Abstract
For over ten years, arrays of interacting single-domain nanomagnets, referred to as artificial spin ices, have been engineered with the aim to study frustration in model spin systems. Here, we use Fresnel imaging to study the reversal process in "pinwheel" artificial spin ice, a modified square ASI structure obtained by rotating each island by some angle about its midpoint. Our results demonstrate that a simple 45° rotation changes the magnetic ordering from antiferromagnetic to ferromagnetic, creating a superferromagnet which exhibits mesoscopic domain growth mediated by domain wall nucleation and coherent domain propagation. We observe several domain-wall configurations, most of which are direct analogues to those seen in continuous ferromagnetic films. However, charged walls also appear due to the geometric constraints of the system. Changing the orientation of the external magnetic field allows control of the nature of the spin reversal with the emergence of either one- or two-dimensional avalanches. This property of pinwheel ASI could be employed to tune devices based on magnetotransport phenomena such as Hall circuits.
Highlights
Artificial spin ice (ASI) systems have been used as a route to new physical phenomena and to gain insight into fundamental physics
In order to characterize the behavior of the pinwheel ASI, we first look at the behavior of the Ising net magnetization of the individual pinwheel units, where each unit is formed by the four nearest-neighbor islands
Pinwheel ASI provides an example of how a simple geometry modification can dramatically affect the magnetic properties of a spin ice array
Summary
Artificial spin ice (ASI) systems have been used as a route to new physical phenomena and to gain insight into fundamental physics. We use Lorentz transmission electron microscopy (LTEM)[23] to directly visualize the magnetization reversal process in a pinwheel ASI array in the presence of a static externally applied magnetic field Under such conditions, the system behaves as a superferromagnet, that is, an ensemble of macrospins with collective ferromagnetic behavior.[24] Our superferromagnet has coherent domain growth and shrinking as opposed to the chain avalanche reversal seen in square ASI.[25,26] The different magnetic domains seen in pinwheel ASI are separated by domain walls, some of which behave much like the classical ferromagnetic Neé l domain walls in continuous films. These properties of pinwheel ASI offer a possible avenue to design functional materials exploiting the emergent magnetic spin textures and controllable reversal dimensionality of the system
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