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