凝聚态物理和材料研究中的一个重大挑战是在现有物理框架基础上进行新奇物态的发现与设计。为此目的，在单一相中实现多种传统上不相容的性质共存是一条有吸引力的途径。这其中值得注意的一个例子是铁电性与铁磁性，两者的组合促成了具有强磁电耦合的多铁性材料的诞生。该材料在存储与传感技术领域有着广泛应用前景的。类似地，由通常在绝缘材料相伴的电极化与金属性这一对看似不相容的性质组合得到的极化金属，在最近因为其潜在特性也吸引了大量的研究兴趣。受到多铁系统中磁电耦合效应的启发，将磁性（时间反演对称性破缺）引入到极化金属中具有重要科学意义，通过耦合的磁与电极化序有希望在金属体系中诱导出新奇物相。在本论文中，我们通过材料的设计，成功合成了一种全新的准二维性过渡金属氧化物Ca3Co3O8，并以此为材料基础实现了铁磁极化金属这一新奇电子态。在该化合物中，包括空间反演对称性破缺（极化）、金属性以及铁磁性在内的三种性质和谐地统一在一起。在结构上，该材料因为氧空位序的存在形成了单层氧四面体与双层氧八面体的交替周期性堆垛。在物性上，由于单层四面体的存在，铁磁金属态被限制在双层CoO6八面体中，该材料表现出准二维导电的特性。与此同时，Co离子的位移打破了空间反演对称性。由于空间和时间反演对称性在该材料中的同时破缺，我们在零磁场下发现了本征的磁手征各向异性，该现象伴随着非互易非线性电阻响应。此外，我们在该材料宽阔的温度-磁场相空间发现了多重磁态转变与拓扑霍尔效应，这暗示着包括塞曼相互作用、海森堡相互作用、Dzyaloshinsky-Moriya相互作用以及磁晶各向异性在内的复杂竞争在该材料中产生了新奇的自旋结构。总而言之，我们在实验上设计并实现了本征铁磁极化金属Ca3Co3O8，并从本征非互易电输运与非传统霍尔效应的实验测量出发，发现材料之中具有新奇的磁电特性。我们期望该材料能作为探索由铁磁性、极化与金属态的耦合所诱导的丰富物理现象的模型体系，同时也期望氧空位序的构建方式能为新型氧化物材料的设计提供新方向。
A grand challenge for condensed matter physics and materials research is to discover exotic states of matter. To that end, a promising pathway is to combine multiple properties that are conventionally incompatible into a single-phase material. A notable example is the combination of ferroelectricity and ferromagnetism, which has led to the birth of multiferroics with strong magnetoelectric coupling, possessing vast potential applications in memory and sensor technologies. In analogy, polar metals with a combination of two seemingly incompatible properties of polarity typically hosted in insulating materials and metallicity, attract great research interests recently with promising functionalities. Inspired by the promising magnetoelectric coupling in multiferroic systems with coupled polarization and magnetization, it is of fundamental interest to introduce magnetization (remove time-reversal symmetry) into polar metals to access exotic electronic states that might emerge through the coupled magnetic and polar orders. This search has already led to a series of experimental investigations as well as theoretical proposals; however, the experimental demonstration of an intrinsic ferromagnetic polar metal is still extremely scarce.In this thesis, we report the experimental realization of an exotic electronic state through material design in a hitherto-undiscovered quasi-two-dimensional transition metal oxide Ca3Co3O8. In this compound, three incompatible properties including broken inversion symmetry (polarity), metallicity, and ferromagnetism are harmoniously unified. Here, the crystalline structure processes a periodic stacking of oxygen tetrahedral monolayers alternating with octahedral bilayers, forming a perovskite-like structure with ordered oxygen vacancy channels. The ferromagnetic metallic state is confined within CoO6 octahedra bilayers with quasi-two-dimensional nature, while the broken inversion symmetry arises simultaneously from the displacement of Co ions. Due to the breaking of both spatial-inversion and time-reversal symmetries, we observe an intrinsic magnetochiral anisotropy with exotic nonreciprocal electrical resistivity at zero magnetic field, which is a quantum phenomenon beyond the linear framework of Ohm’s law in metals. Besides, we observe multiple magnetic transition and robust topological Hall signal within a wide temperature-field phase space, which indicates the emergent exotic spin structure through the competition of Zeeman interaction, Heisenberg interaction, Dzyaloshinsky-Moriya interaction and magnetic anisotropy.To sum up, we designed and realized an intrinsic ferromagnetic polar metal Ca3Co3O8 and demonstrated the coupling of magnetization and polarization in the form of an exotic remnant nonreciprocal electrical transport and unconventional Hall effect. We envision that this material would serve as a model system to probe a rich spectrum of phenomena with coupled ferromagnetic, polar, and metallic states; and the oxygen vacancy ordering approach demonstrated here paves a pathway to design novel oxides with intriguing magnetic and electronic properties.
