Andy Thomas

Co2MnSi Heusler alloy as magnetic electrodes in magnetic tunnel junctions

As a consequence of the growing theoretical predictions of 100% spin-polarized half- and full-Heusler compounds over the past six years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated Co2MnSi, as a representative of the full-Heusler compound family, as one magnetic electrode into magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co2MnSi at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer, subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co2MnSi, and (3) homogenizes the AlOx barrier. It is shown that the magnetic switching of the ferromagnetic electrodes is well controlled from room temperature down to 10K. The resulting tunnel magnetoresistance-effect amplitude of the Co2MnSi containing magn...

Memristor-based neural networks

The synapse is a crucial element in biological neural networks, but a simple electronic equivalent has been absent. This complicates the development of hardware that imitates biological architectures in the nervous system. Now, the recent progress in the experimental realization of memristive devices has renewed interest in artificial neural networks. The resistance of a memristive system depends on its past states and exactly this functionality can be used to mimic the synaptic connections in a (human) brain. After a short introduction to memristors, we present and explain the relevant mechanisms in a biological neural network, such as long-term potentiation and spike time-dependent plasticity, and determine the minimal requirements for an artificial neural network. We review the implementations of these processes using basic electric circuits and more complex mechanisms that either imitate biological systems or could act as a model system for them. (Some figures may appear in colour only in the online journal)

The Memristive Magnetic Tunnel Junction as a Nanoscopic Synapse-Neuron System

Memristors cover a gap in the capabilities of basic electronic components by remembering the history of the applied electric potentials, and are considered to bring neuromorphic computers closer by imitating the performance of synapses.[1–3] We used memristive magnetic tunnel junctions[4,5] based on MgO to demonstrate that the synaptic functionality is complemented by neuron-like behavior in these nanoscopic devices. The synaptic functionality originates in a resistance change caused by a voltage-driven oxygen vacancy motion[6] within the MgO layer. The additional functionality provided by magnetic electrodes enabled a current-driven resistance modulation due to spin-transfer torque.[7] We showed that a phenomenon known as back-hopping[8–11] leads to repeated switching between two resistance levels accompanied by current spiking, which emulates neuronal behavior. As a result, this remarkably simple system, which is composed of two magnetic layers separated by an insulator, provides a sufficient basis for the fabrication of a complete neural network. According to a simple model, the human brain consists of a network of neurons, axons and synapses. Neurons exchange electrical impulses (i.e., spikes) along their axons, and synapses are the functionalized junctions between axons and neurons. The structure of the network is defined by the spatial arrangement of neurons and axons and by the strength of each synaptic connection. In this framework, one can equate the current configuration of the network with “knowledge” and its modulation with “learning”. In the conventional (Hebbian) concept of learning,[12] synaptic strength is modified by the coincident activity of presynaptic and postsynaptic neurons. In the past few years, a series of experiments has revealed a new picture. These studies suggest that instead of mere coincidence, the precise timing of presynaptic and postsynaptic spikes (i.e., spike-timing-dependent plasticity, STDP) plays a decisive role in determining the type of synaptic modification.[13,14] This modification can lead to a persistent increase or decrease in synaptic strength, commonly referred to as long-term potentiation (LTP) or long-term depression (LTD). Software inspired by neuronal networks is widely used for specific tasks, such as pattern recognition. In addition, hardware

Scaling behavior of the spin pumping effect in ferromagnet-platinum bilayers.

We systematically measured the dc voltage V(ISH) induced by spin pumping together with the inverse spin Hall effect in ferromagnet-platinum bilayer films. In all our samples, comprising ferromagnetic 3d transition metals, Heusler compounds, ferrite spinel oxides, and magnetic semiconductors, V(ISH) invariably has the same polarity, and scales with the magnetization precession cone angle. These findings, together with the spin mixing conductance derived from the experimental data, quantitatively corroborate the present theoretical understanding of spin pumping in combination with the inverse spin Hall effect.

Spin polarization in half-metals probed by femtosecond spin excitation

Knowledge of the spin polarization is of fundamental importance for the use of a material in spintronics applications. Here, we used femtosecond optical excitation of half-metals to distinguish between half-metallic and metallic properties. Because the direct energy transfer by Elliot-Yafet scattering is blocked in a half-metal, the demagnetization time is a measure for the degree of half-metallicity. We propose that this characteristic enables us vice versa to establish a novel and fast characterization tool for this highly important material class used in spin-electronic devices. The technique has been applied to a variety of materials where the spin polarization at the Fermi level ranges from 45 to 98%: Ni, Co(2)MnSi, Fe(3)O(4), La(0.66)Sr(0.33)MnO(3) and CrO(2).

Local Charge and Spin Currents in Magnetothermal Landscapes

A scannable laser beam is used to generate local thermal gradients in metallic (Co2FeAl) or insulating (Y3Fe5O12) ferromagnetic thin films. We study the resulting local charge and spin currents that arise due to the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE), respectively. In the local ANE experiments, we detect the voltage in the Co2FeAl thin film plane as a function of the laser-spot position and external magnetic field magnitude and orientation. The local SSE effect is detected in a similar fashion by exploiting the inverse spin Hall effect in a Pt layer deposited on top of the Y3Fe5O12. Our findings establish local thermal spin and charge current generation as well as spin caloritronic domain imaging.

Memristive switching of MgO based magnetic tunnel junctions

Here we demonstrate that both, tunnel magnetoresistance (TMR) and resistive switching (RS), can be observed simultaneously in nanoscale magnetic tunnel junctions. The devices show bipolar RS of 6% and TMR ratios of about 100%. For each magnetic state, multiple resistive states are created depending on the bias history, which provides a method for multibit data storage and logic. The electronic transport measurements are discussed in the framework of a memristive system. Differently prepared MgO barriers are compared to gain insight into the switching mechanism.

Room-temperature preparation and magnetic behavior of Co2MnSi thin films

Our study presents experimental results on Co2MnSi thin-film preparation and resulting magnetic properties of the Co2MnSi Heusler alloy. The focus of our work is on the important role of the microstructure and the magnetic properties relationships of Co2MnSi thin films prepared using dc magnetron sputtering. We examined the microstructure evolution determined with x-ray diffraction for various substrates, e.g., MgO, SrTiO3, Si and SiO2, at different substrate temperatures. Polycrystalline growth observed at high substrate temperatures is independent of the nature and orientation of the substrate. These films show soft magnetic behavior at a net magnetization of 4.12μB. In contrast, textured growth is obtained at room temperature by introducing a vanadium seed layer. These samples are magnetically harder but possess a magnetization of 0.25μB only. This behavior indicates a two phase film consisting of an amorphous and textured volume. Consequently, sputtering at low argon pressure at high temperature resul...

Large tunnel magnetoresistance in tunnel junctions with Co2MnSi∕Co2FeSi multilayer electrode

Two kinds of magnetic tunnel junctions with Co2FeSi electrodes are compared. Using Co2FeSi single layers a tunnel magnetoresistance of 52% is reached, whereas the magnetization of the Co2FeSi is only 75% of the expected value. By using [Co2MnSi∕Co2FeSi]x10 multilayer electrodes the magnetoresistance can be increased to 114% and the full bulk magnetization is reached. All junctions show an inverse tunnel magnetoresistance, when the electrons are tunneling from the Co–Fe into the Heusler compound electrode. This results from a special band structure feature of full Heusler compounds, which is robust even for atomic disorder in the films.

Evolution of the dielectric breakdown in Co/Al2O3/Co junctions by annealing

The temperature and dielectric stability of magnetic tunnel junctions are important requirements for magnetic memory devices and their integration in the semiconductor process technology. We have investigated the changes of the tunneling magnetoresistance (TMR), the barrier properties (height, thickness, and asymmetry) and the dielectric stability upon isochronal annealing up to 410 °C in Co/Al2O3/Co junctions with an artificial antiferromagnet as a pinning layer. Besides a small decrease of the TMR signal after annealing up to 230 °C, a strong decrease between 300 and 350 °C is found. According to Auger and transmission electron microscopy investigations, this decrease is mainly due to interdiffusion of the metallic layers. The dielectric breakdown is characterized by voltage ramp experiments. The size-averaged breakdown voltage improves from 1.35 V for the as prepared junctions to 1.55 V by annealing at 300 °C. At higher temperatures the breakdown voltage decreases strongly to 0.8 V (at 380 °C). Simulta...