.5＜x＜0.55) In this chapter,the isothermal magnetoresistance(MR) with magnetic field(H) parallel and perpendicular to the ab plane is systematically studied on the single crystal Na_0.52CoO₂ with charge ordering at～50 K and an in-plane ferromagnetism below 25 K.The isothermal MR behavior with H//ab plane and that with H⊥ab plane are quite different.When H//ab plane,the MR is always negative and the in-plane ferromagnetic behavior is enhanced,while the MR with H⊥ab plane changes from negative to positive with decreasing temperature or increasing H,and the in-plane ferromagnetic behavior is suppressed.A striking feature is that the MR with H⊥ab plane shows a hysteresis behavior below 25 K,which is absent for the case of H//ab plane.These results provide strong evidence for a spin-flop transition of small moments of Co^3.5-δ sites induced by H⊥ab plane,leading to a metamagnetic transition for small moments of Co^3.5-δsites.Such complex magnetism suggests an unconventional superconductivity in Na_xCoO₂system because the charge-ordering Na_xCoO₂ around x = 0.5 has been considered to be the parent compound of superconductivity.3.Rearrangement of sodium ordering and its effect on physical properties in the Na_xCoO₂ systemIn this chapter,We systematically study Raman spectroscopy of Na_xCoO₂ single crystals with 0.37＜x＜0.80.The Raman shift of Alg mode is found to be linearly dependent on Na content,while the Raman shift of Elg mode has an abnormal shift to high frequency around x = 0.5.The abnormal shift is ascribed to the occurrence of Na rearrangement in O1 structure.Temperature-dependent Raman spectrum for x = 0.56 sample shows that Na rearrangement transition from O1 structure to H1 structure occurs around 240 K.Electronic transport and susceptibility for the sample with x = 0.56 show a response to the Na rearrangement transition from O1 to H1 structure and that different Na ordering pattern causes distinct physical properties.These results give a direct evidence to prove Na ordering effect on physical properties of Co-O plane.4.Evidence for local moments by electron spin resonance study of polycrystalline LaFeAsO_1-xF_x(x = 0 and 0.13)In this chapter,the temperature dependence of electron spin resonance was studied in the oxypnictide superconductors LaFeAsO_1-xF_x(x = 0 and 0.13).In the samples,the ESR signal indicates that the g factor and peak-to-peak linewidth strongly depend on temperature,especially at low temperatures.It indicates a strong-coupling picture with existence of local moment.The dependence mentioned above gradually attenuates and tends to saturation around room temperature.This behavior could be ascribed to the "bottleneck" effect due to coupling of local moments and itinerant electrons.In addition,a Curie-Weiss-type behavior is also observed in temperature dependent spin susceptibility.Our results strongly support the existence of local moments in these materials,while their origin is still unclear.The results also indicate strong magnetic frustration in this system,and the magnetic fluctuation mechanism for superconductivity is suggested.5.Magnetic phase diagram in Eu_1-xLa_xFe₂As₂ single crystalsIn this chapter,we have systematically measured resistivity,susceptibility and specific heat under different magnetic fields(H) in Eu_1-xLa_xFe₂As₂ single crystals.It is found that a metamagnetic transition from A-type antiferromagnetism to ferromagnetism occurs at a critical field for magnetic sublattice of Eu²⁺ ions.The jump of specific heat is suppressed and shifts to low temperature with increasing H up to the critical value,then shifts to high temperature with further increasing H.Such behavior supports the metamagnetic transition.Further,a two-folding in-plane magnetization is found in antiferromagnetic state which is similar with spin density wave(SDW) order.In contrast,isotropic in-plane magnetization is presented in both ferromagnetic and paramagnetic state.Moreover,ferromagnetic state becomes more easily established under external field with suppressing SDW order.An anisotropic exchange model is proposed to understand our findings.Our results suggest that metamagnetism of Eu²⁺ ions is somewhat related to SDW order.Finally,detailed H-T phase diagrams for x = 0 and 0.15 crystals are given,and possible magnetic structure is proposed.6.Giant anisotropy of the magnetoresistance and the 'spin valve' effect in antiferromagnetic Nd_2-xCe_xCuO₄In this chapter,we have studied anisotropic magnetoresistance(MR) and magnetization with a rotating magnetic field(B) within the CuO₂ plane in lightly doped AF Nd_2-xCe_xCuO₄.A giant anisotropy in the MR is observed at low temperature,below 5 K.The c-axis resistivity can be tuned over about one order of magnitude just by changing the B direction within the CuO₂ plane,and a scaling behavior for the out-of-plane and in-plane MR is found.A 'spin valve' effect is proposed for explaining the giant anisotropy of the out-of-plane MR and the evolution of the scaling parameters with the external field.It is found that the field-induced spin-flop transition of the Nd³⁺ layer under high magnetic field is the key to understanding the giant anisotropy.These results suggest that a novel entanglement of charge and spin dominates the underlying physics.