复杂磁性材料中磁性竞争效应的理论研究

THEORETICAL STUDY OF MAGNETIC COMPETITION EFFECT IN COMPLEX MAGNETIC MATERIALS

罗志恒

11849402

硕士

物理学

张文清

2020-05-29

2020-07-15

哈尔滨工业大学

深圳

过渡金属关联电子材料因其自旋、电荷、轨道、晶格等自由度相互耦合与竞争效应而呈现出丰富的磁性质等物性，发展新型自旋电子学器件迫切需要优良的磁性载体材料，可以给下一代信息存储和量子计算技术带来革命性的突破。其中，庞磁阻（又称“超巨磁阻”，Colossal Magneto-Resistance, CMR）材料的电阻率对外加磁场有敏感响应，可被用于超高密度的磁性存储；半金属（Half metal）材料的一个自旋通道为金属性、另一个自旋通道为绝缘性，从而可以产生100%的高效极化电流，可被用于磁隧道结（Magnetic Tunnel Junction, MTJ）。庞磁阻性质和半金属性质来源于复杂的磁性竞争效应。以Mn基钙钛矿（例如La1-xSrxMnO3）为代表的庞磁阻材料中存在复杂的铁磁性（Ferromagnetism, FM）与反铁磁性（Antiferromagnetism, AFM）的竞争，微观上与Mn-3d电子的轨道序密切相关。以Mn基Heusler（例如Cu2MnAl）为代表的半金属合金材料通常表现出类似于单质铁的铁磁性，但Heusler同时具备较强的化合物成键性质，可以产生100%的高效自旋极化。本论文分别选取过渡金属氧化物SrRuO3和铁基Heusler合金，利用第一性原理方法研究其丰富磁性质以及微观上复杂的磁性作用机制。首先，块体SrRuO3是较为稀有的4d电子氧化物金属，晶体结构是常见的钙钛矿结构，可以与SrTiO3等衬底构成高质量的异质结，并且SrRuO3异质结能够发生有趣的“金属-绝缘”相变。本论文将研究决定SrRuO3/SrTiO3异质结金属相与绝缘相竞争的微观机制，发现了晶体结构畸变的重要作用，微观上解释了单层SrRuO3表面结构的绝缘性与单层SrRuO3超晶格的金属性。第二，Full-Heusler半金属合金通常表现出很强的铁磁性，本论文发现铁基Fe2TiSb体系具有更加稳定的反铁磁耦合，本论文理论研究了微观磁性竞争机制，解释了这一“反常磁耦合”现象。本论文对理解SrRuO3和Fe2TiSb体系的磁性质提供了重要的理论信息。密度泛函第一原理方法通常不能可靠描述过渡金属d电子的关联性质，被广泛诟病。本论文研究发现最新的SCAN交换关联泛函（Strongly Constrained and Appropriately Normed, SCAN）更好地抓住了关联性质，非常适用于SrRuO3和Fe2TiSb两类弱磁性体系。这是本文在理论方法上的特色之处。总之，复杂磁性材料微观上的复杂、微妙磁性竞争效应一方面是材料研究的困难，另一方面给材料的物性调控带来巨大的空间。本论文对SrRuO3和Fe2TiSb两个磁性材料的理论研究揭示了若干有趣的物理现象，将为未来的相关研究提供重要的参考信息。

Transition metal and correlated electronic materials exhibit rich magnetic properties such as magnetic properties due to their spin, charge, orbital, lattice and other degrees of freedom coupling and competitive effects. The development of new spintronics devices urgently requires excellent magnetic carrier materials, which can revolutionize the next generation of information storage and quantum computing technologies. The resistivity of a colossal magnetoresistance(CMR) material has a sensitive response to the external magnetic field and can be used for ultra-high-density magnetic storage; one spin channel of a half-metal material is metallic and the other is insulating, thus generating 100% highly efficient polarization currents that can be used for magnetic tunnel junctions(MTJ).Colossal magnetoresistance and half-metal properties are derived from complex magnetic competition effects. In the colossal magnetoresistive materials represented by Mn-based perovskites (e.g., La1-xSrxMnO3), there is a complex competition between ferromagnetism(FM) and antiferromagnetism(AFM), which is closely related to the orbital order of Mn-3d electrons on the microscopic level. The half-metal alloy materials represented by Mn-based Heusler (e.g., Cu2MnAl) usually show ferromagnetism similar to elemental Fe, but Heusler also has strong compound bonding properties, which can produce 100% efficient spin polarization.In this thesis, the transition metal oxide SrRuO3 and the Fe-based Heusler alloy were selected, and the first-principles method was used to study its rich magnetic properties and microscopically complex magnetic interaction mechanism. First of all, bulk SrRuO3 is a relatively rare 4d electron oxide metal. The crystal structure is a common perovskite structure. It can form a high-quality heterojunction with SrTiO3 and other substrates, and SrRuO3 heterojunction can produce interesting "metal -Insulation" phase transition. In this thesis, we will study the microscopic results that determine the competition between the SrRuO3/SrTiO3 heterojunction metal phase and the insulating phase, and discover the important role of crystal structure distortion. Microscopically explain the insulator of the single-layer SrRuO3 surface structure and the metal of single-layer SrRuO3 superlattice. Second, Full-Heusler half-metal alloys usually exhibit strong ferromagnetism. This thesis found that the Fe-based Fe2TiSb system has a more stable antiferromagnetic coupling. This thesis theoretically studies the microscopic magnetic competition mechanism and explains this "abnormal magnetic coupling" phenomenon. This thesis provides important theoretical information for understanding the magnetic properties of SrRuO3 and Fe2TiSb systems.The density functional theory of the first-principle method usually cannot reliably describe the correlation properties of d electrons of transition metals, and is widely criticized. The research in this thesis found that the latest SCAN(Strongly Constrained and Appropriately Normed) exchange correlation functional better grasps the correlation properties, and is very suitable for SrRuO3 and Fe2TiSb two types of weak magnetic systems. This is the innovation of this article in theory and method.In short, the complex and subtle magnetic competition effect of complex magnetic materials on the one hand is the difficulty of materials research, on the other hand, it brings huge space for the regulation of physical properties of materials. The theoretical study of SrRuO3 and Fe2TiSb magnetic materials in this thesis reveals several interesting physical phenomena and will provide important reference information for future related research.

第一性原理 自旋电子学 半金属 复杂磁性 磁性竞争

first principle spintronics half-metal complex magnetism magnetic competition

中文

联合培养

南方科技大学

罗志恒. 复杂磁性材料中磁性竞争效应的理论研究[D]. 深圳. 哈尔滨工业大学,2020.