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Séminaire Magnétisme non conventionnel

Jeudi 23 janvier 2020 à 14h00,
Salle Remy Lemaire, K223

Orateur : Ursula Bengaard Hansen (ILL)
"Domain wall dynamics in a near-Ising ferromagnetic chain CoCl2*2D2O"


In this presentation, I will discuss the magnetic excitations of a near-Ising ferromagnet in a longitudinal field.
The near Ising ferromagnetic chain has a characteristic excitation spectrum. In zero field the elementary excitations are clusters of overturned spins, in a pure Ising system the excitation energy of these clusters are degenerate and hence independent of the number of flipped spins, the cost of creating a spin cluster comes only from the domain walls at either end of the cluster. This degeneracy can be lifted by inter-chain interactions, non Ising terms in the Hamiltonian or magnetic fields.
It has been proposed, that the dynamics of the domain walls of the cluster excitations in the presence of a longitudinal magnetic field, can be interpreted as a magnetic analogue of electronic Bloch Oscillations [1], which is the phenomena where a charged particle in a periodic lattice starts to oscillate when a constant electric field is applied. It has been predicted [2] that the signatures of these Magnetic Bloch Oscillations may be observed using inelastic neutron scattering.
We have studied the dynamic behaviour of the cluster excitations in CoCl2*2D2O at the MACS neutron spectrometer [3]. Here I will present our results and how we are able to account for the different features in the excitation spectrums using a Random-Phase Approximation [4], that also accounts for the non one-dimensional magnetic interactions that are nevertheless still present in CoCl2*2D2O.

[1] F. Bloch, Zeitschrift für Physik 52, 555 (1929) [2] S. Shinkevich and O. Syljuåsen, PRB 85, 104408 (2012) [3] U. B. Hansen, arXiv preprint arXiv:1906.11554, 2019 [4] J. Jensen, PRB, 97(2), 024423 (2018)

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