二维铁磁材料Fe3GeTe2的电子自旋输运研究

信息科学的进步离不开微电子器件技术的进步，而微电子器件技术的发展不仅要归功于微纳加工工艺的不断发展，更离不开拥有优异性能的新材料的开发和利用。如今，二维层状材料成为自旋电子学研究的新方向，而近年来Fe₃GeTe₂，一种新型二维铁磁材料，由于其独特的特性受到广泛关注，成为了二维材料研究的热门领域之一，被认为具有广阔的发展前景。

在本论文中，我们以Fe₃GeTe₂作为研究对象，通过微纳结构加工技术，制作了Fe₃GeTe₂的霍尔器件和Nb/Fe₃GeTe₂/Nb异质结器件，研究了Fe₃GeTe₂在低温和磁场环境下的自旋输运性质。具体而言，我们进行了以下工作：

基于Fe₃GeTe₂的霍尔器件，对其纵向电阻和霍尔电阻进行了测量。通过分析纵向电阻对温度的依赖关系，我们确认Fe₃GeTe₂的居里温度（T_C）约为205 K。随着温度降低，Fe₃GeTe₂首先会表现出金属性，但在低温下（小于~25 K）表现出类似半导体的电阻特性。这种性质的变化可能是由于低温下Fe₃GeTe₂中出现反铁磁性使磁性散射增强所致。此外，霍尔电阻在T_C之下表现出了显著的反常霍尔效应。随着温度升高，矫顽场逐渐降低，自发反常霍尔电阻却显示出先升后降的变化。我们认为这进一步佐证了低温下Fe₃GeTe₂中反铁磁性和铁磁性的共存。磁滞回线的高矩形性还表明Fe₃GeTe₂具有很强的垂直磁各向异性，易磁化轴垂直于Fe₃GeTe₂表面。

在Nb/Fe₃GeTe₂/Nb异质结器件的微分电阻测量中，我们发现在Fe₃GeTe₂和Nb之间构成的铁磁/超导界面上存在有受到强烈抑制的安德烈夫反射。在Nb的超导相变温度以下，这在一定偏压（V）范围内产生了一个较宽的微分电阻平台（|V|<Δ_L），且Δ_L的值与Nb的超导能隙相当吻合。更为重要的是，我们还在微分电阻谱内观察到了另一个更窄的微分电阻平台（|V|<Δ_S），这意味着Nb的超导性与Fe₃GeTe₂的铁磁性的耦合，在异质结的界面上诱导出了一个新的超导相，其超导能隙的大小决定了Δ_S。除此以外，我们还研究了安德烈夫反射对器件磁阻的影响，根据所测微分电阻谱及相关理论估算了Fe₃GeTe₂的自旋极化率。

我们的工作有助于加深人们对Fe₃GeTe₂的自旋输运性质的理解。对基于Fe₃GeTe₂的超导异质结中的超导-铁磁相互作用的研究，还揭示出一个新的界面超导相的存在，这可能有助于未来人们对自旋三重态超导的研究。

The progress of information technology relies not only on the progress of microelectronic device technology, but also on the development and utilization of new materials with excellent performance. Nowadays, a new research direction in the field of spintronics is the study of two-dimensional layered materials, especially Fe₃GeTe₂. As a new type of two-dimensional ferromagnetic materials, it has received intense attention due to its unique physical properties and become one of the hot areas of two-dimensional materials research.

In this thesis, we aim to study the magnetotransport properties of Fe₃GeTe₂.We have fabricated Fe₃GeTe₂ Hall devices and Nb/Fe₃GeTe₂/Nb heterojunction devices by micro-nano fabrication techniques. We have systematically investigated the transport properties of Fe₃GeTe₂ under different temperatures and magnetic fields. Specifically, we have carried out the following work.

Based on the Hall devices, we have measured the longitudinal and Hall resistance of Fe₃GeTe_2. From the temperature dependence of resistance, the Curie temperature (T_C) is determined to be around 205 K. An upturn of resistance below 25 K is also noticed, which might arise from the enhanced magnetic scattering due to the emergence of antiferromagnetism (AFM) in Fe₃GeTe₂ at low temperatures. Besides, the measured Hall resistance exhibits prominent anomalous Hall effect (AHE) below T_C. With increasing temperatures, the coercive field decreases, but the spontaneous AHE resistance shows a non-monotonic dependence on the temperature, implying the coexistence of AFM and ferromagnetism (FM) in Fe₃GeTe_2. The high degree of squareness of the hysteresis loop further reveals the perpendicular magnetic anisotropy in Fe₃GeTe_2.

In the differential resistance measurement of Nb/Fe₃GeTe₂/Nb heterojunction devices, we have observed strongly suppressed Andreev reflections at the interface between ferromagnetic Fe₃GeTe₂ and superconducting Nb. It gives rise to a wide differential resistance plateau in the bias (V) region with |V|<Δ_L below the superconducting transition temperature of Nb and the value Δ_L of coincides well with the superconducting gap of Nb. More interestingly, another narrower differential resistance plateau |V|<Δ_S is also observed in the differential resistance spectra. It’s believed that the coupling of FM from Fe₃GeTe₂ and superconductivity (SC) from Nb results in a new superconducting phase at the junction interface with the superconducting gap characterized by Δ_S. Besides that, we have also studied the impact of the interface Andreev refection on the device magnetoresistance and estimated the spin polarization ratio of Fe₃GeTe₂.

Our work helps to deepen our understanding of the spin transport properties of Fe₃GeTe₂. The study of the interplay between SC and FM in Fe₃GeTe₂-based ferromagnet-superconductor heterostructures even reveals the presence of a new interface superconducting phase, which might be of great interest to the field of triplet superconductivity.

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