Jungtaek Kim

Distribution of magnetic domain pinning fields in Ga 1 − x Mn x As ferromagnetic films

Using the angular dependence of the planar Hall effect in GaMnAs ferromagnetic films, we were able to determine the distribution of magnetic domain pinning fields in this material. Interestingly, there is a major difference between the pinning field distribution in as-grown and in annealed films, the former showing a strikingly narrower distribution than the latter. This conspicuous difference can be attributed to the degree of non-uniformity of magnetic anisotropy in both types of films. This finding provides a better understanding of the magnetic domain landscape in GaMnAs that has been the subject of intense debate.

Phase transitions and molecular motions in a model biomembrane (C12H25NH3)2SnCl6

Abstract Two successive phase transitions in bis-n-dodecylammonim hexachlorostannate, (n-C12H25NH3)2SnCl6, were studied by means of 1H NMR. From the temperature dependent dipolar splitting, the low and the high temperature transition were attributed to an order–disorder transition and a conformational transition of the rigid hydrocarbon chain, respectively. The spin-lattice relaxation rate in the low and the intermediate temperature phase was well fitted with four types of molecular motions, and the ammonium group was revealed to dictate the order–disorder transition temperature.

Electrical properties of the Sm2Ti2O7 thin films for metal-insulator-metal capacitor applications

A homogeneous crystalline Sm2Ti2O7 (ST) phase was formed in films grown at temperatures ranging between 100 and 200 °C and subsequently annealed at 900 °C. The ST film had a large dielectric constant of 58, which is similar to that of ST ceramics. The leakage current density of the ST film was low and the Poole-Frenkel emission was suggested as being the leakage current mechanism. The ST film had a negative quadratic voltage coefficient of capacitance (VCC), possibly due to the dipolar relaxation. The 100-nm-thick ST film had a high capacitance density of 5.2 fF∕μm2 with a low leakage current density of 1.34 nA∕cm2 at 2 V. Its quadratic and linear VCCs were −99.5 ppm∕V2 and 11 ppm/V, respectively, with a low temperature coefficient of capacitance of 135 ppm∕°C at 100 kHz. These results confirmed the potential for the ST film to be used as a high performance metal-insulator-metal capacitor.

Quantitative analysis of the angle dependence of planar Hall effect observed in ferromagnetic GaMnAs film

The angle dependence of the planar Hall effect has been analyzed based on the magnetic free energy including the magnetic anisotropy and the Zeeman effects. The Zeeman effect dominated the magnetic anisotropy in high field and only a single energy minimum is shown in free energy over entire field angle, which leads to the coherent rotation of the magnetization in the form of a single domain state. When the field strength is reduced below 300Oe, multiple energy minima appear in the angle dependence of free energy due to the increase in the relative importance of magnetic anisotropy. In the low field region, reorientation of magnetization experiences abrupt transition between the free energy minima. The pinning fields obtained from the analysis showed systematic dependence on the strength of external field, which was used to rotate magnetization. We understood such pinning energy dependence in terms of the difference in the free energy density profile for the different field strengths.

Single and multidomain characteristics of GaMnAs investigated by magnetotransport measurements

We have investigated the magnetization reorientation process of GaMnAs ferromagnetic films by changing external field direction in planar Hall effect (PHE) measurement. While the angular dependences of PHE data taken with clockwise and counterclockwise under strong magnetic field (i.e., above 400Oe) are completely overlapped without hysteresis, they are significantly different under small magnetic field (i.e., below 50Oe) by exhibiting nonabrupt hysteresis. We have analyzed such angular dependence of PHE using the magnetic free energy based on Stoner-Wohlfarth model. The behavior observed under the high field was well understood in terms of coherent rotation of magnetization in the form of single domain. However, the nonabrupt hysteric behavior observed with low field cannot be explained by a single domain picture and requires involvement of multidomain structures.

Nematic order and nuclear magnetic relaxation in PCH-3 liquid crystal

Proton NMR was employed to study the nematic phases upon heating and upon cooling in PCH-3 (trans-4 (4-propylcyclohexy) benzonitrile). Our systematic measurements of the nematic order parameter and spin relaxations reveal the microscopic nature of the thermal hysteresis in this relatively low molecular weight nematic liquid crystalline material. 2001 Published by Elsevier Science B.V.

Electron-nuclear spin control in charged semiconductor quantum dots by electrical currents through micro-coils

We have fabricated micrometer-sized single-turn coils on top of charged CdSe/ZnSe quantum dot heterostructures by lithographical techniques. Current injection creates magnetic fields in the some 10 mT range, strong enough to modulate the hyperfine interaction. The very low coil inductance allows for generation of fast field transients. We demonstrate local control of the resident electron spin as well as read-out of the nuclear spin state on the 10 ns time scale by electrical current pulses.

Effects of humidity and depurination on the charge transport in DNA films

We have investigated the humidity and depurination (creating apurinic sites) effects on the electrical conduction in the λ-DNA films at different relative humidities and depurination times. The DNA films were found to follow the polaron hopping model, and the polaron hopping distance turned out to decrease at the higher relative humidity, more water molecules supposedly being adsorbed on the film. The activation energy showed little dependence on the depurination time at the higher relative humidity, whereas at the lower humidity it increased sharply with increasing depurination time.

Effect of shape anisotropy on the magnetization reversal process of (Ga,Mn)As ferromagnetic semiconductors

We have carried out the planar Hall effect measurement on two different sizes of Hall bar devices fabricated from GaMnAs ferromagnetic films. The detailed information on the magnetic anisotropy properties including the shape anisotropy field were obtained from the angular dependence of planar Hall resistance. The shape anisotropy field per magnetization in the device with 10μm channel width was about 39G. Though the value of shape anisotropy is small, it affects the magnetization reversal processes of the 10μm Hall device by showing steplike feature in the angular dependence of switching fields.

Reduction of Defects in GaN on Reactive Ion Beam Treated Sapphire by Annealing

Previous studies showed that reactive ion beam (RIB) pretreatment of sapphire prior to GaN deposition results in the reduction of dislocation density in the GaN film. It was also found that an amorphous phase remained at the interface region after the GaN deposition at high temperature. Annealing was performed to obtain the structural change due to the recrystallization of the remaining amorphous phase, and the effect on the electrical properties of the GaN thin film on RIB treated sapphire (0001) substrate. DCXRD spectra and Hall mobility of the specimen were studied as a function of the annealing time at 1000°C in N 2 atmosphere. For the annealed specimen, FWHM of DCXRD decreased and the mobility increased. The annealed specimen was compared with a not annealed sample by TEM. A decrease of lattice strain and a reduction of the dislocation density about 56-59% was observed. The present results clearly show that the combination of RIB pretreatment and proper post annealing conditions improve the properties of GaN films grown by MOCVD.