Description
Arrays of nanoscale magnetic elements, each acting as a single mesospin, are the building blocks of artificial spin systems in which the mesospin and lattice geometry can be used to design emergent mesoscale magnetic order. The geometry of the mesospins coupled with their spacing determine their magnetic dimensionality and temperature dependent interactions which define the global ordering and thermally driven dynamics of the array [1].
Here, we focus on Ising chains of elliptical islands as well as square artificial spin ice (SASI) structures. The different mesospin gaps generate varying coupling strengths at the vertices which compete with the thermally active Fe/Pd base material to drive the collective array behaviour. As a function of increasing temperature, the rate of island reversals or flips increases, introducing defects into the arrays and reducing the magnetic correlations over characteristic timescales. Direct imaging using PEEM is limited for fluctuating systems, so here we use a different approach and combine coherent magnetic scattering with x-ray photon correlation spectroscopy (XPCS) to probe the spatial and temporal variations of the magnetic super-structure. We present the dynamic evolution of Ising chains and SASI arrays at different length scales using coherent scattering and record the speckles produced from the dynamic array configurations. In the SASI, the ground state is ordered with a magnetic configuration comprising two mesospins pointing in and two mesospins pointing away from a vertex. This configuration results in a doubling of the charge unit cell and magnetic scattering being found in-between the diffraction peaks from the long-range order of the islands. Island reversals drive the system away from an ordered ground state with excited vertex states giving scattering around the structural Bragg peaks. The intensity and spatial extent of the magnetic scattering is measured across the temperature window between blocking temperature (fixed by the Zeeman and shape anisotropy) of the mesospin and the Curie temperature of the Fe/Pd. In this temperature window, thermodynamically metastable phases occur [2-5], which include both intra- and inter-Island magnetic excitations. This ongoing study yields new insights into the dynamics of magnetic excitations in these arrays, with both high spatial and temporal resolution.
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