Room: Park Tower 8216
Amanatullah Khan - University of California, Irvine
Non-reciprocity of propagating signal carriers is extremely important for the development and application of microwave signals processing. A signal medium is non-reciprocal when there is a difference in the signal propagating forward or backwards in a geometrically symmetric configuration, this includes amplitude, wavelength, and phase differences. Currently, non-reciprocal devices based on bulk Yttrium Iron Garnet (YIG) films are used often, but their bulk sizes are impractical for on-chip communications. In order to apply YIG based devices on the integrated chip level, non-reciprocity must be studied on ultra-thin YIG films. This has not been done yet. Furthermore, chargeless signal carriers like spin waves in ferromagnetic insulators are growing in interest in both academia and industry as an energy efficient, more powerful alternative to the current paradigms in industrial nano-technology. Through nano-fabrication processes we have designed and created short wavelength spin wave antennae on ultra-thin YIG films (40 nm). With the use of a Vector Network Analyzer, we conducted broadband ferromagnetic resonance measurements on the sample. This involves sending microwave current through one antenna and reading the inductive pick-up from propagating spin waves (which generate an oscillating local field) at the other. We sweep frequency from 100 MHz to 8GHz at fixed intervals of magnetic field pointing in plane of the film and perpendicular to the direction of propagation. The fields swept were from -0.15 T to 0.15 T in increments of 0.8 mT. We remove non-magnetic noise by looking at the normalized derivative of our spectra. Surprisingly, we have observed non-reciprocity in bare thin-film YIG beyond that of just amplitude (which was expected). We have observed that for forward and reverse transmission the group velocities were 560 m/s and 516 m/s respectively. This is surprising because there is not any notable layer on top of or below the YIG film to warrant this result. We believe that this may be due to varying surface anisotropies between the top (forward propagating) and bottom (reverse) of the film, possible sources of these may be due to some difference in magnetic pinning or edge effects on the orbitals on both sides. This surprising result emerged while we are pursuing a gated system where there will be a bias electric field across the film between antennae. Future research involves studying the field effects on non-reciprocality of spin waves in ultra-thin film YIG and other potential spin wave media. This is a promising result towards a voltage controlled, non-reciprocal, on-chip spin-wave device. References: A. V. Chumak et al., Nature Physics 11, 453 (2015) M. Jamali et al., Scientific Reports 3, 3160 (2013) M. Kostylev, Jour. of Appl. Phys. 115, 233902 (2014) H. Maier-Flaig et al., Rev. of Sci. Instr. 89, 076101 (2018)
Funder Acknowledgement(s): I thank Y. J. Chen, H. K. Lee, A. A. Jara, and C. Safranski for their exceptional mentorship in micromagnetic measurements and nanofabrication as my former seniors. I especially thank Professor Krivorotov for his guidance and mentorship as my advisor. Finally I thank Tao Liu and Mingzhong Wu at Colorado State University for providing me the high quality films required for this project. Funding provided by NSF EFRI NewLAW, #EFMA-1641989 grant awarded to I. N. Krivorotov
Faculty Advisor: Ilya Krivorotov, firstname.lastname@example.org
Role: Designed the antenna/detector. Nanofabricated the devices. Optimized nano-device recipe. Measured the scattering matrix that yielded the non-trivial non-reciprocity. Optimized measurement technique for better signal to noise ratio.