Abstract
We report a comprehensive study on the role of the free layer thickness (tF) in electric-field controlled nanoscale perpendicular magnetic tunnel junctions (MTJs), comprising of free layer structure Ta/Co40Fe40B20/MgO, by using dc magnetoresistance and ultra-short magnetization switching measurements. Focusing on MTJs that exhibits positive effective device anisotropy (Keff), we observe that both the voltage-controlled magnetic anisotropy (ξ) and voltage modulation of coercivity show strong dependence on tF. We found that ξ varies dramatically and unexpectedly from ∼−3 fJ/V-m to ∼−41 fJ/V-m with increasing tF. We discuss the possibilities of electric-field tuning of the effective surface anisotropy term, KS as well as an additional interfacial magnetoelastic anisotropy term, K3 that scales with 1/tF2. Voltage pulse induced 180° magnetization reversal is also demonstrated in our MTJs. Unipolar switching and oscillatory function of switching probability vs. pulse duration can be observed at higher tF, and agrees well with the two key device parameters — Keff and ξ.
Highlights
INTRODUCTIONPerpendicular magnetic tunnel junction (pMTJ) is a key component for spin transfer torque magnetic random access memory (STT-MRAM) – a viable non-volatile technology with attractive features like fast writing speed, high density integration, and high endurance.[1,2,3,4,5,6,7,8,9] While standby power is dramatically reduced because of non-volatility of magnetic tunnel junctions (MTJs), the writing energy utilizing STT is at best sub-pJ/bit which is still larger than their semiconductor memory counterparts.[10,11,12] One of the alternative low power writing schemes in MTJs is by means of voltage controlled magnetic anisotropy (VCMA).[13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28] Write energies as low as 6 fJ/bit has been demonstrated using this scheme.[25,29]
One of the alternative low power writing schemes in magnetic tunnel junctions (MTJs) is by means of voltage controlled magnetic anisotropy (VCMA).[13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]
Our MTJs are in perpendicular configuration for the free layer thickness range 0.72 nm< tF
Summary
Perpendicular magnetic tunnel junction (pMTJ) is a key component for spin transfer torque magnetic random access memory (STT-MRAM) – a viable non-volatile technology with attractive features like fast writing speed, high density integration, and high endurance.[1,2,3,4,5,6,7,8,9] While standby power is dramatically reduced because of non-volatility of MTJs, the writing energy utilizing STT is at best sub-pJ/bit which is still larger than their semiconductor memory counterparts.[10,11,12] One of the alternative low power writing schemes in MTJs is by means of voltage controlled magnetic anisotropy (VCMA).[13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28] Write energies as low as 6 fJ/bit has been demonstrated using this scheme.[25,29]. By incorporating Ta/CoFeB/MgO as the free layer in pMTJs, we investigate — i) CoFeB thickness dependent VCMA at the single domain limit using sub-100 nm MTJs and ii) consequence of thickness dependence on the precessional magnetization switching. We determine that VCMA is, surprisingly, strongest near the higher end of the thickness spectrum which is quite unexpected for an interfacial phenomenon.[30]
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