Hybrid structure with a ferromagnetic film and an array of magnetic molecules for deep-nanoscale reprogrammable magnonics

TL;DR

This work introduces a deep-nanoscale, reprogrammable magnonic platform formed by decorating a YIG film with a regularly spaced Mn$_{12}$ single-molecule magnets array. Through micromagnetic and FEM analyses, the authors demonstrate resonant dipolar coupling between propagating spin waves and molecular moments, producing an anti-crossing gap that suppresses transmission and is tunable via external field, molecular density, arrangement, and AFM clustering. The results show the system can operate in the GHz regime with controllable gap position and width, enabling dense, reprogrammable magnonic networks with potential neuromorphic and quantum-magnonic interfaces. The work outlines practical routes to fabrication and orientation control and highlights the key role of inter-molecular spacing in determining coupling strength, offering a viable path toward hardware neural networks at the nanoscale.

Abstract

Miniaturization is an essential element in the development of information processing technologies and is also one of the main determinants of the usability of the tested artificial neural networks. It is also a key element and one of the main challenges in the development of magnonic neuromorphic systems. In this work, we propose a new platform for the development of these new spin-wave-based technologies. Using micromagnetic simulations, we demonstrate that magnetic molecules regularly arranged on the surface of a thin ferromagnetic layer enable resonant coupling of propagating spin waves with the dynamics of the molecules' magnetic moments, opening a gap in the transmission spectrum up to 150 MHz. The gap, its width, and frequency can be controlled by an external magnetic field or the arrangement of molecules on the ferromagnetic surface. Furthermore, the antiferromagnetic arrangement of the magnetic moments of molecules or clusters of molecules allows for control of the gap's position and width. Thus, the proposed hybrid structure offers reprogrammability and miniaturization down to the deep nanoscale, operating frequencies in the range of several GHz, key properties for the implementation of artificial neural networks.

Figures (7)

• Figure 1: Schematic illustration of the investigated hybrid magnonic system. The system consists of a 10 nm thick YIG film (shown in blue) with an array of Mn$_{12}$ single-molecule magnets (SMMs) deposited on its surface (shown as blue ellipses). The external magnetic field $H_{\text{ext}}$ is applied perpendicular to the spin wave propagation direction, establishing the Damon-Eshbach configuration. Spin waves are excited by a microwave antenna (shown as gold stripe) and propagate along the $x$-direction through the YIG film. The magnetic molecules are arranged in a square lattice with lattice constant $a$ and are positioned 1 nm above the YIG surface to avoid direct exchange coupling while maintaining dipolar interaction.
• Figure 2: Spin wave transmission through the hybrid magnonic system obtained from time-domain simulations. (a) Dispersion relation of SWs in a YIG film decorated with a square array of Mn$_{12}$ molecules, showing the characteristic anti-crossing gap at 2.75 GHz. The inset displays the signal collected solely from the layer of molecules, confirming the coupling between the Mn$_{12}$ and YIG. (b) Evolution of SW transmission along the $x$-axis in the YIG layer. The inset shows the signal from the layer of molecules as a function of $x$, indicating energy transfer from the YIG to the molecules. (c) The transmission spectra at four selected positions: before ($x=7.0$$\mu$m), within ($x=9.0$ and $10.5$$\mu$m), and after ($x=14.0$$\mu$m) the array of molecules. For all figures, SWs are excited at $x=6$$\mu$m, and the molecular array occupies the region between $x=7.75$$\mu$m and $x=13.75$$\mu$m. The bias magnetic field $\mu_0 H_0 = 10$ mT is along the $y$ axis.
• Figure 3: The dispersion relation of SWs in a hybrid structure around the anti-crossing between the DE SW mode and the molecule resonance for the molecules modeled as either cuboids (blue lines) or spheres (orange lines). Despite their different shapes, both geometries maintain the same total magnetic moment per molecule and the same distance between the molecular center and the YIG surface. The simulations were performed in the frequency domain using FEM. The bias magnetic field is $\mu_0 H_0=10$ mT.
• Figure 4: (a) Dispersion relation of SWs in the hybrid system for the two values of the bias magnetic field in the DE geometry. (b) The coupling strength $\Delta f$ (full dots and the left-hand scale axis) between the resonances of the molecules and the propagating SWs in YIG in the DE geometry (filled red squares) and the BV geometry (filled green squares) as a function of the external magnetic field strength. The empty squares indicate the wavenumber at which the anti-crossing occurs (empty dots and the right-hand scale axis).
• Figure 5: (a) The dependence of the resonance frequency of two molecules on the distance between them, showing the influence of direct dipolar interactions. (b) The coupling strength $\Delta f$ between the DE mode and the magnetic molecules as a function of intermolecular distance. Black points represent values extracted from numerical simulations and the red line shows the theoretical prediction from CMT. The bias field is set to zero.