Sunjae Chung

Carrier-mediated antiferromagnetic interlayer exchange coupling in diluted magnetic semiconductor multilayers Ga1-xMnxAs/GaAs:Be.

We report the antiferromagnetic (AFM) interlayer exchange coupling between Ga0.97Mn0.03As ferromagnetic semiconductor layers separated by Be-doped GaAs spacers. Polarized neutron reflectivity reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers is AFM. When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the spacers are undoped, suggesting that the observed AFM coupling is mediated by doped charge carriers.

Effect of Be doping on the properties of GaMnAs ferromagnetic semiconductors

We have studied two series of molecular beam epitaxy grown Ga1−xMnxAs epilayers with several different Be doping levels. Two Mn concentrations x were chosen for this study: 0.03 and 0.05, and these values were maintained constant in each series. These samples were characterized by using SQUID and magnetotransport measurements. A systematic increase of the Curie temperature TC was observed in SQUID measurements on the series of Ga1−xMnxAs with x=0.03. The resistivity measured at zero magnetic field shows a local maximum near the Curie temperature, reflecting the effects of critical scattering near TC. The observed increase of TC in Ga1−xMnxAs for this low range of x can be explained by the increase of the free carrier concentrations in the system arising from Be doping. However, in the series of Ga1−xMnxAs with the higher concentration of Mn (x=0.05), the measurements reveal that the TC systematically decreases with increasing Be doping level. We discuss this effect in terms of a fundamental limitation of ...

Precise Investigation of Domain Pinning Energy in GaMnAs Using Planar Hall Effect and Magnetoresistance Measurements

The planar Hall effect (PHE) and magnetoresistance (MR) measurements have been carried out on a GaMnAs ferromagnetic semiconductor. The PHE and MR spectra exhibit interesting two-step magnetization switching behavior arising from the magnetic anisotropy properties of the system. By fitting the angle-dependent planar Hall resistance (PHR) data taken at 5 kG with the Stoner-Wohlfarth model, the cubic and uniaxial anisotropy constants were independently obtained. The anisotropy constants lead to the precise determination of easy axis direction, which turns out to be in good agreement with the easy axis determined from the angular plot of the switching field. The domain pinning energies were further obtained by fitting the angle dependence of the switching field, including the effect of uniaxial anisotropy

Possible indication of interlayer exchange coupling in GaMnAs/GaAs ferromagnetic semiconductor superlattices

The effects of free carriers in the nonmagnetic spacer layer on the magnetic properties of GaMnAs/GaAs ferromagnetic semiconductor superlattices (SL) were investigated. In the SL system, one of the ...

Carrier carrying capacity of one-step grown suspended carbon nanotube bridge with carbon nanotube contact electrodes: For practical one-dimensional electronics

The authors report the carrier transport and electrical breakdown behaviors of micron-long-channel suspended carbon nanotube (CNT) of carbon-based one-dimensional junction with CNTs as drain and source electrodes. The structure consisted of bundle-type CNT electrode–a CNT channel–bundle-type CNT electrode, produced by one-step in situ direct growth via a unique diluted magnetic nanothick film catalyst at low temperature. The unique suspended all-one-dimensional CNT-based junction provides some insights into recent reports that an electrical breakdown of CNTs can be induced not by the contact problem but by the nonhomogeneously Joule heating along nanotube without release of heat into contacts by the observed breakdown at midlength of a 1–2μm long channel. The high current carrying all-CNT junction with bridging yield of 90% and stable operation at fixed voltage level can contribute into realization of practical integrated nanoelectronics such as interconnector and transistor via junction formation of one-...

Four discrete Hall resistance states in single-layer Fe film for quaternary memory devices

We report the realization of four distinct magnetic states using single layers of Fe at room temperature. When the Fe film was grown on the vicinal surface of GaAs, the fourfold-symmetric magnetization along crystallographic direction give rise to four distinct Hall resistance values due to the different combination of planar and anomalous Hall effects for the given direction. Each Hall resistance state can be written reproducibly by the sequence of field pulses and was remained constant at the written state for more than 2 h, which brings the idea of a quaternary memory device much closer to practical implementation.

Tunable quaternary states in ferromagnetic semiconductor GaMnAs single layer for memory devices

The authors show that staggered asymmetric planar Hall resistance (PHR) loops observed in GaMnAs ferromagnetic semiconductor films with biaxial in-plane magnetic anisotropy result in four stable resistance states at zero magnetic field owing to the formation of a stable multidomain structure during magnetization reversal. The values of PHR can be systematically controlled by changing the direction and the scanning range of the applied magnetic field. The possibility of a quaternary memory device based on the observed four PHR states is demonstrated by obtaining consistent results in the writing process using appropriate sequences of magnetic field pulses.

Investigation of weak interlayer exchange coupling in GaMnAs/GaAs superlattices with insulating nonmagnetic spacers

A robust long-range antiferromagnetic coupling between ferromagnetic Ga 0.97Mn 0.03As layers has previously been realized via insertion of nonmagnetic Be-doped GaAs spacers between the magnetic lay ...

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.

Vertical gradient of magnetic anisotropy in the ferromagnetic semiconductor (Ga,Mn)As film

The properties of magnetic anisotropy of GaMnAs films along the growth direction were studied by Hall effect measurements. The magnetic anisotropy fields were obtained by analyzing the angular dependence of the planar Hall resistance and the anomalous Hall resistance for in-plane and out-of plane components of the Hall signal, respectively. The magnetic anisotropy fields obtained by this process were used to construct a three-dimensional magnetic free energy diagram, which clearly shows that the out-of-plane characteristics of magnetic anisotropy become more pronounced as we approach the bottom part of the film.