Micromagnetic Modeling of Thin Film Segmented Medium for Microwave-Assisted Magnetic Recording
In this dissertation, a systematic modeling study has been conducted to investigate the microwaveassisted magnetic recording (MAMR) and its related physics. Two different modeling approaches including effective field modeling and recording signal-to-noise ratio (SNR) modeling has been conducted to understand the MAMR mechanism on segmented thin film granular medium. First the background information about perpendicular magnetic recording (PMR) and its limitation has been introduced. The motivation of studying MAMR is to further improve the recording area density capacity (ADC) of the hard disk drive (HDD) and to overcome the theoretical limitation of PMR. The development of recording thin film medium has also been discussed especially the evolvement of the multilayer composite medium. Since the spin torque oscillator (STO) is the essential component in MAMR, different STO structures have been discussed. The relation between STO setting (thickness, location and frequency) to the ac field distribution has also been explored. In effective field modeling, both head configuration and medium structure optimization have been investigated. The head configuration study includes the effective field distribution in relation to the fieldgeneration- layer thickness, location, and frequency. Especially an interesting potential erasure is detected due to the imperfect circularity of the ac field. Several approaches have been proposed to prevent the erasure. Meanwhile, notched and graded segmentation structure have been compared through effective field analysis in terms of the field gradient and track width. It has been found that MAMR with notched Hk distribution is able to achieve both high field gradient and narrow track width simultaneously. In recording SNR modeling, first the behavior of MAMR with single layer medium has been studied and three phases have been discovered. As proceed to the multi-layer medium, a practical issue which is MAMR with insufficient ac field power and high medium damping has been introduced. Since the fabrication of STO with high ac power is highly difficult, the issue has been investigated from the medium side which is through an optimized medium structure, the provided ac field can be utilized more efficiently. It has been found that more segmentation on upper part of the grain to fit the ac field yields more efficient ac field power usage. Following this scenario, the graded and notched segmentation structure have been studied in terms of SNR and track width. The traditional dilemma between recording SNR and track width in the conventional PMR is partially solve using MAMR with notched segmentation structure.