Amy Eden they/them

Motivation

Building on recent breakthroughs in on-chip hybrid magnon–photon systems based on Yttrium Iron Garnet (YIG) and superconducting resonators [1,3], this PhD project aims to expand the material platform for hybrid quantum systems based on magnonics beyond YIG.

While YIG has long been the benchmark for magnonic applications due to its exceptionally low spin-wave damping and long coherence times [1], its reliance on nonstandard substrates and complex fabrication methods limits scalability and integration with superconducting quantum circuits and CMOS-compatible technologies [2]. Moreover, YIG’s isotropic magnetic response and narrow tunability range restrict the design flexibility of hybrid quantum devices.

Exploring alternative magnetic materials may offer new opportunities to tailor spin–photon coupling, enhance frequency tunability, and achieve new coupling regimes. Such materials could also bring additional functionalities, including electrical gating, magneto-optical effects, or topological spin textures, to hybrid quantum architectures.

This project is motivated by the vision of developing a versatile material platform for cavity spintronics and quantum hybrid technologies, enabling coherent information transfer between photons, magnons, and potentially other quasiparticles such as phonons or excitons.

Aims

The primary aim of the project is to overcome the incompatibility issues of YIG by exploring different materials to achieve strong magnon–photon coupling in a system integrated on a chip, specifically at sub-Kelvin temperatures.

Specific aims included:

1. Developing a method for integrating magnetic materials with superconducting quantum technologies using Plasma Focused Ion Beam (PFIB) technologies.
2. Achieving strong coupling between superconducting resonators and magnetic resonance modes of high-quality magnetic materials while maintaining low loss and reducing the system scale to the micrometer range.
3. Developing a comprehensive theoretical framework to understand the coupling in hybrid systems by investigating the electromagnetic properties of both the resonator and the magnetic samples.
4. Employing an accurate electromagnetic perturbation theory to predict the resonant hybrid mode frequencies for any field configuration within the cavity resonator.

Methodology

The project will utilise both advanced fabrication techniques for on-chip integration and a rigorous theoretical approach based on electromagnetism.

Fabrication and Measurement:

• Superconducting planar devices are to be fabricated from 100 nm-thick niobium nitride (NbN) films deposited on intrinsic silicon substrates. NbN is selected for its high critical field values, ensuring stable operation under applied magnetic fields [4].
• Plasma Focused Ion Beam (PFIB) technology will be employed for high-precision processing and integration of the chosen magnetic materials. Microstructures will be milled from bulk or thin-film sources into rectangular cuboid or tailored geometries (typically 5–50 µm in dimension), optimised for coupling to the resonator field.
• A nanomanipulator will be used to precisely position and affix the PFIB-processed magnetic samples onto prefabricated NbN microwave devices via localised platinum (Pt) deposition. The samples will be placed at the magnetic antinode of the lumped-element resonator’s inductive line, where the oscillating microwave magnetic field is maximised [3].
• Ferromagnetic resonance (FMR) spectra and magnon–photon coupling measurements will be conducted at milliKelvin temperatures (down to 80 mK) using an adiabatic demagnetisation refrigerator equipped with a superconducting solenoid to apply a tunable static magnetic field. The microwave transmission parameter (S_{21}) will be recorded using a vector network analyser (VNA) to identify hybridised resonant modes and quantify coupling strength.

Theoretical Framework:

• The project employs an electromagnetic perturbation theory combining the Landau–Lifshitz equation with Maxwell’s equations to describe the interaction between superconducting resonator fields and magnetic resonance modes.
• The model incorporates the dynamic magnetic susceptibility tensor, accounting for sample geometry, anisotropy, and demagnetisation effects.
• Assuming small magnetic perturbation and a negligible dielectric contribution, the framework enables the calculation of hybrid mode frequencies and coupling strengths based on cavity parameters and stored energy ratios.
• This approach can be applied to various magnetic materials and geometries, providing a predictive tool for understanding and optimising magnon–photon coupling in hybrid quantum systems.

Impact

This research extends on-chip hybrid magnon–photon systems beyond YIG, advancing materially diverse and scalable cavity spintronic platforms for quantum technologies.

Key Objectives and Expected Achievements:

• Develop a general integration framework for coupling superconducting resonators with various magnetic materials using PFIB-based microfabrication.
• Demonstrate strong magnon–photon coupling across multiple materials while maintaining low loss and high cooperativity.
• Apply and extend electromagnetic perturbation theory to predict hybrid mode frequencies and coupling strengths without empirical parameters.
• Understand how material properties (damping, anisotropy, spin density) affect coupling, coherence, and device performance.

Future Significance:

• Enable customisable hybrid quantum devices with tunable coupling through material selection.
• Guide the design of scalable quantum components such as spin-based memories, microwave–optical transducers, and hybrid logic elements.
• Provide a predictive theoretical framework for optimising complex magnetic systems and novel materials in cavity spintronics and quantum information science.

References

[1] P. G. Baity, D. A. Bozhko, R. Macˆedo, W. Smith, R. C. Holland, S. Danilin, V. Seferai, J. Barbosa, R. R. Peroor, S. Goldman, et al. Strong magnon–photon coupling with chip-integrated yig in the zero-temperature limit. Applied Physics Letters, 119(3), 2021.

[2] B. Lenk, H. Ulrichs, F. Garbs, and M. M¨unzenberg. The building blocks of magnonics. Physics Reports, 507(4-5):107–136, 2011.

[3] R. Macedo, R. C. Holland, P. G. Baity, L. J. McLellan, K. L. Livesey, R. L. Stamps, M. P. Weides, and D. A. Bozhko. Electromagnetic approach to cavity spintronics. Physical Review Applied, 15(2):024065, 2021.

[4] A. Banerjee, R. M. Heath, D. Morozov, D. Hemakumara, U. Nasti, I. Thayne, and R. H. Hadfield. Optical properties of refractory metal based thin films. Optical Materials Express, 8(8):2072–2088, 2018.