Low magnetisation alloys for in-plane spin transfer torque devices

Abstract

Magnetic properties of thin films of three in-plane alloy systems, namely, Co40Ni40−xCrxB20, Co40Fe40−xNixB20, and Co40Fe40−xCrxB20 are investigated with a view to reducing the critical current density (jc) for spin transfer torque switching in MgO barrier magnetic tunnel junction nanopillars. In all three systems, the saturation magnetisation, Ms, decreases with increasing Cr or Ni substitution. The Co40Fe40−xCrxB20 alloy system is found to be the best in terms of reducing Ms, while maintaining a high tunnel magnetoresistance ratio. Ferromagnetic resonance experiments revealed that the damping coefficient of CoFeCrB alloy remains unchanged with increasing Cr content, but jc is reduced by a factor of four to 4.9 × 105 A/cm2 by using a Co40Fe32.7Cr7.3B20 free layer while maintaining a magnetoresistance of around 60 %.