Kungwon Rhie

A key to room-temperature ferromagnetism in Fe-doped ZnO: Cu

Successful synthesis of room-temperature ferromagnetic semiconductors, Zn1−xFexO, is reported. The essential ingredient in achieving room-temperature ferromagnetism in bulk Zn1−xFexO was found to be additional Cu doping. A transition temperature as high as 550 K was obtained in Zn0.94Fe0.05Cu0.01O; the saturation magnetization at room temperature reached a value of 0.75μB per Fe. A large magnetoresistance was also observed below 100 K.

Magnetic-field-controlled reconfigurable semiconductor logic

Logic devices based on magnetism show promise for increasing computational efficiency while decreasing consumed power. They offer zero quiescent power and yet combine novel functions such as programmable logic operation and non-volatile built-in memory. However, practical efforts to adapt a magnetic device to logic suffer from a low signal-to-noise ratio and other performance attributes that are not adequate for logic gates. Rather than exploiting magnetoresistive effects that result from spin-dependent transport of carriers, we have approached the development of a magnetic logic device in a different way: we use the phenomenon of large magnetoresistance found in non-magnetic semiconductors in high electric fields. Here we report a device showing a strong diode characteristic that is highly sensitive to both the sign and the magnitude of an external magnetic field, offering a reversible change between two different characteristic states by the application of a magnetic field. This feature results from magnetic control of carrier generation and recombination in an InSb p–n bilayer channel. Simple circuits combining such elementary devices are fabricated and tested, and Boolean logic functions including AND, OR, NAND and NOR are performed. They are programmed dynamically by external electric or magnetic signals, demonstrating magnetic-field-controlled semiconductor reconfigurable logic at room temperature. This magnetic technology permits a new kind of spintronic device, characterized as a current switch rather than a voltage switch, and provides a simple and compact platform for non-volatile reconfigurable logic devices.

Ferromagnetism in Cr-doped Ge

We have fabricated Cr-doped bulk Ge single crystal using the vertical gradient solidification method. The material shows ferromagnetic ordering at 126 K, as determined from temperature-dependent magnetization and resistance measurements. A sample with x=0.01 was p type with nh=3×1017 cm−3 at 350 K. The measured magnetic moment per Cr was 0.83μB at 5 K.

Ferromagnetic properties in Cr, Fe-doped Ge single crystals

We have fabricated Cr and Fe-doped bulk Ge single crystal using the vertical gradient solidification method. The Cr-doped Ge single crystal showed ferromagnetic ordering at 126 K, as determined from temperature dependent magnetization and resistance measurements. The measured magnetic moment per Cr was 0.83 μB at 5 K. On the other hand, Fe0.05Ge0.95 showed ferromagnetic ordering at 233 K. The coersive field was 80 Oe at 5 K.

Local Hall effect in hybrid ferromagnetic/semiconductor devices

The authors have investigated the magnetoresistance of ferromagnet-semiconductor devices in an InAs two-dimensional electron gas system in which the magnetic field has a sinusoidal profile. The magnetoresistance of their device is large. The longitudinal resistance has an additional contribution which is odd in applied magnetic field. It becomes even negative at low temperature where the transport is ballistic. Based on the numerical analysis, they confirmed that their data can be explained in terms of the local Hall effect due to the profile of negative and positive field regions. This device may be useful for future spintronic applications.

Magnetic bead counter using a micro-Hall sensor for biological applications

Micro-Hall sensors have been fabricated, and various numbers of micron-size magnetic beads have been placed within the sensor area. The Hall resistances measured at room temperature are found to be proportional to the number of the beads, and are in good agreement with the numerically simulated results presented in this study. Our sensors are designed to measure the number of beads between zero and full-scale signals for a given number range of interest. The effects of miniaturizing the beads and sensors to nanoscale are also discussed.

Enhanced tunneling magnetoresistance and thermal stability of magnetic tunnel junction by rapid thermal anneal

Abstract The effect of rapid thermal anneal (RTA) has been investigated on the properties of an FeMn exchange-biased magnetic tunnel junction (MTJ) using magnetoresistance and I – V measurements and transmission electron microscopy (TEM). The tunneling magnetoresistance (TMR) in an as-grown MTJ is found to be ∼27%, while the TMR in MTJs annealed by RTA increases with annealing temperature up to 300°C, reaching ∼46%. TEM images reveal that the interface of Al 2 O 3 layer for the annealed MTJ has changed into a relatively clear morphology, as compared to that for the as-grown MTJ. The oxide barrier parameters are found to vary abruptly with annealing time within a few ten seconds. Our results demonstrate that the present RTA enhances the thermal stability of MTJs.

TMR of double spin-valve type AF/FM/I/FM/I/FM/AF magnetic tunneling junctions

An unusually large enhancement of TMR at 77 K was observed in double barrier tunnel junctions (DBTJ). This is explained with extended Jullieres model which yields a twice larger TMR value. When the spin coherence length is much smaller at higher temperature, DBTJ is shown to work as a series of two single barrier tunnel junctions.

An electrical switching device controlled by a magnetic field-dependent impact ionization process

An abrupt change of conductance at a threshold magnetic field was observed in a device consisting of a nonmagnetic narrow-gap semiconductor. The conductance varies more than 25 times as the magnetic field increases. The threshold magnetic field can be tuned using a bias voltage from zero to several hundred Gauss. This large magnetoconductance effect is caused by the magnetic field-dependent impact ionization process. A theoretical model is proposed, and calculations based on this model simulate the experimental results closely. This device may be a good candidate for an electrical switching device controlled by a magnetic field.

Temperature dependence of magnetoresistance for tunnel junctions with high-power plasma-oxidized barriers: Effects of annealing

Magnetic tunnel junctions (MTJ) were fabricated with high oxygen-plasma power and the effects of annealing on the temperature dependence of tunneling magnetoresistance (TMR) were investigated experimentally. As grown, TMR increases, peaks around 160 K, and decreases with increasing temperature from 80 K to 300 K. When MTJs are annealed, Tmax, the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain the temperature dependence of TMR, the difference of conductance between parallel and antiparallel alignments of magnetizations as a function of temperature is also analyzed. The shifts of Tmax due to annealing are described phenomenologically with spin-dependent transfer rates of electrons through the barrier.