Low-loss magnonic crystals based on nanometer-thick YIG films

Abstract

Magnonics utilizing spin waves for information transport, storage, and processing offers an alternative for CMOS-based technology free of detrimental Joule heating at high operation frequency [1]. The realization of magnonic devices requires active control of spin waves and low transmission losses. Magnonic crystals, i.e., metamaterials with periodically modulated magnetic properties, made of yttrium iron garnet (YIG) are promising candidates for performing such tasks because of ultralow magnetic damping [2]. Thus far, several types of micrometer-thick YIG magnonic crystals have been explored including crystals made of periodic shallow airgrooves, metallic stripe arrays on top of a YIG film, and width-modulated YIG structures [2]. On the other hand, programmable control of spin-wave transmission in micrometer-thick YIG films has been demonstrated using current-carrying meander structures [3], optical absorbers [4], and strain coupling to a piezoelectric layer [5]. However, micrometer-thick YIG magnonic crystals with a large number of crystal units are not compatible with miniaturized on-chip device integration. In addition, magnonic crystals based on thick YIG films exhibit small bandgaps in the range 5 MHz – 50 MHz, which limits the manipulation of propagating spin waves. Low-loss nanomagnonics requires smaller YIG magnonic crystals with larger tunable bandgaps [6].Here, we present the experimental realization of magnonic crystals made of only 2 - 4 discrete 260-nm-thick YIG stripes with robust bandgaps up to 200 MHz for Damon-Eshbach spin waves [7]. The YIG stripes of the magnonic crystal are separated by airgrooves or grooves filled by CoFeB. Compared to micrometer-thick YIG films, the opening of larger bandgaps with less crystal units in arrays of thin YIG stripes is explained by strong Bragg reflection on the individual scatterers. Moreover, efficient dipolar coupling between the YIG stripes via in-plane dynamic fields facilitates low-loss transmission in the allowed bands of the magnonic crystal. The bandgaps of the nanometer-thick YIG magnonic crystals are tunable through a variation of the groove depth, lattice constant, and film thickness [8]. For instance, the bandgaps of a 260-nm-thick YIG crystal widen upon an increase of the groove depth from half to full film thickness. Importantly, low-loss spin-wave transmission in the allowed bands of the magnonic crystal is hardly affected by the patterning of fully discrete YIG stripes. Downscaling of the YIG film thickness to 35 nm decreases the bandgap size through a flattening of the spin-wave dispersion relation. We show that a reduction in the lattice constant effectively compensates for this trend. For 65-nm-thick YIG crystals, we realized 100–200 MHz bandgaps by decreasing the lattice constant from 50 µm to 10 µm. Finally, we show that robust bandgaps up to 220 MHz in combination with low-loss spin-wave transmission at allowed frequencies are attained in a 45-nm-thick YIG crystal with a lattice constant of 2 µm (see Fig. 1). We anticipate that further downscaling of YIG magnonic crystals to the nanoscale is possible through an optimization of the fabrication process and efficient spin-wave excitation over a broad range of wave vectors. **