Artificial Spin Ices: Magnetization Dynamics and X-ray Orbital Angular Momentum Generation

Artificial spin ices (ASI) are patterned arrays of nanoscale, ferromagnetic elements that can serve as model systems and are also being considered for applications in energy-efficient computing.  Our group has studied permalloy, square ASIs with and without programmed topological defects using resonant soft X-ray techniques.  These include X-ray photon correlation spectroscopy, Bragg coherent diffraction imaging, and X-ray photoemission electron microscopy.  These studies revealed the importance of superdomain-wall nucleation, annihilation, and motion for understanding equilibrium fluctuations of ASIs near room temperature.  They also showed that square ASIs with even-order topological defects can equilibrate into single-domain antiferromagnetic ground states that impart orbital angular momentum (OAM) to scattered X-rays.  The X-ray OAM beams can be switched on and off with changes in temperature and applied magnetic field and may prove to be a first step toward reconfigurable X-ray optics.  In addition to affecting X-ray OAM generation, the charge of the topological defect also influences both frustration and fluctuations of the ASIs. 

This work is the product of a collaboration among researchers including J. Woods^1,2, X. Chen^1,3,5 B. Farmer¹, S. Dhuey³, R. Chopdekar³, R. Koch³, A. Scholl³, S. Kevan³, A. Tremsin⁴, C. Mazzoli⁵, S. Wilkins⁵, W. Hu⁵, A. Barbour⁵, I. Robinson⁵, W. Kwok², L. De Long¹, and S. Roy³.

¹ University of Kentucky, ² Argonne National Laboratory, ³ Lawrence Berkeley National Laboratory, ⁴ University of California, Berkeley, and ⁵ Brookhaven National Laboratory