烧绿石铱氧化物的磁性、电性和磁热效应的研究
发布时间：2018-08-25 18:36
【摘要】：烧绿石结构氧化物(A_2B_2O_7)由于晶格结构所支持的几何阻挫性质引起丰富的新奇现象而被广泛关注,而烧绿石结构的铱氧化物由于Ir~(4+)离子的低自旋(S=1/2)、强自旋-轨道耦合等性质引起了研究者的注意,金属-绝缘转变、巨磁阻效应、霍尔效应、绝缘态中的畴壁电导等现象的发现掀起了烧绿石铱氧化物的研究热潮。近些年来对该体系的中子衍射实验、μSR等实验证明该体系在金属-绝缘转变的温度附近发生Ir~(4+)离子磁矩的短程序,随着温度的降低,在低温下由于强的自旋-轨道耦合进一步发生A位稀土离子的磁矩长程有序,同时证明了A位子晶格和Ir~(4+)离子构成的子晶格形成All-in-all-out的基态磁结构。关于该体系的磁有序已经进行了多种方法的探测与证明,那么基于磁有序的转变是否会引起材料中具有大的磁热效应?由于表征磁热效应强度的参量磁熵变与材料体系的有序度有直接的关系,这启发了我们探测是否可以将烧绿石铱氧化物作为磁制冷的功能材料,同时关于该体系作为功能材料这方面的研究非常少。为此,本文中将主要涉及到烧绿石铱氧化物R_2Ir_2O_7(R=Nd,Dy和Ho)磁热效应,探究其是否适用于磁制冷功能材料,在研究该体系的磁热效应之前,对R_2Ir_2O_7(R=Nd,Dy和Ho)的磁性、输运、热力学等基本性质给出表征。首先,本文从测量的电阻率数据分析表明Nd_2Ir_2O_7存在半金属-绝缘体的转变,Dy_2Ir_2O_7和Ho-_2Ir_2O_7存在半导体-绝缘体的转变,转变温度分别为34 K,125 K,137 K。Dy_2Ir_2O_7和Ho_2Ir_2O_7的高温顺磁性下的载流子类型可以用变程跳跃模式来描述,同时利用公式拟合给出了能隙宽度分别为4 K、10 K、11 K(R=Nd,Dy和Ho)。其次,得到R_2Ir_2O_7(R=Nd,Dy和Ho)的转变温度后,进行了热磁曲线和磁滞回线的测量,尤其关注了金属-绝缘转变温度附近时的磁性行为,发现在金属-绝缘转变温度附近伴随着零场冷却和带场冷却热磁曲线的分叉行为,且低温下的磁化强度明显增强说明存在铁磁性相互作用,基于这一猜测,接下来进行变温测量该体系的磁滞回线,其在低温下有一弱的铁磁相互作用,同时通过对零场冷却的热磁曲线进行居里-外斯拟合证明该体系中主要存在反铁磁性相互作用,因此得出结论:该体系在低温下是铁磁性相互作用和反铁磁性相互作用共存体。同时得出低的阻挫指数说明该体系为弱阻挫体系,阻挫会抑制体系中的长程有序度,那么弱阻挫体的有序会被释放。接着我们进行了比热数据的测量,在低温下比热出现Schottky异常,该异常源于稀土离子的晶体场劈裂所致,体系在低温下进入长程有序的状态,但是在金属-绝缘转变温度附近并没有测到明显的异常,猜测原因有两条:一、体系中Ir~(4+)离子的磁矩过弱,由于测试精确度并不能测量到Ir~4离子的短程序;二、该体系在金属-绝缘转变温度附近并未形成Ir4+离子的短程序。尽管未确定该体系在转变温度时Ir~(4+)离子否是进入短程有序,但是随着温度的降低伴随着稀土离子的长程有序,我们进行了磁热效应的表征,无论通过等温磁化曲线计算所得、或者通过比热数据积分之差所得,都在低温下同时出现较大的磁熵变值,该温度对应着体系的长程有序,Nd_---2Ir_2O_7在34 K并未出现任何磁熵变的异常,结合比热数据同时未在此处出现异常的行为,说明该体系在转变温度附近并未Ir~(4+)离子的短程有序,尽管这一结果与一些文献中的报道又出入,但是同时文献中也给出制备样品的方法决定了该体系样品的一些性质。通过比热数据和等温磁熵变的数值进行计算得到该体系的绝热温升,相比较合金的磁熵变值,该体系的磁熵变偏小,但是本文中计算给出的绝热温升数值与大量合金中的绝热温升数值是相当的。这一研究更深入的探讨了体系中的磁有序的来源,随着温度的降低,磁结构从无序的3-in-1-out或者2-in-2-out进入更有序的和all-in-all-out的磁结构,即所谓的磁有序是磁结构的转变,同时通过磁热效应的表征证明了我们的猜测:R_2Ir_2O_7(R=Nd,Dy和Ho)是一种非常好的磁制冷功能材料,通过卡诺循环可以将其作为低温下的制冷工质。
[Abstract]:The pyrochlore-structured oxides (A_2B_2O_7) have attracted much attention due to their geometric barrier properties supported by lattice structures. Iridium oxides with pyrochlore structure have attracted much attention due to their low spin (S=1/2) and strong spin-orbit coupling properties, such as metal-insulating transition, giant magnetoresistance effect, Huo et al. In recent years, neutron diffraction experiments and mu-SR experiments on the system have proved that the short-range order of the magnetic moment of Ir~ (4+) ions occurs near the temperature of the metal-insulating transition. With the decrease of temperature, the strong self-dependence occurs at low temperature. The spin-orbit coupling further leads to the long-range magnetic ordering of A-site rare earth ions. It is also proved that the A-site lattice and the sublattice of Ir~ (4+) ions form the all-in-all-out ground-state magnetic structure. Because the parametric magnetic entropy change, which characterizes the intensity of magnetocaloric effect, is directly related to the degree of ordering of the material system, this enlightens us to explore whether pyrochlore iridium oxide can be used as a functional material for magnetic refrigeration, and there is very little research on the system as a functional material. The magnetocaloric effect of pyrochlore iridium oxide R_2Ir_2O_7 (R=Nd, Dy and Ho) and its applicability to magnetic refrigeration functional materials are discussed. The magnetic, transport and thermodynamic properties of R_2Ir_2O_7 (R=Nd, Dy and Ho) are characterized before the magnetocaloric effect of the system is studied. Firstly, the resistivity data measured show that Nd_2Ir_2O_7 is suitable for magnetic refrigeration functional materials. Semimetallic-insulator transition exists in Dy_2Ir_2O_7 and Ho-_2Ir_2O_7. Transition temperatures are 34 K, 125 K, 137 K. Dy_2Ir_2O_7 and Ho_2Ir_2O_7, respectively. Carrier types under high-temperature paramagnetism can be described by variable-range jump mode. The energy gap widths are 4 K, 10 K, respectively. 11 K (R = Nd, Dy and Ho). Secondly, after obtaining the transition temperatures of R_2Ir_2O_7 (R = Nd, Dy and Ho), the thermomagnetic curves and hysteresis loops were measured, especially the magnetic behavior near the metal-insulating transition temperature. It was found that the bifurcation behavior of the thermomagnetic curves was accompanied by zero-field cooling and field cooling near the metal-insulating transition temperature. Based on this conjecture, the hysteresis loop of the system is measured at variable temperature, which has a weak ferromagnetic interaction at low temperature. Curie-Weiss fitting of the thermomagnetic curve of zero field cooling proves that the main antiferromagnetic interaction exists in the system. It is concluded that the system is a coexistence of ferromagnetic and antiferromagnetic interactions at low temperatures. The low resistance index indicates that the system is a weak frustration system, and frustration inhibits the long-term order of the system, so the order of the weak frustration body will be released. The Schottky anomaly of specific heat occurs at low temperature, which is caused by the cleavage of crystal field of rare earth ions. The system enters a long-range ordered state at low temperature. However, there is no obvious anomaly near the metal-insulating transition temperature. It is speculated that there are two reasons. First, the magnetic moment of Ir~ (4+) ions in the system is too weak, because the measurement accuracy is not good. The short-range order of Ir~ (4+) ions was measured. Second, the short-range order of Ir~ (4+) ions was not formed near the metal-insulating transition temperature. Although it was not determined whether the Ir~ (4+) ions entered the short-range order at the transition temperature, we characterized the magnetothermal effect with the decrease of temperature accompanied by the long-range order of the rare earth ions. On the basis of the calculation of isothermal magnetization curves or the difference between the integrals of specific heat data, a large change of magnetic entropy occurs simultaneously at low temperatures. The temperature corresponds to the long-range order of the system. Nd_ - - 2Ir_2O_7 does not show any abnormal change of magnetic entropy at 34 K. Combined with the specific heat data, there is no abnormal behavior at the same time. The short-range order of Ir~ (4+) ions is not found near the transition temperature. Although the results are different from those reported in some literatures, the method of preparing samples is also given in the literatures, which determines some properties of the samples. Comparing the magnetic entropy of the alloys, the magnetic entropy of the system becomes smaller, but the adiabatic temperature rise calculated in this paper is similar to that of a large number of alloys. Ordered and all-in-all-out magnetic structures, so-called magnetic ordering, are transformations of magnetic structures. Our hypothesis is proved by the magnetocaloric effect that R_2Ir_2O_7 (R=Nd, Dy and Ho) is a very good magnetic refrigeration functional material, which can be used as refrigerant at low temperature through the Carnot cycle.
【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB64
本文编号：2203750
[Abstract]:The pyrochlore-structured oxides (A_2B_2O_7) have attracted much attention due to their geometric barrier properties supported by lattice structures. Iridium oxides with pyrochlore structure have attracted much attention due to their low spin (S=1/2) and strong spin-orbit coupling properties, such as metal-insulating transition, giant magnetoresistance effect, Huo et al. In recent years, neutron diffraction experiments and mu-SR experiments on the system have proved that the short-range order of the magnetic moment of Ir~ (4+) ions occurs near the temperature of the metal-insulating transition. With the decrease of temperature, the strong self-dependence occurs at low temperature. The spin-orbit coupling further leads to the long-range magnetic ordering of A-site rare earth ions. It is also proved that the A-site lattice and the sublattice of Ir~ (4+) ions form the all-in-all-out ground-state magnetic structure. Because the parametric magnetic entropy change, which characterizes the intensity of magnetocaloric effect, is directly related to the degree of ordering of the material system, this enlightens us to explore whether pyrochlore iridium oxide can be used as a functional material for magnetic refrigeration, and there is very little research on the system as a functional material. The magnetocaloric effect of pyrochlore iridium oxide R_2Ir_2O_7 (R=Nd, Dy and Ho) and its applicability to magnetic refrigeration functional materials are discussed. The magnetic, transport and thermodynamic properties of R_2Ir_2O_7 (R=Nd, Dy and Ho) are characterized before the magnetocaloric effect of the system is studied. Firstly, the resistivity data measured show that Nd_2Ir_2O_7 is suitable for magnetic refrigeration functional materials. Semimetallic-insulator transition exists in Dy_2Ir_2O_7 and Ho-_2Ir_2O_7. Transition temperatures are 34 K, 125 K, 137 K. Dy_2Ir_2O_7 and Ho_2Ir_2O_7, respectively. Carrier types under high-temperature paramagnetism can be described by variable-range jump mode. The energy gap widths are 4 K, 10 K, respectively. 11 K (R = Nd, Dy and Ho). Secondly, after obtaining the transition temperatures of R_2Ir_2O_7 (R = Nd, Dy and Ho), the thermomagnetic curves and hysteresis loops were measured, especially the magnetic behavior near the metal-insulating transition temperature. It was found that the bifurcation behavior of the thermomagnetic curves was accompanied by zero-field cooling and field cooling near the metal-insulating transition temperature. Based on this conjecture, the hysteresis loop of the system is measured at variable temperature, which has a weak ferromagnetic interaction at low temperature. Curie-Weiss fitting of the thermomagnetic curve of zero field cooling proves that the main antiferromagnetic interaction exists in the system. It is concluded that the system is a coexistence of ferromagnetic and antiferromagnetic interactions at low temperatures. The low resistance index indicates that the system is a weak frustration system, and frustration inhibits the long-term order of the system, so the order of the weak frustration body will be released. The Schottky anomaly of specific heat occurs at low temperature, which is caused by the cleavage of crystal field of rare earth ions. The system enters a long-range ordered state at low temperature. However, there is no obvious anomaly near the metal-insulating transition temperature. It is speculated that there are two reasons. First, the magnetic moment of Ir~ (4+) ions in the system is too weak, because the measurement accuracy is not good. The short-range order of Ir~ (4+) ions was measured. Second, the short-range order of Ir~ (4+) ions was not formed near the metal-insulating transition temperature. Although it was not determined whether the Ir~ (4+) ions entered the short-range order at the transition temperature, we characterized the magnetothermal effect with the decrease of temperature accompanied by the long-range order of the rare earth ions. On the basis of the calculation of isothermal magnetization curves or the difference between the integrals of specific heat data, a large change of magnetic entropy occurs simultaneously at low temperatures. The temperature corresponds to the long-range order of the system. Nd_ - - 2Ir_2O_7 does not show any abnormal change of magnetic entropy at 34 K. Combined with the specific heat data, there is no abnormal behavior at the same time. The short-range order of Ir~ (4+) ions is not found near the transition temperature. Although the results are different from those reported in some literatures, the method of preparing samples is also given in the literatures, which determines some properties of the samples. Comparing the magnetic entropy of the alloys, the magnetic entropy of the system becomes smaller, but the adiabatic temperature rise calculated in this paper is similar to that of a large number of alloys. Ordered and all-in-all-out magnetic structures, so-called magnetic ordering, are transformations of magnetic structures. Our hypothesis is proved by the magnetocaloric effect that R_2Ir_2O_7 (R=Nd, Dy and Ho) is a very good magnetic refrigeration functional material, which can be used as refrigerant at low temperature through the Carnot cycle.
【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB64
【参考文献】
相关期刊论文 前3条
1 杜永平;刘慧美;万贤纲;;5d过渡金属氧化物中的奇异量子物性研究[J];物理学报;2015年18期
2 陈远富,陈云贵,滕保华,唐永柏,付浩,唐定骧,涂铭旌;磁制冷发展现状及趋势:Ⅱ磁制冷技术[J];低温工程;2001年02期
3 施颖,梁敬魁,刘泉林,陈小龙;X射线粉末衍射法测定未知晶体结构[J];中国科学(A辑);1998年02期
相关硕士学位论文 前1条
1 梁丹丹;烧绿石型铱氧化物中的阻挫磁性及电子结构研究[D];安徽工程大学;2016年
，本文编号：2203750
本文链接：https://www.wllwen.com/guanlilunwen/gongchengguanli/2203750.html
