Title: Magnetization dynamics of weakly interacting sub-100 nm square artificial spin ices,
Abstract: Artificial Spin Ice (ASI), consisting of a two dimensional array of nanoscale
magnetic elements, provides a fascinating opportunity to observe the physics of
out of equilibrium systems. Initial studies concentrated on the static, frozen
state, whilst more recent studies have accessed the out-of-equilibrium dynamic,
fluctuating state. This opens up exciting possibilities such as the observation
of systems exploring their energy landscape through monopole quasiparticle
creation, potentially leading to ASI magnetricity, and to directly observe
unconventional phase transitions. In this work we have measured and analysed
the magnetic relaxation of thermally active ASI systems by means of SQUID
magnetometry. We have investigated the effect of the interaction strength on
the magnetization dynamics at different temperatures in the range where the
nanomagnets are thermally active and have observed that they follow an
Arrhenius-type Néel-Brown behaviour. An unexpected negative correlation of
the average blocking temperature with the interaction strength is also
observed, which is supported by Monte Carlo simulations. The magnetization
relaxation measurements show faster relaxation for more strongly coupled
nanoelements with similar dimensions. The analysis of the stretching exponents
obtained from the measurements suggest 1-D chain-like magnetization dynamics.
This indicates that the nature of the interactions between nanoelements lowers
the dimensionality of the ASI from 2-D to 1-D. Finally, we present a way to
quantify the effective interaction energy of a square ASI system, and compare
it to the interaction energy calculated from a simple dipole model and also to
the magnetostatic energy computed with micromagnetic simulations.