The Study On The Magnetism And Related Properties Of Spin Frustrated Cobalt Based Oxide Ca₃Co₂O₆

Due to the competition between the different internal interactions,spin frustrated materials often have the degenerate ground states.Owing to the strong degeneracy,small perturbations,such as thermal disturbance,magnetic field,or chemical composition variation,can make a great change in the physical properties of these materials.Quasi-one-dimensional spin chain compound Ca3Co2O6 and double perovskite compound A2CoMnO6(A = rare-erath element)are two typical frustrated materials,which exhibt a lot of peculiar physical phenomena,such as,spin density wave,spin glass state,magnetization steps,and so on.In this thesis,we select Ca3Co2O6 and A2CoMnO6 materials as our research objects,and their structural,magnetic,electronic and thermal transport properties are systematically studied.We attempt to conclude the law of magnetic evolution,explore the correlations between the magnetic and electronic properties,and got a deep understanding of the spin frustrated properties of these materials.We hope our work could provide a guideline for the design of new spin frustrated materials.The main contents of this thesis are as follows:(1).We successfully tune the magnetic exchange interactions in Ca3Co2O6 material by magnetic element doping at Ca site.The results indicate that the intra-chain magnetic exchange constant J decreases with the increase of Sm content.When the doping content reaches the critical point x = 0.5,the main magnetic exchange interaction changes from ferromagnetic to antiferromagnetic,and a new antiferromagnetic transition appears.Dy-doping at Ca site will also reduce the strength of ferromagnetic interaction of Ca3Co2O6.The long range magnetic transition is completely suppressed when the doping content reaches x = 0.4.Unlike the Sm-doped samples,there is no new magnetic transition can be observed in the Dy-doped samples.(2).The Ca3Co2O6 single crystals are grown by flux method and the magnetization step phenomenon is systemically investigated.It is found that the low-temperature magnetization steps are closely related to the magnetic history during the cooling process.We also find that the strength of ferromagnetic interaction can be reduced and the magnetization steps can be suppressed through appropriate Cu-doping.The induced Cu2+ ions can reduce the resistivity due to the increase of carrier concentration.Moreover,the doped Cu2+ ions will lead to a part of Co3+ transfer into Co4+,which enhances the spin entropy of materials,and then increases the thermopower.The decreased resistivity and enhanced thermopower give rise to the large enhancement in the figure of merit.The room-temperature figure of merit of the x = 0.6 sample is about 60 times larger than that of the un-doped sample.(3).The carrier type tunning of Ca3Co2O6.It is found that the appropriate Pb-doping at Ca-site and F-doping at O-site in Ca3Co2O6 can change the main conduction type from p-type to n-type.Moreover,a part of Co3+ ions will transfer into Co2+,which leads to the decrease of intra-chain ferromagnetic interaction.For the Pb-doped samples,the particle size and sample density increase due to the effect of the induced liquid phase PbO during the sintering process,which leads to the decrease of resistivity.Whereas F-doping has a great influence on the thermopower of Ca3Co2O6.The thermopower of F-doped sample can be tuned from 752.7 ?V·K-1(x = 0.2)to-333.1 ?V·K-1(x = 0.4).(4).We successfully synthesized the Y2Co1-xMn1+xO6 and La2CoIrO6 samples by solid-state-reaction method and their structural,magnetic and dielectric properties have been systemically investigated.The results indicate that the magnetic interactions and magnetization steps of Y2Co1-xMn1+xO6 are closely related to its chemical composition.With the increase of Mn and the decrease of Co content,the strength of ferromagnetic interaction decreases and the magnetization steps are gradually suppressed.Compared to Y2CO1-xMn1+xO6,La2CoIrO6 sample exhibts more abundant magnetic properties.With the decrese of temperature,La2CoIrO6 undergoes a paramagnetic-ferromagnetic transition at TC?90 K,followed by a reentrant spin-glass transition at lower temperatures.Moreover,we find that the dielectric relaxation behavior and magnetodielectric phenomenon also exist in La2CoIrO6 at low temperatures.