磁性氧化物磁输运和磁热效应的研究
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摘要
自旋电子学是近些年来在半导体电子学和磁电子学基础上发展起来的一门新兴交叉学科。丰富的物理内涵、明确的应用目标以及广阔的市场前景,自旋电子学已成为当今凝聚态物理和材料科学领域最为关注的方向之一。其中,巨磁电阻的发现及在自旋器件上的应用是最有重大影响力的成果。非易失性存储器和高密度磁存储器的诞生,不仅给基础研究注入了活力,更为市场带来了巨大的经济效益。巨磁电阻研究的关键在于如何获得更为实用的低场室温磁电阻效应。具有高自旋极化率的半金属材料,如掺杂锰氧化物,CrO_2,Fe_3O_4等成为研究者的首选。无论是提高半金属材料的内禀磁电阻,还是外禀磁电阻,都有着极大的研究和应用潜力。研究表明,半金属颗粒复合体中,颗粒边界对低场磁电阻产生和增强有重要作用,调节颗粒边界势垒已成为颗粒体系中磁电阻增强的有效实验途径。此外,掺杂锰氧化物中,3d/4d/5d过渡族金属氧化物材料日益受到人们的关注。在这类新型的磁电阻氧化物中,4d或5d金属离子较3d金属离子具有宽的d轨道和电子巡游特性,d电子与氧的2p电子存在较强的杂化作用。自旋,轨道和晶格间的相互耦合,引起材料中大的电、磁响应以及巨磁电阻等丰富物理现象,为磁电阻材料研究范围的拓宽以及强关联电子体系中物理性质的探讨提供了新的实验依据。
另一方面,磁致冷技术的飞速发展使得凝聚态物理工作者越来越关注磁性材料的磁热效应。传统的气体制冷存在众多的缺点,相比之下,磁制冷具有熵密度高、体积小、噪音小、无污染、高效低耗等独特优势。磁制冷研究的关键在于获得室温附近大的磁热效应。传统的金属钆(Gd)以及近年来报道的Gd_5(Ge_(1-x)Si_x)_4和La(Fe_(13-x)Si_x)等合金都是具有大磁熵变的磁性材料。然而,这些材料中稀有金属的昂贵,化学性质的不稳定,居里温度单一,磁滞与热滞现象严重等因素,使得磁制冷技术的应用步履维艰。值得注意的是,具有庞磁电阻的掺杂锰氧化物同样表现出了大的磁熵变效应,这一发现大大拓宽了磁制冷工质的研究范畴。掺杂锰氧化物具有价廉、制备简单、居里温度在较大温区范围内可调等优点,使得其在磁制冷研究中关注程度日趋增加。但这一材料也有其明显不足之处,如磁性能受制备工艺和掺杂影响较大。因此,研究者仍在通过提高制备工艺等方法,努力探索具有实用价值的磁熵材料。
综合上述两方面的研究背景,本文的研究主要分为二个部分。一,磁性氧化物磁输运性质的研究,包括半金属CrO_2和Fe_3O_4颗粒边界磁电阻效应的研究,以及3d/4d过渡族金属氧化物Sr电磁性能,磁性相变和磁电阻的研究。二,研究了半金属CrO_2的磁热效应,以及多晶SrRu_(1-x)Mn_xO_3中磁熵变随体系磁性相变的变化关系。主要研究内容包括:
1.磁性氧化物磁输运性质的研究
(1)通过调节颗粒边界势垒状态,获得了室温Fe_3O_4颗粒中增强的磁电阻和磁阻抗效应。系统研究了球磨Fe_3O_4粉末颗粒的电阻率、磁性能、磁电阻、磁阻抗与颗粒边界势垒的关系。磁电阻和磁阻抗效应在优化的颗粒边界势垒样品中出现极大值。室温磁阻抗效应在5000 Oe磁场下从-7.0%增加到了-12.3%,提高了近76%。采用阻抗谱分析方法,将颗粒体系中晶粒、晶界和颗粒边界电阻加以分离,区分出晶粒和颗粒边界对磁输运性质的贡献。通过非线性I-V曲线计算得到各样品中边界势垒高度和势垒宽度,详细讨论了磁输运与颗粒边界势垒状态的关系。因此,这一增强的交流磁输运特性为高频自旋电子器件的开发提供实验依据。
(2)首次通过化学反应方法,改善半金属CrO_2颗粒表面天然的Cr_2O_3势垒层,获得了低场磁电阻的显著增强。相成分和表面态的测试结果表明,强氧化剂KMnO_4能有效地去除或调整半金属CrO_2颗粒表面绝缘的Cr_2O_3层,从而得到一系列具有不同表面态的CrO_2颗粒,最大磁电阻达到MR=-33%,相比于未处理样品提高了近22%。并对具有不同表面态CrO_2颗粒的电、磁性能、磁电阻效应和颗粒间势垒性质作了较为系统的分析和讨论。探索了一条用化学反应法调整颗粒表面势垒状态而获得低场磁电阻增强效应的有效实验途径。
(3)首次在实验上制备了具有正自旋极化率CrO_2和负自旋极化率Fe_3O_4的二组元半金属颗粒复合体,研究了具有特殊微观结构半金属颗粒复合体的磁输运特性,并探讨了体系逾渗阈值、磁电阻效应与微观结构的关系。我们发现随着CrO_2组分的变化,复合体的电导发生逾渗现象,在逾渗阈值φ_c=0.15(CrO_2组分浓度)处出现磁电阻的极小值。详细研究了复合体逾渗阈值处电导和磁输运机制,分析了由异号自旋极化率的CrO_2和Fe_3O_4组成的颗粒复合体磁电阻效应的特征。
(4)首次系统研究了3d/4d过渡族金属氧化物SrRu_(1-x)Mn_xO_3(0≤x≤1)磁性相变和磁输运性质,发现了这一体系中低温-41%的磁电阻效应和居里温度附近-35%的磁电阻峰值,率先报道了多晶SrRu_(1-x)Mn_xO_3中磁相变图。3d金属离子Mn~(4+)对4d钙钛矿氧化物SrRuO_3的B位掺杂,驱使体系从金属性巡游铁磁体转变为绝缘性反铁磁体。X-射线光电子能谱(XPS)测试表明,多晶样品中Mn和Ru离子主要以Mn~(4+)和Ru~(4+)价态为主,但存在少量的Mn~(3+)和Ru~(5+)离子价态。详细研究了体系中多种磁性相的共存和相互竞争,如Mn~(3+)-Mn~(4+)(Ru~(5+))铁磁性相互作用,Mn-Ru铁磁性超交换作用,Mn~(4+)-Mn~(4+)及Ru-Ru反铁磁相互作用,以及由此导致的多晶SrRu_(1-x)Mn_xO_3中复杂的磁性相变行为。系统分析了不同温度下电、磁性质和磁场导致的磁性相变特征,对低温和居里温度处产生大的磁电阻效应物理机制作了深入探讨。
2.磁性氧化物中磁热效应的研究
(1)首次研究了半金属CrO_2颗粒中的磁熵变△S_M和绝热温度变化△T_(ad),发现具有二级相变的CrO_2材料表现出大的低场磁热效应:在1.5 T磁场下,磁熵变△S_M达到-5.1 J/kg-K,绝热温度变化达到△T_(ad)=2.0 K。这一低场磁熵变在磁性氧化物中是比较可观的。同时,还系统研究了不同颗粒尺寸CrO_2的磁热效应。运用分子场理论,朗道模型和德拜近似等方法,分析了具有二级相变CrO_2材料中磁熵的来源,包括电子自旋,磁滞伸缩,电子熵变对整个磁熵变的贡献。
(2)首次研究了4d巡游金属性铁磁体SrRuO_3和多晶SrRu_(1-x)Mn_xO_3体系中磁热效应与磁性相变的关系。结果表明,6.5 T磁场下,巡游铁磁体SrRuO_3在居里温度160K处存在△S_M=-2.5 J/kg-K的磁熵变和△T_(ad)=3.1 K的绝热温度变化。相对制冷率达到RCP=70 J/kg。由于相变温度处SrRuO_3晶格常数存在较大的变化,运用德拜近似计算,发现具有二级相变的SrRuO_3材料晶格熵变在整个熵变中的重要性。
B位Mn离子的掺入,体系低温下出现自旋玻璃/团簇玻璃的磁性态,高温(-200K)出现反铁磁/顺磁的转变。体系磁熵变强烈依赖于磁性相变,不同磁性相变温度处表现出系统的变化。在铁磁-顺磁转变温度T_c处和自旋冻结温度T_f处均出现了负磁熵变△S_M的峰值,而在反铁磁相变温度T_N附近,出现了正的磁熵变△S_p。详细探讨了体系中磁熵变的物理机制以及Mn的掺入导致磁熵变随磁性能的变化关系。
In recent years, spintronics is a new branch of condensed matter physics based on the development of semiconductor electronics and magnetoelectronics. Spintronics lies in the area of current scientific interest in physics and material sciences due to abundant physical conceptions and applied potentials. An important result is the discovery of giant magnetoresistance with the application in spintronics devices. Non-volatile magnetic computer memory (MRAM) is expected to have a large economic impact, and has been significantly focused on its experimental research. The key of giant magnetoresistance (GMR) is how to obtain a large low-field magnetoresistance (MR) at room temperature. Half-metal materials with high spin polarization, such as CrO_2, Fe_3O_4 and doped manganites, have been paid much attention in enhanced extrinsic MR and intrinsic MR. Theoretical and experimental results indicate that enhanced low-field MR originates from the grain boundary or particle boundary effect in half-metal granular composite. Improving the properties of boundary barrier is an effective method to obtain enhanced MR. Furthermore, 3d/4d/5d transition metal perovskite oxides are of lasting interests due to unique physical phenomena (such as colossal magnetoresistance, percolation, phase separation etc.) that result from highly correlated d-band electrons and strong electron-lattice coupling. Compared to 3d transition metal oxides, metallic conductivity is rather more common among 4d and 5d transition metal oxides. In strongly correlated electron systems different degrees of freedom, such as the spin, orbitals, and lattice deformations, are inextricably coupled, which lead to large electronic, magnetic responses and giant magnetoresistance etc.
On the other hand, rapid development of a new magnetic refrigeration technology, based upon the magnetocaloric effect (MCE), has attracted an immense increase in interest in magnetic materials. Magnetic refrigeration exhibits more considerable advantages than conventional vapor-cycle refrigeration, such as high energy efficiency, small volume, ecological cleanliness, etc. Until recently, a gadolinium (Gd) rare-earth metal and Gd_5(Ge_(1-x)Si_x)_4 series with large MCE have been considered as the most active magnetic refrigerant in room-temperature magnetic refrigerators, but its usage is somehow limited because the expensive cost, chemical metastable, and large thermal and field hysteresis. Therefore, research in the magnetic cooling field has been focused on the search for new materials that are cheaper but displaying large MCEs.
Based on above discussed background, the thesis is composed of two parts. Firstly, magnetotransport properties in half-metallic CrO_2 and Fe_3O_4 particles are studied. At the same time, magnetic phase transition and magnetoresistance are widely discussed in 3d/4d transition metal oxides SrRu_(1-x)Mn_xO_3. Secondly, the magnetocaloric effect is investigated in half-metal CrO_2 nano-rods. Also, we have addressed the effect of Mn doping on magnetic phase transition and magnetocaloric effect in polycrystalline SrRu_(1-x)Mn_xO_3.
The main results of our study are listed as follows:
1. The study of magnetotransport properties in magnetic oxides.
(1) Enhanced Magnetoresistance (MR) and magnetoimpedance (MI) effect are obtained in ball milled Fe_3O_4 particles by improving particle boundary properties. The resistivity, magnetic properties, magnetoresistance, magnetoimpedance effect and properties of boundary barriers are studied systematically. MR and MI exhibit the maximum values when t=350 hour, which are strongly related to the barrier properties of grain boundaries. The MI value increases from -7.0% to-12.3% at room temperature and under 5000 Oe. The complex impedance analysis is effectively used to evaluate the contribution of grain and grain boundary to conduction process in Fe_3O_4 powders. Based on Simmons model, the changes of barrier thickness and barrier height with respect to ball milling time are obtained from a series of non-linear current-voltage (I-V) curves. We have a detailed discussion on magnetotransport with barrier properties, which maybe develop new high frequency spintronics devices.
(2) The enhanced magnetoresistance is obtained in CrO_2 powder compact by an oxidization reaction process. An aqueous potassium permanganate (KMnO_4) is used to react with the CrO_2 particles coated naturally with Cr_2O_3 layer. The experiment indicates that the strong oxidant can effectively adjust thickness of the natural Cr_2O_3 layer, and thereby change the surface state of the CrO_2 particles. An optimal reaction process yields an obvious increase up to -33% in magnetoresistance at a temperature of 5 K for the chemical treated CrO_2 powder, compared to MR=-27% for the original CrO_2 powder. The magnetic and magnetotransport properties are systematically investigated with the variation of barrier properties. The simple chemical approach has a potential to achieve an enhanced magnetoresistance in a metallic particle system by adjusting the surface state of the magnetic nanoparticle.
(3) The present work is originally intended to investigate the magnetoresistance in the binary half-metal CrO_2/Fe_3O_4 composites with opposite spin polarization. A detailed investigation on percolation, magnetoresistance and microstructure has been studied. Experimental results indicate that the conductivity exhibits an abrupt increase when CrO_2 volume fraction is close to 0.15, where a percolation behavior happens. Magnetoresistance exhibits a minimum value in the vicinity of percolation threshold at a temperature of 77 K. Moreover, we have addressed a detailed discussion on the mechanism of magnetoresistance near the percolation region in the binary half-metal granular CrO_2/Fe_3O_4 composite with opposite sign of spin polarizations.
(4) The magnetic phase transition and large magnetoresistance are firstly studied in 3d/4d transition metal oxides SrRu_(1-x)Mn_xO_3 (0≤x≤1). A large low temperature magnetoresistance (MR) of -41% at 10 K, which is the largest MR reported in Mn-doped SrRuO_3. The large MR=-35% at T_c is also observed in MR-T curves for sample x=0.55. Importantly, we firstly report the magnetic phase diagram in polycrystalline SrRu_(1-x)Mn_xO_3. Substitution of Mn ions for Ru drives the system from ferromagnetic state SrRuO_3, to an antiferromagnetic state SrMnO_3. The measurement of x-ray photoelectron spectroscopy supports that small amount of Mn~(3+) and Ru~(5+) exists in present samples. The presence of combined ions with different valence states may result in a variety of magnetic interactions including Mn~(3+)-Mn~(4+) DE ferromagnetic interaction, Mn~(3+)-Ru~(4+)(Ru~(5+)) FM superexchange interaction, Mn~(4+)-Mn~(4+) and Ru-Ru AFM interaction. Therefore, the magnetic phase transitions with x are more complicated and pronounced than those in single crystals. Furthermore, the systematical studies in magnetic and magnetotransport properties at different temperature have been discussed. The mechanisms of large magnetoresistance at different magnetic states are addressed.
2. The study of magnetocaloric effect in magnetic oxides.
(1) The magnetic entropy change△S_M and adiabatic temperature change△T_(ad) of half-metallic CrO_2 particles is firstly investigated by superconducting quantum interference device (SQUID) measurements. The experimental results indicate that the acicular CrO_2 particles yield a large△S_M=-5.1 J/kg-K and△T_(ad)=2.0 K at an applied field of 15 kOe near Curie temperature. The observations are one of the best results measured ever among those magnetic oxides, and also can be comparable to that for a pure metal Gd. At the same time, the magnetocaloric effect in different CrO_2 granular has been studied. In order to make a further study of MCE in half-metal CrO_2 nano-rods, molecular field model, Landau theory and Debye approximation are used to unveil origins of the magnetic entropy change associated with a second phase transition in the system including the contribution of spin, magnetostriction and electronic entropy change.
(2) The magnetocaloric effect (MCE) in 4d itinerant ferromagnet SrRuO_3 has been firstly investigated by superconducting quantum interference device (SQUID). The experiment indicates that SrRuO_3 exhibits magnetic entropy chang△S_M=-2.5 J/kg-K and adiabatic temperature change△T_(ad)=3.1 K at an applied field of 6.5 T and near Curie temperature (-160 K). The second order phase transition for SrRuO_3 not only makes the large magnetic entropy change retain over a broad temperature range of 28 K, but also leads to a large relative cooling power (RCP) of 70 J/kg. Based on the analysis of Debye approximation, the present work demonstrates the importance of the lattice entropy change in SrRuO_3 material due to a remarkable change in lattice deformation at T_c.
With the Mn doping, magnetic state exhibits a phase transition from ferromagnetic state through spin glass/cluster glass to antiferromagnetic state. The magnetic entropy change△S_M presents a negative value, peaking at both Curie temperature T_c and spin freezing temperature T_f. Significantly, a positive magnetic entropy change△S_p is observed at N(?)el temperature T_N. Therefore, we have addressed a detailed investigation on the mechanism of magnetic entropy changes with the variation of magnetic properties.
另一方面,磁致冷技术的飞速发展使得凝聚态物理工作者越来越关注磁性材料的磁热效应。传统的气体制冷存在众多的缺点,相比之下,磁制冷具有熵密度高、体积小、噪音小、无污染、高效低耗等独特优势。磁制冷研究的关键在于获得室温附近大的磁热效应。传统的金属钆(Gd)以及近年来报道的Gd_5(Ge_(1-x)Si_x)_4和La(Fe_(13-x)Si_x)等合金都是具有大磁熵变的磁性材料。然而,这些材料中稀有金属的昂贵,化学性质的不稳定,居里温度单一,磁滞与热滞现象严重等因素,使得磁制冷技术的应用步履维艰。值得注意的是,具有庞磁电阻的掺杂锰氧化物同样表现出了大的磁熵变效应,这一发现大大拓宽了磁制冷工质的研究范畴。掺杂锰氧化物具有价廉、制备简单、居里温度在较大温区范围内可调等优点,使得其在磁制冷研究中关注程度日趋增加。但这一材料也有其明显不足之处,如磁性能受制备工艺和掺杂影响较大。因此,研究者仍在通过提高制备工艺等方法,努力探索具有实用价值的磁熵材料。
综合上述两方面的研究背景,本文的研究主要分为二个部分。一,磁性氧化物磁输运性质的研究,包括半金属CrO_2和Fe_3O_4颗粒边界磁电阻效应的研究,以及3d/4d过渡族金属氧化物Sr电磁性能,磁性相变和磁电阻的研究。二,研究了半金属CrO_2的磁热效应,以及多晶SrRu_(1-x)Mn_xO_3中磁熵变随体系磁性相变的变化关系。主要研究内容包括:
1.磁性氧化物磁输运性质的研究
(1)通过调节颗粒边界势垒状态,获得了室温Fe_3O_4颗粒中增强的磁电阻和磁阻抗效应。系统研究了球磨Fe_3O_4粉末颗粒的电阻率、磁性能、磁电阻、磁阻抗与颗粒边界势垒的关系。磁电阻和磁阻抗效应在优化的颗粒边界势垒样品中出现极大值。室温磁阻抗效应在5000 Oe磁场下从-7.0%增加到了-12.3%,提高了近76%。采用阻抗谱分析方法,将颗粒体系中晶粒、晶界和颗粒边界电阻加以分离,区分出晶粒和颗粒边界对磁输运性质的贡献。通过非线性I-V曲线计算得到各样品中边界势垒高度和势垒宽度,详细讨论了磁输运与颗粒边界势垒状态的关系。因此,这一增强的交流磁输运特性为高频自旋电子器件的开发提供实验依据。
(2)首次通过化学反应方法,改善半金属CrO_2颗粒表面天然的Cr_2O_3势垒层,获得了低场磁电阻的显著增强。相成分和表面态的测试结果表明,强氧化剂KMnO_4能有效地去除或调整半金属CrO_2颗粒表面绝缘的Cr_2O_3层,从而得到一系列具有不同表面态的CrO_2颗粒,最大磁电阻达到MR=-33%,相比于未处理样品提高了近22%。并对具有不同表面态CrO_2颗粒的电、磁性能、磁电阻效应和颗粒间势垒性质作了较为系统的分析和讨论。探索了一条用化学反应法调整颗粒表面势垒状态而获得低场磁电阻增强效应的有效实验途径。
(3)首次在实验上制备了具有正自旋极化率CrO_2和负自旋极化率Fe_3O_4的二组元半金属颗粒复合体,研究了具有特殊微观结构半金属颗粒复合体的磁输运特性,并探讨了体系逾渗阈值、磁电阻效应与微观结构的关系。我们发现随着CrO_2组分的变化,复合体的电导发生逾渗现象,在逾渗阈值φ_c=0.15(CrO_2组分浓度)处出现磁电阻的极小值。详细研究了复合体逾渗阈值处电导和磁输运机制,分析了由异号自旋极化率的CrO_2和Fe_3O_4组成的颗粒复合体磁电阻效应的特征。
(4)首次系统研究了3d/4d过渡族金属氧化物SrRu_(1-x)Mn_xO_3(0≤x≤1)磁性相变和磁输运性质,发现了这一体系中低温-41%的磁电阻效应和居里温度附近-35%的磁电阻峰值,率先报道了多晶SrRu_(1-x)Mn_xO_3中磁相变图。3d金属离子Mn~(4+)对4d钙钛矿氧化物SrRuO_3的B位掺杂,驱使体系从金属性巡游铁磁体转变为绝缘性反铁磁体。X-射线光电子能谱(XPS)测试表明,多晶样品中Mn和Ru离子主要以Mn~(4+)和Ru~(4+)价态为主,但存在少量的Mn~(3+)和Ru~(5+)离子价态。详细研究了体系中多种磁性相的共存和相互竞争,如Mn~(3+)-Mn~(4+)(Ru~(5+))铁磁性相互作用,Mn-Ru铁磁性超交换作用,Mn~(4+)-Mn~(4+)及Ru-Ru反铁磁相互作用,以及由此导致的多晶SrRu_(1-x)Mn_xO_3中复杂的磁性相变行为。系统分析了不同温度下电、磁性质和磁场导致的磁性相变特征,对低温和居里温度处产生大的磁电阻效应物理机制作了深入探讨。
2.磁性氧化物中磁热效应的研究
(1)首次研究了半金属CrO_2颗粒中的磁熵变△S_M和绝热温度变化△T_(ad),发现具有二级相变的CrO_2材料表现出大的低场磁热效应:在1.5 T磁场下,磁熵变△S_M达到-5.1 J/kg-K,绝热温度变化达到△T_(ad)=2.0 K。这一低场磁熵变在磁性氧化物中是比较可观的。同时,还系统研究了不同颗粒尺寸CrO_2的磁热效应。运用分子场理论,朗道模型和德拜近似等方法,分析了具有二级相变CrO_2材料中磁熵的来源,包括电子自旋,磁滞伸缩,电子熵变对整个磁熵变的贡献。
(2)首次研究了4d巡游金属性铁磁体SrRuO_3和多晶SrRu_(1-x)Mn_xO_3体系中磁热效应与磁性相变的关系。结果表明,6.5 T磁场下,巡游铁磁体SrRuO_3在居里温度160K处存在△S_M=-2.5 J/kg-K的磁熵变和△T_(ad)=3.1 K的绝热温度变化。相对制冷率达到RCP=70 J/kg。由于相变温度处SrRuO_3晶格常数存在较大的变化,运用德拜近似计算,发现具有二级相变的SrRuO_3材料晶格熵变在整个熵变中的重要性。
B位Mn离子的掺入,体系低温下出现自旋玻璃/团簇玻璃的磁性态,高温(-200K)出现反铁磁/顺磁的转变。体系磁熵变强烈依赖于磁性相变,不同磁性相变温度处表现出系统的变化。在铁磁-顺磁转变温度T_c处和自旋冻结温度T_f处均出现了负磁熵变△S_M的峰值,而在反铁磁相变温度T_N附近,出现了正的磁熵变△S_p。详细探讨了体系中磁熵变的物理机制以及Mn的掺入导致磁熵变随磁性能的变化关系。
In recent years, spintronics is a new branch of condensed matter physics based on the development of semiconductor electronics and magnetoelectronics. Spintronics lies in the area of current scientific interest in physics and material sciences due to abundant physical conceptions and applied potentials. An important result is the discovery of giant magnetoresistance with the application in spintronics devices. Non-volatile magnetic computer memory (MRAM) is expected to have a large economic impact, and has been significantly focused on its experimental research. The key of giant magnetoresistance (GMR) is how to obtain a large low-field magnetoresistance (MR) at room temperature. Half-metal materials with high spin polarization, such as CrO_2, Fe_3O_4 and doped manganites, have been paid much attention in enhanced extrinsic MR and intrinsic MR. Theoretical and experimental results indicate that enhanced low-field MR originates from the grain boundary or particle boundary effect in half-metal granular composite. Improving the properties of boundary barrier is an effective method to obtain enhanced MR. Furthermore, 3d/4d/5d transition metal perovskite oxides are of lasting interests due to unique physical phenomena (such as colossal magnetoresistance, percolation, phase separation etc.) that result from highly correlated d-band electrons and strong electron-lattice coupling. Compared to 3d transition metal oxides, metallic conductivity is rather more common among 4d and 5d transition metal oxides. In strongly correlated electron systems different degrees of freedom, such as the spin, orbitals, and lattice deformations, are inextricably coupled, which lead to large electronic, magnetic responses and giant magnetoresistance etc.
On the other hand, rapid development of a new magnetic refrigeration technology, based upon the magnetocaloric effect (MCE), has attracted an immense increase in interest in magnetic materials. Magnetic refrigeration exhibits more considerable advantages than conventional vapor-cycle refrigeration, such as high energy efficiency, small volume, ecological cleanliness, etc. Until recently, a gadolinium (Gd) rare-earth metal and Gd_5(Ge_(1-x)Si_x)_4 series with large MCE have been considered as the most active magnetic refrigerant in room-temperature magnetic refrigerators, but its usage is somehow limited because the expensive cost, chemical metastable, and large thermal and field hysteresis. Therefore, research in the magnetic cooling field has been focused on the search for new materials that are cheaper but displaying large MCEs.
Based on above discussed background, the thesis is composed of two parts. Firstly, magnetotransport properties in half-metallic CrO_2 and Fe_3O_4 particles are studied. At the same time, magnetic phase transition and magnetoresistance are widely discussed in 3d/4d transition metal oxides SrRu_(1-x)Mn_xO_3. Secondly, the magnetocaloric effect is investigated in half-metal CrO_2 nano-rods. Also, we have addressed the effect of Mn doping on magnetic phase transition and magnetocaloric effect in polycrystalline SrRu_(1-x)Mn_xO_3.
The main results of our study are listed as follows:
1. The study of magnetotransport properties in magnetic oxides.
(1) Enhanced Magnetoresistance (MR) and magnetoimpedance (MI) effect are obtained in ball milled Fe_3O_4 particles by improving particle boundary properties. The resistivity, magnetic properties, magnetoresistance, magnetoimpedance effect and properties of boundary barriers are studied systematically. MR and MI exhibit the maximum values when t=350 hour, which are strongly related to the barrier properties of grain boundaries. The MI value increases from -7.0% to-12.3% at room temperature and under 5000 Oe. The complex impedance analysis is effectively used to evaluate the contribution of grain and grain boundary to conduction process in Fe_3O_4 powders. Based on Simmons model, the changes of barrier thickness and barrier height with respect to ball milling time are obtained from a series of non-linear current-voltage (I-V) curves. We have a detailed discussion on magnetotransport with barrier properties, which maybe develop new high frequency spintronics devices.
(2) The enhanced magnetoresistance is obtained in CrO_2 powder compact by an oxidization reaction process. An aqueous potassium permanganate (KMnO_4) is used to react with the CrO_2 particles coated naturally with Cr_2O_3 layer. The experiment indicates that the strong oxidant can effectively adjust thickness of the natural Cr_2O_3 layer, and thereby change the surface state of the CrO_2 particles. An optimal reaction process yields an obvious increase up to -33% in magnetoresistance at a temperature of 5 K for the chemical treated CrO_2 powder, compared to MR=-27% for the original CrO_2 powder. The magnetic and magnetotransport properties are systematically investigated with the variation of barrier properties. The simple chemical approach has a potential to achieve an enhanced magnetoresistance in a metallic particle system by adjusting the surface state of the magnetic nanoparticle.
(3) The present work is originally intended to investigate the magnetoresistance in the binary half-metal CrO_2/Fe_3O_4 composites with opposite spin polarization. A detailed investigation on percolation, magnetoresistance and microstructure has been studied. Experimental results indicate that the conductivity exhibits an abrupt increase when CrO_2 volume fraction is close to 0.15, where a percolation behavior happens. Magnetoresistance exhibits a minimum value in the vicinity of percolation threshold at a temperature of 77 K. Moreover, we have addressed a detailed discussion on the mechanism of magnetoresistance near the percolation region in the binary half-metal granular CrO_2/Fe_3O_4 composite with opposite sign of spin polarizations.
(4) The magnetic phase transition and large magnetoresistance are firstly studied in 3d/4d transition metal oxides SrRu_(1-x)Mn_xO_3 (0≤x≤1). A large low temperature magnetoresistance (MR) of -41% at 10 K, which is the largest MR reported in Mn-doped SrRuO_3. The large MR=-35% at T_c is also observed in MR-T curves for sample x=0.55. Importantly, we firstly report the magnetic phase diagram in polycrystalline SrRu_(1-x)Mn_xO_3. Substitution of Mn ions for Ru drives the system from ferromagnetic state SrRuO_3, to an antiferromagnetic state SrMnO_3. The measurement of x-ray photoelectron spectroscopy supports that small amount of Mn~(3+) and Ru~(5+) exists in present samples. The presence of combined ions with different valence states may result in a variety of magnetic interactions including Mn~(3+)-Mn~(4+) DE ferromagnetic interaction, Mn~(3+)-Ru~(4+)(Ru~(5+)) FM superexchange interaction, Mn~(4+)-Mn~(4+) and Ru-Ru AFM interaction. Therefore, the magnetic phase transitions with x are more complicated and pronounced than those in single crystals. Furthermore, the systematical studies in magnetic and magnetotransport properties at different temperature have been discussed. The mechanisms of large magnetoresistance at different magnetic states are addressed.
2. The study of magnetocaloric effect in magnetic oxides.
(1) The magnetic entropy change△S_M and adiabatic temperature change△T_(ad) of half-metallic CrO_2 particles is firstly investigated by superconducting quantum interference device (SQUID) measurements. The experimental results indicate that the acicular CrO_2 particles yield a large△S_M=-5.1 J/kg-K and△T_(ad)=2.0 K at an applied field of 15 kOe near Curie temperature. The observations are one of the best results measured ever among those magnetic oxides, and also can be comparable to that for a pure metal Gd. At the same time, the magnetocaloric effect in different CrO_2 granular has been studied. In order to make a further study of MCE in half-metal CrO_2 nano-rods, molecular field model, Landau theory and Debye approximation are used to unveil origins of the magnetic entropy change associated with a second phase transition in the system including the contribution of spin, magnetostriction and electronic entropy change.
(2) The magnetocaloric effect (MCE) in 4d itinerant ferromagnet SrRuO_3 has been firstly investigated by superconducting quantum interference device (SQUID). The experiment indicates that SrRuO_3 exhibits magnetic entropy chang△S_M=-2.5 J/kg-K and adiabatic temperature change△T_(ad)=3.1 K at an applied field of 6.5 T and near Curie temperature (-160 K). The second order phase transition for SrRuO_3 not only makes the large magnetic entropy change retain over a broad temperature range of 28 K, but also leads to a large relative cooling power (RCP) of 70 J/kg. Based on the analysis of Debye approximation, the present work demonstrates the importance of the lattice entropy change in SrRuO_3 material due to a remarkable change in lattice deformation at T_c.
With the Mn doping, magnetic state exhibits a phase transition from ferromagnetic state through spin glass/cluster glass to antiferromagnetic state. The magnetic entropy change△S_M presents a negative value, peaking at both Curie temperature T_c and spin freezing temperature T_f. Significantly, a positive magnetic entropy change△S_p is observed at N(?)el temperature T_N. Therefore, we have addressed a detailed investigation on the mechanism of magnetic entropy changes with the variation of magnetic properties.
引文
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