在尺寸受限下4d氧化物Sr4Ru3O10的新颖磁行为研究

The 4d perovskites are strongly correlated system involving complex interactions between the charge, spin, orbit and lattice degrees of freedom, have received extensive attention in the condensed matter physics. (Sr,Ca)n+1RunO3n+1 is one of the representatives in the 4d correlated system. It almost covers every ordered state known in the condensed matter physics just by isovalent Ca substitution for Sr or change of the stacking number n of the RuO6 octahedron. Investigating the mechanism of the phenomenon is not only benefit to a better understanding of the complex interactions in the strongly correlated system, but also conducive to the development and design of functional materials. . Sr4Ru3O10 is an interesting example in the (Sr, Ca)n+1RunO3n+1 system for it contains an unique magnetic ground state. It shows a ferromagnetic (FM) transition at a Curie temperature Tc=105K, followed by an additional magnetic transition, namely the metamagnetic transition, at a temperature Tm=50K. Though the metamagnetic transition has been studied for a long time, its origin is still unclear and remains elusive. . We have studied the magnetic nature of Sr4Ru3O10 in the nanometer scale for the first time by transport measurement recently. Results show that the change of thickness has little effect on the FM transition, but can modulate the metamagnetic transition significantly. Furthermore, we have observed a large spin-valve effect, which usually occurs only in the FM/non-FM/FM sandwich structure, on the transverse magnetoresistance when the magnetic field is applied in the ab plane. The emergence of these new phenomena in the nanometer scale provides an opportunity for a better understanding of the unique magnetic property of Sr4Ru3O10.. In this proposal, we will investigate the unique magnetic property of Sr4Ru3O10 in the nanometer scale deeply, such as study the origin of the metamagnetic transition, the mechanism of the observed "spin-valve" effect, and explore the possible new physics when the thickness of Sr4Ru3O10 is only of several unit cells, etc. Relevant experimental results will not only provide deeper understanding of Sr4Ru3O10, but also conducive to the investigation of (Sr, Ca)n+1RunO3n+1 system.

4d钙钛矿关联体系中存在多种自由度间的复杂相互作用，一直是凝聚态物理领域研究的热点与难点。(Sr,Ca)n+1RunO3n+1是其中的代表，该体系通过改变Ca的掺杂浓度或RuO6八面体的堆垒个数所呈现出来的物理特性几乎涵盖了目前凝聚态物理中所观测的所有现象。理解该体系中各现象的机理，不仅有利于更深入的了解关联体系中的复杂相互作用，而且有利于功能材料的开发与设计。Sr4Ru3O10是该体系中的典型成员，但其所具有的奇异磁特性-变磁转变的机理仍不清楚。课题组首次将Sr4Ru3O10的厚度推进到纳米尺度，发现其变磁转变严重的受到厚度的调制，并且出现奇异的“自旋阀”行为。这些现象的出现，为理解 Sr4Ru3O10的奇异磁行为提供了契机。本课题中，我们提出深入的研究纳米尺度Sr4Ru3O10中的奇异磁行为，包括其变磁转变发生的原因、“自旋阀”出现的机理，以及探索其在几个原胞厚度下可能出现的新物理。

4d钌基钙钛矿氧化物关联电子体系中存在多种自由度，包括电荷、自旋、轨道以及晶格间的复杂相互作用，蕴含着丰富的物理现象，一直是凝聚态物理领域研究的热点。深入理解这些材料物性的机理，对探索更多的新型功能材料以及器件具有指导意义。项目以纳米尺度厚度4d钙钛矿氧化物Sr4Ru3O10单晶为研究对象，以理解该材料中的第二个磁转变、变磁转变和横向磁阻的类“自旋阀”效应等的机理，以及探索其在超薄尺度下的物性为主要研究内容。项目实施基本按原计划进行，完成了预期研究目标，取得了预期研究成果。获得的主要研究成果如下：.1）优化和发展了针对4d钌基钙钛矿氧化物的机械剥离制备方法，实现了不同厚度尺度Sr4Ru3O10单晶样品的机械剥离制备，最薄达到2个原胞厚度，并探明了各厚度尺度Sr4Ru3O10单晶的输运特性；.2）系统的建立起了Sr4Ru3O10的铁磁转变温度以及第二个磁转变温度随厚度变化的依赖关系，发现该材料中的铁磁转变温度与厚度无关，但第二个磁转变温度随着厚度的减薄而降低，当厚度薄于3个原胞厚度时，第二个磁转变被完全抑制；.3）发现纳米尺度厚度Sr4Ru3O10单晶的ab面内存在铁磁序，探明了该铁磁序的易磁化方向以及其随温度的演化关系，指出了该铁磁序的起源，并解释了纳米尺度厚度Sr4Ru3O10横向磁阻中出现的类“自旋阀”行为的机理；.4）阐明了Sr4Ru3O10中的第二个磁转变以及变磁转变的物理机理，本质上均来源于材料本身极强的自旋晶格耦合。研究结果表明控制该材料的晶格自由度可以有效控制其磁性能，这对开发新概念功能器件具有指导意义；.5）成功合成了Fe掺杂Sr4Ru3O10单晶，发现仅仅1%的Fe掺杂，就会完全反转其磁特性，并解释了元素掺杂引起磁特性反转的物理机制。.6）研究了纳米尺度厚度Fe掺杂Sr4Ru3O10样品在不同温度和外加磁场下的电输运特性，提出Fe掺杂Sr4Ru3O10可以作为制备自旋电子学功能器件的候选材料。