Simulation Of High-Field Magnetization,ESR,and The Doping Effect Of Skew-Chain Compounds T₂V₂O₇(T=Ni,Co)

Low-dimensional magnetic materials are of great interest to condensed matter physicists due to their novel physical phenomena and quantum effects.Among them,the quasi-one-dimensional(1D)skew-chain T₂V₂O₇(T=Ni,Co)exhibits a coexistence of classical magnetic phase transitions and fractional magnetization plateaus and might be model materials for exploring quantum behaviors in classical antiferromagnets.The relevant research about this is absent,which is a theoretical challenge.In this paper,the magnetic phase transitions and magnetic excitations of T₂V₂O₇ are studied by high-field magnetization,ESR,and neutron diffraction,then the origins of these phenomena are interpreted via the first principles,exact diagonalization,and quantum Monte Carlo simulation.The main contents of the work are as follows:1.A brief introduction about the history of 1D spin chains and several typical 1D spin chain models are present.Then,the applications of high-field magnetization and ESR in the 1D spin chain are introduced.Finally,review the progress of the research on 1D skew-chain T₂V₂O₇(T=Ni,Co)and put forward the basis and the significance of this doctoral dissertation.2.The instruments involved in this work are described,with an emphasis on the basic principles of pulsed magnetic field magnetization and ESR systems.Besides,the theoretical calculation methods and their principles are briefly described and reliability tests are performed.3.The theoretical simulation of the high-field magnetization process of Co₂V₂O₇ is presented in details.Firstly,a brief overview of the magnetization plateaus is presented.When the field is along the b-axis,the wide 1/2 and narrow 3/4 magnetization plateaus at5.4–11.6 T and 15.7 T,respectively,are observed.For the a and c axes,there is only a sign of 1/2-like magnetization plateau.Then,the result of ESR reveals g=3.95 and the effective spin of S_eff=1/2.Neutron powder diffraction demonstrates a complicated non-collinear magnetic structure.The first-principles calculations show that the antiferromagnetic(AFM)inter-and intra-chain exchange interactions are comparable.Finally,by building a coupled skew-chain model,the magnetization curve is simulated by exact diagonalization and quantum Monte Carlo,and the reasonable exchange interaction parameters are obtained.The results show that the broad and stable quantum magnetization plateau originates from the cross of spin energy levels,the competing exchange interactions,and the strong quantum spin fluctuations.4.The magnetization process of Ni₂V₂O₇ polycrystalline samples is studied by using a 100 T magnetic field.The wide 1/2 magnetization plateau(11.5 T?30.4 T)and the wide 3/4 magnetization plateau(57.8 T?85.3 T)are observed.The magnetization plateau widths and critical magnetic fields are significantly larger than those of Co₂V₂O₇.The results of the exact diagonalization show that the exchange interactions of Ni₂V₂O₇ are larger than that of Co₂V₂O₇,and the strength of the interactions is mainly controlled by the size of the spin quantum number.5.The magnetic excitations of Ni₂V₂O₇ and Co₂V₂O₇ are studied in details.For Ni₂V₂O₇,ESR spectra confirm the presence of the AFM ordering and spin-flop transitions.Besides,a special resonance mode softens at the critical magnetic field of 1/2magnetization plateau.For Co₂V₂O₇,ESR spectra also confirm the presence of the AFM ordering and spin-flop transitions.A resonance mode corresponding to the transitions between the ground state and the excited state is observed,indicating that the energy gap between them is 660 GHz.In the system where classical and quantum magnetism coexists,the complex ESR modes cannot be described by the conventional AFM resonance theory with easy-axis anisotropy within the framework of the two-sublattice mean-field model.6.The non-magnetic Zn²⁺ions and magnetic Ni²⁺ions are selected to dope Co₂V₂O₇samples and the resultant effect on the magnetic phase transitions is investigated.For Co_2-xZn_xV₂O₇,with the doping of Zn²⁺ion,the AFM exchange interactions between the Co²⁺ion in the parent phase are partially destroyed.As a result,the spin-flop transition shifts towards the lower field,the 1/2 magnetization plateau gradually broadens,and the3/4 magnetization plateau gradually disappears.For Co_2-xNi_xV₂O₇,with the Ni²⁺ion doped,the new exchange coupling mechanism and single-ion anisotropy are introduced.As a result,the spin-flop transition moves towards the lower field,1/2 magnetization plateau shifts towards the higher field,and 3/4 magnetization plateau gradually disappears.