| Résumé: | A recent advance in the study of emergent magnetic monopoles was the discovery that
monopole motion is restricted to dynamical fractal trajectories [J. N. Hallén et al.,
Science 378, 1218 (2022)], thus explaining the characteristics of magnetic monopole
noise spectra [R. Dusad et al., Nature 571, 234 (2019); A. M. Samarakoon et al., Proc.
Natl. Acad. Sci. U.S.A. 119, e2117453119 (2022)]. Here, we apply this novel theory
to explore the dynamics of field-driven monopole currents, finding them composed
of two quite distinct transport processes: initially swift fractal rearrangements of local
monopole configurations followed by conventional monopole diffusion. This theory
also predicts a characteristic frequency dependence of the dissipative loss angle for
AC field–driven currents. To explore these novel perspectives on monopole transport,
we introduce simultaneous monopole current control and measurement techniques
using SQUID-based monopole current sensors. For the canonical material Dy2Ti2O7,
we measure Φ(t), the time dependence of magnetic flux threading the sample when a
net monopole current J(t) = Φ̇ (t)∕0 is generated by applying an external magnetic
field B0(t). These experiments find a sharp dichotomy of monopole currents, separated
by their distinct relaxation time constants before and after t ~600 μs from monopole
current initiation. Application of sinusoidal magnetic fields B0(t) = Bcos(t) generates
oscillating monopole currents whose loss angle
(
f
)
exhibits a characteristic transition
at frequency f ≈ 1.8 kHz over the same temperature range. Finally, the magnetic noise
power is also dichotomic, diminishing sharply after t ~600 μs. This complex phenomenology represents an unprecedented form of dynamical heterogeneity generated by the
interplay of fractionalization and local spin configurational symmetry.
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