We report on the crossover from the thermal to the athermal regime of an artificial spin ice formed from a square array of magnetic islands whose lateral size, 30 nm $\times$ 70 nm, is small enough that they are dynamic at room temperature. We used resonant magnetic soft x-ray photon correlation spectroscopy as a method to observe the time-time correlations of the fluctuating magnetic configurations of spin ice during cooling, which are found to slow abruptly as a freezing temperature of $T_0=178\pm 5$ K is approached. This slowing is well described by a Vogel-Fulcher-Tammann law, implying that the frozen state is glassy, with the freezing temperature being commensurate with the strength of magnetostatic interaction energies in the array. The activation temperature, $T_{\mathrm{A}}=40\pm 10$ K, is much less than that expected from a Stoner-Wohlfarth coherent rotation model. Zero-field-cooled/field-cooled magnetometry reveals a freeing up of fluctuations of states within islands above this temperature, caused by variation in the local anisotropy axes at the oxidised edges. This Vogel-Fulcher-Tammann behavior implies that the system enters a glassy state upon freezing, which is unexpected for a system with a well-defined ground state.

We study the magnetic properties of weakly disordered Co films with in-plane uniaxial magnetocrystalline anisotropy. The growth sequence used allowed the controlled introduction of grain orientation disorder. Above a threshold disorder level, we observe an anomalous magnetic reversal near the nominal hard axis; while the behavior in all other field orientations is barely affected. A two-grain model explains the anomaly as the occurrence of nonuniform magnetization states near the hard axis, a fact that is experimentally confirmed by Kerr microscopy.

Designing and constructing model systems that embody the statistical mechanics of frustration is now possible using nanotechnology. We have arranged nanomagnets on a two-dimensional square lattice to form an artificial spin ice, and studied its fractional excitations, emergent magnetic monopoles, and how they respond to a driving field using X-ray magnetic microscopy. We observe a regime in which the monopole drift velocity is linear in field above a critical field for the onset of motion. The temperature dependence of the critical field can be described by introducing an interaction term into the Bean-Livingston model of field-assisted barrier hopping. By analogy with electrical charge drift motion, we define and measure a monopole mobility that is larger both for higher temperatures and stronger interactions between nanomagnets. The mobility in this linear regime is described by a creep model of zero-dimensional charges moving within a network of quasi-one-dimensional objects.

We report on the magnetic properties and anisotropic magnetoresistance behavior of engineered bicomponent array of “holes” with alternating diameters embedded in a continuous permalloy film. We observed that the magnetization reversal is significantly altered when compared to a homogeneous array of antidots, and exhibits strong dependence on the permalloy thickness. Our results demonstrate that the use of two antidot sublattices greatly enhances the parameters available for engineering the behavior of antidot nanostructures.