Ajey P. Jacob

Electric-field-controlled ferromagnetism in high-Curie-temperature Mn0.05Ge0.95 quantum dots

Electric-field manipulation of ferromagnetism has the potential for developing a new generation of electric devices to resolve the power consumption and variability issues in todays microelectronics industry. Among various dilute magnetic semiconductors (DMSs), group IV elements such as Si and Ge are the ideal material candidates because of their excellent compatibility with the conventional complementary metal-oxide-semiconductor (MOS) technology. Here we report, for the first time, the successful synthesis of self-assembled dilute magnetic Mn(0.05)Ge(0.95) quantum dots with ferromagnetic order above room temperature, and the demonstration of electric-field control of ferromagnetism in MOS ferromagnetic capacitors up to 100 K. We found that by applying electric fields to a MOS gate structure, the ferromagnetism of the channel layer can be effectively modulated through the change of hole concentration inside the quantum dots. Our results are fundamentally important in the understanding and to the realization of high-efficiency Ge-based spin field-effect transistors.

Single Crystalline Ge1-xMnx Nanowires as Building Blocks for Nanoelectronics

Magnetically doped Si and Ge nanowires have potential application in future nanowire spin-based devices. Here, we report a supercritical fluid method for producing single crystalline Mn-doped Ge nanowires with a Mn-doping concentration of between 0.5-1.0 atomic % that display ferromagnetism above 300 K and a superior performance with respect to the hole mobility of around 340 cm(2)/Vs, demonstrating the potential of using these nanowires as building blocks for electronic devices.

Performance of Magnetic Quantum Cellular Automata and Limitations Due to Thermal Noise

Operation parameters of magnetic quantum cellular automata are evaluated for the purposes of reliable logic operation. The dynamics of the nanomagnets is simulated via the Landau-Lifshitz-Gilbert equations with a stochastic magnetic field corresponding to thermal fluctuations. It is found that in the macrospin approximation, the switching speed does not change under scaling of both size and distances between nanomagnets. Thermal fluctuations put a limitation on the size of nanomagnets: when we consider a majority gate that features a biaxial anisotropy as a stabilizing mechanism and a uniform clocking field, the gate error rate becomes excessive for nanomagnets smaller than about 200 nm at room temperature.

Tadpole shaped Ge0.96Mn0.04 magnetic semiconductors grown on Si

Magnetic and structural properties of a Ge0.96Mn0.04 thin film grown on Si has been investigated by transmission electron microscopy and superconducting quantum interference device. Tadpole shaped coherent GeMn clusters induced by spinodal decomposition were revealed in the film. Although these coherent clusters are dominant, Mn5Ge3 precipitates can be still detectable, contributing to a complex ferromagnetism. The Ge buffer layer, by relieving the misfit strain between Si and Ge, can significantly reduce the density of lattice defects in the subsequent GeMn layer. Our findings unveil a particular morphology of GeMn clusters, which would contribute to better understand the GeMn system.

Dispersion in magnetostatic CoTaZr spin waveguides

Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion and up to three different modes are identified. Attenuation lengths of the order of 3 μm are several orders of magnitude shorter than that predicted from eddy currents in these thin wires.

Dispersion and spin wave “tunneling” in nanostructured magnetostatic spin waveguides

Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion. The measured attenuation lengths, of the order of 3 μm, are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires. Spin waves effectively “tunnel” through air gaps, produced by focused ion beam etching, as large as 1.5 μm.

Optical properties of InAs quantum dots grown on patterned Si with a thin GaAs buffer layer

InAs quantum dots (QDs) were grown on patterned Si substrates with a thin GaAs buffer using SiO2 as a mask by molecular beam epitaxy. GaAs was firstly selectively grown on the exposed Si surface with feature size around 250nm. The InAs QDs were selectively grown on top of the GaAs. Low temperature photoluminescence (PL) measurements show strong optical activity in the wavelength range from 900to1050nm. The temperature dependent measurement of the PL response indicates that, for temperatures over 110K, the carrier escape from quantum dots leads to quenching of the signal. The PL results demonstrate that using nanostructures, it is possible to integrate high quality direct gap III-V materials on Si with high optical activity, leading to potentially new optoelectronic applications on Si and other convenient substrates which are lattice mismatched to InAs and other III-V materials.

Logic Devices with Spin Wave Buses - an Approach to Scalable Magneto-Electric Circuitry

We analyze spin wave-based logic circuits as a possible route to building reconfigurable magnetic circuits compatible with conventional electron-based devices. A distinctive feature of the spin wave logic circuits is that a bit of information is encoded into the phase of the spin wave. It makes possible to transmit information as a magnetization signal through magnetic waveguides without the use of an electric current. By exploiting sin wave superposition, a set of logic gates such as AND, OR, and Majority gate can be realized in one circuit. We present experimental data illustrating the performance of a three-terminal micrometer scale spin wave-based logic device fabricated on a silicon platform. The device operates in the GHz frequency range and at room temperature. The output power modulation is achieved via the control of the relative phases of two input spin wave signals. The obtained data shows the possibility of using spin waves for achieving logic functionality. The scalability of the spin wave-based logic devices is defined by the wavelength of the spin wave, which depends on the magnetic material and waveguide geometry. Potentially, a multifunctional spin wave logic gate can be scaled down to 0.1μm 2 . Another potential advantage of the spin wave-based logic circuitry is the ability to implement logic gates with fewer elements as compared to CMOS-based circuits in achieving same functionality. The shortcomings and disadvantages of the spin wave-based devices are also discussed.

Spin Wave Logic Circuit on Silicon Platform

We analyze spin wave-based logic circuits for information transmission and processing in integrated magneto-electric circuits. A bit of information is encoded into the phase of the spin wave propagating in the magnetic waveguide referred to as spin wave bus. It makes possible to transmit and process a number of bits at constant time. By exploiting sin wave superposition, a set of logic gates such as AND, OR, and Majority gate can be realized in one circuit. This paper presents the realization of a spin-wave based logic circuit fabricated on a silicon platform. The prototype micrometer scale circuit operates in the GHz frequency range and offers exciting potential as a multifunctional logic gate that could be scaled down to 0.1 mum2 with energy per bit as low as 10-18 J. Another potential advantage of the spin wave-based logic circuitry is the ability to implement logic gates with fewer elements as compared to CMOS- based circuits in achieving same functionality. The shortcomings and disadvantages of the spin wave-based devices are discussed.

Logic Devices with Spin Wave Buses: Potential Advantages and Shortcoming

The authors analyze the performance and estimate the efficiency of the spin-wave based logic devices. The main potential advantage of using spin wave for information transmission and processing is that a bit of information can be transmitted without use of an electric current. Spin-wave based devices may find an application as an interface between the electronic and magnetic circuits. The authors show the schematics of the integration of the spin-wave based devices with conventional electron-based circuit. The input information is received in the form of voltage pulses. Next, it is converted into the spin wave signals, where logic state 1 and 0 corresponds to the phases of the spin waves. Spin-wave based device accomplishes computation using spin wave signals and provides the output on the form of voltage pulses.