二维铁磁材料 CrSiTe3的磁性表征及调控研究

   磁性材料构成了现代信息存储技术的基石，其中二维磁性材料具有维度低、柔韧、无悬挂键等优点，使其不仅成为研究在二维极限厚度下基本磁结构的理想模型，也构成了微尺度自旋电子器件、柔性电子学器件以及高密度堆叠磁存储器件的基石。为了向实际应用迈进，具有很强磁各向异性和更高居里温度的硬铁磁材料成为首选。然而，大部分的二维磁性材料居里温度相对较低，磁滞回线表现为多磁畴的软铁磁特性。本论文以居里温度为32.8 K的二维软铁磁材料CrSiTe₃为研究对象，开展了改变层内以及层间磁性原子间的耦合作用调控体系磁性的实验研究，这对研究磁交换相互作用控制磁性的内在机理和拓宽二维磁性材料的应用领域具有深远的科学意义和实用价值。本论文的主要研究工作概述如下：

（1）自主搭建了反射磁圆二色性测量系统和拉曼光谱系统，与金刚石对顶砧技术相结合，实现了材料在极低温、高压、强场条件下的原位综合表征。

（2）成功制备出CrSiTe₃单晶样品。测量结果表明，制备的CrSiTe₃单晶属R3（No. 148）空间群。CrSiTe₃体材料表现出铁磁性，易磁化轴沿c轴方向，居里温度（T_c ）为32.8 K。其磁性主要来源于材料中的三价Cr⁺³离子。

（3）研究了二维铁磁材料CrSiTe₃的磁性随厚度的演化。实验上利用反射磁圆二色性（RMCD）系统测量了170 nm-1.5 nm厚度的CrSiTe₃材料的磁滞回线。实验结果表明厚度大于10 nm的样品都表现为具有多磁畴的软铁磁特性。但随着厚度的减小饱和磁场随之减小，而当厚度减小到7-8 nm时，CrSiTe₃材料从软铁磁态转变成了具有单磁畴的硬铁磁态。而当样品厚度在3 nm以下时，铁磁性消失。拉曼光谱测量结果表明，材料减薄晶体结构不变。硬铁磁的出现主要归因于样品厚度在10 nm以下限制了磁畴壁的形成而表现为单磁畴态。实验结果还表明当厚度小于5 nm时，材料的居里温度明显降低，这暗示着材料由3D铁磁向2D铁磁体的过渡。利用实验数据拟合出临界厚度约为4.69 nm。

    （4）研究了静水压对二维铁磁材料CrSiTe₃铁磁性的调控。实验上结合了金刚石对顶砧（DAC）技术和原位高压磁圆二色性测量系统研究了CrSiTe₃材料在压力作用下铁磁性的演化过程。研究结果表明，压强小于4 GPa时，体系表现出与常压下同样的软铁磁特性。压强在4 GPa～8 GPa时，体系由软铁磁态转变成硬铁磁态，并伴随着矫顽场和居里温度的显著升高。

    （5）第一性原理计算研究了压力增强CrSiTe₃铁磁性的内在机理。平均场理论表明铁磁性系统的居里温度正比于原子间的磁交换相互作用。计算结果揭示，系统中的第一、第二和第三最近邻交换相互作用J₁、J₂、J₃ 因子以及层间最近邻交换相互作用因子J₄在压力作用下增大，其中J₁的作用最为显著，是CrSiTe₃材料体系铁磁性随压强增强的主导因素。

Magnetic materials constitute the cornerstone of modern data storage technologies. In particular, two-dimensional (2D) magnetic materials have recently attracted enormous attention due to their extraordinary physical properties, such as low dimensionality, flexibility, free of dangling bonds, and semiconductor properties. It does not only provide a platform to study the basic magnetic structure and ordering in the 2D limit, but also provides raw material library in the manufacture of microscale spintronic devices, flexible electronics, and high-density stacked magnetic memory devices. For practical applications, hard ferromagnetic states with strong magnetic anisotropy and higher Curie temperatures are preferred properties. However, most of the 2D magnetic materials show a soft ferromagnetic behavior with a relatively low Curie temperature and many multiple domains. In this thesis, we focus on a 2D soft ferromagnet CrSiTe₃, with a Curie temperature of 32.8 K. By modulating the interlayer and intralayer coupling, we studied the evolution of magnetic properties and explored the plausible physical mechanism. Our studies have far-reaching scientific value for exploring fundamental physics in 2D magnets and open up a new possibility for searching high-temperature magnets in the 2D limit. The main research results are summarized as follows:

(1) Home-made reflection magnetic circular dichroism measurement system and Raman spectroscopy system, combined with diamond anvil technology, realized in-situ comprehensive characterization of materials under extremely low temperature, high pressure and strong field conditions.

(1) CrSiTe₃ single crystals were successfully grown. The measurement results show that the CrSiTe₃ single crystal belongs to the R3(No. 148) space group. It exhibits ferromagnetic property, with an easy axis along the c axis direction, and a Curie temperature (T_c) of 32.8 K. Its ferromagnetism mainly comes from the trivalent Cr⁺³ ions.

(2) We have studied the evolution of magnetism by decreasing the thickness in 2D ferromagnet CrSiTe₃. In the experiment, the magnetism of CrSiTe₃ with the thickness from 170 nm to 1.5 nm was measured using the reflected magnetic circular dichroism (RMCD). The results show that samples with a thickness greater than 10 nm exhibit soft ferromagnetic properties with many multiple domains, and the corresponding saturation magnetic field decreases as the thickness decreases. when the thickness approaches 8 or 7 nm, it transforms from a soft-ferromagnetic state to a hard-ferromagnetic state with a single magnetic domain; but below 3 nm, the ferromagnetism disappears. The appearance of hard-ferromagnetic is mainly attributed to the fact that the thickness of the sample is below 10 nm, which limits the formation of magnetic domain walls and exhibits a single magnetic domain state. Moreover, the thickness-dependent Curie temperature also shows a magnetic transition from three-dimensional to two-dimensional ferromagnetic ordering, with a critical thickness of around 4.7 nm.

(3) We have studied the evolution of ferromagnetism in CrSiTe₃ by hydrostatic pressure. The ferromagnetism of CrSiTe₃ flake under pressure was studied experimentally by combining diamond counter-anvil (DAC) technology and in-situ high-pressure magnetic circular dichroism spectroscopy. The results show that when the pressure less than 4 GPa, CrSiTe₃ flake exhibits soft ferromagnetic properties which are the same as that at normal pressure. When the pressure is between 4 GPa and 8 GPa, it changes from a soft ferromagnetic state to a hard ferromagnetic state, accompanied by a significant increase of the coercive field and Curie temperature.

(4) Density functional theory (DFT) calculations to explain the enhancement of ferromagnetic properties under external pressure. The Mean-field theory reveals the proportional relationship between the exchange interaction and transition temperature. Our results also show that the first, second, and third nearest-neighbor intralayer exchange interactions J₁、J₂、J₃ factors and the interlayer nearest-neighbor exchange interaction factor J₄, increase with increasing pressure, among which J₁ dominates the magnetic exchange interaction and contributes to the enhancement of ferromagnetic properties.

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