Effect of interdot dipolar coupling on the magnetic properties of permalloy nano-cylinders

Square arrays of nanometric cylindrical dots (radius 100 nm), with interdot spacing variable in the range 50–800 nm, have been patterned by e-beam lithography, starting from a 50 nm thick permalloy film. A detailed magneto-optical Kerr effect study revealed that on reducing the interdot separation below 200 nm, there is a marked decrease of the vortex nucleation and annihilation fields, as well as an increase of the susceptibility. These results are interpreted in terms of the influence of interdot dipolar coupling, on the basis of micromagnetic simulations with either open or periodic boundary conditions. The coupling also affects the high-frequency magnetic properties, as seen by the modification of the spin-wave spectrum measured by Brillouin light scattering. In particular, a broadening of the discrete resonance modes and a shift to higher frequency for some of them is evident for the specimens with small interdot spacings.