Magnonic Crystals without the "pit"-falls

Magnonics research, the current hot topic in spintronics, is concerned with structures, devices and circuits that use spin currents carried by magnons - the quanta of spin waves. Analogous to electric currents, magnon-based currents can be used to carry, transport and process information without the drawbacks inherent to modern electronics, such as dissipation of energy due to Ohmic losses. A 2D magnonic crystal is created by adding an artificial periodicity of the order of the magnon wavelength (~micron) to a magnetic thin film. Similar to photonic crystals, the artificial periodicity modifies the wave propagation and allows for the formation of band gaps. What makes spin waves favourable for technological applications is the magnetic "index of refraction", which can easily be modified by modulating the saturation magnetisation (M). Due to its exceptionally low damping, even in thin films, Yttrium Iron Garnet (YIG) is considered the most prominent material in this field being widely used in spin pumping experiments and research on magnonics. The ultra-low damping allows for long spin lifetimes and propagation lengths. Two recent works in the field have opened up new areas previously not considered. The first (NPhys 4175) has shown that amorphous YIG (a-YIG) has exceptionally long spin lifetimes even though there is no long range ferromagnetic order (M ~ 0). The second work (Sci Rep 20827) demonstrates that a-YIG can be recrystallized to give exceptionally low damping and near bulk magnetizations (M ~ 140 emu/cc).

Based on our preliminary work and the findings in Nat. Phys 4175 we propose a studentship to perform research on developing amorphous / crystalline YIG 2D magnonic crystals. Key to the work is understanding spin hydrodynamics in the amorphous magnetic insulator depending on which of two different glassy phases the material is in. This leads to behaviour described by either a speromagnet or a correlated spin glass, with one predicted to be more favourable for spin transport. Growth and anneal techniques will be studied to obtain the different amorphous phases and spin lifetimes measured to test the theory. Unlike all other work in the field, in which the magnonic crystal is formed by physically removing material (lithography) from a thin film to make "anti-dots" or "pits", we will also attempt to periodically modulate the magnetization of a-YIG by rapid annealing micron sized circular areas using the groups IR laser or a high intensity electron beam. Annealing will fully recrystallize the a-YIG locally to produce the 2D magnonic crystal, without the pits and associated edge roughness found in lithographed samples. The crystal will be characterized structurally and chemically using TEM and X-ray absorption spectroscopy providing the student training at leading international facilities (SuperSTEM, DLS).