Engineering of domain wall pinning in ferromagnetic nano-structures for magnetic memory applications.

• Main Author: Low, Seow Chin.
• Other Authors: Lew Wen Siang
• Format: Final Year Project
• Language: English
• Published: 2010
• Subjects: DRNTU::Science::Physics::Electricity and magnetism
• Online Access: http://hdl.handle.net/10356/40804
• Institution: Nanyang Technological University

Magnetic domain walls in ferromagnetic nanowires are the focus of intense interest both from a fundamental perspective and due to their potential applications. Magnetic logic and memory devices based on domain wall motion and manipulation have been recently proposed and experimentally demonstrated. A key issue in the application of domain wall devices is the ability to manipulate the domain wall configurations using magnetic field or spin polarized current. As such, a detailed knowledge of the domain wall nucleation and propagation is needed. In addition, although IBM had demonstrated the feasibility of data storage in a racetrack, it was limited to 2D planar racetrack geometry. The 3D racetrack memory is currently in its infancy stage with limited proposals on a suitable medium for the 3D magnetic racetrack. In this work, a systematic investigation of transverse domain wall pinning at notches in a nanowire via numerical simulation was presented. The pinning and depinning of domain wall at notches of different geometries and dimensions was studied in detail. The simulation results reveal that the pinning of a domain wall depends on the winding number of the wall at the notch. For all the notches and domain walls investigated, only walls with winding number of +1/2 approaching the notch were pinned. A non-monotonic variation of the depinning field, with a maximum field of 177Oe occurring at a notch width to track height (WN/HT) ratio of 0.65 was observed for positive notches. For WN/HT > 1.3, the depinning field reduces to zero. Interestingly, it was observed that the depinning field reaches a constant maximum value when the notch height to track height HN/HT > 0.33. The synthesis of compositionally modulated ferromagnetic cylindrical nanowires was also reported. The nanowires were grown via electro-chemical deposition in AAO template. By exploiting the differential etch rate of Ni95Fe5 and Ni87Fe13 in HNO3, modulated nanowires with periodic notches were fabricated. The composition of the nanowires was determined to be Ni95Fe5 and Ni87Fe13, the former being the magnetic element and the latter the notch. The magnetic properties of the modulated nanowires were characterized using both AGFM and MFM measurement. From the MFM image, we can visually confirm that the notches adopt the vortex configuration at remanence. The experimental results were substantiated using micromagnetic simulations. From the experimental results, we propose that our modulated periodic etched nanowires can be a possible candidate as the medium for the racetrack memory.