One goal of this proposal is to independently confirm and then extend the surprising report by a German group that dynamic Bose-Einstein Condensation (BEC) of magnons (the elementary excitations of magnetic systems) can occur under appropriate conditions in thin films of the ferrimagnetic compound Yttrium Iron Garnet (YIG). In parallel we propose to expand our earlier studies on patterned magnetic nanostructures to include objects displaying topologically nontrivial magnetization distributions such as vortices, skyrmions, and Dirac strings. Ordinarily BEC of magnons would be forbidden since their number is not constrained: the inelastic collisions among magnons cause them to appear and disappear and number conservation is a requirement of (equilibrium) BEC. However it has been argued that elastic collisions rapidly bring the magnons into a dynamic thermal equilibrium at much faster time scales; this would allow BEC. What is both surprising and important is that the BEC phenomenon is reported to occur at room temperature. If room temperature BEC is real (confirmed) the potential for applications of what has historically been the largely academic field of BEC (in systems such as liquid helium, cold atoms/molecules, excitons, and even photons), would be greatly expanded since one could then apply modern lithographic tools to make arrays of interacting condensates, possibly allowing the development of room-temperature devices based on quantum superimposed macroscopic condensates with programmable couplings. Since there is no guarantee that the reported observations will be reproducible, the research has a high risk factor; however if the phenomena is real, with the extensions we propose the impact is expected to be transformative. The second component proposed involves microwave characterizations of unusual collective modes of magnetic nanostructures that support topologically non-trivial magnetization distributions. The structures are patterned by e-beam lithography both in-house by our group and by our collaborators. Here we would exploit our recent expertise on the detection of propagating and localized modes in patterned arrays of holes (in continuous films) and patterned but non-contacting (and to varying degrees magnetically coupled) objects such as discs and bars. The latter can be constructed to yield magnetically frustrated systems, the so-called spin ice arrays that have recently received much attention. However most groups focus on static field-history dependent phenomena whereas our group is one of the few studying microwave dynamic responses. Particular exotic objects for study include the collective oscillations of isolated structures supporting unusual topological distributions of their magnetization such as vortices, coupled vortices and, in anti-ferromagnetically coupled sandwiched structures, merons. Discs coupled laterally through an underlying vertically aligned film are predicted to support and even more exotic topologicasl entity, a skyrmion. Such structures will be fabricated and their mode structure probed. In addition we have developed and will improve on patterned electromagnetic elements that allow direct coupling between the essentially uniform (centimeter wavelength) microwave fields to the short wavelength magnons in magnetic thin films (and patterned nanostructures in general), this miss-match being a possible stumbling block in developing more sophisticated magnetic devices. Current magnetic devices are largely restricted to data storage involving the orientation of individual nanomagnets, as in
|Effective start/end date||9/1/15 → 8/31/22|
- Department of Energy (DE-SC0014424-0005)
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