Reconfigurable magnonics using self-biased reprogrammable nanomagnetic structures INVITED

Abstract

Magnetic materials are at the hearts of billions of high frequency communication devices and on-chip integration of magnetic microwave devices is of paramount importance for efficient utilization of the huge electromagnetic (EM) spectrum beyond 5G. Magnonics – a sub-field of spintronics – employs magnetic waves and oscillations in magnetic thin films and patterned nanostructures for processing GHz to sub-THz EM signal [1-2]. Magnetic ferrites are conventionally used in such magnetic microwave devices. However, due to the need for external bias magnetic field and incompatibility with complementary metal oxide semiconductor (CMOS) process, ferrites are not suitable for miniaturized microwave components. On the other hand, metallic magnetic thin films and nanostructures have great potential for efficient engineering of microwaves. Although, the need for bias field remains a bottleneck. We have been working on solutions where the requirement of the bias-field is absent. We have successfully demonstrated several strategies for achieving such device architecture by utilizing self-biased nanomagnets with reprogrammable remanent states which are associated with distinct magnetization dynamics [2-11].To illustrate reconfigurable microwave properties, we have shown in Fig 1 as an example, a pair of dipolarly coupled rhomboid nanomagnets (RNMs). Two different remanent states (↑↑ and ↑↓) are obtained by utilizing a simple field initialization (HI) scheme: applying a saturating field along short and long axes of the RNMs with subsequent removal of the field (Fig. 1(a)). Due to different distributions of the stray fields for the two remanent states, the effective fields (Fig. 1(b)) are different which manifest in the shift of the microwave spectra for the two remanent states. Measurement of microwave properties from a single isolated nanomagnetic pair was obtained by using micro-focused Brillouin light scattering (micro-BLS) experiment (Fig. 1(c)) and the micro-BLS spectra are shown in Fig. 1(d). This strategy for reconfigurable magnonic responses is expanded to 2D arrays of the RNMs for different magnetic configurations: antiferromagnetic-type and ferrimagnetic-type artificial magnonic crystals. The maximum amount of the shift in the microwave responses is achieved by using multilayer isolated as well as dipolar-coupled networks of RNMs. Beyond, RNM shape, we have also demonstrated reconfigurable microwave operation based on other strategic shapes with multiple remanent states, such as arrow-, C-, L- and S-type nanomagnets and zigzag nanowires.Furthermore, we emphasize that such self-biased nanomagnets are also suitable for magnonic device applications where the propagation and gating of magnons – quanta for spin waves – are of primary interest. Conventionally, a large bias magnetic field is used in order to satisfy an orthogonal orientation between spin wave propagation and magnetization direction for efficient transmission characteristics (known as surface geometry or Damon-Eshbach mode). The need of such bias field hinders device integration on-chip for magnonic waveguides. Therefore, we have proposed and demonstrated a self-biased magnonic waveguide based on dipolar-coupled but physically separated chain of RNMs (Fig. 2). A prototype of such magnonic devices is shown in Fig. 2(a) where a microwave antenna (input) excites the spin waves and the spin waves are detected using micro-BLS technique with a focused laser spot (output). In order to gate the spin wave propagation, we have flipped a RNM in the waveguide and it is marked as ‘gate’ in the scanning electron microscopy (SEM) image (Fig. 2(b)). Such waveguide design enables one to have two different remanent states: ferromagnetically ordered (FO) and FO with a defect (FO*) when field initialized (same scheme as mentioned above) along the width and the length of the waveguide, respectively. FO and FO* remanent magnetic states are shown in Fig. 2(b) using magnetic force microscopy (MFM) images. Gating of the spin wave propagation has been shown by plotting 2D spatial profiles of the spin wave intensities using micro-BLS technique which clearly indicate gated magnon flow for the FO* state in comparison to the FO state. Thus, one can also achieve binary gating of output signal amplitude using such self-biased nanomagnetic waveguide which is remarkable as it also eliminates the need for bias field.In this presentation, we would like to emphasize the need for bias-free reconfigurable magnonics along with recent developments and future roadmap. **