Study On Physical Properties And Doping Effects In The J_eff=1/2 Antiferromagnet Sr₂IrO₄

Coupling of charge,spin,orbit,and lattice is the basis for generating many novel states of matter,e.g,spin correlation produces magnetism,spin-charge coupling generates magnetoresistance,spin-phonon coupling causes superconductivity,and spin-orbit coupling induces multiferroicity.In general,the spin-orbit coupling is often treated as perturbations.The situation is continued until the discovery of magnetically induced multiferroicity.In the past decade,spin-orbit coupling has been found to be essential to electronic structure of topological materials and 5d heavy element compounds.In 5d transition metal oxides,spin-orbit coupling is comparable with other fundamental interactions such as electron correlation and Hund's coupling.Therefore,the delicate competition among them induces new physics and new effects inaccessible in 3d compounds.In this thesis,we will systematically study physical property of Sr₂IrO₄ which is a model system with strong spin-orbit coupling.The major contents and innovations are summarized as following:(1)Firstly,history of strongly correlated electron system is reviewed briefly,and the Mott-Hubbard model,charge-transfer model,spin-orbit coupling theory are illustrated.Then,a series of novel phenomena arising from spin-orbit coupling will be introduced.Subsequently,physical properties and latest research works on Sr₂IrO₄ are presented.At last,main contents of the thesis are shown.(2)A series of single crystal samples Sr_xIrO₄(1.4?x?2.2)were synthesized via flux method.With decreasing x,the resistivity of Sr_xIrO₄ decreases significantly,which is mainly caused by the increase of hole concentration and Ir-O-Ir bond angle?.On the contrary,the long range antiferromagnetic order looks robust against the Sr-defects,and there is no apparent variation for T_N.In addition,a magnetic field driven insulator-metal transition is observed in Sr₂IrO₄,which is probably related to a spin scattering mechanism.(3)A series of single crystals(Sr_1-xRE_x)₂IrO₄(0?x?0.05,RE=La,Nd,Sm,Y)were prepared using the flux method.It is found that La-doping can easily triggermetal-insulator transition,and destroy the long-range antiferromagnetic order,due to collapse of the Mott gap and increase of the Ir-O-Ir bond angle?.In addition,with decreasing the radius of rare earth ions of(Sr_0.98RE_0.02)₂IrO₄,the Néel temperature is moved to higher temperature,and resistivity is enhanced gradually approaching the parent compound.The results show that charge and lattice are essential in tuing the physical properties of Sr₂IrO₄.(4)Sr₂(Ir_1-xGa_x)O₄(0?x?0.09)single crystals were prepared through flux method.Upon increasing,resistivity of Sr₂(Ir_1-xGa_x)O₄ is suppressed significantly,and an insulator-to-metal transition is induced as?0.05.It is attributed to the increase of hole concentration,decrease of spin-orbit coupling,and increase of Ir-O-Ir bond angle?.On the contrary,the long range antiferromagnetic order is robust against Ga-doping,and T_N just shows slight decrease as x is increased.Furthermore,remarkable fourfold anisotropic magnetoresistance is evidenced in all samples,resulting from magnetocrystalline anisotropy energy.It is noted that the anisotropic magnetoresistance in the samples with metallic transport can still be as large as approximately 1%,which is sizable in comparison with those in the antiferromagnetic metals reported so far.Our results show that Sr₂(Ir_1-xGa_x)O₄system could be good candidate for antiferromagnetic spintronics.(5)Detailed magnetotransport measurements have been performed on Sr₂IrO₄single.It is found that the anisotropic magnetoresistance of Sr₂IrO₄ can be as giant as?160%at T=35 K,arising from the cooperation of GMR-like effect and magnetocrystalline anisotropy.This is the largest value that has been reported so far.Moreover,nonvolatile memory effect is evidenced in the tiny Ga-doped sample Sr₂Ir_0.99Ga_0.01O₄.Our results show that multi-mechanism combination is one of the effective ways to improve the antiferromagnetic electronic effect.