Chirality control via double vortices in asymmetric Co dots

Reproducible control of the magnetic vortex state in nanomagnets is of critical importance. We report on chirality control by manipulating the size and/or thickness of asymmetric Co dots. Below a critical diameter and/or thickness, chirality control is achieved by the nucleation of single vortex. Interestingly, above these critical dimensions chirality control is realized by the nucleation and subsequent coalescence of two vortices, resulting in a single vortex with the opposite chirality as found in smaller dots. Micromagnetic simulations and magnetic force microscopy highlight the role of edge-bound halfvortices in facilitating the coalescence process.Comment: 15 pages, 4 figure

Controlling magnetization reversal in Co/Pt nanostructures with perpendicular anisotropy

We demonstrate a simple method to tailor the magnetization reversal mechanisms of Co/Pt multilayers by depositing them onto large area nanoporous anodized alumina (AAO) with various aspect ratios, A = pore depth/diameter. Magnetization reversal of composite (Co/Pt)/AAO films with large A is governed by strong domain-wall pinning which gradually transforms into a rotation-dominated reversal for samples with smaller A, as investigated by a first-order reversal curve method in conjunction with analysis of the angular dependent switching fields. The change of the magnetization reversal mode is attributed to topographical changes induced by the aspect ratio of the AAO templates.Comment: 12 pages, 3 figure

Quantitative Decoding of Interactions in Tunable Nanomagnet Arrays Using First Order Reversal Curves

To develop a full understanding of interactions in nanomagnet arrays is a persistent challenge, critically impacting their technological acceptance. This paper reports the experimental, numerical and analytical investigation of interactions in arrays of Co nanoellipses using the first-order reversal curve (FORC) technique. A mean-field analysis has revealed the physical mechanisms giving rise to all of the observed features: a shift of the non-interacting FORC-ridge at the low-H$_c$ end off the local coercivity H$_c$ axis; a stretch of the FORC-ridge at the high-H$_c$ end without shifting it off the H$_c$ axis; and a formation of a tilted edge connected to the ridge at the low-H$_c$ end. Changing from flat to Gaussian coercivity distribution produces a negative feature, bends the ridge, and broadens the edge. Finally, nearest neighbor interactions segment the FORC-ridge. These results demonstrate that the FORC approach provides a comprehensive framework to qualitatively and quantitatively decode interactions in nanomagnet arrays.Comment: 19 pages, 4 figures. 9 page supplemental material including 3 figure

Bit patterned media with composite structure for microwave assisted magnetic recording

Patterned magnetic nano-structures are under extensive research due to their interesting emergent physics and promising applications in high-density magnetic data storage, through magnetic logic to bio-magnetic functionality. Bit-patterned media is an example of such structures which is a leading candidate to reach magnetic densities which cannot be achieved by conventional magnetic media.Patterned arrays of complex heterostructures such as exchange-coupled composites are studied in this thesis as a potential for next generation of magnetic recording media. Exchange-coupled composites have shown new functionality and performance advantages in magnetic recording and bit patterned media provide unique capability to implement such architectures. Due to unique resonant properties of such structures, their possible application in spin transfer torque memory and microwave assisted switching is also studied.This dissertation is divided into seven chapters. The first chapter covers the history of magnetic recording, the need to increase magnetic storage density, and the challenges in the field. The second chapter introduces basic concepts of magnetism. The third chapter explains the fabrication methods for thin films and various lithographic techniques that were used to pattern the devices under study for this thesis. The fourth chapter introduces the exchanged coupled system with the structure of [Co/Pd] / Fe / [Co/Pd], where the thickness of Fe is varied, and presents the magnetic properties of such structures using conventional magnetometers. The fifth chapter goes beyond what is learned in the fourth chapter and utilizes polarized neutron reflectometry to study the vertical exchange coupling and reversal mechanism in patterned structures with such structure. The sixth chapter explores the dynamic properties of the patterned samples, and their reversal mechanism under microwave field. The final chapter summarizes the results and describes the prospects for future applications of these structures

Chirality control via double vortices in asymmetric Co dots

Reproducible control of the magnetic vortex state in nanomagnets is of critical importance. We report onchirality control by manipulating the size and/or thickness of asymmetric Co dots. Below a critical diameterand/or thickness, chirality control is achieved by the nucleation of a single vortex. Interestingly, above thesecritical dimensions, chirality control is realized by the nucleation and subsequent coalescence of two vortices,resulting in a single vortex with the opposite chirality as found in smaller dots. Micromagnetic simulations andmagnetic force microscopy highlight the role of edge-bound half vortices in facilitating the coalescence process