Magnetic relaxation in exchange-coupled Co/CoO bilayers measured with ac-anisotropic magnetoresistance

The complex ac-susceptibility of field-cooled Co/CoO bilayers has been measured by perturbing the magnetization with a small (5 Oe) ac field, and detecting the response through anisotropic magnetoresistance using a double modulation technique. Samples of Ta(3 nm)/Co(8 nm)/CoO(3 or 10 nm)/Cu(3 nm) grown by sputter deposition on an amorphous silicon nitride substrate have been studied using a frequency of 100 Hz. The relaxation of the exchange anisotropy in response to the perturbation allows the activation energies and relaxation times of the CoO grains to be investigated. Both the real and imaginary response show prominent thickness-dependent peaks at the blocking temperature, as well as a peak near 80 K which is insensitive to CoO film thickness. The measurements are well-described by a generic linear relaxation model of the anisotropy field, using a bimodal distribution of activation energies for the CoO grains.

Magnetic relaxation in exchange-coupled Co/CoO bilayers measured with ac-anisotropic magnetoresistance Journal Articles