Magnetoelectric Properties Of Multiferroic Materials RMnO₃ And Ni₃V₂O₈ Under Pulsed Magnetic Fields

Due to the existence of magnetic and ferroelectric orders,multiferroics have rich physical properties and wide application prospects.High magnetic field is an important method to study magnetic structures and spin mutual interactions in materials,and explore novel physical phenomena,so it plays a more and more remarkable role in the field of studying multiferroics.In this doctoral dissertation,we performed a systematic study on three typical multiferroic materials.In the materials,rare-earth manganites RMnO₃?R=Ho,Er,Yb?,having hexagonal crystal structure,are type-I multiferroics;while DyMnO₃ and Ni₃V₂O₈,having orthorhombic crystal structure,belong to type-II ones.On the other hand,RMnO₃?R=Ho,Er,Yb?and Ni₃V₂O₈ respectively possess triangular and Kagomémagnetic structures,therefore they also belong to typical frustrated magnetic materials.These different physical attributes make them exhibit rich magnetic transitions,quantum phase transitions and exotic magnetoelectric properties under high magnetic fields.The primary contents are as follows,1.We briefly introduce the research background of multiferroics,then systematically present basic physical properties and research progresses of hexagonal RMnO₃?R=Ho,Er,Yb?,orthorhombic RMnO₃?R=Eu-Dy?and KagoméNi₃V₂O₈.2.We briefly introduce growth methods of single crystals and high-magnetic-field devices.We grew single crystals of hexagonal HoMnO₃ using optical floating zone technique,and constructed magnetization and electric polarization measurement systems in high magnetic fields.3.We systematically studied magnetic transitions,high-field H-T phase diagrams,magnetic structures and magnetic symmetries in hexagonal RMnO₃?R=Ho,Er,Yb?.As magnetic fields are applied along c axis,RMnO₃?R=Ho,Er,Yb?show rich magnetic-field-induced phase transitions.ErMnO₃ undergoes three successive transitions at 0.8,12 and 28T;YbMnO₃ undegoes two transitions at 2.8 and 25 T;while HoMnO₃ undergoes transitions at 0.85,1.29,2.1,2.33,8 and 41 T.By the analysis of magnetization processes in ErMnO₃,we revealed that magnetic reorientation of representation?₂ induces the magnetic transitions at 12 T,i.e.,from a one-third plateau to a fully polarized phase.Accordingly,YbMnO₃ and HoMnO₃ have the similar magnetization process.YbMnO₃ undergoes the transition of?₂?ferromagnetic state at 28 T,while HoMnO₃ undergoes it across two transitions at 8 and 41 T.In addition,we further revealed that magnetic ground states of three compounds can be all described by the representation?₁.By the analysis of magnetization jumps and completed high-field phase diagram,we finally got the information about magnetic structures in different magnetic ordered phases and magnetic symmetries,etc.4.We studied magnetic transitions,magnetic-field-induced ferroelectric states,dynamic process of ferroelectric domains,and temperature-and degree-dependent magnetoelectric properties of orthorhombic DyMnO₃ via systematical high-field magnetization and electric polarization measurements.As magnetic fields are applied along b axis,high-field magnetization process shows two magnetic transitions at 1.4 and 36 T,while electric polarization data?E//a?reveals many different magnetic ordered or ferroelectric states between 1.4 and 36 T.By measurements in varying temperatures,we constructed high-field magnetoelectric phase diagram of DyMnO₃,and found that two main ferroelectric states of low-and high-field gradually separate with increasing temperatures.In addition,high-field ferroelectric state includes more phase-transition features.Different from low-field one,it is sensitive to degree variation of magnetic fields.These experimental results reveals that two ferroelectric states arise from different physical microcosmic mechanisms.We speculate that high-field state is probably attributed to common contributions of Mn³⁺and Dy³⁺ions,and mainly arises from exchange striction mechanism.5.We systematically studied high-field magnetoelectric properties of Ni₃V₂O₈,including magnetic-field-induced ferroelectric states,high-field magnetoelectric phase diagram,magnetoelectric history and magnetoelectric memory effect.We found that a-axis magnetic fields induce two ferroelectric states respectively at 2-11 T and 19-24 T,and electric polarization is along b axis.With the b-axis bias electric fields,the ferroelectric state in 19-24 T can be well controlled,while the ferroelectric state exhibits unusual magnetoelectric properties:when magnetic fields are below 11 T,the electric polarization is frozen even in opposite bias fields;while above 11 T,the polarization is released and undergoes a two-step reversal by tuning bias electric fields.We ascribe these phenomena to magnetoelectric memory effect of Ni₃V₂O₈,i.e.,nonpolar ground state preserves the information of ferroelectric state.Our study firstly reveals high-field properties of magnetoelectric memory effect,and well explains the origin of the memory effect by“nucleation-pinning-depinning”mechanism.