Yttrium iron garnet nano films: epitaxial growth, damping, spin pumping, and magnetic proximity effect
Date
2014
Authors
Sun, Yiyan, author
Wu, Mingzhong, advisor
Patton, Carl, committee member
Field, Stuart, committee member
Reising, Steven, committee member
Celinski, Zbigniew, committee member
Journal Title
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Volume Title
Abstract
Recently, a new research field called magnetic insulator-based spintronics opened the door to a large amount of potential applications in the electronics industry. In this field, low-damping materials in the nanometer scale are critically needed for both fundamental studies, such as spin pumping, and device applications, such as spin-torque nano-oscillators. Yttrium iron garnet (YIG) materials are the best candidate among other materials. There is a critical demand for high-quality nanometer-thick YIG films. This dissertation reports experimental studies on YIG films with the thickness ranged from several nanometers to several hundreds of nanometers. Firstly, the feasibility of low-damping YIG nano films growth via pulsed laser deposition (PLD) techniques is demonstrated. A 5-nm-thick YIG film, for example, shows a peak-to-peak ferromagnetic resonance (FMR) linewidth of <10 Oe at 10 GHz. Optimization of PLD control parameters and post-deposition annealing processes and surface modification by ion beam etching for the realization of high-quality films are discussed in detail. The second main topic is on spin pumping and magnetic proximity effects in YIG nano films. Specifically, the dissertation touches on (1) the spin pumping efficiency of YIG nano films and (2) damping enhancement in YIG nano films due to Pt capping layers. Knowing the efficiency of spin angular momentum transfers across YIG/normal metal (NM) interfaces is critical to the use of YIG films for spintronics. Under subtopic (1), the spin transfer efficiency at YIG/NM interfaces is determined through the measurement of spin pumping-caused additional damping in YIG nano films. A fairly large portion of recent studies on YIG-based spintronics made use of a Pt capping layer either as a detector to measure spin currents or as a spin-current source. Work under subtopic (2), however, indicates that the growth of a Pt capping layer onto a YIG film can result in a significant damping enhancement in the YIG film. Fortunately, this damping can be completely suppressed simply by the addition of a thin Cu spacer in-between the YIG and Pt films. The interpretation of the observed damping enhancement in terms of the magnetic proximity effect in the Pt film is presented. The last topic addresses the growth of high-quality YIG thin films on metallic substrates. It is demonstrated that one can grow YIG thin films on Cu via the use of a protection layer of high entropy alloy nitrides. The YIG films showed a peak-to-peak FMR linewidth of about 1.1 Oe at 9.45 GHz. This work provides implications for the future development of YIG thin film-based monolithic devices for high frequency processing.
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Subject
ferromagnetic resonance
thin film
magnetic proximity
spin pumping
damping
Yttrium iron garnet