Increased magnetic damping of permalloy upon Cr implantation

The understanding and the modification of the magnetic damping processes in thin ferromagnetic films has gained considerable interest in recent years. In addition to applied research on ultrafast magnetization reversal [1-3] also studies concerning the basic damping phenomena [4,5] and their modification by means of adjacent non-magnetic layers [6] or alloying [7] have been reported. Also the use of ion implantation to modify magnetic properties on a local scale has been addressed [8].

Here we report on investigations of the static and dynamic magnetic properties of 20 nm thick permalloy films as prepared and after 30 keV Cr implantation with different Cr fluences. Due to the doping the Curie temperature of the permalloy film decreases with the implantation fluence. At an averaged Cr concentration of about 7% (≈ 1.4·10^16 Cr/cm^2) the Curie temperature drops below room temperature. The uniaxial anisotropy, induced by a magnetic field during deposition, decreases but persists with increasing Cr fluence as indicated by the 2-fold symmetry of the magnetic remanence as a function of in-plane angle measured by magneto-optic Kerr effect magnetometry.

In order to investigate the magnetization damping pulsed inductive microwave magnetometry (PIMM) has been performed for various applied magnetic fields. In Fig. 3 the magnetization traces for the as-deposited permalloy film as well as the Cr-implanted samples are shown for an applied field of 10 Oe. Upon Cr implantation the PIMM amplitude is decreased consistent with a decrease in the uniaxial anisotropy AND a reduction in saturation magnetization. This results in a decrease of the precession frequency which is easily observed. However, the magnetic damping, i. e. the relaxation of the precession, is also altered by the Cr implantation. A detailed analysis of the damping parameter leads to 7-fold increase of the damping for only 4% of Cr doping. In order to clarify the basic mechanism for the enhancement the chemical and structural contributions to the magnetic damping parameter are separated.

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