NB : horaire avancé à 10h30.
Abstract:
Neutrons are an invaluable probe to study magnetic defects in bulk materials. I will give a few examples of neutron experiments investigating local defects or larger-scale ones in several ferromagnetic-like media, either metallic or insulating.
In Fe1-xCrx alloys, Cr atoms repel each other as near neighbors at low concentration (below xc=0.11), while they attract each other above xc [1]. This unique case in nature yields a subtle interplay between magnetic and chemical short range orders [2]. In dilute alloys, the moments locally cant around Cr impurities, a precursor effect of the frustrated reentrant spin glass phases observed at higher Cr content.
In similar reentrant spin glasses, vortex like-texture can be observed by small angle neutron scattering, as shown in Ni0.79Mn0.21 single crystal [3]. Monte-Carlo simulations allow us to identify their differences with the frustrated skyrmions predicted in ordered chiral magnets.
Static displacements due to lattice defects are well known in irradiated materials. We recently discovered that they also exist in the frustrated pyrochlore Pr2Zr2O7, induced by Pr/Zr site inversion. The local strains promote a new state of matter described as a quantum spin liquid. It is characterized by short range antiferromagnetic correlations between quadrupolar moments, and a peculiar dynamics preserving spin fluctuations down to T=0 [4].
References:
[1] Short range order inversion in a transition alloy
I. Mirebeau, M. Hennion, G. Parette Phys. Rev. Lett 53, 687(1984); Phys; Rev. B 82, 104203 (2010)
[2] Magnetic and atomic short range order in Fe1−x Crx alloys,
I. Mirebeau, V. Pierron-Bohnes, C. Decorse, E. Rivière, Chu-Chun Fu, Kangming Li, G. Parette, submitted (2019).
[3] Spin textures induced by quenched disorder in a reentrant spin glass: vortices versus frustrated skyrmions,
I. Mirebeau, N. Martin et al, Phys; Rev. B 98, 014420 (2018)
[4] Disorder and quantum spin ice, N. Martin et al, Physical Review X 7, 041028(2017)
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