Graduate Thesis Or Dissertation
 

Acoustically Assisted Magnetic Recording

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9g54xm648

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  • In the past few decades, magnetic recording has been used as a dominant solution for massive data storage due to its large capacity, excellent reliability and low cost. Nowadays, exponentially increasing user-generated information is creating a huge demand as well as great challenges for high density data storage solutions. For high density magnetic recording, or more specifically, for hard disk drives, the bit size of the recording medium has to be reduced. This requires the medium to have an increased coercivity to avoid data loss due to thermal instability. However, data recording in a high coercivity medium is challenging due to the limited practicable write field. Therefore, the requirements of high coercivity for data stability and low coercivity for writeability are conflicting. During the last 15 years, enormous research and engineering efforts have been invested in heat-assisted magnetic recording and bit-patterned media, endeavoring to solve this contradiction. In this dissertation we take an alternative approach and demonstrate that acoustic wave, or strain wave, could be used to address the tradeoff between writeability and stability. This technique is called acoustically assisted magnetic recording. The physics behind this technique is based on the inverse magnetostriction effect or Villari effect, by which the coercivity of a magnetostrictive material can be modified by strain. In operation, a surface acoustic wave is applied to a recording medium, where the resulting acoustic strain temporarily lowers the coercivity (thus makes it magnetically soft), enabling data to be recorded with a lower write field. The medium regains its high coercivity after the acoustic wave is removed and thus becomes magnetically hard for data stability. An experimental device consisting of an interdigitated transducer is designed and fabricated on a piezoelectric quartz substrate for generating the surface acoustic wave. Galfenol film with high magnetostriction is deposited as the recording medium on the same substrate. The acoustic wave propagates in the galfenol film and the resulting strain is measured by laser interferometry. In the proof-of-principle experiments for acoustically assisted magnetic recording, data are recorded in a strained magnetostrictive medium with a smaller write field than when it is unstrained. The required write field strength is lowered by ~10% with ~100 ppm strain applied to the medium. A curved acoustic transducer is developed to focus the acoustic strain so large change in coercivity can be achieved in the focal spot and to demonstrate that an individual bit can be selectively written in the medium. In hard disk drives, rather than the low coercivity galfenol film, high coercivity material is used as the recording medium. L1₀ phase Fe₅₀Pt(₅₀-x)Pdx thin film is a promising candidate for next-generation high coercivity recording media and its magnetostriction is characterized regarding to the application of acoustically assisted magnetic recording. It is found that the magnetostriction of Fe₅₀Pt(₅₀-x)Pdx film is dependent on the Pd content. This suggests that the magnetostriction can be controlled and thus optimized by adjusting the film composition. Also, to apply the acoustically assisted magnetic recording technique to the practical hard disk drives, the acoustic transducer should be integrated in the write head and the generated acoustic wave needs to be air coupled from the head to the recording medium. An electromagnetic acoustic transducer is developed to investigate the possibility of coupling the wave through electromagnetic interaction. The investigation of mechanical-magnetic interaction between acoustic waves and magnetostrictive films also leads to applications other than magnetic recording. We demonstrate that various and controllable magnetization patterns such as periodic stripes (10 μm periodicity) and single dot (3 μm diameter) could be written in the magnetostrictive film by creating different acoustic interferences. This could be potentially used in magnetic particle manipulation, spatial light modulation and magnonic signal processing. Also, a surface acoustic wave magnetic field sensor using galfenol thin film is successfully demonstrated. Here, based on the ΔE effect, the velocity of acoustic wave propagating in the galfenol film is measured to determine the magnetic field. Since galfenol is highly magnetostrictive, a maximum velocity change of 6.4% is achieved, much larger than previously reported results in similar systems.
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