Transition from strongly collective to completely isolated ultrafast magnetization dynamics in two-dimensional hexagonal arrays of nanodots with varying inter-dot separation
A hexagonally arranged array of ferromagnetic nanodots is particularly interesting because it offers the highest areal density of features achievable using modern nanofabrication techniques. They are important for high density magnetic storage, memory, logic, sensors and magnonic crystals. However, understanding the collective static and dynamic magnetic properties by varying the inter-dot separations in such a lattice has yet not been fully explored. Here, we demonstrate transition from a strongly collective to a completely isolated magnetization dynamics via various weakly collective dynamics by systematically varying the inter-dot separation of circular Ni80Fe20 nanodots of 100 nm in diameter (d) arranged in hexagonal lattice. Time-resolved Kerr microscopy has been exploited to study the ultrafast magnetization dynamics of the arrays with varying inter-dot separation (S) between 30 nm and 390 nm. The transition between different collective regimes was identified from sudden change in frequency values and number of modes in the frequency spectra. This was further supported by the bias field variation of the frequency of various spin wave modes and simulated mode profiles. The latter clearly showed the variation in the nature of the spatial distribution of the collective modes in the arrays in different collective regimes. The observations are imperative for selection of correct values of inter-dot separation in hexagonal arrays of nanodots for various applications.