Effect of dipolar interactions on the ferromagnetic resonance properties in arrays of magnetic nanowires

The microwave absorption properties of arrays of magnetic nanowires electrodeposited in nanoporous membranes have been studied using a strip transmission line configuration. From the measurements, the ferromagnetic resonance properties of the magnetic wires have been extracted at frequencies ranging from 100 MHz to 40 GHz for various angles between the wire axis and an external static magnetic field up to 8 kOe. We report on extensive data obtained on the ferromagnetic resonance properties in arrays of nanowires of Ni, NiFe, and NiFeCo alloys with various diameters and packing densities. In such arrays, apart from the shape anisotropy, a strong dipolar coupling between wires is present which can be modulated by the wire density. Both ferromagnetic resonance spectra and hysteresis loops show that this coupling acts as an additional uniaxial anisotropy favoring an easy axis perpendicular to the wires. In particular, it is shown that the easy axis of magnetization can be tuned parallel or perpendicular to the wire axis by changing the wire packing density. Furthermore, ferromagnetic resonance occurs even at zero field and the resonance frequency is fixed by the saturation magnetization of the material independent of the packing density. (C) 2001 American Institute of Physics.