Artificial frustrated spin models – FRUSTRATED

We propose to investigate, experimentally and theoretically, the physics of magnetic frustration and magnetic monopoles in artificial arrays of nanomagnets. The strength of this project relies on a close collaborating work between theoreticians, experts in spin models and micromagnetism, and experimentalists, having a strong background in the elaboration, characterization and study of magnetic nanostructures. To the best of our knowledge, such a consortium does not exist worldwide. Although there is a large, internationally recognized, French community involved in the physics of magnetic frustration and magnetic monopoles, there is no team in France addressing this physics via a micromagnetic approach. Looking at the recent works based on artificial arrays of nanomagnets, and at their large impact in the field, we are convinced that this project is a great opportunity to set up a competitive consortium, bringing together scientists from the communities of magnetic frustration, statistical physics and micromagnetism. Several scientific issues currently hotly debated in the community will be addressed in this project, and experiments will be systematically confronted to theoretical predictions.
1) Approaching the ground state: “real time” investigations
Based on our recent results, we will re-explore pioneer works that have been published these last few years on artificial kagomé spin ice systems. In particular, to go beyond the state-of-the-art knowledge, we plan to study the role of the network size, geometry and boundaries on the occurrence of disordered / ordered spin ice phases. Since the choice of the demagnetizing procedure to approach the ground state configuration is crucial in these systems, we will image and simulate in “real time” how the networks accommodate frustration while they are demagnetized.
2) Classical magnetic monopoles: how charged defects move
We will also study the physics of magnetic monopoles in these kagomé arrays of nanomagnets, which is currently a fast moving field of research. One interesting experimental observation in the recent literature is that monopoles essentially move along “straight” lines in the networks, and do not follow random paths. But this surprising feature has not been investigated so far.
3) Unexplored 2D and 2.5D spin models
We will then extend these studies to other spins systems showing magnetic frustration, for which no theoretical prediction and no experimental realization exist yet. In particular, we will focus our work on XY and q-state Potts models, on antiferromagnetically coupled Ising spin systems, and on an artificial slab of a three-dimensional structure. As the ground state spin configuration in these spin models is still unknown, this part of the project is challenging and innovative.
4) From statistical physics to micromagnetism
Finally, particular attention will be paid to the comparison between theoretical and experimental results in order to determine whether or not frustrated micromagnetic systems are well described by spin models of statistical physics. Indeed, spin ice models are based on important assumptions that are clearly not satisfied in artificial arrays of nanomagnets. In other words, up to now, concepts of spin physics are directly applied to micromagnetic objects, which are characterized by their remanence, switching field, anisotropy, etc. But this crucial point remains unaddressed by the community.