Experimental Study On The Magnetocaloric,magnetoelectric,and Magnetoelastic Effects Of Ni3V₂O₈ And GdMnO₃ In High Magnetic Fields

The pulsed high magnetic field is an important experimental means in material science with the advantages of high magnetic field intensity,fast field-sweeping rate and non-contact induction.As an extreme experimental condition,the measurements under the pulsed high magnetic field are extremely challenging,due to the short measuring time,strong electromagnetic interference,significant magnet vibration and other unfavorable factors.In recent years,people have made continuous efforts to develop new pulsed-field measurement techniques,such as adiabatic magnetocaloric effect,AC specific heat,and optical fiber Bragg grating magnetostriction techniques.On the other hand,the research on the multiferroic materials are in full swing.The mutual coupling among multiple orders（magnetic order,ferroelectric order,and ferroelastic order）,makes the multiferroic materials interesting with rich physical phenomena under magnetic fields.However,related research under high magnetic fields remain scarce due to the extreme field conditions,and some basic physical issues are still unclear.Therefore,it is of scientific significance to develop new pulsed-field measurement techniques and carry out studies on multiferroic materials under high fields.One of our main tasks is to design and develop an adiabatic magnetocaloric effect measurement under pulse fields.By using a variety of new pulsed-field measurement techniques,we also systematically studied magnetocaloric,magnetoelectric and magnetoelastic effects of multiferroic materials Ni₃V₂O₈and Gd Mn O₃in high fields.This thesis is organized as follows:The first chapter summarizes the research history and development of multiferroic materials,as well as the research progress of field-induced multiferroic materials Ni₃V₂O₈and Gd Mn O₃.In addition,we will also introduce the measurement of the magnetocaloric effect and its application in the study of magnetic phase transitions.The second chapter presents the high magnetic field facilities and several related pulsed-field measurement techniques.Among them,we focus on the adiabatic magnetocaloric effect and AC spe-cific heat techniques under pulse fields.In the third chapter,an introduction concerning the adiabatic magnetocaloric effect measurement system in Wuhan national high magnetic field center is given.Based on the newly built system,sys-tematic magnetocaloric effect measurements on Ni₃V₂O₈in pulsed magnetic fields up to 50 T are performed.In addition,we measured AC specific heat of Ni₃V₂O₈under magnetic fields up to 35 T,by using the world’s largest DC generator in the University of Tokyo,Japan.Through the two advanced magnetocaloric measurement techniques,we refined the magnetic phase transitions and build the H-T magnetic phase diagram of Ni₃V₂O₈,which clarified the long standing controversy in previous reports.Our research also shows that the adiabatic magnetocaloric effect and AC specific heat measurements are more sensitive to magnetic phase transitions without distortion of the phase boundaries,compared to the conventional magnetization measurements.The fourth chapter is about our progress in the measurements of polarization with rotating field direction in ac plane,and the multiferroic properties of Ni₃V₂O₈in high magnetic fields.We reveal the angle dependent low-field（<12 T）and high-field（>18 T）ferroelectric phases（FE）.Among them,the low-field phase continuously exists at|θ|≤50°（θis the angle between the magnetic field and the a axis）,but suddenly disappears at|θ|>50°.However,the high-field phase gradually disappears near|θ|～0°,and symmetrically appears in the interval of|θ|≤43°.By symmetry analysis,we found that characteristic angles（50°and 0°）are closely related to the ladder-like kagome lattice and magnetic structure in Ni₃V₂O₈.The study also found that the low-field and high-field ferroelectric phases mainly arise from the spiral spin arrangement of spine and cross-tie,respectively.The electric polarization is expressed as P∝e_ij×（S_i×S_j）.Owing to the strong magnetic frustration,cross-tie spins remain disordered in low-field ferroelectric phase,and exhibit magnetic order and ferroelectricity in higher fields than the low one.We further propose that the high field FE phase originates from the canted conical spin spirals structure formed under high field.In the fifth chapter,we performed systematic studies on the continuous magnetic and ferroelec-tric phase transitions of orthorhombic Gd Mn O₃up to 60 T.When the magnetic field is along the b axis（H//b）,the magnetization measurements reveal four continuous magnetic phase transitions at～1 T,15 T,42 T and 53 T,indicating the complex interaction between Gd and Mn spins.Electric polarization measurements reveal that magnetic-field-induced ferroelectric phases are in～1-15 T and42-53 T.The polarization direction along the a axis（P//a）indicates that the spin spiral order in ab plane.Magnetostriction measurements prove that there are strong spin-lattice coupling in Gd Mn O₃,which indicates that the high-field ferroelectric phase originates from the breaking of spatial inversion symmetry.By comparing the magnetization,polarization,and magnetostriction results（H//a and H//c）,we found that the low-field（<15 T）magnetization jump is related to the Gd spins.On the other hand,its ferroelectric state is derived from the Gd and Mn spins,whereas the high-field（>15 T）mag-netization increase is caused by Mn spins.Different from the spin current mechanism corresponding to the low-field phase,the high-field ferroelectric phase is likely derived by the exchange striction with collinear orders of the Mn spins.Finally,a summary of this thesis is made and perspectives on the future work are proposed.