Magnetoelectric Properties Of Vanadium Oxide R₂V₂O₇（R=Co,Ni） In Pulsed High Magnetic Fields

Multiferroic and low-dimensional frustration are frontier science in material and condensed matter physics.In multiferroic material,various orders coexist and couple with each other leading to intriguing magnetoelectric and magnetoelastic effects.On the other hand,multiferroic material may also have a degenerate ground state and show excited quantum states due to the low dimensionality and magnetic frustration.Therefore,the combination of multiferroicity and frustration makes the system to be an ideal model to study the correlation between these interesting physical properties.In this work,we focus our interest on the vanadium oxides R2V2O7(R=Co,Ni),which are typical quasi-onedimensional frustrated materials examined by crystallographic and magnetic investigations.These two materials are also a new family compound of type-II multiferroic through electric-polarization measurements.We prepared high-quality polycrystalline and single crystal R2V2O7(R=Co,Ni)and measured magnetization,electric polarization,dielectric constant,magnetrocaloric effect in pulsed high magnetic fields.The whole thesis is organized as follows:Chapter 1.We introduce the background of multiferroic and the physical phenomena such as field-induced multiferroicity and Bose-Einstein condensation of magnons.Recent high magnetic field study on multiferroic materials is also reviewed.Chapter 2.We introduce the pulsed high-magnetic-field facility including experimental techniques,methods and principles of magnetization,electric polarization,magnetocaloric effect and dielectric constant.Chapter 3.Here we report high-field study on polycrystalline R2V2O7(R=Co,Ni).Highquality ceramic samples are prepared by the solid-state reaction.High-field magnetization reveals that Co2V2O7 and Ni2V2O7 exhibit obvious 1/2 magnetization plateau at 7-12 T and 8-30 T,respectively.In addition,both materials exhibit magnetic-field induced ferroelectricity with a maximum value of 50-60 μC/m2.The low-and high-field ferroelectric(FE)phases are located on two sides of the magnetization plateau.Through comprehensive magnetization and polarization measurements,we construct magnetoelectric H-T phase diagrams of two compounds.For Ni2V2O7,a 3/4 magnetization plateau in the region of high magnetic fields is observed.In case of Co2V2O7,a strong magnetoelectric coupling is revealed.These experimental findings evidence that this system is a multifunctional material with both quantized plateau and multiferroicity.Chapter 4.We study the field-induced transitions and multiferroicity in Co2V2O7 single crystals.The crystals of R2V2O7(R=Co,Ni)are grown by a flux method with high quality.High-field magnetization shows that Co2V207 exhibits an anisotropic magnetization plateau.For H ‖ b,the 1/2 plateau is robust whereas for H ‖a or H‖ c the plateau is largely smeared that the M(H)curve merely shows a bend around the 1/2 plateau.We make an in-depth study of magnetic transitions,phase boundary and H-T phase diagram of Co2V2O7 single crystals by means of electric polarization,dielectric constant and magnetocaloric effect.For H ‖ a or H ‖ c,the low-and high-field FE phases are connected.However for H ‖ b,the low-and high-field FE phase are well separated by the magnetization plateau,indicating that the quantum plateau can fully suppress the ferroelectricity.We also study the H-T phase diagram and the multiferroicity of Ni2V2O7 single crystals in pulsed high magnetic fields.Different from Co2V207,Ni2V2O7 has an isotropic 1/2 magnetization plateau which appears in the field range of 8-30 T.An overall H-T phase diagram is obtained via specific heat,susceptibility,magnetization and electric polarization measurements.Further magnetoelectric investigation on Ni2V2O7 single crystals is highly desired and on progress.Chapter 5.We explore successive electric-polarization switches in Co2V2O7 single crystal by application of a magnetic field along the b axis.In this field direction,a 1/2-magnetization plateau is seen at 5.4-11.6 T.The electric polarization measurements show that polarization reversal appears at～5 T from-P ‖ b to+P ‖ b,leading to an irreversible magnetoelectric history involved with a memory effect,while the polarization flop in fields of 12-17 T is identified by a transition from the P ‖ b to P ‖(ac)plane,different from previous reports for Mn-based materials.This is the first observation that both polarization reversal and polarization flop can take place in the same compound and the same field direction.These intriguing magnetoelectric phenomena are owing to the unique nature of the skew-chain-like magnetic structure of Co2V2O7 and can be understood by a change in symmetry of the magnetic order in applied fields.The emergent ferroelectricities deviating from the half-plateau state may arise from magnon Bose-Einstein condensation in this quantum magnet.Chapter 6.A summary and prospect.