UC Riverside

Magnetic skyrmions offer various advantages compared to domain wall based memory applications. A skyrmion based shift register is analyzed that uses either ferromagnetic or synthetic anti-ferromagnetic skyrmions. The energy requirement to shift a ferromagnetic (FM) skyrmion is less than that required to create or annihilate a skyrmion in conventional race track memory. Two different types of skyrmion shift register structures are investigated. They differ in the geometry used to confine a skyrmion into an individual cell. One structure uses a rectangular channel combined with a modification of the gate metal above the channel to define individual cells. In the the second structure, the channel width is modified into an hourglass type shape, and the narrow regions of the channel serve to confine the skyrmion to an individual cell. The effects of the shape of the structures, the thickness of the top heavy metal layer, the gap between the gate regions, and the presence of disorder and finite temperature are analysed using micromagnetic simulations. Synthetic antiferromagnetic (SAFM) skyrmions provide higher velocity and reduced switching energy compared to ferromagnetic skyrmions, and they can be stable above room temperature. All of the necessary phase diagrams for obtaining stable skyrmions are presented, which can help to select the correct ferromagnetic material and heavy metalcombinations to obtain skyrmions with desired diameters.

To stabilize skyrmions, the interfacial Dzyaloshinskii-Moriya interaction (iDMI) plays a vital role. The interface between a ferromagnet (FM) or antiferromagnet (AFM) and a heavy metal (HM) results in an antisymmetric exchange interaction known as the interfacial Dzyaloshinskii-Moriya interaction (iDMI) which favors non-collinear spin configurations. The iDMI is responsible for stabilizing noncollinear spin textures such as skyrmions in materials with bulk inversion symmetry. Interfacial DMI values have been previously determined theoretically and experimentally for FM/HM interfaces, and, in this work, they are calculated for the metallic AFM MnPt and the insulating AFM NiO. The heavy metals considered are W, Re, and Au. Values for the iDMI, exchange, and anisotropy constants are determined for different AFM and HM thicknesses. The iDMI values of the MnPt heterolayers are comparable to those of the common FM materials, and those of NiO are lower. In few-layer films of (001) MnPt, the high spin orbit coupling of the Pt layers can give rise to a small DMI in the absence of a HM layer.

Magnetic properties have also been determined for a new family of two-dimensional ferromagnetic materials. The recent demonstration of MoSi2N4 and its exceptional stability to air, water, acid, and heat has generated intense interest in this family of two-dimensional (2D) materials. Among these materials, monolayers of NbSi2N4, VSi2N4, and VSi2P4 are semiconducting, easy-plane ferromagnets with negligible in-plane magnetic anisotropy. They thus satisfy a necessary condition for exhibiting a dissipationless spin superfluid mode. The Curie temperatures of monolayer VSi2P4 and VSi2N4 are determined to be above room temperature based on Monte Carlo and density functional theory calculations. The magnetic moments of VSi2N4 can be switched from in-plane to out-of-plane by applying tensile biaxial strain or electron doping.

Besides magnetic materials, topological materials also offer various prospective applications. There have been also a tremendous interests in finding 1D and 2D materials. To facilitate identifying materials of interest, 27 one dimensional and 300 two dimensional topological materials are identified using several databases. 1D and 2D topological insulators and semimetals are identified. 1D and 2D topological materials which have been experimentally demonstrated, are also identified.