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.

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 degrees 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 Delta(pi) gap and the Delta(sigma) gap. (C) 2012 American Institute of Physics. [doi:10.1063/1.3671792]

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. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3670972]

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.

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.

The bulk magnetic moment and the element specific magnetic moment of Co 2 FeAl 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 Co 2 FeAl and Co-Fe electrodes.

MgO-based magnetic tunnel junctions with up to 230% tunnel magnetoresistance ratio at room temperature and up to 345% at 13 K are prepared. The lower electrode is either exchange-biased or free, while the top electrode is free or an exchanged-biased artificial ferrimagnet, respectively. Additionally, a pseudo-spin-valve (hard-soft switching) design with two unpinned electrodes is used. Inelastic electron-tunneling spectra for each of these systems show a strong variation in the zero-bias anomaly with a reduced peak for some of the junctions. At voltages around 200 mV additional structures are found, which are not known from junctions with lower magneto resistance, such as alumina-based junctions. We discuss the spectra for the different electrode types and compare our findings with respect to barrier material and magnetoresistance ratio.

We present tunnel magnetoresistance values of up to 147% at room temperature and 273% at 13K for MgO-based magnetic tunnel junctions with Co2FeAl and Co–Fe electrodes. The magnetic moment and coercive field were examined as a function of the annealing temperature by alternating gradient magnetometer investigations. This is compared with X-ray diffraction studies of the same samples and all results are contrasted to similar layer stacks based on the Heusler compound Co2MnSi.

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.

The transport properties of Co2MnSi/AlOx/Co-Fe magnetic tunnel junctions showing a tunnel magnetorestistance of 95% at low temperatures are discussed with respect to temperature-dependent magnetic moments at the Co2MnSi/AlOx interface and electronic band structure effects. These junctions show a considerably larger temperature and bias voltage dependence of the tunneling magnetoresistance compared to Co-Fe-B/AlOx/Co-Fe-B junctions, although the effective spin polarization of Co2MnSi (66%) is larger than Co-Fe-B (60%). Especially, the tunnel magnetoresistance of the Co2MnSi based junctions becomes inverse for large bias voltages. With increasing atomic disorder of the interfacial Co2MnSi its magnetic moments decrease and show a stronger temperature dependence. Even for the best atomic ordering achieved the corresponding spin-wave parameters of Mn and Co at the Co2MnSi/AlOx interface are significantly larger than expected for Co2MnSi bulk and also larger than the spin-wave parameters of Co and Fe at a Co-Fe-B/AlOx interface. The influence of enhanced interfacial magnon excitation in the Co2MnSi/AlOx/Co-Fe junctions on their transport properties will be discussed as well as possible origins for the negative tunnel magnetoresistance at high bias voltages.

In this study, we report the relationship between interface roughness, texture and the exchange bias and the interlayer‐Néel coupling fields of the spin‐valve magnetic tunnel junction (SV‐MTJ). SV‐MTJs with the structure: Si(100)/SiO2/buffer/IrMn/CoFeB/AlOx /NiFe/Ta were deposited on two buffers: Cu and Ta/Cu/Ta/Cu, and next the samples were annealed in vacuum and in a magnetic field. XRD analysis reveals that the texture degree of MTJ with Cu buffer is significantly lower than that of the sample with Ta/Cu/Ta/Cu buffer. The enhancement of IrMn and CoFeB texture leads to a higher amplitude of roughness and an increase of exchange bias and Néel coupling fields. The comparison of junctions with CoFe and CoFeB bottom electrodes shows that addition of 5% B in a CoFe layer makes the surface of the pinned layer smoother. This leads to a lower Néel coupling field, which is desirable for application of MTJ in MRAM technology. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)