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

The microwave absorption properties of arrays of magnetic nanowires electro deposited in nanoporous membranes have been studied using a strip transmiss ion 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 ext ernal static magnetic field up to 8 kOe. We report on extensive data obtain ed on the ferromagnetic resonance properties in arrays of nanowires of Ni, NiFe, and NiFeCo alloys with various diameters and packing densities. In su ch arrays, apart from the shape anisotropy, a strong dipolar coupling betwe en wires is present which can be modulated by the wire density. Both ferrom agnetic resonance spectra and hysteresis loops show that this coupling acts as an additional uniaxial anisotropy favoring an easy axis perpendicular t o 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 wi re packing density. Furthermore, ferromagnetic resonance occurs even at zer o field and the resonance frequency is fixed by the saturation magnetizatio n of the material independent of the packing density. (C) 2001 American Ins titute of Physics.