Il-Jae Shin

Electrical spin accumulation with improved bias voltage dependence in a crystalline CoFe/MgO/Si system

We report the electrical spin accumulation with enhanced bias voltage dependence in n-type Si, employing a crystalline CoFe/MgO tunnel contact. A sizable spin signal of ∼4.8 kΩμm2, a spin lifetime of ∼155 ps, and a spin diffusion length of ∼220 nm were obtained at 300 K. The spin signal and lifetime obtained in this system show consistent behavior with the temperature variation irrespective of the bias voltage. Notably, the spin signal exhibits nearly symmetric dependence with respect to the bias polarity, which is ascribed to the improved bias dependence of tunnel spin polarization.

Electrical spin injection and accumulation in CoFe/MgO/Ge contacts at room temperature

We first report the all-electrical spin injection and detection in CoFe/MgO/moderately doped n-Ge contact at room temperature (RT), employing threeterminal Hanle measurements. A sizable spin signal of ~170 k{\Omega} {\mu}m^2 has been observed at RT, and the analysis using a single-step tunneling model gives a spin lifetime of ~120 ps and a spin diffusion length of ~683 nm in Ge. The observed spin signal shows asymmetric bias and temperature dependences which are strongly related to the asymmetry of the tunneling process.

Single crystalline CoFe/MgO tunnel contact on nondegenerate Ge with a proper resistance-area product for efficient spin injection and detection

We report the proper resistance-area products in the single crystalline bcc CoFe/MgO tunnel contact on nondegenerate n-Ge desirable for efficient spin injection and detection at room temperature. The electric properties of the single crystalline CoFe(5.0 nm)/MgO/n-Ge(001) tunnel contacts with an ultrathin MgO thickness of 1.5, 2.0, and 2.5 nm have been investigated by the I-V-T and C-V measurements. Interestingly, the crystalline tunnel contact with the 2.0-nm MgO exhibits the Ohmic behavior with the RA products of 5.20×10−6/1.04×10−5 Ω m2 at ±0.25 V, satisfying the theoretical conditions required for significant spin injection and detection. We believe that the results are ascribed to the presence of MgO layer between CoFe and n-Ge, enhancing the Schottky pinning parameter as well as shifting the charge neutrality level.

Effects of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by in situ annealing

We study the effect of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by high-temperature in situ annealing above 400 °C, circumventing Mn diffusion conventionally caused by postannealing process. The high temperature in situ annealing leads to Ru diffusion at the CoFeB/Ru interfaces, and thereby results in a reduction in tunnel magnetoresistance (TMR). The minimization of Ru diffusion during the in situ annealing provides a large TMR of 294% at room temperature with an exchange-bias field of 280 Oe. In addition, the temperature and voltage dependence of TMR reveals that there is neither significant spin-exchange scattering nor severe impurity-assisted scattering in the MgO barrier.We study the effect of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by high-temperature in situ annealing above 400 °C, circumventing Mn diffusion conventionally caused by postannealing process. The high temperature in situ annealing leads to Ru diffusion at the CoFeB/Ru interfaces, and thereby results in a reduction in tunnel magnetoresistance (TMR). The minimization of Ru diffusion during the in situ annealing provides a large TMR of 294% at room temperature with an exchange-bias field of 280 Oe. In addition, the temperature and voltage dependence of TMR reveals that there is neither significant spin-exchange scattering nor severe impurity-assisted scattering in the MgO barrier.

Perpendicular magnetic tunnel junctions with synthetic ferrimagnetic pinned layer

We have fabricated perpendicular magnetic tunnel junctions (p-MTJs) consisting of CoPt/Ru/CoFeB/MgO/CoFeB/Pt. The tunnel magnetoresistance (TMR) of the p-MTJs characterizes the magnetic configurations of the CoPt/Ru/CoFeB pinned layer. The magnetic moment of the CoFeB, which is naturally in-plane, can be aligned to the out-of-plane direction by an indirect exchange coupling with the hcp CoPt alloy having large perpendicular magnetic anisotropy. When the applied field is relatively smaller than the exchange coupling strength, the p-MTJs show a configurationally negative TMR as a consequence of an antiparallel alignment of the CoPt and CoFeB magnetic moments.

Unconventional Hanle effect in a highly ordered CoFe/MgO/n-Si contact: non-monotonic bias and temperature dependence and sign inversion of the spin signal

We report in this paper the unconventional bias and temperature dependence of the Hanle effect in a highly ordered CoFe/MgO/n-Si contact investigated by means of a three-terminal Hanle method. The spin signal and the effective spin lifetime obtained in this system show non-monotonic behavior with bias and temperature variations. Interestingly, the sign of the spin signal changes significantly with the bias voltage at a low temperature. The sign inversion is presumably ascribed to the contribution of interfacial resonant states formed at the CoFe/MgO interface or bound states in the Si surface during the spin extraction process.

MgO-based magnetic tunnel junctions for spin-transfer-torque random access memory

We illuminate how the tunnel magnetoresistance (TMR) of MgO-based magnetic tunnel junctions (MTJs) is affected by the structure, materials, and fabrication processes. First, we demonstrate a possibility to control detrimental diffusions by separating a step for obtaining a grain-to-grain epitaxy in CoFeB/MgO/CoFeB layers from a step for achieving a high exchange-bias field in the pinned layer. A high TMR and large exchange-bias field can be obtained simultaneously by circumventing Mn diffusion and minimizing Ru diffusion during the annealing process at high temperature. Second, we show that the MTJs consisting of CoFeB/ MgO/ CoFeB/ Ru/ ferromagnet (FM), where FM is Co, Ni, NiFe, CoFe, or CoFeB, can provide a reasonably high TMR and decent thermal stability, presumably useful for the memory applications.

Consistent Relationship Between the Tunnel Magnetoresistance of CoFeB/MgO/CoFeB Junctions and X-Ray Diffraction Properties

We have investigated the effect of Ar pressure during MgO sputtering on the tunnel magnetoresistance (TMR) and resistance area (RA) product of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). The TMR of MTJs with a thin MgO tunnel barrier deposited at different Ar pressures (1.3, 4, 10, and 25 mTorr) shows a consistent relationship with X-ray diffraction (XRD) properties of thick MgO films deposited with the same conditions. The deposition of the MgO-barrier at 1.3 mTorr results in a low TMR ratio and a high RA product due to the disordered MgO barrier and the oxidation of the bottom electrode of the MTJ, while the deposition at 25 mTorr results in a rough MgO barrier, and thereby gives rise to a large shift of switching field of the free layer due to the orange-peel coupling.