Magnetic properties of Co Re hcp(10(1)over-bar0) superlattices

hcp(10 (1) over tilde 0) Co/Re superlattices were grown via magnetron sputtering on Al2O3(11 (2) over bar 0) substrates. The thickness of the Co layers was approximately 1.8 nm with the Re layer thickness varying between 0.5 nm and 3.0 nm. Low angle x-ray reflectivity revealed that for our growth conditions the interfacial roughness is approximately 0.4 nm in each material at each interface. High angle x-ray diffraction, together with off-specular x-ray diffraction, showed that the growth is epitaxial with the [0001] axis in-plane and parallel to the Al2O3[0001] axis. Magnetization measurements indicate the presence of an in-plane uniaxial anisotropy in all samples and antiferromagnetic coupling when the Re layer thicknesses are less than 1.0 nm and close to 2.0 nm. The uniaxial anisotropy was measured via ferromagnetic resonance and determined to be approximately 5 times smaller than in bulk Co for thicker Re layer samples. For thin Re samples, a spin-flop transition causes a competition between the anisotropic magnetoresistance and the giant magnetoresistance when the external field is applied parallel to the easy axis. The most notable consequence is that the magnetoresistance is positive for small fields and negative for large fields when the current is perpendicular to the applied field. We also report a magnetoresistance of similar to 4.5% at 10 K, more than twice the maximum value previously reported for hcp(0001) Co/Re multilayers. Co/Re hcp(10 (1) over tilde 0) superlattices provide a new system whereby the role of in-plane magnetic anisotropy in the magnetoresistance of metallic superlattices can be studied. [S0163-1829(99)00317-3].