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Séminaire Magnétisme non conventionnel

Jeudi 26 Avril à 14h00,
Salle Remy Lemaire K223

Orateur : Rafal WAWRZYNCZAK (ILL Grenoble)
"Classical Spin Nematic Phase in Nearly Stoichiometric Breathing Pyrochlore"

Abstract

The A-site ordered spinel chromates AA’Cr4O8 (i.e. A=Li, A’=Ga, In) realize a so-called breathing pyrochlore lattice, where the B site forms a size-alternating array of cornersharing tetrahedra occupied by Cr3+ (S=3/2) ions. This alternation leads to different exchange interactions, J and J’, along the edges of the large and small tetrahedra, respectively. The degree of breathing can be quantified by Bf = J’/J, whose limiting values 0 and 1 correspond to isolated small tetrahedra and the uniform pyrochlore lattice.

The LiGa1-xInxCr4O8 family has proven to manifest a wide range of magnetic behaviours [1-5] as a function of the substitution (and hence bond disorder), x. The end member compositions LiGaCr4O8 (Bf = 0.6) and LiInCr4O8 (Bf = 0.1) [1] exhibit a series of complex magneto-structural transitions [2,3] resulting in magnetically ordered phases. Recently we have shown that the Néel order is suppressed by even vestigial doping at A’ site ; for x = 0.05, magnetic order is replaced with a second-order transition to a possible classical spin nematic phase at Tf 11 K [5]. Subsequent inelastic neutron scattering experiments reveal a quasi-elastic response at high temperature, as commonly observed in the pyrochlore antiferromagnets. This is replaced in the low temperature phase by an inelastic mode showing a strong resemblance to the broad resonant mode observed in lightly Cd-doped ZnCr2O4 [6]. The structure factor of the feature is consistent with “weathervane” modes on hexagonal antiferromagnetic loops, which are abundant in the nematic state. The finite energy of the excitation is generated by the biquadratic term in the effective magnetoelastic Hamiltonian, which is also responsible for the onset of the nematic state. These findings provide a unique example of a link between the complex low temperature behaviour and the spin dynamics in chromate spinel system. Furthermore, the spin nematic state might be relevant to similar transitions observed in other doped chromate pyrochlores [6,7].

References :

[1] Y. Okamoto, et al., PRL 110, 097203 (2013)
[2] Y. Tanaka , et al., PRL 113, 227204 (2014)
[3] G. Nilsen, et al., PRB 91, 174435 (2015)
[4] Y. Okamoto, et al., JPSJ 84, 043707 (2015)
[5] R. Wawrzyńczak, et al., PRL 119, 087201 (2017)
[6] W. Ratcliff II, et al., PRB 65, 220406(R) (2002)
[7] H. Martinho, et al., PRB 64, 024408 (2001).

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