多铁性氧化物RFe_2O_4的磁电效应及相关物性研究
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摘要
磁电多铁性材料同时具有铁电性和铁磁性,这使得在单相材料中获得巨大的磁电耦合效应成为可能,因而在传感器、记录存储和自旋电子学器件等方向有着广泛的应用前景。近年来,磁诱导铁电性的研究取得了重要的进展,特别是复杂的磁有序结构(如螺旋磁有序等)导致的铁电现象和关联电子效应,使得电极化状态对外加磁场非常敏感。然而,这些材料中磁电的直接耦合往往很弱,或者它的工作温度远低于室温,或者需要几个特斯拉的强磁场来驱动,从而严重制约了器件的实际应用。
在这篇论文中,作者在生长出高质量单晶样品的基础上,致力于发掘多铁性材料RFe_2O_4中磁电耦合的直接实验证据。研究结果揭示了磁冻结态的电子起源,并讨论了冻结温度前后奇异的交换偏置现象和电子关联效应等。基于材料中同时存在的类极化玻璃和类自旋玻璃行为,作者提出了一个多重玻璃的唯象模型来解释低温的磁电冻结态,而该体系在冻结温度以上仍然存在具有短程关联的自旋和极化团簇。此外,论文也记录了作者为改善多铁性陶瓷材料的介电性能所做出的一些努力和探索。
全文共分为七章。
第1章是多铁性材料与磁电效应的概述,包括磁电多铁材料的概念、历史发展和当前的研究热点。
第2章介绍研究过程中使用的实验方法。
第3章介绍了RFe_2O_4系列氧化物中的铁电起源和多铁性研究工作的进展,其中作者的工作包括通过光学浮区熔融技术生长出高质量的YbFe_2O_4单晶,以及在结构、磁和电性质研究中的表现出来的各向异性行为。
第4章研究了多铁性YbFe_2O_4单晶在c方向上磁化的集体冻结和多重玻璃特性。磁的冻结行为和急剧增加的矫顽场密切相关,并且低温磁冻结态的热释电和介电弛豫测量结果确凿的显示了磁化和电极化之间的内禀联系,揭示出磁冻结态的电子起源。这也是多铁性材料YbFe_2O_4中磁电效应的重要证据。
第5章研究YbFe_2O_4中的交换偏置效应。其独特的电荷有序双层结构以及与其他双层结构之间失措的短程关联,决定了该体系的玻璃特征,也形成了假想中的铁磁/反铁磁界面。交换偏置的研究,不仅涉及到铁磁/反铁磁界面处的交换耦合,还应该重点去考虑磁的集体冻结效应。在磁冻结态下磁滞回线的测量过程中,外加电场表现出对磁弛豫或磁矩翻转的促进作用。
第6章研究LuFe_2O_4陶瓷材料中的掺杂和复合效应,介绍了作者的工作对于改善陶瓷介电性能的重要意义。
第7章是本论文的总结,同时也对未来多铁性材料和磁电效应的研究工作做出了展望。
Magnetoelectric multiferroics with coexisting magnetic and ferroelectric orders enable strong magnetoelectric coupling in a single-phase material for many potential applications like sensors, memory, and spintronics. In recent years magnetically induced ferroelectrics have renewed interest in research on magnetoelectric correlations where ferroelectricity is induced by complex magnetic orders like spiral spin orders, and exhibit remarkable changes in electric polarization in response to external magnetic fields. However, direct coupling in these materials just shows a weak magnetoelectric effect, or operates far below room temperature, hindering their practical device applications.
In this thesis, the author devoted his effort to reveal experimental evidence for magnetoelectric coupling in multiferroic RFe_2O_4 based on well-grown single crystals. The results revealed an electronic origin of the magnetic frozen state and a novel exchange bias effect around the freezing temperature. Upon magnetoelectric coupling, the author’s work also presented multi-glass feature on the simultaneous occurrence of a polar glass and a spin glass state, which may open up new approaches towards applications using magnetoelectric effects. Moreover, other efforts were made to improve dielectric performance of ceramic multiferroic materials via doping and composite effects.
The whole thesis consists of seven chapters.
1. Brief overview of multiferroics and magnetoelectric effects.
This chapter aimed at a brief overview of history, progress, and current status of multiferroics and magnetoelectric effects. By these illustrations, the author’s work was urgent and highlighted for further investigations into magnetoelectric coupling.
2. Experimental methods.
This chapter listed the experimental methods applied in the author’s studies.
3. Introduction to recent studies in multiferroic RFe_2O_4 oxides.
This chapter summarized basal concepts and recent advances in studies of the multiferroic RFe_2O_4 family oxides. In the author’s work, high-quality YbFe_2O_4 single crystals were prepared by an optical floating zone technique, and were well characterized in the structure, magnetic, and electric properties, which confirmed the existence of an intrinsic anisotropy of the system.
4. Magnetic freezing and multi-glass state in multiferroic YbFe_2O_4 crystals.
In this chapter, collective freezing in the c-axis magnetization of multiferroic YbFe_2O_4 single crystals had been observed. The freezing behavior coincided well with the remarkable enhancement of coercivity. Pyroelectric current measurements and dielectric response clearly evidenced magnetoelectric coupling effects associated with the magnetic freezing behavior, revealing an electronic origin of the magnetic frozen state. Thus, a plausible multi-glass model had been proposed.
5. Exchange bias effects in YbFe_2O_4 crystals.
In this chapter, exotic exchange bias effects in YbFe_2O_4 had been studied. In consideration of the charge-ordered double layer structure, glass-like scenario was well established on the short-range and frustrated interbilayer interactions responsible for the hypothetical FM/AFM interfaces. The exchange bias was discussed in terms of the collective freezing of magnetization other than the exchange coupling between FM/AFM interfaces.
6. Doping and composite effects in LuFe_2O_4 ceramic materials.
This chapter referred to the effort to improve dielectric performance of LuFe_2O_4 ceramic materials via doping and composite effects.
7. Conclusion.
This chapter made a brief conclusion and an outlook for future multiferroics and magnetoelectric effects.
在这篇论文中,作者在生长出高质量单晶样品的基础上,致力于发掘多铁性材料RFe_2O_4中磁电耦合的直接实验证据。研究结果揭示了磁冻结态的电子起源,并讨论了冻结温度前后奇异的交换偏置现象和电子关联效应等。基于材料中同时存在的类极化玻璃和类自旋玻璃行为,作者提出了一个多重玻璃的唯象模型来解释低温的磁电冻结态,而该体系在冻结温度以上仍然存在具有短程关联的自旋和极化团簇。此外,论文也记录了作者为改善多铁性陶瓷材料的介电性能所做出的一些努力和探索。
全文共分为七章。
第1章是多铁性材料与磁电效应的概述,包括磁电多铁材料的概念、历史发展和当前的研究热点。
第2章介绍研究过程中使用的实验方法。
第3章介绍了RFe_2O_4系列氧化物中的铁电起源和多铁性研究工作的进展,其中作者的工作包括通过光学浮区熔融技术生长出高质量的YbFe_2O_4单晶,以及在结构、磁和电性质研究中的表现出来的各向异性行为。
第4章研究了多铁性YbFe_2O_4单晶在c方向上磁化的集体冻结和多重玻璃特性。磁的冻结行为和急剧增加的矫顽场密切相关,并且低温磁冻结态的热释电和介电弛豫测量结果确凿的显示了磁化和电极化之间的内禀联系,揭示出磁冻结态的电子起源。这也是多铁性材料YbFe_2O_4中磁电效应的重要证据。
第5章研究YbFe_2O_4中的交换偏置效应。其独特的电荷有序双层结构以及与其他双层结构之间失措的短程关联,决定了该体系的玻璃特征,也形成了假想中的铁磁/反铁磁界面。交换偏置的研究,不仅涉及到铁磁/反铁磁界面处的交换耦合,还应该重点去考虑磁的集体冻结效应。在磁冻结态下磁滞回线的测量过程中,外加电场表现出对磁弛豫或磁矩翻转的促进作用。
第6章研究LuFe_2O_4陶瓷材料中的掺杂和复合效应,介绍了作者的工作对于改善陶瓷介电性能的重要意义。
第7章是本论文的总结,同时也对未来多铁性材料和磁电效应的研究工作做出了展望。
Magnetoelectric multiferroics with coexisting magnetic and ferroelectric orders enable strong magnetoelectric coupling in a single-phase material for many potential applications like sensors, memory, and spintronics. In recent years magnetically induced ferroelectrics have renewed interest in research on magnetoelectric correlations where ferroelectricity is induced by complex magnetic orders like spiral spin orders, and exhibit remarkable changes in electric polarization in response to external magnetic fields. However, direct coupling in these materials just shows a weak magnetoelectric effect, or operates far below room temperature, hindering their practical device applications.
In this thesis, the author devoted his effort to reveal experimental evidence for magnetoelectric coupling in multiferroic RFe_2O_4 based on well-grown single crystals. The results revealed an electronic origin of the magnetic frozen state and a novel exchange bias effect around the freezing temperature. Upon magnetoelectric coupling, the author’s work also presented multi-glass feature on the simultaneous occurrence of a polar glass and a spin glass state, which may open up new approaches towards applications using magnetoelectric effects. Moreover, other efforts were made to improve dielectric performance of ceramic multiferroic materials via doping and composite effects.
The whole thesis consists of seven chapters.
1. Brief overview of multiferroics and magnetoelectric effects.
This chapter aimed at a brief overview of history, progress, and current status of multiferroics and magnetoelectric effects. By these illustrations, the author’s work was urgent and highlighted for further investigations into magnetoelectric coupling.
2. Experimental methods.
This chapter listed the experimental methods applied in the author’s studies.
3. Introduction to recent studies in multiferroic RFe_2O_4 oxides.
This chapter summarized basal concepts and recent advances in studies of the multiferroic RFe_2O_4 family oxides. In the author’s work, high-quality YbFe_2O_4 single crystals were prepared by an optical floating zone technique, and were well characterized in the structure, magnetic, and electric properties, which confirmed the existence of an intrinsic anisotropy of the system.
4. Magnetic freezing and multi-glass state in multiferroic YbFe_2O_4 crystals.
In this chapter, collective freezing in the c-axis magnetization of multiferroic YbFe_2O_4 single crystals had been observed. The freezing behavior coincided well with the remarkable enhancement of coercivity. Pyroelectric current measurements and dielectric response clearly evidenced magnetoelectric coupling effects associated with the magnetic freezing behavior, revealing an electronic origin of the magnetic frozen state. Thus, a plausible multi-glass model had been proposed.
5. Exchange bias effects in YbFe_2O_4 crystals.
In this chapter, exotic exchange bias effects in YbFe_2O_4 had been studied. In consideration of the charge-ordered double layer structure, glass-like scenario was well established on the short-range and frustrated interbilayer interactions responsible for the hypothetical FM/AFM interfaces. The exchange bias was discussed in terms of the collective freezing of magnetization other than the exchange coupling between FM/AFM interfaces.
6. Doping and composite effects in LuFe_2O_4 ceramic materials.
This chapter referred to the effort to improve dielectric performance of LuFe_2O_4 ceramic materials via doping and composite effects.
7. Conclusion.
This chapter made a brief conclusion and an outlook for future multiferroics and magnetoelectric effects.
引文
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