[Hybrid] Magnetic Bloch oscillations and domain wall dynamics in a near-Ising ferromagnetic chain

by Dr Ursula Bengaard Hansen (Institut Laue-Langevin)

Monday 21 Nov 2022, 14:30 → 15:30 Europe/Berlin

PH HS 3 (Physics Department)

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 energies 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.
Quantum mechanics predicts that a particle in a periodic potential will undergo oscillatory motion in
response to a constant force. This rather counterintuitive phenomenon, known as Bloch oscillations (BO) [1],
was controversial for a long time, but it has been demonstrated experimentally that charged particles in
periodic lattices subjected to a constant electric field, will undergo an oscillatory motion. Here I will show
that the magnetic analogue of these Bloch oscillations can be realised in a ferromagnetic near-Ising chain. In
this case, the “particles” undergoing oscillatory motion in the presence of a magnetic field are domain walls.
It has been predicted that the signatures of these Magnetic Bloch Oscillations may be observed using
inelastic neutron scattering [2,3].
 

We have studied the dynamic behaviour of the cluster excitations in the ferromagnetic chain CoCl2⋅2D2O at
the neutron spectrometers MACS at NCNR, NIST and ThALES at the ILL [4]. Here I will present our results
and how we can account for the different features in the excitation spectrums using a Random-Phase
Approximation [5], which 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] J. Kyriakidis and D. Loss, PRB 58, 5568, (1998)
[3] S. Shinkevich and O. Syljuåsen, PRB 85, 104408 (2012)
[4] U. B. Hansen et al., Nature Communications 13, 2547 (2022)
[5] J. Jensen et al., PRB, 97(2), 024423 (2018)