Volker Drewello

Time-resolved measurement of the tunnel magneto-Seebeck effect in a single magnetic tunnel junction

Recently, several groups have reported spin-dependent thermoelectric effects in magnetic tunnel junctions. In this paper, we present a setup for time-resolved measurements of thermovoltages and thermocurrents of a single micro- to nanometer-scaled tunnel junction. An electrically modulated diode laser is used to create a temperature gradient across the tunnel junction layer stack. This laser modulation technique enables the recording of time-dependent thermovoltage signals with a temporal resolution only limited by the preamplifier for the thermovoltage. So far, time-dependent thermovoltage could not be interpreted. Now, with the setup presented in this paper, it is possible to distinguish different Seebeck voltage contributions to the overall measured voltage signal in the μs time regime. A model circuit is developed that explains those voltage contributions on different sample types. Further, it will be shown that a voltage signal arising from the magnetic tunnel junction can only be observed when the laser spot is directly centered on top of the magnetic tunnel junction, which allows a lateral separation of the effects.

Electric breakdown in ultrathin MgO tunnel barrier junctions for spin-transfer torque switching

Magnetic tunnel junctions for spin-transfer torque (STT) switching are prepared to investigate the dielectric breakdown. Intact and broken tunnel junctions are characterized by transport measurements prior to transmission electron microscopy analysis. The comparison to our previous model for thicker MgO tunnel barriers reveals a different breakdown mechanism arising from the high current densities in a STT device: instead of local pinhole formation at a constant rate, massive electromigration and heating leads to displacement of the junction material and voids are appearing. This is determined by element resolved energy dispersive x-ray spectroscopy and three dimensional tomographic reconstruction.

Co/Pt multilayer-based magnetic tunnel junctions with perpendicular magnetic anisotropy

Temperature-dependent transport measurements of magnetic tunnel junctions with perpendicularly magnetized Co/Pt electrodes are presented. Magnetization measurements of the Co/Pt multilayers are performed to characterize the electrodes. The interface magnetization of the Co layers at the Pt interface is estimated in dependence of the annealing temperature. The effect of the annealing temperature on the tunneling magnetoresistance effect of the magnetic tunnel junctions (MTJs) is investigated. Tunneling magnetoresistance ratios of about 19% at room temperature are attained and two well-defined switching fields are observed. The tunneling magnetoresistance of Co/Pt based tunnel junctions changes by a factor of 1.9 if cooled to 13 K. The results are compared to measurements of MTJs with Co-Fe-B electrodes and in-plane anisotropy.

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

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 d2I/dV2 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.

Direct imaging of the structural change generated by dielectric breakdown in MgO based magnetic tunnel junctions

MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown. This can be explained within a simple model assuming a relationship between the current density at the breakdown and the rate of pinhole formation.

Tunnel magnetoresistance in alumina, magnesia and composite tunnel barrier magnetic tunnel junctions

Abstract Using magnetron sputtering, we have prepared Co–Fe–B/tunnel barrier/Co–Fe–B magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia, and magnesia–alumina bilayer systems. The highest tunnel magnetoresistance ratios we found were 73% for alumina and 323% for magnesia-based tunnel junctions. Additionally, tunnel junctions with a unified layer stack were prepared for the three different barriers. In these systems, the tunnel magnetoresistance ratios at optimum annealing temperatures were found to be 65% for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The similar tunnel magnetoresistance ratios of the tunnel junctions containing alumina provide evidence that coherent tunneling is suppressed by the alumina layer in the composite tunnel barrier.

Magnon excitation and temperature dependent transport properties in magnetic tunnel junctions with Heusler compound electrodes

Magnetic tunnel junctions were prepared with the Heusler compounds Co2FeAl, Co2FeSi, and Co2MnSi as the soft magnetic electrode. The Co2MnSi electrodes had a multilayer design that used either the Co2FeAl or the Co2FeSi compound as a buffer material. Pinned Co-Fe was used as the hard reference electrode. The electronic transport characteristics were analyzed by tunneling spectroscopy. The dependence of sample properties on the buffer material was of interest, especially the gap in the minority density of states of the Heusler electrode. The temperature dependence of the transport properties was also investigated.

Dielectric breakdown and inelastic electron tunneling spectroscopy of top and bottom pinned Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions

The time dependent dielectric breakdown in Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions was investigated by voltage ramp experiments. The measurements were done for two types of junctions: one set of junctions had exchange biased (pinned) bottom electrodes and one set had exchange biased (pinned) top electrodes with an additional artificial ferrimagnet. We found a significant polarity dependence in the dielectric breakdown: top as well as bottom pinned tunnel junctions showed higher breakdown voltage when the top electrode was biased positively compared to negative bias. In contrast to this the differential resistance (dV/dI)−V spectra revealed an asymmetry for the top pinned junctions which was reversed in comparison to the bottom pinned system. This indicates that both asymmetries have different origins. Additionally the bottom pinned junctions showed in general slightly lower breakdown voltages and stronger magnon excitation in the inelastic electron tunneling (d2I/dV2)−V spectra than the top pinned j...

Transmission Electron Microscopy Analysis of Tunnel Magneto Resistance Elements with Amorphous CoFeB Electrodes and MgO Barrier

Magnetic tunnel-junctions consisting of CoFeB/ MgO/ CoFeB trilayers have been of great interest in research just recently. Due to their high magnetoresistance they are a promising candidate for the fabrication of spintorque MRAM devices. For future writing concepts like current-induced magnetic switching, magnetic tunnel-junctions (MTJs) with thin barriers are necessary to provide sufficient high current densities. In such elements the tunnel magneto resistance (TMR) is strongly dependent on the electron transmission at the metal/oxide interfaces. Therefore the quality of the interfaces is of great significance and has to be optimized on the nanoscale.