【作者】 陈永亮；

【导师】 赵勇；

【作者基本信息】 西南交通大学 ， 材料学， 2010， 博士

【摘要】 本论文旨在研究四元ZrCuSiAs-型化合物RETMAsO (RE=Sm, Gd, Ce; TM=Fe, Co)中的超导电性和磁性质,以更加深入、全面地理解该体系的电子行为与超导电性和磁性质的关联,并在此基础上探索新的超导及磁性材料。文中主要涉及REFeAsO体系中RE位、Fe位及O位的元素掺杂对材料晶体结构、微观形貌以及超导电性的影响。此外,详细研究了铁基超导材料相关体系RECoAsO (RE=Sm, Gd, Ce)化合物中丰富的磁性质以及Fe和F掺杂对SmCoAsO磁性质的影响。第一章对铁基超导体研究的背景和意义、发展现状以及本论文的主要研究目的和内容进行了综述。第二章重点介绍了本论文中所涉及的各种制备及表征手段。第三章着重研究了5d过渡金属元素Ir在SmFeAsO和GdFeAsO体系中掺杂导致的超导电性,两者的超导电性基本相似,但是两种材料对掺杂量的敏感程度明显不同。SmFe1-xIrxAsO体系对掺杂量较为敏感,但GdFe1-xIrxAsO体系超导范围比SmFe1-xIrxAsO宽。除此之外,GdFe1-xIrxAsO体系最高超导转变温度(Tc=18.9 K)高于SmFe1-xIrxAsO体系(Tc=17.3 K)。这一行为与在F掺杂REFeAsO体系中最高超导转变温度行为不同。这说明O位电子掺杂与Fe位电子掺杂导致超导电性的机制可能有所不同。论文第四章中利用纳米尺寸的REF3作为所制备样品中F的来源,开发出一种低温制备REFeAsO1-xFx超导材料的方法。利用常规固相反应法及低温方法分别制备了SmFeAsO1-xFx和GdFeAsO1-xFx两个铁基超导体系,并比较其性能的差异。随着掺F量的增加,体系的晶格参数逐渐减小,超导转变温度升高。当掺杂量达到一定程度时,系统掺杂趋于饱和,出现相边界。实验证实,低温方法制备的样品晶格常数的减小和超导转变温度的升高程度均大于常规制备方法同组分的样品,制备所得样品的纯度及最高超导转变温度也有相应的提高,说明新的低温制备方法比常规固相反应法更具高效性及实用性。第五章选用不同的金属材料作为包套材料,利用原位粉末套管法成功研制了SmFeAsO0.8F0.2超导线材,并研究了不同的包套材料对线芯的影响,及其所制得线材的性能。研究发现,Cu包套材料在高温下扩散进入线芯直接破坏其成相,所制备线材线芯材料不具有超导电性。而以Ta为包套材料的SmFeAsO0.8F0.2超导线材超导转变温度可达52.5 K,并且在30K以下温度范围内表现出较大的颗粒内临界电流密度。而且在Jc-H曲线中观察到了峰效应,说明铁基超导线材料具有较大的应用前景。第六章主要研究了SmFeAsO0.8F0.2和SmFeAsO中的掺杂效应。用离子半径较小的稀上元素Y部分取代SmFeAsO中的Sm,造成了晶格收缩和自旋密度波转变温度的降低,但未诱导出超导电性；由于负化学压力效应,Y掺杂对SmFeAsO0.8F0.2的超导转变具有抑制作用。由此可知,晶格收缩或化学压力只是影响超导电性,不会直接导致超导电性。Zn在SmFeAsO0.8F0.2中Fe位的掺杂导致其晶格发生了膨胀,Tc被强烈的抑制,并且随Zn掺入量的增加,体系由超导体转变为半导体。第七章主要研究了RECoAsO (RE=Sm, Gd, Ce)的磁性及输运性质。输运测量表明,三种化合物都属于金属导体,而且室温电阻较小。在不同温度和外加磁场下,SmCoAsO和GdCoAsO均表现出极为丰富的磁性质,包括巡游铁磁性、反铁磁性、混磁性,顺磁性及亚铁磁性等。另外,在反铁磁转变温度以下观察到了变磁性相变,此相变主要源自于倾斜(canting)的自旋结构。而CeCoAsO却表现为硬铁磁性,而且在低温下发生了连续的铁磁转变。第八章主要研究了Co位Fe掺杂和O位F掺杂对SmCoAsO磁性质和输运性质的影响。Fe掺杂体系中存在铁磁和反铁磁两相共存的现象,Fe的掺杂使体系的反铁磁性受到抑制,铁磁性加强。在SmCoAsO1-xFx体系中,F掺杂加强了两个Co子晶格间的相互作用,稳定了高温铁磁态,使Tc增加。通过以上研究表明SmCoAsO的磁性质主要源自于Co-As层或Co子晶格的相互作用。输运性质研究表明,SmCo1-xFexAsO和SmCoAsO1-xFx两体系表现为相似的特征,电阻率随温度的降低而减小,具有金属导体特征。另外,在SmCoi1-xFexAsO体系中可能存在变磁性量子临界终点。更多还原

【Abstract】 In this dissertation, the superconductivity and magnetism of the quaternary -ZrCuSiAs type compounds RETMAsO (RE=Sm, Gd, Ce; TM=Fe, Co) have been investigated, in order to understand the correlations between the electronic behaviors and the superconductivity and magnetism more comprehensively. On the basis of these researches, exploration of the new superconductors has also been on progress. The main contents involved are presented as follows:effects of chemical doping at RE, Fe and O sites on the crystal structure, microstructure and the superconductivity of REFeAsO; the magnetism of RECoAsO (RE= Sm, Gd, Ce) compounds and the effect of Fe doping at Co site on the magnetic properties of SmCoAsO.In Chapter 1, the research background, importance, development of the iron-based superconductors, as well as the main purpose and content in this dissertation has been introduced.In Chapter 2, related preparation and measurement method of the samples has been clarified in detail.In Chapter 3,5d-transition metal Ir doping-induced superconductivity in the SmFeAsO and GdFeAsO systems has been investigated. The changes of the electrical properties for these two systems proved to be consistent with each other. However, the superconductivity depending on the Ir doping content shows different behavior in SmFeAsO and GdFeAsO system. SmFe1-xIrxAsO system is more sensitive to the Ir doping level whereas the GdFe1-xIrxAsO system shows superconductivity in a wider range of doping concentration. The highest superconducting transition in GdFe1-xIrxAsO system reaches 18.9 K which is higher than 17.3 K for SmFe1-xIrxAsO system. This highest Tc behavior is opposite to that in the F-doped REFeAsO systems, indicating a different superconducting mechanism between the doping at Fe site and O site. More investigation is needed to clarify the differences.A new low-temperature preparation process, in which the nano-scaled REF3 has been used as the starting materials, has been developed for REFeAsO1-xFx superconductors in Chapter 4.A series of SmFeAsO1-xFx and GdFeAsO1-xFx samples have been synthesized using both of the traditional solid state method and the present low temperature method. With increasing F content, the lattice constants shrunk and the superconducting transition temperature increased in both systems. A gradually saturation of the lattice constant and transition temperature are observed, leading to the appearance of the phase booundary. It is evident that the samples, prepared using low-temperature method, possess relatively larger shrinkage of the lattice constants and increase of Tc, higher top Tc and phase purities than the ones with the same doping level prepared by the traditional solid state method, suggesting that the low-temperature preparing process is more effective and practical in synthesizing the F-doped REFeAsO superconductors.In Chapter 5, Different sheathed (Cu/Nb and Ta) SmFeAsO0.8F0.2 wires have been fabricated by an in situ powder-in-tube method. The main investigations have been focused on the properties of the wires with different sheaths. Cu was observed diffusing into the cores under high temperature, leading to a fuzzy boundary between the sheath and the core. This diffusion destroyed the superconducting phase of SmFeAsO0.8F0.2-However, superconductivity at Tc=52.5 K. has been detected in the Ta-sheathed SmFeAsO0.8F0.2core. High intragrain Jc up to 2×106 A cm-2, a severe weak-link effect and a peak effect with a strongly temperature-dependent peak field Hpear has been observed in the Jc-H curves over the range 10-40 K, indicating that the iron-based superconductors might become another kind of competitive material for application.Effects of chemical doping on the superconductivity of SmFeAsO0.8F0.2 and SmFeAsO have been discussed in Chapter 6. Y doping at Sm site in SmFeAsO induced the shrinkage of the lattice parameters and the decrease of the TSDW without superconducting transition. The superconductivity of Sm1-xYxFeAsO0.8F0.2 is suppressed by Y doping for the negative pressure effect. It is concluded that the shrinkage of the lattice parameters and chemical pressure effect would just affect superconductivity but not induce superconductivity. The lattice parameters of SmFeAsO0.8F0.2 have been stretched by Zn doping at Fe site whereas the Tc has been sharply suppressed. With increasing Zn doping contents, the SmFe1-xZnxAsO0.8F0.2 compounds changes from superconductor to semiconductor.In Chapter 7, the magnetic and transport properties of RECoAsO (RE=Sm, Gd,Ce) have been investigated. All of the compounds present to be metallic behavior with low resistivity at room temperature. Under various temperatures and magnetic fields, multiple magnetic properties including ferromagnetism, antiferromagnetism, paramagnetism and ferrimagnetism etc., have been observed. Furthermore, metamagnetic transition caused by the spin canting has also been detected below the antiferromagnetic transition temperature in SmCoAsO and GdCoAsO. However, CeCoAsO shows different magnetism. It presents to be hard ferromagnetism with successive ferromagnetic transition under low temperature.In Chapter 8, effects of Fe doping on the magnetic and transport properties of SmCoAsO have been investigated. The ferromagnetism and antiferromagnetism coexists in the SmCo1-xFexAsO system. With Co partially replaced by Fe, the antiferromagnetism was suppressed whereas the ferromagnetism was enhanced. The F doping in SmCoAsO enhanced the interactions between two Co sublattices, stabilized ferromagnetic state under high temperature, inducing the increase of Tc. The investigations above revealed that the magnetism in SmCoAsO is originated from the interactions between the Co-As layers or Co the sublattices. The transport properties indicated that both of the SmCo1-xFexAsO and SmCoAsO1-xFx systems possess metalic characteristics.更多还原