Large in-plane anisotropies in Co/Re superlattices: What's happening at the interface?

We have performed magnetization and 36 GHz ferromagnetic resonance measurements as a function of in-plane angle and Co layer thickness in order to determine the interface anisotropies in epitaxial hcp (101̄0) Co(x)/Re(1.2nm) superlattices (1.0nm<x<3.5nm), whose Co c axis lies in the film plane. The linear dependence of magnetization upon inverse Co layer thickness indicates the bulk of the Co layer has a magnetization of 1390 emu/cm ³, and nonmagnetic Co within 0.5 nm of each Re interface. The angular variation in the resonance position yields out-of-plane, and first and second order uniaxial in-plane anisotropies. These coefficients are plotted as a function of the inverse magnetic Co layer thickness (1/t _Co), yeilding their interface and volume contributions. The volume out-of-plane term is 10.7×10 ⁶erg/cm ³, which compares favorably with the 12.3×10 ⁶erg/cm ³ expected from shape anisotropy. Despite the c axis being in plane, a significant out-of-plane interface energy of -0.074 erg/cm ² was detected, which favors out-of-plane magnetization. The first order uniaxial in-plane term is independent of t _Co. Its average value of 1.2×10 ⁶erg/cm ³ is a factor of 4 smaller than that observed in macroscopic Co. The second order in-plane uniaxial term does vary linearly with 1/t _Co, yielding volume and interface contributions of 1.3×10 ⁵erg/cm ³ (a factor of 10 smaller than that observed in macroscopic Co and -7×10 ^-4erg/cm ², respectively).