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Damping mechanisms in magnetic recording materials & microwave-assisted magnetization reversal

Abstract

Understanding the damping of magnetization precession in magnetic recording materials is of both fundamental and practical significance. From the practical perspective, the relaxation processes not only set a natural limit to the time of magnetization switching which determines recording data rates, but also play critical roles in advanced magnetic recording techniques such as microwave-assisted magnetic recording and two-dimensional magnetic recording. Experimental and theoretical studies of magnon-electron scattering and two-magnon scattering (TMS) contributions to magnetization relaxations in magnetic recording head and media materials were conducted for the first time in this dissertation. The accuracy of ferromagnetic resonance (FMR) measurements was increased by the use of vector network analyzer (VNA) FMR techniques. Working equations of the grain-to-grain TMS and grain boundary TMS processes were developed based on the TMS models of Krivosik and Mo, and were applied to understand the relaxation mechanisms in various recording-related thin film materials. The dependences of the FMR behavior and relaxation rates on the external field orientation, the microwave frequency, and the temperature were investigated experimentally in the following three domains: the exchange-coupled composite media, the free layers of tunnel magneto-resistance readers, and FeCo alloy films for future writers. The theoretical models were used to analyze the experimental data and to understand the relaxation mechanisms. Microwave-assisted magnetization reversal (MAMR) is considered as a promising mechanism for further increasing the recording area density and pushing it beyond the super-paramagnetic limit. The MAMR operation was demonstrated with a 700-Gbit/in2 perpendicular media sample in this thesis study. For microwaves with frequencies close to the FMR frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.

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Subject

recording
media
microwave
damping
magnetics

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