Kwang-Seok Kim

Variation in the properties of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing

Variation in the quality of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta=300°C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV∕dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB∕MgO∕CoFeB junction.

Junction area dependence of breakdown characteristics in magnetic tunnel junctions

Breakdown characteristics of the magnetic tunnel junctions (MTJ) with different junction areas of S=200 μm2 and S=0.5 μm2 are investigated under constant voltage stress. The breakdown process is found to be quite different for the two junction areas. For the large junctions with S=200 μm2, magnetoresistance (MR) ratio decreases gradually with increasing time of constant voltage stress and lasts for 10–30 min. The low frequency 1/f noise power also increases with increasing stress time due to the Johnson and shot noises, caused by current flowing through ohmic shorts or pinholes across tunnel barrier. Meanwhile, the junctions with S=0.5 μm2 show abrupt junction breakdown after stress time of 1–7 h. There is no significant change in both MR ratio and its bias dependence during the voltage stress. In particular, soft-breakdown events are observed before total breakdown occurs. The junction breakdown of small junction area is of intrinsic nature of the junction while the one of large junction area is due to e...

Role of interface traps on breakdown process of a magnetic tunnel junction

Time dependent dielectric breakdown (TDDB) measurements were carried out for magnetic tunneling junctions (MTJs) with different electron trap densities at the interface between a bottom electrode and an insulating barrier. The TDDB shows a strong bias-polarity dependence, which becomes bigger with increasing trap density. In addition, the current creep before total dielectric breakdown consistently shows bias-polarity dependence. The polarity dependence of the breakdown in MTJs with an ultrathin tunneling barrier (15–20A) is believed to be caused by precursor effect of the traps at the bottom interface, which enhances the trap generation rate in a tunneling barrier, resulting in acceleration of the breakdown process.

Magnetic field dependent noise in magnetic tunnel junction

The measurements of voltage fluctuations at room temperature are performed for magnetic tunnel junction, patterned by photolithography to have 20×10 μm2 in junction area. The observed noise power follows 1/f behavior for the whole applied magnetic field (H) range (−30 Oe⩽H⩽30 Oe) and shows strong field dependence. The sharp increase in noise power spectral density is observed near a transition region from antiparallel to parallel magnetic state in two electrodes during field ramping down. In addition, the reduction of noise power density is accompanied by the reduction of magnetoresistance ratio after thermal annealing at 300 °C for 700 s in the same junction. It is conjectured that the observed reduction of the noise level in our junction is due mainly to magnetic origin, such as thermally activated domain wall motion, rather than electrical origin.

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.

Stress polarity dependence of breakdown characteristics in magnetic tunnel junctions

Time-dependent dielectric breakdown (TDDB) measurements under constant voltage stress with positive and negative bias polarities are carried out for magnetic tunnel junctions (MTJs) with different oxidation status (under-, optimal, and overoxidation). We found that there is significant polarity dependence in the TDDB and speculated that the polarity dependence is due to both intrinsic and extrinsic origins. Optimally oxidized MTJs with positive bias on the top electrode show shorter times to breakdown (tBD’s) and lower barrier height than with negative bias, indicating that asymmetric band structure, in part, causes the polarity dependence. On the other hand, under- and overoxidized MTJs show much shorter tBD’s than optimally oxidized one and show a higher 1∕f noise power density for positive bias than for negative bias, indicating that the polarity dependence is also, in part, due to the interface states, which acts like precursors for the dielectric breakdown.

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.

Control of the Degree of Out-of-Plane Anisotropy of CoFe/Tb Multilayer by Applying an External Inplane Field During Film Deposition

We investigated the out-of-plane anisotropy of Co50Fe50/Tb multilayer film with changing the thickness of Tb layer and applying an inplane filed of 200 Oe during film deposition. A strong out-of-plane anisotropy was found only for the multilayer with small thickness of Tb layer (tTb = 1 nm) and was suppressed significantly by the application of an inplane magnetic field of 200 Oe during deposition, resulting in an inplane anisotropy. It was also found that the application of the magnetic field caused the enhanced intermixing between CoFe and Tb layers to form Co-Tb and Fe-Tb alloys at the interface of the layers. While the origin of the out-of-plane anisotropy and the change of the out-of-plane to inplane anisotropy with the application of inplane field of 200 Oe are not understood completely, it seems that there is a strong correlation between the out-of-plane anisotropy and the formation of the alloys and the stress at the interface.

Dependence of magnetic tunnel junction's reliability on oxidation condition

Time-dependent dielectric breakdown measurements under constant voltage stress were carried out for magnetic tunnel junctions (MTJs), prepared by different oxidation techniques. Insulating barriers in MTJs were fabricated by oxidation of a predeposited Al layer with different oxidation techniques, such as conventional O2 plasma, off-axis O2 plasma, Ar-mixed O2 plasma, and off-axis Ar-mixed O2 plasma oxidations. The time to breakdown (TBD) of the MTJs was estimated from Weibull failure distribution plot. The estimated TBD when 63% of the junctions formed by conventional O2 plasma oxidation failed was estimated to be about 55 s. The TBD increased up to about 250, 750, and 2,500 s for the junctions of off-axis, Ar-mixed, and off-axis Ar-mixed O2 plasma oxidation, respectively. We believe that the enhanced reliability of MTJs is due to the reduction of plasma damage during oxidation process, resulting in low electron trap density in the Al2O3 tunneling barrier. Low-frequency 1/f noise power density was measur...