Oliver Schebaum

Dielectric breakdown in Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junction

The time-dependent dielectric breakdown has been investigated in Co–Fe–B/MgO/Co–Fe–B junctions by voltage ramp experiments and focused on its dependence on the barrier thickness, junction area, polarity of the applied voltage, ramp speed, and annealing temperature. The results suggest that the breakdown voltage strongly depends both on the polarity of the applied voltage and the annealing temperature. Magnetic tunnel junctions (MTJs) with positive bias on the top electrode show higher breakdown voltage than MTJs with negative bias. We found that there is a significant decrease in the breakdown voltage when the annealing temperature is increased above 350 °C.

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.

Spin-polarized tunneling in MgO-based tunnel junctions with superconducting electrodes

We prepared magnetic tunnel junctions with one ferromagnetic and one superconducting Al-Si electrode. Pure cobalt electrodes were compared with a Co-Fe-B alloy and the Heusler compound Co2FeAl. The polarization of the tunneling electrons was determined using the Maki-Fulde model and is discussed along with the spin-orbit scattering and the total pair-breaking parameters. The junctions were post-annealed at different temperatures to investigate the symmetry filtering mechanism responsible for the giant tunneling magnetoresistance ratios in Co-Fe-B/MgO/Co-Fe-B junctions.

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.

Direct measurement of the spin polarization of Co2FeAl in combination with MgO tunnel barriers

It is a truth universally acknowledged that a Heusler compound in possession of a good order must be in want of a high spin polarization. In the present work, we investigated the spin polarization of the Heusler compound Co2FeAl by spin polarized tunneling through a MgO barrier into a superconducting Al–Si electrode. The measured spin polarization of P=55% is in good agreement with the previously obtained tunnel magnetoresistance values and compared to the data by other groups.

X-Ray Absorption and Magnetic Circular Dichroism Studies of Co

The bulk magnetic moment and the element specific magnetic moment of Co2FeAl thin films were examined as a function of annealing temperature by alternating gradient magnetometer (AGM) and X-ray absorption spectroscopy (XAS)/X-ray magnetic circular dichroism (XMCD), respectively. A high magnetic moment can be achieved for all annealing temperatures and the predicted bulk and interface magnetic moment of about 5 ¿B are reached via heating. We will also present tunnel magnetoresistance (TMR) values of up to 153% at room temperature and 260% at 13 K for MgO based magnetic tunnel junctions (MTJs) with Co2FeAl and Co-Fe electrodes.

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The magnetic anisotropy and transport properties of superconducting MgB2 thin films on MgO (100) substrates were studied. The films were prepared by rf/dc-magnetron cosputtering and with in situ annealing temperatures of 650 °C. The film orientation was measured by X-ray diffractometry, which revealed a c-axis orientation of the MgB2 films. The critical onset temperature without field cooling is 15.5 K. We found a critical field of 14.73 T parallel to the film plane and 10.79 T perpendicular to the film plane from transport measurements of the dependence of the applied magnetic field. Differential conductance measurements of a lateral MgB2/Fe/MgB2 junction show the Δπ gap and the Δσ gap.