燒綠石銥氧化物的磁性、電性和磁熱效應(yīng)的研究
發(fā)布時(shí)間：2018-08-25 18:36
【摘要】：燒綠石結(jié)構(gòu)氧化物(A_2B_2O_7)由于晶格結(jié)構(gòu)所支持的幾何阻挫性質(zhì)引起豐富的新奇現(xiàn)象而被廣泛關(guān)注,而燒綠石結(jié)構(gòu)的銥氧化物由于Ir~(4+)離子的低自旋(S=1/2)、強(qiáng)自旋-軌道耦合等性質(zhì)引起了研究者的注意,金屬-絕緣轉(zhuǎn)變、巨磁阻效應(yīng)、霍爾效應(yīng)、絕緣態(tài)中的疇壁電導(dǎo)等現(xiàn)象的發(fā)現(xiàn)掀起了燒綠石銥氧化物的研究熱潮。近些年來對該體系的中子衍射實(shí)驗(yàn)、μSR等實(shí)驗(yàn)證明該體系在金屬-絕緣轉(zhuǎn)變的溫度附近發(fā)生Ir~(4+)離子磁矩的短程序,隨著溫度的降低,在低溫下由于強(qiáng)的自旋-軌道耦合進(jìn)一步發(fā)生A位稀土離子的磁矩長程有序,同時(shí)證明了A位子晶格和Ir~(4+)離子構(gòu)成的子晶格形成All-in-all-out的基態(tài)磁結(jié)構(gòu)。關(guān)于該體系的磁有序已經(jīng)進(jìn)行了多種方法的探測與證明,那么基于磁有序的轉(zhuǎn)變是否會(huì)引起材料中具有大的磁熱效應(yīng)?由于表征磁熱效應(yīng)強(qiáng)度的參量磁熵變與材料體系的有序度有直接的關(guān)系,這啟發(fā)了我們探測是否可以將燒綠石銥氧化物作為磁制冷的功能材料,同時(shí)關(guān)于該體系作為功能材料這方面的研究非常少。為此,本文中將主要涉及到燒綠石銥氧化物R_2Ir_2O_7(R=Nd,Dy和Ho)磁熱效應(yīng),探究其是否適用于磁制冷功能材料,在研究該體系的磁熱效應(yīng)之前,對R_2Ir_2O_7(R=Nd,Dy和Ho)的磁性、輸運(yùn)、熱力學(xué)等基本性質(zhì)給出表征。首先,本文從測量的電阻率數(shù)據(jù)分析表明Nd_2Ir_2O_7存在半金屬-絕緣體的轉(zhuǎn)變,Dy_2Ir_2O_7和Ho-_2Ir_2O_7存在半導(dǎo)體-絕緣體的轉(zhuǎn)變,轉(zhuǎn)變溫度分別為34 K,125 K,137 K。Dy_2Ir_2O_7和Ho_2Ir_2O_7的高溫順磁性下的載流子類型可以用變程跳躍模式來描述,同時(shí)利用公式擬合給出了能隙寬度分別為4 K、10 K、11 K(R=Nd,Dy和Ho)。其次,得到R_2Ir_2O_7(R=Nd,Dy和Ho)的轉(zhuǎn)變溫度后,進(jìn)行了熱磁曲線和磁滯回線的測量,尤其關(guān)注了金屬-絕緣轉(zhuǎn)變溫度附近時(shí)的磁性行為,發(fā)現(xiàn)在金屬-絕緣轉(zhuǎn)變溫度附近伴隨著零場冷卻和帶場冷卻熱磁曲線的分叉行為,且低溫下的磁化強(qiáng)度明顯增強(qiáng)說明存在鐵磁性相互作用,基于這一猜測,接下來進(jìn)行變溫測量該體系的磁滯回線,其在低溫下有一弱的鐵磁相互作用,同時(shí)通過對零場冷卻的熱磁曲線進(jìn)行居里-外斯擬合證明該體系中主要存在反鐵磁性相互作用,因此得出結(jié)論:該體系在低溫下是鐵磁性相互作用和反鐵磁性相互作用共存體。同時(shí)得出低的阻挫指數(shù)說明該體系為弱阻挫體系,阻挫會(huì)抑制體系中的長程有序度,那么弱阻挫體的有序會(huì)被釋放。接著我們進(jìn)行了比熱數(shù)據(jù)的測量,在低溫下比熱出現(xiàn)Schottky異常,該異常源于稀土離子的晶體場劈裂所致,體系在低溫下進(jìn)入長程有序的狀態(tài),但是在金屬-絕緣轉(zhuǎn)變溫度附近并沒有測到明顯的異常,猜測原因有兩條:一、體系中Ir~(4+)離子的磁矩過弱,由于測試精確度并不能測量到Ir~4離子的短程序;二、該體系在金屬-絕緣轉(zhuǎn)變溫度附近并未形成Ir4+離子的短程序。盡管未確定該體系在轉(zhuǎn)變溫度時(shí)Ir~(4+)離子否是進(jìn)入短程有序,但是隨著溫度的降低伴隨著稀土離子的長程有序,我們進(jìn)行了磁熱效應(yīng)的表征,無論通過等溫磁化曲線計(jì)算所得、或者通過比熱數(shù)據(jù)積分之差所得,都在低溫下同時(shí)出現(xiàn)較大的磁熵變值,該溫度對應(yīng)著體系的長程有序,Nd_---2Ir_2O_7在34 K并未出現(xiàn)任何磁熵變的異常,結(jié)合比熱數(shù)據(jù)同時(shí)未在此處出現(xiàn)異常的行為,說明該體系在轉(zhuǎn)變溫度附近并未Ir~(4+)離子的短程有序,盡管這一結(jié)果與一些文獻(xiàn)中的報(bào)道又出入,但是同時(shí)文獻(xiàn)中也給出制備樣品的方法決定了該體系樣品的一些性質(zhì)。通過比熱數(shù)據(jù)和等溫磁熵變的數(shù)值進(jìn)行計(jì)算得到該體系的絕熱溫升,相比較合金的磁熵變值,該體系的磁熵變偏小,但是本文中計(jì)算給出的絕熱溫升數(shù)值與大量合金中的絕熱溫升數(shù)值是相當(dāng)?shù)�。這一研究更深入的探討了體系中的磁有序的來源,隨著溫度的降低,磁結(jié)構(gòu)從無序的3-in-1-out或者2-in-2-out進(jìn)入更有序的和all-in-all-out的磁結(jié)構(gòu),即所謂的磁有序是磁結(jié)構(gòu)的轉(zhuǎn)變,同時(shí)通過磁熱效應(yīng)的表征證明了我們的猜測:R_2Ir_2O_7(R=Nd,Dy和Ho)是一種非常好的磁制冷功能材料,通過卡諾循環(huán)可以將其作為低溫下的制冷工質(zhì)。
[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.
【學(xué)位授予單位】：中國科學(xué)院大學(xué)(中國科學(xué)院物理研究所)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TB64
本文編號：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.
【學(xué)位授予單位】：中國科學(xué)院大學(xué)(中國科學(xué)院物理研究所)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TB64
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