Enhancing nonreciprocal filtering through surface modification of ferrimagnetic insulators

Abstract

To address the significant reduction in magnetostatic surface waves (MSSW) nonreciprocity as ferrimagnetic insulators are scaled to the nanometer level, this study demonstrates an effective interface engineering methodology through targeted surface oxidation of 100 nm yttrium iron garnet (YIG) thin films. By implementing a sequential chemical treatment using FeCl3 and piranha solutions, we introduce a 30-nm-deep oxygen gradient near the YIG surface and increase the surface Fe3+/Fe2+ ratio from 2.7 to 7.3. This surface modification effectively restored the asymmetric dynamic dipolar stray fields associated with MSSW precession, which are essential for nonreciprocity, while simultaneously reducing the Gilbert damping coefficient from 8.2 × 10−4 to 4.8 × 10−4. Experimental characterization of the resulting filter prototype at 6.9 GHz reveals that surface treatment enhances the nonreciprocal isolation from 1.1 to 16.2 dB, narrows the signal passband from 200 to 70 MHz, and achieves optimized microwave transmission with an insertion loss of 4.2 dB. Together with semi-analytical modeling, these results show that surface oxidation converts a chemical depth gradient into direction-dependent effective magnetic parameters, thereby enhancing MSSW nonreciprocity through direction-dependent frequency response and dynamic-loss asymmetry. This work establishes a physically grounded interface engineering route for high-isolation, low-loss nonreciprocal components in integrated radio frequency and microwave front-end systems.