Abstract
We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-MRAM applications. We start by studying a p-MTJ consisting of a bottom synthetic Co/Pt reference layer and a synthetic FeCoB/Ru/FeCoB storage layer covered with an MgO layer. We first investigate the evolution of RKKY coupling with Ru spacer thickness in such a storage layer. The coupling becomes antiferromagnetic above 0.5 nm and its strength decreases monotonously with increasing Ru thickness. This contrasts with the behavior of Co-based systems for which a maximum in interlayer coupling is generally observed around 0.8 nm. A thin Ta insertion below the Ru spacer considerably decreases the coupling energy, without basically changing its variation with Ru thickness. After optimization of the non-magnetic and magnetic layer thicknesses, it appears that such a FeCoB/Ru/FeCoB synthetic storage layer sandwiched between MgO barriers can be made stable enough to actually be used as hard reference layer in single or double magnetic tunnel junctions, the storage layer being now a single soft FeCoB layer. Finally, we realize Pt- or Pd-free robust perpendicular magnetic tunnel junctions, still keeping the advantage of a synthetic reference layer in terms of reduction of stray fields at small pillar sizes.
Highlights
We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-Magnetic Random Access Memories (MRAM) applications
This allows making a major loop on SAF2 without modifying the bottom SAF1 configuration at low fields
We have presented the magnetic properties of perpendicular junctions in which the bottom reference is made of a “hard” Synthetic AntiFerromagnet (SAF) with Co/Pt multilayers (SAF1) and the storage layer is a “soft” SAF made of FeCoB/(Ta)/Ru/FeCoB (SAF2)
Summary
We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-MRAM applications. The most commonly used structures consist of a single CoFeB storage layer and a reference layer including a Co/Pt or Co/Pd multilayer-based Synthetic AntiFerromagnet (SAF)[5,7]. SAF structures are composed of two magnetic layers antiferromagnetically coupled through a metallic spacer, generally Ru, thanks to RKKY coupling[8,9]. The SAF reference layer comprises a CoFeB layer in contact with the MgO barrier to increase the TMR value thanks to the body-centered cubic (bcc) structure that it acquires after annealing[10].
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