All-Optical Excitation and Detection of Picosecond Dynamics of Ordered   Arrays of Nanomagnets with Varying Areal Density
  We have demonstrated optical excitation and detection of collective precessional dynamics in arrays of coupled Ni80Fe20 (permalloy) nanoelements with systematically varying areal density by an all-optical time-resolved Kerr microscope. We have applied this technique to precisely determine three different collective regimes in these arrays. At very high areal density, a single uniform collective mode is observed where the edge modes of the constituent elements are suppressed. At intermediate areal densities, three nonuniform collective modes appear and at very low areal density, we observe noncollective dynamics and only the centre and edge modes of the constituent elements appear. 
Bivas Rana, Semanti Pal, Saswati Barman, Yasuhiro Fukuma, YoshiChika   Otani, and Anjan Barman
cond-mat.mes-hall cond-mat.mtrl-sci