Microwave absorption of nanoscale CoNi powders

Ferromagnetic resonance (FMR) and microwave magnetoabsorption can be expected to be powerful tools for characterization of magnetic nanoparticles. In the present work, we have studied the FMR and magnetic field dependence of nonresonant microwave losses in CoNi nanoparticles of about 45, 100, and 200 nm caliper sizes prepared by the polyol technique. Contrary to our expectation, FMR lines, although wide, show that the particles are not truly spherical. Subsequent scanning and transmission electron microscopy investigations have confirmed this finding. Further, as before, all the powders are found to have large zero field absorption which reduces on application of a direct current (dc) field parallel to the microwave magnetic field but is relatively insensitive to a dc field if the powder is located in the microwave electric field, hence defining a magnetoimpedance for the system. (c) 2006 American Institute of Physics.