Three-Dimensional Artificial Spin-Ice

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.

The world is now ever-reliant on digital data storage as businesses and consumers store more information. It has been estimated that the total amount of digital information stored worldwide is approximately 300 exobytes and this is set to increase by a factor of 50 in the next ten years (Source: IDC Digital Universe study). Today the vast majority of the worlds information (>40%) is stored on magnetic hard disk drives, and it is fundamental research into magnetic materials that has allowed the massive increase in areal density (Factor of >1000). New technologies such as patterned media and heat-assisted magnetic recording (HAMR) promise to maintain a steady increase in areal density over the next ten years but ultimately the computer hard drive will hit a plateau in data storage due to the fact that data is stored only within two dimensions.

Future technologies based upon magnetic materials, such as magnetic racetrack memory, have been proposed and these rely on magnetic materials that are nanostructured in all three dimensions. In order to understand the potential of such 3D magnetic storage devices, the fundamental physics of how nanostructured magnetic materials behave and interact in 3D geometries is needed. This work seeks to develop 3D magnetic nanostructures that mimic bulk frustrated materials known as spin-ice. Initial samples, made of simple tetrahedra will provide an insight into how simple 3D magnetic nanostructures behave when placed in close 3D geometries. The work may take important steps in understanding the fundamentals of domain wall movement in complex networks, and may help aid in the realization of 3D magnetic data storage devices. The research therefore has a large potential impact in a society that has increasing storage demands.

The bulk spin-ice materials have recently been shown to be home to quasi-particles that are analogous to magnetic monopoles. At this point in time it is unclear whether these magnetic charges can be controlled in order to make functional devices. This research will study the dynamics of monopole defects, analogous to the monopoles in bulk spin-ice, and determine how their dynamics change when lattice spacing, magnetic moment, or defect density are varied.
Such studies may aid in the understanding of magnetic change transport, ultimately helping the bulk spin-ice community in device design.