Effects of boundary conditions on magnetization switching in kinetic Ising models of nanoscale ferromagnets

Magnetization switching in highly anisotropic single-domain ferromagnets has been previously shown to be qualitatively described by the droplet theory of metastable decay and simulations of two-dimensional kinetic Ising systems with periodic boundary conditions. In this article we consider the effects of boundary conditions on the switching phenomena. A rich range of behaviors is predicted by droplet theory: the specific mechanism by which switching occurs depends on the structure of the boundary, the particle size, the temperature, and the strength of the applied field. The theory predicts the existence of a peak in the switching field as a function of system size in both systems with periodic boundary conditions and in systems with boundaries. The size of the peak is strongly dependent on the boundary effects. It is generally reduced by open boundary conditions, and in some cases it disappears if the boundaries are too favorable towards nucleation. However, we also demonstrate conditions under which the peak remains discernible. This peak arises as a purely dynamic effect and is not related to the possible existence of multiple domains. We illustrate the predictions of droplet theory by Monte Carlo simulations of two-dimensional Ising systems with various system shapes and boundary conditions.Comment: RevTex, 48 pages, 13 figure

Portaria 116/CSE/2017 - Torna pública a composição da banca examinadora para o concurso público, Processo nº 23080.030439/2017-76

We report, for the first time, the MR head response from lithographically patterned perpendicular nickel columns. Electron-beam lithography is used to fabricate arrays of Ni columns, 400 nm tall and 150 nm in diameter spaced 2.1 μm apart, embedded in SiO2. The sample surface is planarized with a chemical mechanical polish. The technique of Scanning Magnetoresistance Microscopy (SMRM), in which a magnetoresistive (MR) head is raster-scanned in contact with a sample, is used to investigate the MR head response from the Ni columns. Single columns can be “read” with a 0-peak MR voltage of 60-70 μV. Unexpectedly, we find that the magnetic field due to the bias current in the MR head is enough to switch the columns during scanning, which results in a “dibit-like” MR response, By scanning in the presence of a small (~21 Oe) external magnetic bias field, the columns can be imaged in either their “up” or “down” magnetic states