# Design and Modelling of a Novel Integrated Photonic Device for Nano-Scale Magnetic Memory Reading
## 1 Introduction  
In the modern world, exponentially increasing generation of data and its handling require novel technologies that perform faster and more energy efficiently. To answer this need, optical components are being used in combination with electronic circuitry to improve the speed and bandwidth of data communication and telecommunication. For example, optical interconnections that were once a conceptual design suggestion [1] are currently being used in commercial products replacing slow and heat-dissipating electrical signal communication channels [2; 3]. Researchers continue to demonstrate the superior performance circuitry achieved through the integration of photonics into electronics [4; 5; 6; 7]. Yet, these advances require back-and-forth signal conversion between optical and electrical domains, which happens to be the new bottleneck in data communication and processing. Addressing this problem requires establishing novel functionalities in photonic devices that will enable a seamless conversion. Furthermore, (integrated) photonics is lacking a simple and fast non-volatile memory function. A huge potential is anticipated for future devices that enable direct inter-conversion of data between the photonic and magnetic (memory) domain without any intermediate electronics steps,cutting down on time and energy costs. This study works with existing non-volatile magnetic memory material technology used in electronics: multilayered ferromagnetic thin-film layers. When the multilayered magnetic material is used as memory material, writing bits into the magnetic memory could be facilitated by recent advances in so-called all-optical switching of magnetization [8,9]. Reading out magnetic bits back into the photonic domain could be achieved via a nonreciprocal magneto-optical process [10, 11, 12], while dynamic, on-the-fly reading of magnetic bits could be facilitated by racetrack memory concept [9,13]. In a racetrack memory, magnetic domains (memory bits) move while the material that carries the magnetic domains remain stationary [14,15]. Previously, domain wall velocities up to 1000 ms\({}^{-1}\) were demonstrated [16,17]. It is in this spirit that our paper focuses on the functionality of on-chip optical reading of magnetic memory processed as ultrathin magnetic claddings on photonic waveguides. To our best knowledge, this is the first study which explores the possibility of on-chip, all-optical magnetic memory reading functionality. State-of-the-art non-volatile magnetic memory such as spin-transfer torque magnetic random-access memory (STT-MRAM) relies on ferromagnetic multilayered ultrathin films with perpendicular magnetic anisotropy (PMA), in which the magnetization vector is perpendicular to the film plane [18]. Such PMA films turn out to be essential for the advanced schemes used to electrically control the magnetic memory elements but are also known for their relatively large magneto-optical efficiency. A simple layer stack that hosts all relevant physical mechanisms is Ta(4)/Pt(2)/Co(1)/Pt(2)/Co(1)/Pt(2) where the numbers in parenthesis are thickness in nm. Bringing this memory component to the proximity of light confined in a waveguide in a photonic device setting gives rise to magneto-optic interactions, specifically the Magneto-optic Kerr Effect (MOKE). MOKE causes a change in the polarization state of light (Kerr rotation and ellipticity), which changes sign when the magnetization direction of the memory component is flipped [11,12]. In a photonic waveguide context, this gives rise to partial mode conversion between TE and TM modes, which potentially enables reading of the memory bit. However, the MOKE signal is intrinsically small in amplitude, a typical Kerr rotation is around 0.05\({}^{\circ}\) for films with an in-plane magnetization in free-space optics [19,20]. In order to increase the efficiency of the mode conversion, we propose the use of PMA magnetic claddings, which have not been seriously addressed yet in a photonic perspective.