First-Principles Study on Magnetism and its Manipulation in Two-Dimensional Magnetic Materials

Since the first experimental verification in 2017, two-dimensional (2D) van der Waals (vdW) magnetic materials have emerged as a research hotspot in the field of spintronics. These materials are promising for use in magnetic tunnel junctions, spin tunneling field-effect transistors, and spin valves, yet the diversity of 2D magnetic materials currently available is relatively limited. Challenges such as weak magnetic anisotropy and low Curie temperatures ( 𝑇c) impede their wider utility. Thus, effectively modulating their magnetic properties is essential for expanding their application fields. Based on the principle of magneto-electric coupling, applying electric field has been proven as an effective approach to control magnetism. Constructing 2D vdW ferromagnetic (FM)/ferroelectric heterostructures (HSs) offers a more diverse platform and addresses the issue of volatility in electric field control. However, the underlying physical mechanism governing the control of 2D magnetic materials by ferroelectric substrates is not fully understood. Meanwhile, the complex magnetic interactions and phase transitions in 2D magnetic materials require further in-depth study. In this dissertation, the magnetic modulation of monolayer (ML) CrX3 (X = I, Br, Cl) and 1T-CrTe2, as influenced by vdW substrate, strain, and electronic correlation, is investigated using first-principles methods based on density functional theory. Additionally, the significance of biquadratic exchange interactions (BQEI) and Kitaev interactions in ML 1T-CrTe2, as well as the dependency of magnetism on stacking orders in bilayer (BL) 1T-CrTe2 are explored. Specific findings include: A series of novel HSs by stacking the well-known 2D magnetic materials CrX3 with the polar vdW material AlN are created. These structures effectively enhance the 𝑇c of CrX3 (~10 K) and realize the tunability of their magnetocrystalline anisotropy (MCA) by flipping the polarization direction of AlN. Moreover, CrX3 undergoes a transition from semiconductor to semimetal. A general rule is proposed to guide the selection of suitable substrates to enhance perpendicular magnetic anisotropy in the CrX3 system, which has significant implications for the design principles of interface-based magnetic modulation. Despite various control methods that can enhance the transition temperature of magnetic materials, many 2D magnetic materials still exhibit 𝑇c far below room temperature, significantly limiting their applications. 1T-CrTe2, which displays FM behavior at nearly room temperature (~310 K), has attracted significant interests recently. To address the controversy surrounding the magnetic properties of 1T-CrTe2 at 2D limits, this study thoroughly investigates the intricate interplay among magnetic interactions, strain, and electronic correlation within this material. Through energy simulations based on spin Hamiltonians, it is found that BQEI plays a crucial role in stabilizing collinear magnetic configurations. Meanwhile, the interplay between Kitaev interaction and single ion anisotropy can help understand the experimentally observed canting spin orientation in ML 1T-CrTe2. Monte Carlo simulations reveal that ML 1T-CrTe2 undergoes a series of complex phase transitions as the magnetic frustration strength changes. The possible noncollinear phases due to magnetic frustration are suppressed by BQEI in ML 1T-CrTe2. However, nonzero Dzyaloshinskii-Moriya interaction is induced in BL 1T-CrTe2 due to the inversion symmetry breaking in each sublayer, leading to the topological nontrivial magnetic merons (and antimerons) in AA stacking BL 1T-CrTe2. Additionally, different interlayer stacking orders alter the magnetic exchange coupling in 1T-CrTe2, further impacting their magnetic ground state. These findings fill a research gap in the magnetic control of BL 1T-CrTe2 and provide a theoretical groundwork for further investigating its complex magnetic behavior. This research represented by 2D magnetic materials CrX3 and 1T-CrTe2 holds significant implications in exploring the microscopic mechanisms of magnetic control and opens new possibilities for the application of 2D magnetic materials in spintronic devices.

自2017年首次实验验证以来，二维范德华磁性材料已成为自旋电子学领域的研究热点。尽管在磁隧道结、自旋隧道场效应晶体管及自旋阀等器件中具备广泛的应用潜力，但当前可用的二维磁性材料种类仍相对有限，并存在磁各向异性弱以及居里温度低的特点。这些因素都制约了二维磁性材料在实际中的应用。因此，为了扩大这些材料的应用领域，迫切需要有效地调控其磁性性质。

基于磁电耦合的原理，通过电场诱导磁性有序已经被证明是一种有效的磁性调控途径。构建二维范德华铁磁/铁电异质结提供了更为多样化的平台，并且可以解决电场调控中易失性的问题。然而，铁电衬底对于二维磁性材料调控的物理机制尚不完全明确。与此同时，二维磁性材料内部复杂的磁性相互作用和相变过程仍需要深入研究。

本文采用基于密度泛函理论的第一性原理计算方法，研究了范德华异质结、应力以及电子关联作用对单层CrX3（X = I, Br, Cl）和1T-CrTe2的磁性调控。此外，还探究了biquadratic交换相互作用和Kitaev相互作用在单层1T-CrTe2中的重要性，以及堆垛方式对双层1T-CrTe2的磁性调控。具体如下：

通过将备受瞩目的二维磁性材料CrX3与极性范德华材料AlN堆叠，构造出一种全新的异质结构。这一方案显著提高了CrX3的居里温度，同时通过翻转AlN的极化方向成功地实现了对CrX3磁晶各向异性的调控。此外，CrX3的电子结构也呈现出从半导体到半金属的转变。在此基础上，提出了一个可用于指导如何选择适当的衬底以增强CrX3体系中的垂直磁各向异性的通用规则。这一成果为解释界面相互作用对磁性的调制提供了坚实的理论支持。

然而，尽管存在多种调控手段可以提升磁性材料的转变温度，众多二维磁性材料的转变温度依然远低于室温，显著制约了其在器件上的应用。表现出近室温铁磁性的1T-CrTe2因而受到关注。针对1T-CrTe2在低维极限下磁性性质存在的争议，深入研究了该体系中磁性相互作用与应变和电子关联之间的复杂依赖关系，并从多个角度解释了其中的物理机制。通过基于自旋哈密顿量对能量的模拟，发现在该体系中，biquadratic交换相互作用在稳定共线磁构型方面起到至关重要的作用。通过分析Kitaev相互作用与单粒子各向异性的相互竞争，解释了实验上在单层1T-CrTe2中观测到的倾斜的易磁化轴方向。这一研究不仅解答了之前理论计算和实验中存在的诸多争议，为深入认识这一极具潜力的室温铁磁材料提供了参考，也为实验研究者提供了可靠的磁性调控途径。

通过蒙特卡洛模拟发现，单层1T-CrTe2在磁阻挫强度发生变化时，其磁性会经历一系列复杂的相转变。单层1T-CrTe2中由于磁阻挫可能导致的非共线磁构型被biquadratic交换相互作用抑制。而在双层1T-CrTe2中，由于空间反演对称性破缺导致了非零的Dzyaloshinskii-Moriya相互作用，最终在AA堆垛方式中稳定了拓扑非平庸的磁半子构型。同时，不同的层间堆垛方式改变了1T-CrTe2的磁交换耦合。这一发现填补了关于双层1T-CrTe2磁性调控的研究空白，为深入研究该体系中更为复杂的磁性行为提供了理论依据。

上述以新兴二维磁性材料CrX3和1T-CrTe2为代表的研究，对于探索磁性调控的微观物理机制具有重要的意义，为二维磁性材料在自旋电子学器件的应用开辟了新的可能性。

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