In physics, frustration appears in a system when it is impossible to minimise all pairwise interactions simultaneously. Frustration exists in some particular rare-earth based compounds, such as spin ices . Their internal frustration gives rise to unusual properties, like a residual entropy at low temperature or the presence of monopole-like excitations . However, experimental techniques are unable to probe each spin individually in these compounds.
In 2006, Wang and coworkers opened a new way for studying magnetically frustrated spin systems . Using e-beam lithography, one can make arrays of nanomagnets with the desired design. The state of each nanomagnet can then be probed individually in real space at room temperature using magnetic imaging (eg. Magnetic Force Microscopy). In this context, the square geometry received a considerable interest, since it is closely related to condensed matter spin ice compounds. But for geometrical reasons, this system orders instead of showing a disordered low energy manifold
In this contribution, we explain how to bring back the massive ground state degeneracy in the square array of nanomagnets. We present the first experimental evidence of a Coulomb phase in this system . We also report the presence of magnetic monopoles defects within the Coulomb phase. This study makes a new step toward a direct study of the dynamic of monopoles excitations (e.g. creation, annihilation or diffusion processes).
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Yann Perrin, Nicolas Rougemaille, and Benjamin Canals, "Massively degenerated ground state manifold in artificial square ice (Conference Presentation)," Proc. SPIE 10357, Spintronics X, 103573D (Presented at SPIE Nanoscience + Engineering: August 10, 2017; Published: 21 September 2017); https://doi.org/10.1117/12.2276988.5583428467001.
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