Giant Perpendicular Magnetic Anisotropy Enhancement in MgO-Based Magnetic Tunnel Junction by Using Co/Fe Composite Layer

Abstract

Magnetic tunnel junctions (MTJ) with perpendicular magnetic anisotropy (PMA) [1] form the basis of the spin-transfer torque magnetic random-access memory (STT-MRAM), which is non-volatile, fast, dense, and has quasi-infinite write endurance and low power consumption. In the usual CoFeB|MgO-based MTJ, the cell diameter must be kept above ~30 nm [2], otherwise, the bit becomes thermally unstable due to insufficient PMA. Being able to increase the PMA of the storage layer opens a way to further downsize scaling. In magnetic recording media, this is typically done by introducing heavy metals like Pt or Pd [3], which increases the spin-orbit coupling parameter. However, in the context of STT-MRAM, this would increase the storage layer Gilbert damping and therefore the switching current [4]. To avoid this, multilayers exhibiting bulk PMA based on purely 3d metallic elements were developed [5,6] such as (Fe/Ni) or (Co/Ni) based multilayers. However, these structures are intrinsically complex to fabricate to get large PMA and often do not withstand the required annealing temperature ~400°C.Based on density functional theory (DFT) calculations, we propose a rather simple design of MTJ with greatly enhanced PMA up to several mJ/m2 in the form Fe(n)Co(m)Fe(n)|MgO [Fig. 1]. This design leverages the well-known interfacial PMA of Fe|MgO along with a strain-induced bulk PMA discovered within bcc Co [plot in Fig. 1]. This strain in bcc Co is induced by the underlying layer. The storage layer can be several nm thick and still exhibit perpendicular magnetization. We perform DFT calculations of the effective PMA (the sum of magnetocrystalline energy EMCA and demagnetizing energy Edd) for different Fe and Co thicknesses and compare with the case of pure Fe, as shown in Fig. 2(a). Unlike in Fe, where the effective PMA decreases with Fe thickness and becomes negative for thicknesses above 11 MLs, in the proposed structure, we see a steady increase of PMA vs. Co thickness.The tunneling magnetoresistance (TMR) estimated from the extended Julliere model is around 300%, comparable with that of the pure Fe|MgO case.After studying the electronic structure of strained bcc Co, we can explain why the large PMA emerges. Calculations based on the second-order perturbation theory [7] reveal that strain leads in bcc Co to a significant shift in the energies of dyz and dz2 minority-spin bands further away from the Fermi level. This is shown to cause an overall increase in PMA. Replacing the middle Fe layers with Co also decreases the negative demagnetizing energy [the magnetocrystalline and demagnetizing energies separately are shown in Fig. 2(b)].The PMA enhancement can result in a possible downscaling of the cell area by 200% to 300% if an atomically sharp Fe/Co interface can be fabricated. In the case of interfacial mixing, the PMA is reduced by a few 10%. The impact is excessive only in the thinnest Fe2Co3Fe2/MgO structure, where the drop is about 70%. The thicker structures are far more robust against interdiffusion: in Fe3Co4Fe3/MgO, the drop is only 22%.In conclusion, we propose bcc Fe(3ML)Co(4ML)Fe(3ML) as a storage layer for MgO-based double-barrier MTJ with greatly enhanced perpendicular magnetic anisotropy. **