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

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.Comment: 3 pages,4 figures, 1 tabl

Temperature and bias voltage dependence of Co/Pd multilayer-based magnetic tunnel junctions with perpendicular magnetic anisotropy

Temperature- and bias voltage-dependent transport measurements of magnetic tunnel junctions (MTJs) with perpendicularly magnetized Co/Pd electrodes are presented. Magnetization measurements of the Co/Pd multilayers are performed to characterize the electrodes. The effects of the Co layer thickness in the Co/Pd bilayers, the annealing temperature, the Co thickness at the MgO barrier interface, and the number of bilayers on the tunneling magneto resistance (TMR) effect are investigated. TMR-ratios of about 11 % at room temperature and 18.5 % at 13 K are measured and two well-defined switching fields are observed. The results are compared to measurements of MTJs with Co-Fe-B electrodes and in-plane anisotropy

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

Tunneling spectroscopy of magnetic tunnel junctions

Drewello V. Tunneling spectroscopy of magnetic tunnel junctions. Bielefeld (Germany): Bielefeld University; 2010.In this thesis, magnetic tunnel junctions based on MgO barriers have been prepared and investigated. Different aspects were discussed, in order to understand the physical limitations to the TMR ratio of the MTJs and, hence, their performance as a basis of spintronic devices

Evidence for strong magnon contribution to the TMR temperature dependence in MgO based tunnel junctions

Drewello V, Schmalhorst J-M, Thomas A, Reiss G. Evidence for strong magnon contribution to the TMR temperature dependence in MgO based tunnel junctions. PHYSICAL REVIEW B. 2008;77(1): 014440.We have prepared MgO based magnetic tunnel junctions which show up to 143% tunneling magnetoresistance (TMR) ratio at room temperature and 205% at 12 K. This TMR temperature dependence is mainly caused by a strong temperature dependence in the antiparallel magnetic state, while in the parallel state the change of condunctance is small. We found that a modified version of the magnon excitation model may be applied to these MgO magnetic tunnel junctions. If the thermal smearing of the tunneling electron's energy is included it is possible to fit the temperature dependence. We will show the results for our data and we have also tested our model successfully on data from other publications

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

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

Drewello V, Ebke D, Schäfers M, Kugler Z, Reiss G, Thomas A. Magnon excitation and temperature dependent transport properties in magnetic tunnel junctions with Heusler compound electrodes. Journal of Applied Physics. 2012;111(7): 07C701

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

Khan AA, Schmalhorst J-M, Thomas A, Drewello V, Reiss G. Dielectric breakdown and inelastic electron tunneling spectroscopy of top and bottom pinned Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions. Journal of Applied Physics. 2009;105(8):083723.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 (d(2)I/dV(2))-V spectra than the top pinned junctions. Possible reasons for these correlations are discussed