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Spectroscopy of Spin Excitations in sub-100nm Magnetic Structures using High Electron Mobility Transistor Photodetection

Final Report Summary - SPECTROSPIN (Spectroscopy of Spin Excitations in sub-100nm Magnetic Structures using High Electron Mobility Transistor Photodetection)

The increase in information storage density on magnetic hard drives meets a number of technological challenges. Magnetic bits must be engineered from high anisotropy materials as their size approach the super-paramagnetic limit. In addition, the increase in access rates to individual bits, now in excess of several GHz, requires a detailed understanding of the high frequency dynamics of ultra-small magnets. Microwaves induce spin excitations in the bulk and increasingly at the surface of magnetic elements as magnets become smaller and the surface to bulk ratio increases. Confined spin waves can in principle by harnessed to engineer faster read/write. Surface spin waves are strongly depend on the geometry of magnets whose size and dimensions determine their boundary conditions, the number and frequency of oscillation modes and the coupling between modes. The present project has investigated surface spin waves in magnets which are smaller than the wavelength of light and as a result are too small to be investigated using state of the art magneto-optic techniques such as Brillouin Light Scattering.

The novelty of our approach has been to use high mobility two dimensional electron gases formed in GaAs/AlGaAs quantum well heterostructures as photodetectors of the stray magnetic field emitted by spin waves. We have fabricated a range of sub-100nm magnets in different geometries at the surface of two-dimensional electron gases and measured the photo-voltage induced by spin wave excitations. In this way, we were able to perform photo-voltage spectroscopy of surface spin waves and pick up changes in magnetization equivalent to a few Bohr magnetons. We also report for the first time spin wave excitations in individual sub-100nm magnets
When bar magnets are magnetized along the short axis, microscopically inhomogeneous magnetic fields form near pole surfaces. This spatially varying magnetic field creates magnetic quantum wells which confine two dimensional dipolar edge spin waves near poles. When bar magnets are magnetised along their long axis, boundary conditions on Maxwell’s equations allow spin waves to propagate along Damon-Eshbach modes. These modes were observed as a distinct series of resonant cusps in the photo-voltage as a function of the external magnetic field. In sub-100nm magnets, this picture is strongly modified by the interactions of edge spin waves which overlap in the bulk of nanomagnets. We modelled the individual modes and the magnetic susceptibility using OMMPH micromagnetic simulations. We found a qualitative agreement between the calculated resonances in the susceptibility and those observed in photovoltage spectra. The spatial extent of confined modes
is strongly dependent of frequency in part because of spin waves become confined in three dimensions at the magnet size decreases. This observation is beyond currently available analytical models. More work is underway to provide a full quantitative explanation to the photovoltage spectra using numerical simulation. The symmetric/antisymmetric nature of photovoltage spectra was associated with the predominance of the real/imaginary parts of the susceptibility or equivalently between the dephasing of the dynamic magnetization and the microwave magnetic field. Cobalt dot geometries magnetised transversely produced muted spin resonance spectra. We strain engineered magnetic vortex dots and studied their switching in the presence of a perpendicular magnetic field.

In summary, we have successfully demonstrated photovoltage spectroscopy as an ultrasensitive technique for probing spin dynamics in sub-100nm magnets. On such small scales, discrete spin waves are still formed but are confined in 3D. As the magnet size approaches the decay length of the dipolar magnetic field and the exchange length, bulk states vanish and edge states begin to interact through the bulk. The impact of our work to magnetic storage is to show that bits of information
may be read/written at extremely high frequencies, 30-110GHz, using nanomagnets engineered so that their surface spin wave modes match these frequencies.