Titanium Nitride as a Seed Layer for Heusler Compounds

Titanium nitride (TiN) shows low resistivity at room temperature, high thermal stability and thus has the potential to serve as seed layer in magnetic tunnel junctions. High quality TiN thin films with regard to the crystallographic and electrical properties were grown and characterized by X-ray diffraction and 4-terminal transport measurements. Element specific X-ray absorption spectroscopy revealed pure TiN in the bulk. To investigate the influence of a TiN seed layer on a ferro(i)magnetic bottom electrode, an out-of-plane magnetized Mn2.45Ga as well as in- and out-of-plane magnetized Co2FeAl thin films were deposited on a TiN buffer, respectively. The magnetic properties were investigated using a superconducting quantum interference device (SQUID) and anomalous Hall effect (AHE) for Mn2.45Ga. Magneto optical Kerr effect (MOKE) measurements were carried out to investigate the magnetic properties of Co2FeAl. TiN buffered Mn2.45Ga thin films showed higher coercivity and squareness ratio compared to unbuffered samples. The Heusler compound Co2FeAl showed already good crystallinity when grown at room temperature

Inverted spin polarization of Heusler alloys for new spintronic devices

A new magnetic logic overcomes the major limitations of field programmable gate arrays while having a 50% smaller unit cell than conventional designs utilizing magnetic tunnel junctions with one Heusler alloy electrode. These show positive and negative TMR values at different bias voltages at room temperature which generally adds an additional degree of freedom to all spintronic devices

Structural and magnetic properties of Co-Mn-Sb thin films

Thin Co-Mn-Sb films of different compositions were investigated and utilized as electrodes in alumina based magnetic tunnel junctions with CoFe counter electrode. The preparation conditions were optimized with respect to magnetic and structural properties. The Co-Mn-Sb/Al-O interface was analyzed by X-ray absorption spectroscopy and magnetic circular dichroism with particular focus on the element-specific magnetic moments. Co-Mn-Sb crystallizes in different complex cubic structures depending on its composition. The magnetic moments of Co and Mn are ferromagnetically coupled in all cases. A tunnel magneto resistance ratio of up to 24 % at 13K was found and indicates that Co-Mn-Sb is not a ferromagnetic half-metal. These results are compared to recent works on the structure and predictions of the electronic properties.Comment: 8 pages, 9 figure

Insights into ultrafast demagnetization in pseudo-gap half metals

Interest in femtosecond demagnetization experiments was sparked by Bigot's discovery in 1995. These experiments unveil the elementary mechanisms coupling the electrons' temperature to their spin order. Even though first quantitative models describing ultrafast demagnetization have just been published within the past year, new calculations also suggest alternative mechanisms. Simultaneously, the application of fast demagnetization experiments has been demonstrated to provide key insight into technologically important systems such as high spin polarization metals, and consequently there is broad interest in further understanding the physics of these phenomena. To gain new and relevant insights, we perform ultrafast optical pump-probe experiments to characterize the demagnetization processes of highly spin-polarized magnetic thin films on a femtosecond time scale. Previous studies have suggested shifting the Fermi energy into the center of the gap by tuning the number of electrons and thereby to study its influence on spin-flip processes. Here we show that choosing isoelectronic Heusler compounds (Co2MnSi, Co2MnGe and Co2FeAl) allows us to vary the degree of spin polarization between 60% and 86%. We explain this behavior by considering the robustness of the gap against structural disorder. Moreover, we observe that Co-Fe-based pseudo gap materials, such as partially ordered Co-Fe-Ge alloys and also the well-known Co-Fe-B alloys, can reach similar values of the spin polarization. By using the unique features of these metals we vary the number of possible spin-flip channels, which allows us to pinpoint and control the half metals electronic structure and its influence onto the elementary mechanisms of ultrafast demagnetization.Comment: 17 pages, 4 figures, plus Supplementary Informatio

Seebeck Effect in Magnetic Tunnel Junctions

Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, i.e., the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In that respect, it is the analog to the tunneling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configuration are in the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. Experimentally, we realized 8.8 % magneto-Seebeck effect, which results from a voltage change of about -8.7 {\mu}V/K from the antiparallel to the parallel direction close to the predicted value of -12.1 {\mu}V/K.Comment: 16 pages, 7 figures, 2 table

Chemical and Magnetic Interface Properties of Tunnel Junctions With Co 2 MnSi/Co 2 FeSi Multilayer Electrode Showing Large Tunneling Magnetoresistance

Transport, as well as chemical and magnetic interface properties of two kinds of magnetic tunnel junctions (MTJs) with Co 2 FeSi electrode, Al-O barrier, and Co-Fe counter electrode, are investigated. For junctions with Co 2 FeSi single-layer electrodes, a tunnel magnetoresistance of up to 52% is found after optimal annealing for an optimal Al thickness of 1.5 nm, whereas the room temperature bulk magnetization of the Co 2 FeSi film reaches only 75% of the expected value. By using a [Co 2 MnSi/Co 2 FeSi] 10 multilayer electrode, the magnetoresistance can be increased to 114%, corresponding to a large spin polarization of 0.74, and the full bulk magnetization is reached. For Al thickness smaller than 1 nm, the TMR of both kinds of MTJs decreases rapidly to zero. On the other hand, for 2-to 3-nm-thick Al, the TMR decreases only slowly. The Al thickness dependence of the TMR is directly correlated to the element-specific magnetic moments of Fe and Co at the Co 2 FeSi/Al-O interface for all Al thickness. Especially, for optimal Al thickness and annealing, the interfacial Fe moment of the single-layer electrode is about 20% smaller than for the multilayer electrode, indicating smaller atomic disorder at the barrier interface for the latter MTJ

Cobalt-based Heusler compounds in magnetic tunnel junctions

Ebke D. Cobalt-based Heusler compounds in magnetic tunnel junctions. Bielefeld (Germany): Bielefeld University; 2010.Spintronic devices have attracted a lot of attention in recent years due to possible new applications, e.g., a magnetic random access memory (MRAM), logic and sensors. The spin of the electrons is used as an additional degree of freedom in contrast to common electronic devices. The main constituent of many spintronic devices is the magnetic tunnel junction (MTJ) where two ferromagnets are separated by a thin insulating tunnel barrier. The resistance of such a device depends on the magnetic orientation of the ferromagnets. Usually, R_AP (antiparallel) is higher than R_P (parallel) and a tunnel magnetoresistance (TMR) can be defined as TMR = R_AP-R_P / R_P. For small voltages the resistance is connected to the spin dependent density of states (DOS) at the Fermi level of the ferromagnets. Hence, the TMR value is also given by TMR = (2 P1 P2) / (1-P1 P2) with the spin polarization P1,2. Therefore, materials with a high spin polarization are eligible for applications. A half metallic behavior, i.e., they are 100 percent spin polarized at the Fermi level E_F which has been theoretically predicted for some oxide compounds such as Fe3O4 and CrO2, perovskites (e.g. LaSrMnO3), zinc-blende-type CrAs and Heusler compounds. In particular, Co-based Heusler compounds are promising materials for spintronic applications due to the required high Curie temperatures T_C. Here a Heusler compound is given by the composition X2YZ and a crystallographic L2_1 structure exists. X and Y are transition metal elements and Z is a group III, IV or V element. In 2004, room temperature TMR ratios of more than 100 percent were reported for MgO-based MTJs. Recently Ikeda presented TMR ratios of over 600 percent at room temperature and over 1100 percent at low temperatures for a single MgO tunnel barrier. With the concept of a double barrier system these values can be increased and TMR ratios of more than 1000 percent at room temperature have been reported. High room temperature TMR ratios have also been reported for MTJs containing Heusler compounds as electrodes: 217 percent for Co2MnSi and very recently 386 percent for Co2Fe0.5Al0.5Si. The latter was grown by using molecular beam epitaxy in place of sputtering deposition. However, sputtering is the preferred and established method for industrial applications. From a technological point of view, the aim is also to achieve high TMR ratios by sputtering. The actuality of this topic can by recognized by recent press releases. For example, Toshiba announced the development of a spin transport electronics based metal oxide semiconductor field-effect transistor (MOSFET) cell with a full Heusler compound. However, the predicted half-metallicity for Heusler compounds should lead to much higher TMR ratios. Nevertheless, one has to meet two challenges to achieve half metallicity: 1. crystallization of the Heusler electrode(s) in L2_1 structure 2. coherent interfaces of the Heusler compound and the MgO tunnel barrier In this work we have investigated different Co-based Heusler compounds. We have integrated them into so called half junctions to investigate the crystal growth and magnetic properties of the Heusler electrode and into full MTJs for the transport properties. We describe the optimization of a required seed layer system to induce the preferred (001) texture of the Heusler thin films. Furthermore, we have optimized the Heusler layer in an attempt to achieve a high atomic ordering, represented by a high magnetic moment and a maximum (001) texture. We investigated the transport properties of the full junctions at room temperature and low temperature (13K) respectively, and discuss them in terms of annealing temperature, bias voltage and temperature dependence. Finally, the industrial applicability and integration of Heusler compound electrodes into conventional GMR/TMR systems will be verified. Consequently, Heusler junctions prepared by Singulus NDT GmbH will be compared to our samples. In particular, the growth properties of the Heusler layer will be addressed to determine differences within the sputtering process of the Heusler thin films

Tunneling spectroscopy and magnon excitation in Co2FeAl/MgO/Co-Fe magnetic tunnel junctions

Ebke D, Drewello V, Schäfers M, Reiss G, Thomas A. Tunneling spectroscopy and magnon excitation in Co2FeAl/MgO/Co-Fe magnetic tunnel junctions. APPLIED PHYSICS LETTERS. 2009;95(23):232510.Magnetic tunnel junctions with the Heusler compound Co2FeAl as the soft electrode are prepared. Pinned Co-Fe is used as the hard reference electrode. The junctions show a high tunnel magnetoresistance ratio of 273% at 13 K. The electronic transport characteristics are investigated by tunneling spectroscopy-dI/dV and d(2)I/dV(2) are discussed. In the parallel magnetic state the tunneling spectra are asymmetric with respect to the bias voltage, with a pronounced bias-independent region. In the antiparallel state the dependence on bias voltage is much stronger and the curves are symmetric. The findings can be explained with a gap in the minority density of states of Co2FeAl. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3272947

Electric field induced reversible tuning of resistance of thin gold films

Dasgupta S, Kruk R, Ebke D, Hütten A, Bansal C, Hahn H. Electric field induced reversible tuning of resistance of thin gold films. JOURNAL OF APPLIED PHYSICS. 2008;104(10):103707.The change in resistance of nanostructured metals with respect to an applied field is believed to be due to a change in carrier concentration and hence a linear variation of resistance with the surface charge is expected. In this article, we propose a different approach to explain the resistance variation based on a change in the effective thickness of the film due to a shift of the electron density profile resulting from the applied surface charge. The change in effective thickness together with its effect on surface scattering of electrons account for the majority of the observed variation in resistance. The thin film geometry with different thicknesses and hence a controlled variation of the surface-to-volume ratio allows a deep quantitative understanding and interpretation of the observed phenomena. The model presented in this work shows that a nominal nonlinear response of the resistance of a metal on electrochemically applied surface charge does not necessarily indicate an onset of a redox reaction