Study On Microstructure And Physical Properties Of Multiferroic CuFeO₂

The CuFeO2is of particular interest because of its single-phase multiferroic and strong magnetoelectric coupling.Many of potential applications are dependent on its spin frustration and extraordinary magnetic structure.However,the mechanism of electromagnetic tuning in CuFe02 is still unclear.By combining positron annihilation with other techniques we have characterized the involution of the microstructure,and discuss their effects on the physical properties of CuFe02 in detail.1.CuFe02 ceramics are fabricated by solid-state reaction with different synthetic procedure.The effects of synthetic procedure on microstructure,evolution of defects and magnetic properties of CuFe02 ceramic are studied in detail.The phase structure and morphology of CuFe0-2 ceramics depend on the synthetic procedure are indicated by X-ray diffraction and Scanning electron microscopy results respectively.X-ray photoelectron spectroscopy measurements show the impure Cu2+ ions are present in all CuFeO2 samples.Positron annihilation results reveal that the vacancy defects are present in all samples and the average size of defects and geometry of volume defects increases significantly with the increasing of sintering temperature,while the concentration and distribution of electron density of positron traps remains the same.Magnetic measurements display that the stability of antiferromagnetic phase is inhibited by sintering temperature but not affected by sintering time.The research shows that the stability of antiferromagnetic phase is mostly related with lattice structure,element valence state and the evolution of vacancy defects within samples under different synthetic procedure.2.The structure,crystal defects and magnetic properties of multiferroic Ga-doped CuFeO2 ceramics are studied systemically.Substitution of Ga3+ for Fe3e shrinks the CuFeO2 lattice,decreases the particle size,and causes small liquid phase formation.Positron annihilation spectroscopy demonstrates that all samples contain a considerable number of vacancy defects.The overall defect environment is virtually unaffected by Ga3+ doping,but the open-volume of the defects is redistributed.Magnetic susceptibility measurements show that Ga3+doping enhances the strength of the antferromagnetic interaction between high-spin Fe3+ ions as a result of reduced magnetic correlation length,but decrease the stability of the antiferromagnetic phase.The antiferro-magnetic transition temperature,TN2,decreases from 12 K for x=0 to 8 K for x=0.07.The experimental results show that this destabilization of the antiferromagnetic phase is closely related to the crystal structure and defects.3.Evolution of the microstructure,optical,and magnetic properties have been investigated systematically in multiferroic CuFe1-xSnxO2(x=0-0.05)ceramics.Substitution of Sn4+ for Fe3+ results in expansion of CuFeO2 lattice,and reduces the density of the material,but the metal oxidation states are unchanged.Observation of the optical properties shows that the value of the direct optical band gap(Eg)decreases with increasing Sn doping level,and that the CuFe1-xSnxO2(x=0-0.04)series with values greater than 3.1 eV.Magnetic susceptibility measurements show that Sn4+ doping decreases the Curie-Weiss temperature,but does not affect the stability of the antferromagnetic phase.Magnetization curves show that changes occur in the magnetic interactions and both ferromagnetism and antiferromagnetism co-exist in the Sn4+-doped samples.The changes in the magnetic behavior are closely related to the lattice distortion and charge compensation.4.The microstructure,defect and magnetic properties of multiferroic CuFe1-xGexO2(x=0-0.10)series are studied systematically.The introduction of Ge4+ in Fe3+ sites of CuFeO2 leads to changes in its crystal structure,micro-morphology and surface chemical status.Positron annihilation results show that the concentration of positron traps and the local electron density increases evidently inside CuFe1-xGexO2(x?0.05)series compare with that of un-doped sample.Interestingly,the magnetic hysteresis loops show the magnetic interactions in CuFe02 are changed and obvious ferromagnetism appears in the CuFe1-xGexO2(x?0.05)samples.In addition,the Curie-Weiss temperature saw a fall from-137 eV of x=0 sample to-341 eV of x=0.10 sample,and the translation temperatures TN2 slightly decreases with increasing Ge4+content.The relation between the microstructure and the magnetic behavior are explored in detail.5.The low temperature magnetic ground state of CuFeO2 sample is investigated using the magnetocaloric effect.Measurement of isothermal magnetization curves for different applied magnetic fields near magnetic transition temperatures(16 K and 11 K)shows a reversal in the magnetization trend around 16 K.Arrot plots show that these two phase transitions are accompanied by second-and first-order magnetic phase transitions,respectively.Meanwhile a dominant antiferromagnetic ordering for CuFe02 at T<16 K is observed by the magnetocaloric effect analysis.