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Séminaire Magnétisme non conventionnel

Jeudi 15 Juin 14h00
Salle Remy Lemaire, K223

Orateur : Nicolas MARTIN (LLB, CEA Saclay)
"Spin textures in cubic chiral magnets and re-entrant spin glasses"


In the last decades, a tremendous scientific effort has been devoted to the study of magnetic compounds hosting localized defects such as flux line lattices in type II superconductors, magnetic bubbles in ferromagnetic garnets or skyrmion crystals in chiral magnets. In this context, I will present recent neutron scattering studies of systems where the interplay between different magnetic degrees of freedom prompts the appearance of magnetic textures at the nanoscale.
The first example I shall address is the cubic chiral helimagnet MnGe. Non-isoelectronic substitution of Mn ions is here used to modify the main exchange interactions as well as local anisotropies. It is found that, along with tuning magnetic structure periodicity and ordered magnetic moment, such a procedure leads to the occurrence of a mixed magnetic state where long-period helimagnetism coexists with arrays of magnetic dislocations [1]. This exotic state of matter shares many similarities with the so-called ‘twist-grain boundary’ phase first predicted by de Gennes [2] and later observed in some chiral liquid crystals [3,4].
In a second part, I will describe our experimental and numerical study of a Ni-Mn alloy. Here, interaction frustration drives the system from a strong ferromagnet to a ‘re-entrant’ spin glass (RSG) upon cooling [5]. In the RSG regime, vortex configurations are stabilized under an applied magnetic field and can be directly observed by small-angle neutron scattering [6]. Interestingly, Monte-Carlo simulations confirm that these objects are endowed with non-zero topological charge while their size and density can be tuned by the applied field [7].

[1] N. Martin et al., arXiv:1702.06511 (2016)
[2] P.G. de Gennes, Sol. State Comm. 10, 753 (1972)
[3] J.W. Goodby et al., Nature 337, 449 (1989)
[4] L. Navailles et al., Phys. Rev. Lett. 71, 545 (1993)
[5] See e.g. M. Gabay and G. Toulouse, Phys. Rev. Lett. 47, 201 (1981)
[6] M. Hennion et al., JMMM 54-57 (1986) 121-123
[7] I. Mirebeau et al., in preparation (2017)

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