Three-Dimensional Artificial Spin-Ice

挫折感是指系统内的成对相互作用不能同时满足，并且这种现象在解释从太阳耀斑的产生到水冰内的结合等各种现象方面都很重要。为了研究挫折，物理学家寻找理想的系统，在那里物理元素之间的相互作用，如自旋，可以通过实验直接探测。其中一个凝聚态系统是自旋冰材料，最近成为了密集研究的主题。这些是块状晶体，其中自旋位于关节四面体的角上。每个四面体有四个自旋，每个自旋可以指向四面体的中心，也可以指向四面体的中心。想要与每个相邻自旋首尾相连的自旋被挫败，这导致了被称为冰规则的最小能量配置，其中两个自旋指向四面体的中心，两个自旋指向外面。在四面体上翻转一个自旋会导致一个中心带有有限磁荷的违反冰规则的缺陷。这些缺陷通过磁等效库隆布定律相互作用，并且在矢量场M和H中是磁单极子。磁单极子在自旋冰中的实现是令人兴奋的，并导致了一个新的领域，其中磁荷（磁性）的运动进行了研究。二维纳米结构可以被制造成可以模拟大块自旋冰材料的几何形状，并且这些系统（人工自旋冰）也被证明是由晶格固有的挫折驱动的晶格缺陷和奇异相变的家园。然而，为了捕获散装自旋冰的完整物理模型，需要3D几何形状。本研究将制作三维人工自旋冰结构。制造将进行一种新型的直接激光写入光刻系统，能够雕刻出三维孔内的抗蚀剂在亚微米级。然后，这些孔可以填充有磁性材料，例如镍。结构将被制作成模仿大块自旋冰晶格上磁矩的精确3D几何形状，从而允许两种材料之间的直接类比。磁反转将使用磁力计和显微镜相结合的研究，以区分所产生的现象，从表面和散装。这项工作将探索在3D人工自旋冰系统中创建自旋缺陷的可能性，并探索它们在不同缺陷密度的样品中的动力学。

Frustration is when the pairwise interactions within a system cannot be simultaneously satisfied and the phenomena is important in explaining a diverse range of phenomena, from the production of solar flares to bonding within water-ice. In order to study frustration, physicists look for ideal systems where the interaction between physical elements, such as spin, can be directly probed via experiment. One such condensed matter system that has recently been subject of intense study is the spin-ice materials. These are bulk crystals where spins are located on the corners of joint tetrahedra. There are four spins per tetrahedra, and each of these can point either into or out from the centre of the tetrahedron. The spins, which would like to sit head-to-tail with each of their neighbours, are frustrated and this leads to a minimum energy configuration known as the ice-rules where two spins point into the centre of the tetrahedra and two spins point out. Flipping a single spin on a tetrahedron leads to an ice-rule violating defect with finite magnetic charge in the centre. These defects interact via a magnetic equivalent to Coulombs law and are magnetic monopoles in the vector fields M and H. The realisation of monopoles in spin-ice is exciting and has lead to a new field where the movement of magnetic charges (Magnetricity) is studied. Two-dimensional nanostructures can be fabricated in geometries that can simulate bulk spin-ice materials and these systems (Artificial spin-ice) have also shown to be home to monopole defects and exotic phase transitions that are driven by the frustration intrinsic to the lattice. However, in order to capture a complete physical model of bulk spin-ice, a 3D geometry is required. This study will fabricate 3D artificial spin-ice structures. Fabrication will be carried out with a novel direct laser writing lithography system that is capable of carving out 3D holes within a resist on the sub-micron scale. These holes can then be filled with a magnetic material such as Nickel. Structures will be made such that they mimic the exact 3D geometry of the magnetic moments on a bulk spin-ice lattice, allowing a direct analogy between the two materials. The magnetic reversal will be studied using a combination of magnetometry and microscopy in order to distinguish phenomena arising from the surface and from the bulk. The work will explore the possibility of creating monopole defects in 3D artificial spin-ice systems, and explore their dynamics in samples of varying defect density.