Kazutaka Yamane

Thermal activation effect on spin transfer switching in magnetic tunnel junctions

A calculation of the intrinsic spin transfer switching current Ic0 is realized in the magnetic tunnel junctions (MTJs). Ic0 is determined by the distribution of Ic in repeated measurements, assuming the linear decrease of the thermal stability by increasing the spin-polarized current. The spin transfer torque is found to be proportional to η(I)I with the applied current I and I-dependent spin polarization η(I). Even in case that I is limited less than Ic due to the tunnel barrier breakdown, Ic0 is calculated with an assisting external magnetic field and making better use of the magnetic phase diagram. In our MTJs, the average intrinsic switching current density was 6.4MA∕cm2.

Spin Torque Switching of Perpendicularly Magnetized CoFeB-Based Tunnel Junctions With High Thermal Tolerance

In order to launch spin transfer torque-based magnetoresistive random access memory into mass production below the 40 nm technology node, high performance perpendicularly magnetized magnetic tunnel junctions (pMTJs) are crucial. One of the key issues for pMTJs is to ensure compatibility with conventional back-end-of-line (BEOL) heating process, where thermal tolerances over 350 C for standalone memory and 400 °C for embedded memory are typically required. In this work, we successfully demonstrated high thermal tolerance in CoFeB-based pMTJs with a synthetic antiferromagnet (SAF) pinned structure while keeping stray fields on the free layer low. The annealing temperature (T_anneal) dependence of spin torque switching (STS) and TMR characteristics revealed that the reduction of I_c0, the increase of Δ, and the increase of TMR ratio were achieved by elevating T_anneal from 250 °C up to 380 °C for 30 min. Thus, we attained high STS efficiency (I_c0/Δ)=0.88 (I_c0=69 μA, Δ=78), TMR ratio=120%, RA =12 Ωμm² at T_anneal=380 °C for the pMTJs with dimensions of 44-46 nm in diameter. Moreover, by using specially tailored pMTJs with a SAF pinned structure, we achieved high thermal tolerance up to 400 °C for 30 min while keeping I_c0/Δ=1.12 (I_c0=56 μA, Δ=50), TMR ratio=150% , RA=13 Ωμm².