Grants and Contracts Details
Frustration in physical systems arises when geometrical constraints or competing interactions block their reaching a global energy minimum, and induces non-equilibrium or metastable phases in a variety of amorphous, magnetic, and polymeric systems. The unique behavior of microscopic spin ices derives from frustrated magnetic interactions caused by crystal symmetry, resulting in a large number of lowenergy, nearly-degenerate magnetic configurations that generate a large entropy without magnetic order at low temperatures. These concepts have been generalized to include mesoscale arrays of magnetic dots with frustrated dipole-dipole interactions, such as square artificial spin ice (ASI), which consists of a square array of ferromagnetic wire segments sufficiently small and elongated to be singledomain with bi-stable (Ising) polarization. The segment shape anisotropy and mesoscopic size of ASI generate large thermal barriers (. 105 K) to segment reversal and depress thermal fluctuations, but also ensure their magnetic textures can be directly imaged at variable temperature and magnetic field. Understanding athermal dynamics is extremely challenging, as non-equilibrium systems can be so far from equilibrium that perturbative tools of statistical mechanics fail to predict their dynamics. ASI therefore constitute a novel class of metamaterials for systematic studies of athermal, nonequilibrium dynamics, and their relationship with the non-attainment of a true ground state. We propose a first study of dynamics in a square ASI of composition Ni0.81Fe0.19 using resonant coherent soft x-ray scattering to probe field-driven magnetic reversal, short-range magnetic correlations, equilibration, and dynamics near ordering transitions. A coherent synchrotron beam enables X-ray photon correlation spectroscopy (XPCS), which is a powerful probe of element-specific, spatio-temporal correlations and intermittent dynamics. Our preliminary ALS experiments (with Dr. Sujoy Roy) surprisingly revealed a coherent x-ray beam acquires orbital angular momentum (OAM) while scattering from a ASI. Optical gvorticesh emerge as gdoughnuth-shaped intensity distributions in the resonant diffraction pattern when a field is applied to the ASI along the incident beam direction. The OAM of xray vortex beams can have large integer eigenstates, implying potential for use in high-capacity quantum computation and information transmission. Vortex beams have proven critical for superresolution microscopy in the VIS-NIR spectrum and rotational sensing and actuation. Compared to other, far more complex methods, our method for creating, activating and tuning OAM of soft x-rays with a small applied magnetic field is very appealing. A successful demonstration of control of OAM transfer to x-ray vortex beams by magnetic manipulation of inexpensive and versatile, compact patterned magnetic films could not only enable novel nanotechnologies, such as optical trapping and rotation of molecules, mesoscale rotational sensing, OAM-resolved magnetic spectro-microscopy of highly excited electronic states, and new paradigms for quantum computing, but would also leverage the current ALS-U project at LBNL.
|Effective start/end date||8/1/16 → 7/31/20|
- KY Council on Postsecondary Education: $50,000.00
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