Spin Interaction and Control – Research Topics
Non-linear spin waves in magnetic vortices
Recently, we extensively study nonlinear magnon-magnon scattering in magnetic microstructures with a fully quantized eigenmode spectrum. As a model system, we study micron-sized NiFe disks magnetized in the vortex state. We pattern such disks in omega-shaped microwave antennas. This allows us to excite magnons with a strong, symmetric out-of-plane microwave field. When measuring the magnon spectra with Brillouin light scattering microscopy as a function of the applied microwave frequency we directly observe three-magnon splitting, where the directly excited magnon splits into two new magnons. Due to conservation of energy, the frequencies of the split modes sum up to the excitation frequency. In stark contrast to magnon-magnon scattering studied in continuous films in the past, the rotational symmetry of the magnetization configuration in a vortex state imposes an additional conservation law during the scattering process, which is the conservation of the orbital angular momentum of the participating magnons.
Inside one magnetic disk, not just one of such three-magnon splitting channels exists but many different ones. The complexity of these nonlinear processes and their mutual interaction allow us to do pattern recognition taks and time-series prediction. We follow this direction of reservoir computing using nonlinear magnon-magnon scattering in reciprocal space in the framework of the EU-funded project NIMFEIA.
Spin waves in domain walls
Magnetic textures such as domain walls are promising candidates to manipulate spin waves in nanostructures. One focus of our research is the interaction of spin waves in thin films with Néel type domain walls. These walls have been shown to confine spin waves and, therefore, act as a waveguide which can be easily shifted by applying an external magnetic field. They may also act as phase shifters for spin waves propagating through such walls.
Spin-Hall nano-oscillators
Spin-Hall nano-oscillators are cutting-edge microwave oscillators with significant potential for future applications in communication technologies, serving as emitters and receivers, and in magnonics as sources of spin waves. They enable the conversion of a direct charge current into GHz-range magnetization oscillations. In their simplest form, they consist of a constriction, only a few hundred nanometers wide, composed of a bilayer of ferromagnetic (FM) and nonmagnetic heavy metal (HM) materials. Within these submicron structures, high current densities on the order of 108 A/cm² can be achieved by applying currents of just a few milliamps. In the heavy metal layer (e.g., platinum), the spin-Hall effect generates a pure spin current that flows toward the HM/FM interface and exerts an antidamping torque on the magnetization. This torque can be strong enough to counteract intrinsic damping, leading to magnetic auto-oscillations.
Bridging magnons and solid state quantum bits
Hybrid magnon-quantum spin defects have recently gathered scientific interest for both fundamental and technological applications. By bringing together the rich physics of magnons with the novel characteristics of solid state qubits, new hybrid quantum devices can be developed. While most of the hybrid magnon-quantum spin systems have been developed using Nitrogen-vacancy centers in diamond as the quantum platform, we focus on exploring hybrid architectures in silicon carbide (SiC). SiC is a more technologically mature material, with a shorter path from lab to fab due to the well-established fabrication protocols. Additionally, it hosts a large variety of spin defects with similar optical and quantum properties as the NV center. Despite this, developing a hybrid magnon-quantum system in SiC has proven elusive due to a lack of resonance overlap between the magnon subsystem and the quantum subsystem.
Silicon carbide - the host of qubits
Silicon carbide is a wide-bandgap compound semiconductor material formed by stacking layers of silicon and carbon atoms. Depending on the specific growth process, more than 200 different polytypes of SiC can be formed. Point scale defects naturally ocurring within the crystal structure of SiC have proven to perform as spin qubits, with non-zero spin states that can be readily initialized and read out through optical transitions.
Magnetic vortex - the host of magnons
We utilize confined magnon modes in a magnetic vortex to achieve a resonance overlap with the spin transitions of silicon vacancies in SiC. Above a certain threshold of the microwave driving power, three-magnon scattering sets in. An initial magnon splits spontaneously into two lower frequency magnons, redistributing the energy in the vortex.
Our hybrid platform
By bringing together the magnetic vortex and the silicon vacancy centers in SiC, a previously inexistent frequency overlap appears. Exploiting nonlinear magnon-magnon scattering, specific magnons couple to the silicon vacancy centers, completely decoupled from the direct microwave drive.
