Multiferroicity And Multi-field Coupling Effects In The Metal-organic Framework [（CH₃）₂NH₂]M（HCOO）₃

Multiferroics are defined as the materials exhibiting two or more ferroic properties concurrently,including ferroelectricity,ferromagnetism,and ferroelasticity.In the past decade,these materials have been widely concerned,and have become one of the hotspot research directions in condensed matter physics and materials science.This is associated with their broad prospects in practical applications and their rich physical mechanisms.Furthermore,the magnetoelectric coupling effect and multi-field coupling behavior have received great attention in multiferroics.In recent years,with the rapid development of synthesis theory and technology,metal-organic framework materials have become a new member in multiferroic family.These materials combining metal ions with organic groups exhibit unprecedented structural variety and rich chemical controllability,which compared with inorganics,provides more freedom for designing new multiferroics.Metal-organic frameworks usually have relatively loose structures and large-size framework holes.Their structural configurations are very flexible and changeable.Generally,the long-range magnetic orders come from their frameworks,and the long-range electric orders originate from the polar organic cations in cavities.Although the magnetic and electric properties derive from different structure units,their frameworks and inlaid organic groups are connected by hydrogen bonds,which makes the arrangement and configuration of both entwined closely.Based on these characteristics,some materials are expected to exhibit some interesting coupling effects among magnetism,electricity and lattice.In addition,because of their soft structure,small external pressures can be an effective tool to deform their lattice greatly,thereby modifying their electric and magnetic properties.Above all,metal-organic frameworks are ideal candidates for the study of multi-field coupling effect.This thesis focuses on the perovskite metal-organic frameworks-[（CH₃）₂NH₂]M（HOOC）₃（M=Mn,Co,Fe,Ni）to study their multiferroic properties and multi-field coupling effect.The results are as following:（Ⅰ）Research of multiferroicity and magnetoelectric coupling effect in metal-organic framework-[（CH₃）₂NH₂]Ni（HOOC）₃.By measuring dielectric constants and pyroelectric current,it can be confirmed that a first-order structure transition takes place at 181 K in DMA-Ni,where the system transforms from paraelectric state to ferroelectric state.With further cooling,this material exhibits a spin-canted antiferromagnetic order below 37.6 K and spin-reorientation transition around 15 K.The coexistence of long-range magnetic and ferroelectric orders indicates that[（CH₃）₂NH₂]Ni（HOOC）₃ is a type-Ⅰmultiferroic material below 37.6 K.Moreover,the magnetoelectric coupling effect exists in its paramagnetic state,where the electric polarization can be promoted by increasing the magnetic fields.This behavior is mainly derived from the combined effect of local magnetostriction and piezoelectricity.（Ⅱ）Research of external pressures tuning the order-disorder transition of hydrogen bonds in[（CH₃）₂NH₂]Mn（HOOC）₃.Under ambient pressure,its first-order structure transition takes place in a very narrow temperature range,which results in a huge pyroelectric coefficient peak and a huge thermal expansion near its structure transition temperature,much larger than those of inorganic ferroelectrics.The effect of pressure on hydrogen bond ordering in DMA-Mn has been studied by dielectric and pyroelectric measurement in a piston-cylinder cell.The antiferroelectric state,where ordered hydrogen bonds arrange in antiparallel,is induced by applying certain pressures,while the original ferroelectric state is suppressed intensively,and even disappears.These phenomena imply that the framework has a close relationship with DMA cations.Compressing the framework makes hydrogen bonds favor to arrange in antiparallel.（Ⅲ）Research of external pressures tuning the magnetic properties in[（CH₃）₂NH₂]Fe（HOOC）₃.Under slight pressures,its magnetic ground state maintains the canted antiferromagnetism.Besides,the magnetic phase separation is kept around T_blocking,which leads to the coexistence of canted antiferromagnetic order and single-ion magnet behavior.At this stage,small pressures don’t affect the corresponding magnetic transition temperatures.They mainly suppress the magnetization by manipulating the Dzyaloshinskii-Moriya effect.With increasing external pressures up to 2.26 kbar,the interaction between Fe²⁺ions keeps antiferromagnetic coupling.The pressures increase the crystal field,but suppress the spin-orbital effect.This causes the orbital moments to be quenched greatly.Besides,the magnetic exchange path is also tuned due to the further distortion of framework,which results in the change of the shape of M-T and M-H curves.The original single-ions magnet phase at low temperature is suppressed intensively by external pressures.（Ⅳ）Research of thermal expansion behavior and magnetic-field control of hydrogen-bond order-disorder transition in metal-organic frameworks[（CH₃）₂NH₂]M（HOOC）₃（M=Co,Fe）.Because of their relatively soft structures,the distortions of MO₆（M=Co,Fe）octahedral units are released accompanying with the order-disorder process of hydrogen bonds,which make their frameworks extend largely.These changes lead to huge thermal expansion anomalies in both materials around their transition temperatures.Moreover,by the method of magnetic-field-cooling,high magnetic fields can well tune their hydrogen-bond-ordering temperature in paramagnetic state.The shift direction of transition temperatures is dependent on the ordering arrangement of hydrogen bonds.For[（CH₃）₂NH₂]Fe（HOOC）₃,by strong spin-lattice coupling effect,its magnetostriction property exhibits some abnormal phenomena with the appearance of magnetization resonance quantum tunneling below T_blocking.