The spin torque switching behavior of perpendicular magnetic tunnel junctions consisting of a CoFeB free layer and a CoFeB/Ru/(Co/Pd)n exchanged coupled fixed layer is investigated. At first, the Ru and CoFeB layer thickness is tuned in the CoFeB/Ru/(Co/Pd)n structure to form a ferromagnetically exchange coupled structure with a strong PMA at an annealing treatment of 325 C for 1 h. Then it is shown that that the CoFeB free layer thickness plays an important role in the switching current density. The switching current density decreases with the increase of the CoFeB free layer thickness. A minimum switching current density of 1.87 MA/cm 2 is achieved for a device with 60 nm diameter. The mechanism involved in the switching current reduction with the decrease of CoFeB free layer thickness is also studied.
Bibliographical noteFunding Information:
The authors are grateful for the partial financial support from DARPA STT-RAM program and NSF Nano Fabrication Center at the University of Minnesota. Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the NSF MRSEC program under Award No. DMR-0819885.
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