【作者】 董超；

【导师】 王俊峰；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2021， 博士

【摘要】 脉冲强磁场具有磁场强度高、扫场速度快和感应无接触等优点,是物质科学研究中的重要实验手段。作为一种极端条件,由于测量时间短、电磁干扰强、磁体振动显著等不利因素,脉冲强磁场下的实验测量极具挑战。近年来,人们不断努力发展一些新的脉冲场实验技术,如绝热磁卡测量、交流比热技术、光纤法测量磁致伸缩等。另一方面,在过去十几年里多铁性材料的合成与研究如火如荼,各种铁性（铁电、铁磁、铁弹等）序参量相互耦合,使得多铁性材料在磁场下呈现丰富的物理现象。然而,相关强磁场研究受制于磁场条件而非常有限,一些基本物理问题至今不清楚。因此,发展脉冲场测量技术并开展多铁性材料的强磁场研究具有现实的科学意义。本文主要工作之一是设计和研制了脉冲场下的绝热磁卡测量,并验证了系统的可靠性。采用多种新型的脉冲场测量技术,我们还系统研究了Ni₃V₂O₈和Gd Mn O₃两种多铁性材料在强磁场条件下的磁热、磁电和磁弹效应。本论文研究内容主要包括以下章节:第一章概述了多铁性材料的研究历史和发展现状,以及Ni₃V₂O₈和Gd Mn O₃两种磁致多铁材料的研究进展。此外,我们还介绍了磁热效应测量及在磁相变研究中的应用。第二章介绍了脉冲强磁场装置和本文研究所采用的几种脉冲场实验技术,包括实验原理和测量方法。其中,我们重点介绍了脉冲场下的绝热磁卡测量和交流比热测量技术。第三章中,我们研制和调试了基于武汉强磁场实验装置的绝热磁卡测量系统,并开展了Ni₃V₂O₈的50 T磁卡效应研究。此外,我们利用日本东京大学的世界最大直流发电机,对Ni₃V₂O₈开展了35 T强磁场下的交流比热测量。通过这两种先进的磁热测量技术,我们对Ni₃V₂O₈的磁相变和H-T磁相图进行了重新定义,澄清了文献报道中的有关争论。我们的研究也表明,相比较脉冲场磁化等磁性测量技术,绝热磁卡和交流比热测量具有对磁相变灵敏、不产生畸变的相边界等优势。第四章介绍我们在脉冲场转角电极化测量上的进展,以及Ni₃V₂O₈强磁场下的多铁特性。通过在ac晶面内旋转磁场方向,我们揭示了角度依赖的低场（<12 T）和高场（>18 T）两种铁电相。其中,低场相在|θ|≤50°时连续存在（θ为磁场与a轴夹角）,但在|θ|>50°时突然消失。与之不同,高场相在|θ|～0°附近逐渐消失,以a轴对称出现在|θ|≤43°区间。通过对称性分析,我们发现这种特定的50°和0°的角度依赖关系与Ni₃V₂O₈中梯状kagome晶格与磁结构紧密关联。研究还发现,低场和高场铁电相分别主要来源于spine和cross-tie两种不同格点上螺旋自旋排列P∝e_ij×（S_i×S_j）。由于强的磁阻挫效应,cross-tie上的自旋在低场铁电相中仍然保持无序状态,在强磁场作用下呈现磁有序和铁电性,我们进一步提出了它可能对应的倾斜螺旋锥自旋结构。第五章系统研究了正交锰氧化物Gd Mn O₃在60 T强磁场下的连续磁相变和铁电相变。当磁场沿b轴时（H//b）,磁化测量揭示了～1 T、15 T、42 T和53 T四个连续的磁转变,说明Gd和Mn离子之间具有复杂的相互作用;电极化测量揭示～1-15 T和42-53 T之间分别是磁场驱动的铁电相,极化方向沿a轴（P//a）表明螺旋自旋面为ab面;磁致伸缩测量结果证明Gd Mn O₃中存在强烈的自旋-晶格耦合作用,预示高场铁电相来源于系统的空间反演对称性破缺。通过比较H//a和H//c方向的磁化、电极化和磁致伸缩测量结果,我们发现低场（<15 T）磁化跳跃是由Gd离子的快速磁化导致,其铁电态由两种磁性离子的共同作用产生,而高场（>15 T）磁化增加则是由Mn离子的磁化导致。我们认为,与低场的螺旋自旋序和自旋流机制不同,高场铁电相的物理机制来源于Mn自旋的共线型排列和自旋交换伸缩作用。第六章对全文进行了总结,并对后续工作进行了展望。更多还原

【Abstract】 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.更多还原