Magnetic anisotropy and magnetoresistance of sputtered [(FeTaN)/(TaN)] _n multilayers

We studied the in-plane magnetic anisotropy of rf (radio frequency) sputtered [(FeTaN)/(TaN)] _n multilayers synthesized on Si substrates. In the multilayers where n=5, the FeTaN thickness is fixed at 30 nm and the thickness of TaN, t _TaN, is varied from 0 to 6.0 nm, we observed a clear trend that, with increasing t _TaN, the values of coercivity, grain size, and amplitude of maximum magnetoresistance (MR) of the samples all decrease first and then increase after reaching a minimum when t _TaN is around 2.0-4.0 nm. This trend is also associated with an evolution of in-plane magnetic anisotropy, where the multilayers change from uniaxial anisotropy to biaxial at t _TaN around 4.0 nm and above. We attribute the phenomena to the interlayer coupling effect of FeTaN films as a function of the coupling layer (TaN) thickness, rather than to the thickness dependence observed in single-layered FeTaN films, where the direction of easy axis switches 90° when the film is thicker than 300 nm. The in-plane anisotropy of the [(FeTaN)/(TaN)] _n multilayers also shows signs of oscillation when the number of coupling layers varies. The MR effects observed are mainly due to anisotropy MR (AMR), while the grain size and exchange coupling may also contribute to the change of maximum MR ratios in the multilayers with changing t _TaN.