Spin-ice materials are magnetic interacting systems that are “frustrated” because the spin configuration prevents the magnetic moments from arranging themselves in their lowest-energy configurations. Frustration can lead to complex magnetic ordering and other interesting forms of collective behavior. Inspired by such materials, researchers have been able to create artificial spin ices, in which, instead of atoms with magnetic moments, the spin ice is made up of engineered nanoscale elements that are small enough so that each element consists of a single ferromagnetic domain.
In artificial spin ices, previous research has established the presence of topological defects that can profoundly influence the properties of the system. Such defects include mesoscopic structures that behave similarly to the magnetic monopoles predicted by Dirac in 1931. Now, writing in Physical Review Letters, Sebastian Gliga of Argonne National Laboratory, Illinois, and collaborators, use simulations to show that the signature of these analog monopoles would be well-defined features in the excitation spectrum of the material.
The ability to detect these features, which occur at microwave frequencies, would pave the way for the experimental detection and analysis of topological features in spin ices. In particular, they may help identify an exotic class of defects: pairs of monopoles and antimonopoles (the counterpart of monopoles with opposite polarity) that are linked together by a so-called Dirac string, made up by elements whose magnetization is in the opposite direction with respect to the surrounding lattice. The authors suggest the possibility of using such features for computing devices, in which logical operations are based on the manipulation of such defect modes. – Daniel Ucko