Nicholas Homrocky of Oakland University

Abstract

Surface acoustic waves (SAW) propagate reciprocally within devices that are used in signal processing, and sensory technology. They are efficient, well understood, and used in many practical devices. Our aim is to find optimal coupling of SAW to any spin wave. Here we show SAW can induce excitation of Backward Volume Spin Waves (BVSW) and Surface Spin Waves (SSW) that allow us to work in new areas and create low energy efficient technology. This problem is studied theoretically in a 2-layer system of Yttrium Iron Garnet (YIG) on top of a Gadolinium Gallium Garnet (GGG) substrate. Expressions for the SAW-spin wave coupling rate is obtained using Kalinikos-Slavin theory of dipole-exchange SSW [1] substitution into the magneto-elastic energy density formula. For SAW-SSW coupling, the maximum coupling exceeding 40 MHz is achieved for magnetic field applied at 45⁰ to the direction of wave propagation and SAW frequency ~ 2 GHz. For YIG thicknesses below 500 nm, the SSW profile remains symmetric and the SAW-SSW coupling is reciprocal. While keeping λ smaller, with the increase of YIG thickness, the nonreciprocity of magnetoelastic coupling gradually increases; however, for YIG thickness above 2.2 µm, the SAW-SSW gap becomes wider than the gap between SSW and higher-order standing spin waves, which substantially complicates the spectrum of the hybrid magneto-acoustic waves. The induced SAW non-reciprocity is most clearly observed for YIG films with intermediate thicknesses of 500 nm – 2.2 µm. For SAW-BVSW coupling, the thinner films and larger magnetic fields give the best coupling. With YIG film thickness 250 nm at 3000 Oe internal field we have a coupling rate of ~100 MHz. This will produce a 200 MHz anti-crossing gap in the wave spectrum. However, a balancing act must be made for propagation of BVSW.  The thicker films support faster transport of BVSW in the opposite direction of SAW propagation. The 5 µm thick YIG film has BVSW at 1.7 km/s while the 250 nm has BVSW at 0.4 km/s. This speed is important to note when damping is large in doped materials. Unless we study Bose-Einstein Condensates at the minimum of the BVSW spectrum. The thicker 5µm thick YIG film would require less energy to achieve this considering a lower magnetic field of 1000 Oe is needed compared to thinner films. In these ranges, the SAW/Spin Wave coupling rates allows one to use these phenomena for several practical applications. 

References

[1] B A Kalinikos & A N Slavin 1986 J. Phys. C: Solid State Phys. 19 7013

Biography:

Nicholas Homrocky is an Applied and Computational Physics Ph.D. student at Oakland University. He helps as a teaching assistant to undergraduate physics courses year-round. His research focuses on the coupling of mechanical acoustic waves to magnetic spin waves, using numerical simulations to find the optimal conditions for certain desired outcomes.

In his personal life, Nicholas has been married for 1 year, living with his wife in an apartment with their mini-poodle. He spends a ton of time with family helping his father-in-law with contracting work, his brother with acting in films, games with his father and friends, and outdoor activities with his mom and her dog.

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