Ung-hwan Pi

Verification on the extreme scalability of STT-MRAM without loss of thermal stability below 15 nm MTJ cell

Scalability of interface driven perpendicular magnetic anisotropy (i-PMA) magnetic tunnel junctions (MTJs) has been improved down to 1X node which verifies STT-MRAM for future standalone memory. With developing a novel damage-less MTJ patterning process, robust magnetic and electrical performances of i-PMA MTJ cell down to 15 nm node could be achieved.

Effect of Fe–O distance on magnetocrystalline anisotropy energy at the Fe/MgO(001) interface

We report first-principles calculations on the magnetocrystalline anisotropy energy (MAE) of an Fe monolayer sandwiched by MgO. We found that by increasing the interlayer distance between Fe and O by about 8% from its equilibrium value, the perpendicular interfacial magnetic anisotropy can be enhanced as high as 2.75 erg/cm2, which is three times larger than that at the equilibrium distance. The analysis of MAE based on the second-order interactions of the spin-orbit coupling shows that the energy position of the majority-spin dz2 orbital is of central importance in determining MAE. Our results suggest that increasing the Fe–O distance in the Fe/MgO system is an important material-design direction for high-performance magnetic memories.

Highly scalable STT-MRAM with 3-dimensional cell structure using in-plane magnetic anisotropy materials

Novel spin transfer torque MRAM cells with three dimensional freelayer structures were suggested for the high density memory below 20nm technology node. By folding the freelayer to a special geometry, the 3D MTJ Cell structure retains large freelayer volume without an increase of cell foot-print, scaling down the MRAM cells even with in-plane magnetic anisotropy materials. From the micromagnetic calculation with Nudged Elastic Band (NEB) method, we confirmed the thermal stability over 60 in 3D MTJ cell with 15×30 nm2 area.

The effect of the static magnetic susceptibility on the spin precession in MgO based magnetic tunnel junction

We studied the steady state precession of the spins in the magnetic tunnel junctions. In the field range below the spin-flop transition of synthetic antiferromagnetic pinned layer, the precession mode depends upon the current bias polarity. Above the spin-flop transition, however, the spin precession shows symmetric behavior on the current bias. The dominant mechanism of the spin precession changes from the spin transfer torque to thermal excitation at around the spin-flop transition. The enhancement of thermal fluctuation above the spin-flop transition is analyzed in relation to the static magnetic susceptibility.