Spin dynamics in magnetic thin films and spintronics devices
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Magnetic materials are the essential parts of advanced magnetic recording and microwave devices. All the applications require the control of the magnetization orientation that is studied as magnetic dynamics.
In a magnetic recording device, such as hard disk drive (HDD), the digital information is stored as the magnetization direction of magnetic domains. To rewrite the information, one needs to reverse the magnetization. With the increase of areal density, the magnetic anisotropy is enhanced to overcome the thermal fluctuations. Consequently, the magnetic switching becomes more difficult. Microwave assisted magnetization reversal (MAMR) is developed to lower the switching field by exciting large angle precession of magnetization. Damping constant, as a parameter describing the dissipation of magnetic materials, plays a critical role in MAMR. In this thesis, I theoretically and experimentally study the damping dependence of MAMR in micro-size magnetic dots. Based on the study, MAMR is more efficient when the damping of the magnetic dot is small.
Magnetic random access memory (MRAM) based on magnetic tunnel junctions (MTJs) is a novel magnetic recording device that has the potential to replace HDD due to its fast read/write rate. Moreover, to keep increasing the recording density, the magnetic media with perpendicular magnetic anisotropy (PMA) replaced the in-plane magnetic recording media. In this work, the magnetic thin films such as Co/Pd multilayers and Ta/CoFeB/MgO were fabricated to be with PMA. Based on the PMA thin films, the MTJs were developed with optimized deposition conditions and post-annealing. We observed above 60% TMR in Ta/CoFeB/MgO/CoFeB based perpendicular junctions. The voltage controlled magnetic anisotropy (VCMA) was demonstrated in those MTJs, which is an efficient technique to control the magnetization via an electric field. The spin transfer torques were also characterized in Ta/CoFeB/MgO structures with PMA.
The microwave response is another useful property of magnetic materials. The ferromagnetic resonance (FMR) of typical ferromagnetic materials like Fe, Co, Ni and their alloys are in the gigahertz range. Therefore, the magnetic thin films have been widely utilized in microwave devices such as bandpass filters, insulators, and oscillators. Conventionally, electromagnets generate a magnetic field to control the resonance frequency, which is bulky, noisy, slow and energy consuming. By realizing the huge effective field due to exchange interaction, a novel scheme was demonstrated to shift the resonance frequency of the optical mode in magnetic bilayers with interlayer exchange interaction. A maximum 20 GHz tunable range was achieved in a spin-valve base spintronics microwave device in the experiment. A dynamic tuning of the resonance frequency as high as 9 GHz was demonstrated via temperature controlled exchange interaction.