Abstract
Magnetic coupling between two CoFeB layers through the W insertion layer is important in the conventional double CoFeB/MgO interface, magnetic tunneling junctions (MTJs) (double-MTJs) with MgO/CoFeB/W/CoFeB/MgO free layer stack because it increases the effective magnetic volume of the free layer. The magnetic coupling energy constant per unit area, Jcpl, between two CoFeB layers through the W layer and the effective perpendicular magnetic anisotropy (PMA) energy constant per unit area, Kefft*, were investigated for conventional double-MTJs with various W insertion layer thicknesses. As the W layer thickness increased, Kefft* increased and Jcpl decreased. There exists a trade-off relationship between Jcpl and Kefft*. In conventional double-MTJs with a single W insertion layer, large values for Jcpl and Kefft* were difficult to obtain simultaneously. To improve this tradeoff, we employed a free layer stack with a thin ferromagnetic layer (ferromagnetic bridge layer: FBL) located in the W insertion layer. In the double-MTJs with FBL annealed at 400 °C, a large Jcpl value of 0.37 mJ/m2 was achieved while maintaining the maximum values of Kefft*. Accordingly, the MTJ with FBL provides an MTJ stack structure for obtaining high thermal stability.
Highlights
There has been significant development in spintransfer-torque magnetoresistive random-access memories (STTMRAM)
The origin of the interfacial perpendicular magnetic anisotropy (IPMA) in the magnetic tunneling junctions (MTJs) is based on the CoFeB/MgO interface, which is attributed to the overlap between the O-pz and transition metal dz2 orbitals hybridized with dxz(yz) orbitals
As Δ is proportional to the effective perpendicular magnetic anisotropy (PMA) constant, the free layer volume in a coherent magnetic rotation model,15 the square root of the effective PMA constant, and the diameter of the free layer in a domain propagation model,16 Δ decreases as the MTJ diameter decreases
Summary
There has been significant development in spintransfer-torque magnetoresistive random-access memories (STTMRAM). This was due to the achievement of the CoFeB/MgO-based magnetic tunneling junction (MTJ) with interfacial perpendicular magnetic anisotropy (IPMA).. The origin of the IPMA in the MTJ is based on the CoFeB/MgO interface, which is attributed to the overlap between the O-pz and transition metal dz orbitals hybridized with dxz(yz) orbitals.. In a report of IPMA MTJ, a single CoFeB/MgO interface structure was employed at a junction diameter of 40 nm.. T1, tFBL, and t2 are thicknesses of FL1, FBL, and FL2, respectively. Keff and Keff are effective perpendicular anisotropy energy densities per volume of FL1 and FL2, respectively
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