New Publication: Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core

Congratulations to David Osuna, whose paper, ‘Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core’, was published this month in Physical Review B. David has recently finished his PhD in the CDT and is now a postdoctoral research fellow at University of Exeter, as part of the Electromagnetic and Acoustic Materials Group (EMAG).

David explains the paper’s topic:

“This was a very fruitful collaboration with Dr. Paul Keatley, finally published after about 3 years of hard work!

Generally speaking, we have modeled and ‘filmed’ oscillations of the atomic magnetic spins in microscopic magnets and related them to other dynamics revealed from simulations. Understanding this type of dynamics as a whole is key to design spintronic devices, that may be essential for processing information in quantum computers, for example.

Abstract

The curling spin wave modes of a ferromagnetic vortex confined to a microscale disk have been directly imaged in response to a microwave field excitation using time-resolved scanning Kerr microscopy. Micromagnetic simulations have been used to explore the interaction of gyrotropic vortex core dynamics with the curling modes observed in the region of circulating in-plane magnetization. Hybridization of the fundamental gyrotropic mode with the degenerate, lowest frequency, azimuthal modes has previously been reported to lead to their splitting and counterpropagating motion, as we observe in our spectra and measured images. The curling nature of the modes can be ascribed to asymmetry in the static and dynamic magnetization across the disk thickness, but here we also present evidence that spiral spin waves emitted by the core can influence the spatial character of higher frequency curling modes for which hybridization is permitted only with gyrotropic modes of the same sense of azimuthal motion. While it is challenging to identify if such modes are truly hybridized from the mode dispersion in a confined disk, our simulations reveal that spiral spin waves from the core may act as mediators of the interaction between the core dynamics and azimuthal modes, enhancing the spiral nature of the curling mode. At higher frequency, modes with radial character only do not exhibit marked curling, but instead show evidence of interaction with spin waves generated at the edge of the disk. The measured spatiotemporal character of the observed curling modes is accurately reproduced by our simulations, which reveal the emission of propagating short-wavelength spiral spin waves from both core and edge regions of the disk. Our simulations suggest that the propagating modes are not inconsequential, but may play a role in the dynamic overlap required for hybridization of modes of the core and in-plane magnetized regions. These results are of importance to the fields of magnonics and spintronics that aim to utilize spin wave emission from highly localized, nanoscale regions of nonuniform magnetization, and their subsequent interaction with modes that may be supported nearby.

Fig. Time sequence of a radial-azimuthal spin wave mode simulated and experimentally imaged in a 2 micrometres diameter, 40 nm thick Permalloy disc with an in-plane RF excitation field at 10.24 GHz. Timestep is approximately 24 ps.

New Publication: An in situ investigation of the thermal decomposition of metal-organic framework NH2-MIL-125 (Ti)

Zahid Hussain

Congratulations to Zahid Hussain for his new paper, ‘An in situ investigation of the thermal decomposition of metal-organic framework NH2-MIL-125 (Ti)’ , recently published in Microporous and Mesoporous Materials.

Zahid explains the paper’s findings:

Metal-organic frameworks (MOFs) are exceptionally porous and highly crystalline coordination polymers. Since the late 1990s, MOFs have been intensively investigated for a large variety of applications such as gas separation and storage, energy storage and conversion, batteries, fuel cells, optoelectronics, sensing, supercapacitors, drug delivery and catalysis. However, many key questions need to be answered to optimize the synthesis of these materials for industrial-scale applications. In this study, we present an in-situ investigation of thermal conversion of a titanium-based MOF, NH2-MIL-125(Ti) under an inert atmosphere. In situ thermal analysis of NH2-MIL-125(Ti) reveals the presence of 3 defined stages of thermal transformation in the following order: phase-pure, highly porous, and crystalline MOF → intermediate amorphous phase without accessible porosity → recrystallized porous phase. The three stages occur from room temperature till 300 °C, between 350 and 550 °C and above ∼550 °C respectively. The derived disc-like particles exhibit a 35% volume shrinkage compared to the pristine MOF precursor. Highly crystalline N and/or C self-doped TiO2 nanoparticles are homogeneously distributed in the porous carbon matrix. The original 3D tetragonal disc-like morphology of the NH2-MIL-125(Ti) remains preserved in derived N and/or C doped TiO2/C composites. This study will provide an in-depth understanding of the thermal conversion behaviour of MOFs to rationally select and design the derived composites for the relevant applications.

The crystalline structure of Ti-MOF, NH2-MIL-125(Ti) and mechanism of its thermal decomposition.

 

 

We would also like to congratulate Zahid on passing his viva, and wish him the best of luck for his future career.