Origin of perpendicular magnetic anisotropy in Co/Ni multilayers and their use in STT-RAM

Synopsis

Magnetic properties of (111)-textured SAF/Cu/FL multilayer film structures were opti-mized by varying individual layer thickness and sputtering conditions. The SAF is a syn-thetic antiferromagnet consisting of Co/Ni multilayers coupled antiferromagnetically across a Ru spacer layer, and the FL is a free layer consisting of a single Co/Ni multilayer. The Co and Ni thicknesses were varied at two different deposition rates to obtain a perpendicular magnetic anisotropy of the 8×[Co/Ni] multilayers ranging from 4×105 to 4.6×105 J/m3.The perpendicular magnetic anisotropy, saturation magnetization, damping and zero-frequency line broadening of the Co/Ni multilayers strongly depend on the number of bilayers. With increasing Cu seed-layer thickness, the texture of the Co/Ni multilayers improves while the grain size and film roughness increase. The increase in grain size results in the reduction of the direct exchange coupling between magnetic grains, which enhances the coercivity of the SAF and the FL. Experimentally measured coercivities of the SAF and FL are com-pared with calculations obtained from a coherent rotation model. The effect of the Co/Ru interface in the SAF on the exchange coupling, and the role of the Co/Cu interface in the magnetoresistance, is also discussed.

Spin-transfer-torque induced switching is investigated in 200 nm diameter circularly shaped, perpendicular magnetized nanopillars. The SAF layer is used as a reference layer to minimize the dipolar field on the free layer. The use of Pt and Pd was avoided to lower the spin-orbit scattering in magnetic layers and intrinsic damping in the free layer, and therefore, reduce the critical current required for spin-transfer-torque switching. The intrinsic Gilbert damp-ing of a continuous 4×[Co/Ni] multilayer film was measured by ferromagnetic resonance to be – = 0.022, which is significantly lower than in Pt or Pd based magnetic multilayers. In zero magnetic field the critical current required to switch the free layer from the parallel to antiparallel (antiparallel to parallel) alignment is 5.2 mA (4.9 mA). Given the volume of the free layer, VFL = 1.01×10−22 m3,the switchingefficiency,Ic￿(VFL × µ0Hc), is 5.28×1020 A/Tm3, twice as efficient as any previously reported device with a similar structure.

Variation in perpendicular magnetic anisotropy of (111) textured Au/N× [Co/Ni]/Au films as a function of number of bilayer repeats N is studied. The ferromagnetic resonance and SQUID magnetometer measurements show that the perpendicular magnetic anisotropy of Co/Ni multilayers first increases with N for N ≤ 10 and then moderately decreases forN > 10. The model we propose reveals that the decrease of the anisotropy for N < 10 is pre-dominantly due to the reduction in the magnetoelastic and magnetocrystalline anisotropies. A moderate decrease in the perpendicular magnetic anisotropy for N > 10 is due to the reduction in the magnetocrystalline and the surface anisotropies. To calculate the contribu-tion of magnetoelastic anisotropy in the Co/Ni multilayers, in-plane and out-of-plane X-ray diffraction measurements are performed to determine the spacing between Co/Ni (111) and (220) planes. The magnetocrystalline bulk anisotropy is estimated from the difference in the perpendicular and parallel g-factors of Co/Ni multilayers that are measured using the in-plane and out-of-plane ferromagnetic resonance measurements. The transmission electron microscopy has been used to estimate the multilayer film roughness. These values are used to calculate the roughness induced surface and magnetocrystalline anisotropy coefficients as a function of N.s.

Keywords: Magnetic anisotropy, giant magneto-resistance, spin transfer torque, magnetron sputtering