Hun-Sung Lee

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.

Temperature and bias dependence of Hanle effect in CoFe/MgO/composite Ge

We have investigated the temperature and bias dependence of the Hanle effect in a composite n-type Ge system consisting of a heavily doped surface layer and a moderately doped Ge substrate, using three-terminal Hanle measurements. A large spin signal of ∼5.1 kΩμm2 and a spin lifetime of ∼105 ps are obtained at 300 K. The spin signal, spin lifetime, and their asymmetries with respect to the bias polarity have been measured over a temperature range from 5 K to 300 K. Intriguingly, an inverted Hanle effect, indicating the sign inversion of spin polarization in Ge, is observed at low temperature.

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.

Energy band structure of the single crystalline MgO/n-Ge(001) heterojunction determined by x-ray photoelectron spectroscopy

We report the energy band structure of the single crystalline MgO/n-Ge(001) heterojunction characterized by x-ray photoelectron spectroscopy. The valence band offset of ΔEV=3.64±0.07 eV with a 1.49±0.02 eV band bending was obtained. Given the experimental band gap of MgO (7.83 eV), a type-I band alignment with a conduction band offset of ΔEC=3.52±0.07 eV is found. The band alignment of the MgO/n-Ge heterojunction including the large band bending was analyzed by a theoretical model taking into account the formation of the interface dipole.

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.

Reduced stochasticity in domain wall motion with increasing pinning density in thin Fe films

We report in this paper the decrease of the stochasticity of magnetization half-reversal time with increasing domain wall (DW) pinning in Fe films investigated by means of time-resolved magneto-optic Kerr microscopy. The domain images in the films reveal that the density of DW pinning sites increases with increasing Fe thickness. However, we found that the stochasticity of the magnetization half-reversal time significantly decreases with increasing DW pinning. The major reason for the reduced stochasticity is shown to be due to a thermally activated DW creep mechanism that becomes dominant during magnetization reversal due to increased DW pinning.

Universal Barkhausen critical scaling behavior observed in NixFe1−x (x = 0−0.5) films

We have investigated the Barkhausen critical scaling behavior of NixFe1−x (x = 0−0.5) films using a magneto-optical microscope, capable of time-resolved domain observation. Real-time direct observations of the domain evolutions in these films revealed that magnetization reversal occurred with a sequence of random Barkhausen jumps. From more than 1000 repeated experiments with each sample, it was found that the distribution of the Barkhausen jump size followed a power-law distribution. The scaling exponent of the power-law distribution was found to have the same value of 1.1, independent of the film composition, revealing a universal critical scaling behavior in these alloy films.

Influence of oxygen contamination on annealing behaviours of spin density and optical gap of amorphous silicon films

Abstract Annealing behaviours of spin density and optical gap for varying degree of oxygen-incorporated a-Si films are investigated. It is found that the defect generation by high temperature annealing is closely related to the oxygen contamination of the film during sample preparation. The relation between spin density and the corresponding optical gap is also studied and it is shown that the width of optical gap is affected both by the presence of silicon-oxygen bonds and the magnitude of spin density. A possible mechanism of the defect generation in conjunction with the annealing measurements of infrared absorption is also discussed.

The pinning role of nonferromagnetic β phase in the domain wall motion of ferromagnetic MnAs film on GaAs(001)

We report the pinning role of the nonferromagnetic β phase in the domain wall motion of ferromagnetic MnAs film on GaAs(001) in the temperature range of 20–35°C. The dependence of the coercivity on the applied field direction at all temperatures was found to follow an inverse cosine law, as expected from a theoretical model considering the domain wall motion in ferromagnetic films. From this analysis, it was found that the domain wall pinning energy density increases as the temperature increases. This is ascribed to the increase in the volume fraction of the nonferromagnetic β phase, resulting in an increase of the domain wall pinning effect during the domain wall motion.