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Vendredi 16 Octobre 2015 à 14h00,
Salle Nevil Mott, D420

Orateur : Ioan Chioar (MCBT)
"Artificial Kagome Spin Networks : From Short-Range Degeneracy towards Dipolar Long-Range Order"

Abtract
Artificial spin networks have been initially proposed as toy-spin models destined for the investigation of magnetic frustration effects in two-dimensional spin lattices [1]. This stands as a rather complementary approach to the study of the magnetic frustration encountered in condensed matter spin ice pyrochlores [2]. Generally fabricated by employing lithography techniques, these arrays of nano-scale magnetic islands can be designed at-will and, combined with the possibility of individually imaging the magnetic degrees of freedom in real space, these systems offer an almost infinite playground for the investigation of competing interactions in magnetostatic frameworks and potential for the experimental discovery of novel and exotic magnetic phases.

The current thesis presents experimental and numerical studies performed on two particular realizations of the kagome array, which can present massively degenerated manifolds and exotic spin textures [3,4]. In particular, dipolar kagome spin ice displays a low-temperature regime characterized by the coexistence of a crystalline phase, associated to classical magnetic charges, and a disordered spin lattice [5–7]. This feature has motivated a quest for experimentally accessing this exotic manifold and we have managed to locally reach it using thermally-active GdCo-based artificial arrays. Secondly, a novel TbCo-based artificial kagome array has been studied, with magnetic moments pointing perpendicular to the lattice plane [8]. Given the difference in higher-order dipolar couplings, this so-called kagome Ising system presents a different low-energy behavior than kagome spin ice and we have experimentally highlighted its incipient stages using demagnetization protocols and magnetic force microscopy. These results enrich the spectrum of non-conventional magnetic phases that can potentially be achieved with such nanostructured systems. However, the dipolar long-range ground-state of kagome Ising remains, so far, unknown. Nevertheless, using Monte Carlo simulations, a ground-state candidate is provided, in good agreement with all results reported so far.

These results contribute to a better understanding of how dipolar interactions drive the behavior of artificial kagome spin networks, encouraging further research into such frustrated systems.

[1] R. F. Wang et al., Nature 439, 303 (2006).
[2] M. J. Harris et al. , Phys. Rev. Lett. 79, 2554 (1997).
[3] I. Syôzi, Prog. Theor. Phys. 6, 306 (1951).
[4] K. Kanô and S. Naya, Prog. Theor. Phys. 10, 158 (1953).
[5] G. Möller and R. Moessner, Phys. Rev. B 80, 140409 (2009).
[6] G.-W. Chern, P. Mellado, and O. Tchernyshyov, Phys. Rev. Lett. 106, 207202 (2011).
[7] M. E. Brooks-Bartlett et al., Phys. Rev. X 4, 011007 (2014).
[8] S. Zhang et al., Phys. Rev. Lett. 109, 087201 (2012).

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