Marc Sacher

Aluminum oxidation by a remote electron cyclotron resonance plasma in magnetic tunnel junctions

The aluminum barrier in exchange biased (Mn–Ir/Co–Fe/AlOx/Py) magnetic tunnel junctions was formed by oxidation with an electron cyclotron resonance plasma source. This technique allows, in contrast to commonly used floating substrate, an independent control of the energy of the ions bombarding the Al film by adjusting a direct current bias voltage at the sample. Here, we show that low energy ions and oxidation times between 50 and 200 s lead to optimum barrier properties and a maximum tunnel magneto resistance (TMR) ratio of 46% at room temperature and 73% at 10 K. Shorter oxidation time gives underoxidized samples with strongly reduced TMR. Ions with energy above 30 eV turn out to strongly overoxidize the sample resulting in large junction resistance and reduced TMR values.

Inverted spin polarization of heusler alloys for spintronic devices

A magnetic logic concept 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 tunneling magnetoresistance values at different bias voltages at room temperature which generally add an additional degree of freedom to all spintronic devices.

Postannealing of magnetic tunnel junctions with ion-bombardment-modified exchange bias

The influence of a postannealing procedure on the transport properties of magnetic tunnel junctions with ion-bombardment-manipulated exchange bias is investigated. The controlled manipulation of the direction of the exchange bias field in magnetic tunnel junctions by He ion bombardment usually is accompanied by a reduction of the tunneling magnetoresistance and an increase in the resistance. Here, we demonstrate that it is possible to reduce these negative effects of the ion bombardment considerably by postannealing without a magnetic field. For optimized combinations of ion dose and postannealing temperature, the tunneling magnetoresistance recovers completely (>50% resistance change) while the exchange bias direction set by the ion bombardement is preserved.

X-ray absorption and magnetic circular dichroism studies of annealed magnetic tunnel junctions

The magnetic and chemical interface properties of Mn–Ir∕Co–Fe∕Al+oxidation∕Ni–Fe magnetic tunnel junctions are investigated for different barrier thickness, oxidation times, and annealing conditions by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. For underoxidized samples the formation of Co–Fe–Al alloy at the lower barrier interface during optimal annealing is observed. For optimally oxidized and overoxidized samples FeOx is formed during oxidation, which is reduced by Mn diffusing to the barrier during annealing. The reduction of FeOx is accompanied by an increase of the interfacial magnetic Fe moment, whereas the Co moments hardly change with the postannealing. Comparison of these results with transport properties of the junctions shows that their polycrystalline structure has to be taken into account to understand the annealing temperature and oxidation state dependence of the tunneling magnetoresistance effect.

Spin resolved photoelectron spectroscopy of [Mn6IIICrIII]3 + single-molecule magnets and of manganese compounds as reference layers

Properties of the manganese-based single-molecule magnet [Mn(6)(III)Cr(III)](3+) are studied. It contains six Mn(III) ions arranged in two bowl-shaped trinuclear triplesalen building blocks linked by a hexacyanochromate and exhibits a large spin ground state of S(t) = 21/2. The dominant structures in the electron emission spectra of [Mn(6)(III)Cr(III)](3+) resonantly excited at the L(3)-edge are the L(3)M(2, 3)M(2, 3), L(3)M(2, 3)V and L(3)VV Auger emission groups following the decay of the primary p(3/2) core hole state. Significant differences of the Auger spectra from intact and degraded [Mn(6)(III)Cr(III)](3+) show up. First measurements of the electron spin polarization in the L(3)M(2, 3)V and L(3)VV Auger emission peaks from the manganese constituents in [Mn(6)(III)Cr(III)](3+) resonantly excited at the L(3)-edge near 640 eV by circularly polarized synchrotron radiation are reported. In addition spin resolved Auger electron spectra of the reference substances MnO, Mn(2)O(3) and Mn(II)(acetate)(2)·4H(2)O are given. The applicability of spin resolved electron spectroscopy for characterizing magnetic states of constituent atoms compared to magnetic circular dichroism (MCD) is verified: the spin polarization obtained from Mn(II)(acetate)(2)·4H(2)O at room temperature in the paramagnetic state compares to the MCD asymmetry revealed for a star-shaped molecule with a Mn(4)(II)O(6) core at 5 K in an external magnetic field of 5 T.

Chemical and Magnetic Interface Properties of Tunnel Junctions With Co

Transport, as well as chemical and magnetic interface properties of two kinds of magnetic tunnel junctions (MTJs) with Co2FeSi electrode, Al-O barrier, and Co-Fe counter electrode, are investigated. For junctions with Co2FeSi 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 Co2FeSi film reaches only 75% of the expected value. By using a [Co2MnSi/Co2FeSi]times10 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 Co2FeSi/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.

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A local manipulation of the exchange bias coupling between antiferromagnetic and ferromagnetic layers by ion bombardment induced magnetic patterning enables a patterning of the magnetization direction of the ferromagnetic layer without considerable structural changes. We show that a magnetic line grating with alternating antiparallel orientations of the magnetization of the ferromagnetic layer acts as a magnetically switchable reflective grating for soft x-ray radiation. A common rotational direction of the magnetization at all boundaries between bombarded and not bombarded lines is shown by magnetic force microscopy measurements. Scattering at the edges of the bombarded lines might be responsible for the observed interference pattern.

MnSi/Co

The transport properties of Co∕Al2O3∕Co magnetic tunnel junctions with ion-irradiated tunneling barrier are reported. The irradiation by He+ and Ar+ with energies ranging from 15to105eV takes place in situ after oxidation of the 1.4‐nm-thick Al layer. For both ion species the area resistance of the junctions increases strongly with ion energy, simultaneously the tunneling magnetoresistance is reduced. But the energy dependence of both properties is different for He+ and Ar+ irradiations. Additionally the bias voltage dependence of the tunneling magnetoresistance is deteriorated with increasing ion energy especially for Ar+ irradiation. These experimental results are discussed with respect to the energy-dependent penetration depth of He+ and Ar+ and their energy loss in the barrier.

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The Heusler alloy Co2MnSi with theoretically predicted 100% spin polarization has been successfully implemented in Co2MnSi∕AlOx∕Co7Fe3 magnetic tunnel junctions recently. Here we investigated 100 nm thick Co2MnSi films grown on a vanadium buffer and capped by a thin alumina film. This stack is similar to the lower electrode including the tunneling barrier of magnetic tunnel junctions. By soft x-ray absorption spectroscopy and magnetic circular dichroism in fluorescence and total electron yield detection we determined differences between the chemical and magnetic properties of the Co2MnSi thin films and the Co2MnSi∕AlOx interface. Whereas MnO was formed directly at the interface during plasma oxidation of the Al layer, the interfacial Co was not oxidized and its x-ray absorption line shape at the L2,3 edge was similar to the line shape of bulk Co2MnSi. The atomically ordered bulk Co2MnSi showed characteristic differences in the x-ray absorption spectra in comparison with the disordered samples: in the orde...

FeSi Multilayer Electrode Showing Large Tunneling Magnetoresistance

The ionic single-molecule-magnet [MnIII6CrIII]3 with corresponding three counterions has been deposited on different surfaces and studied with respect to its structure and its electronic and magnetic properties. This is the first time that spin polarization of photoelectrons ejected by means of circularly polarized synchrotron radiation has been measured in a single-molecule-magnet.