Low-Damping Ferromagnetic Resonance in Electron-Beam Patterned, High-$Q$ Vanadium Tetracyanoethylene Magnon Cavities

TL;DR

This paper demonstrates the successful patterning and characterization of low-loss, high-$Q$ vanadium tetracyanoethylene (V[TCNE]) magnon cavities at micron scales, highlighting their potential for advanced microwave and quantum applications.

Contribution

It introduces a novel fabrication process for patterned V[TCNE] films with maintained low-loss properties at micron scales, expanding their applicability in microwave and quantum technologies.

Findings

Patterned V[TCNE] films retain low-loss characteristics down to 25 microns.

A rich spectrum of spin-wave modes is observed, indicating high-quality magnetic excitations.

The study provides detailed parameters like exchange stiffness and insights into mode behavior.

Abstract

Integrating patterned, low-loss magnetic materials into microwave devices and circuits presents many challenges due to the specific conditions that are required to grow ferrite materials, driving the need for flip-chip and other indirect fabrication techniques. The low-loss ($\alpha = 3.98 \pm 0.22 \times 10^{-5}$), room-temperature ferrimagnetic coordination compound vanadium tetracyanoethylene ($\mathrm{V[TCNE]}_x$) is a promising new material for these applications that is potentially compatible with semiconductor processing. Here we present the deposition, patterning, and characterization of $\mathrm{V[TCNE]}_x$ thin films with lateral dimensions ranging from 1 micron to several millimeters. We employ electron-beam lithography and liftoff using an aluminum encapsulated poly(methyl methacrylate), poly(methyl methacrylate-methacrylic acid) copolymer bilayer (PMMA/P(MMA-MAA)) on sapphire and silicon. This process can be trivially extended to other common semiconductor substrates. Films patterned via this method maintain low-loss characteristics down to 25 microns with only a factor of 2 increase down to 5 microns. A rich structure of thickness and radially confined spin-wave modes reveals the quality of the patterned films. Further fitting, simulation, and analytic analysis provides an exchange stiffness, $A_{ex} = 2.2 \pm 0.5 \times 10^{-10}$ erg/cm, as well as insights into the mode character and surface spin pinning. Below a micron, the deposition is non-conformal, which leads to interesting and potentially useful changes in morphology. This work establishes the versatility of $\mathrm{V[TCNE]}_x$ for applications requiring highly coherent magnetic excitations ranging from microwave communication to quantum information.