Switching characteristics and magnetoresistance of Co-based multilayered perpendicular magnetic tunnel junctions
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1198-E02-08
Switching characteristics and magnetoresistance of Co-based multilayered perpendicular magnetic tunnel junctions Zeenath.R.Tadisina, Anusha.Natarajarathinam and Subhadra.Gupta, MINT Center, University of Alabama, Box 870209, Tuscaloosa, AL 35487 USA ABSTRACT Perpendicularly magnetized magnetic tunnel junctions (pMTJ) using Co based multilayers as free and reference layers have been optimized for perpendicular magnetic anisotropy (PMA) and high tunneling magnetoresistance (TMR). The effect of seed layers, Co thickness and Co/Ni thickness ratio on the anisotropy of these multilayer films has been studied. Intermixing of these multilayers was analyzed by local electrode atom probe (LEAP). The effect of Co thickness, Pd thickness and number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]n multilayer films have been studied for both free and reference layers. The magnetic behavior of these PMA systems was studied by alternating gradient magnetometer. CoFeB/MgO/CoFeB trilayers sandwiched between the PMA multilayer material systems were studied. The transport properties of the patterned MTJ stacks were measured by PPMS from 10K to 400 K. A maximum TMR of 10% at 10K (5-10% at 300 K) was obtained for these perpendicular MTJ’s (pMTJ), regardless of whether they were magnetically annealed for MgO-CoFeB crystallization or not, indicating that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering. INTRODUCTION Magnetic tunnel junctions (MTJ’s) with perpendicular magnetic anisotropy (PMA) materials have been extensively studied for applications such as high density read heads and magnetoresistive random access memory (MRAM)1-5. The phenomenon of spin transfer (ST) switching applied to magnetic tunnel junctions with PMA has generated considerable interest in applications such as spin torque transfer random access memory (STT-RAM)1. In order to have high density MRAM, the MTJ cell size should be very small, which works in favor of STTRAM, because the switching current scales down as the MTJ size shrinks. A low switching current and a large thermal stability factor are the two important issues for realizing high density STT-MRAM. The theoretical expressions predict that lower critical currents for switching and high thermal stability can be achieved by using perpendicular magnetic anisotropy materials1. The thermal stability factor, Δ, is expressed as: KV . (1) k BT where K is the anisotropy, V the volume, kB Boltzmann’s constant, and T the absolute temperature. The critical current, IC can be expressed as6: 2e (2) IC M SV H eff where is the Gilbert damping coefficient, the spin-transfer efficiency factor (maximum value=1) and Heff is an effective field, which for in-plane materials is
H eff
HK 2 MS
H eff
HK
(3) and the effective field is dominated by the shape anisotropy term. For PMA materials with an out-of-plane anisotropy H K 4 M S the effective field is:
4 MS
(4) Thus in PMA-based MTJ’s the
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