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Adhesion and spreading of cells strongly depend on the properties of the underlying surface, which has significant consequences in long-term cell behavior adaption. This relationship is important for the understanding of both biological functions and their bioactivity in disease-related applications. Employing our magnetic lab-on-a-chip system, we present magnetoresistive-based real-time and label-free detection of cellular phagocytosis behavior during their spreading process on particle-immobilized sensor surfaces. Cell spreading experiments carried out on particle-free and particle-modified surfaces reveal a delay in spreading rate after an elapsed time of about 2.2h for particle-modified surfaces due to contemporaneous cell membrane loss by particle phagocytosis. Our associated magnetoresistive measurements show a high uptake rate at early stages of cell spreading, which decreases steadily until it reaches saturation after an average elapsed time of about 100 min. The corresponding cellular average uptake rate during the entire cell spreading process accounts for three particles per minute. This result represents a four times higher phagocytosis efficiency compared to uptake experiments carried out for confluently grown cells, in which case cell spreading is already finished and, thus, excluded. Furthermore, other dynamic cell-surface interactions at nano-scale level such as cell migration or the dynamics of cell attachment and detachment are also addressable by our magnetic lab-on-a-chip approach., (Copyright © 2012 Elsevier B.V. All rights reserved.)

The intrinsic dc conductivity of long, individual lambda phage dsDNA molecules has been investigated by ultrasensitive low current-voltage-spectroscopy (IV) under ambient conditions and controlled low humidity inert gas atmosphere on microfabricated metal-insulator-metal gap structures. We found a strong dependence of the measured conductivity on the apparent humidity, which we attribute to capillary condensation of water to the immobilized DNA molecules, giving rise to additional ionic currents. Additional IV-spectroscopy experiments under controlled argon atmosphere always revealed a significant drop in electrical conductivity to 4 x 10(-15)AV(-1)microm(-1), indicating almost no considerable contribution of electrical long range charge transport.

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 junctions. Possible reasons for these correlations are discussed. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions with two MgO barriers separated by a free CoFeB middle electrode have been investigated. As compared to single barrier junctions, the magnetoresistance increase due to annealing was smaller for these systems. The Néel coupling induced hysteresis shift of the middle electrode decreased, while coercivity and exchange bias field of the pinned ferromagnetic layers increased by increasing annealing temperature. For decreasing thickness of the middle electrode we observed that the contribution of the magnetoresistance of the upper barrier decreases probably due to the islandlike growth mode of CoFeB on MgO. Besides we found an increasing hysteresis shift of the middle electrode and additional dips in the major loops caused by magnetic interactions of the free middle electrode with the top pinned layer. [ABSTRACT FROM AUTHOR]

The transport properties of magnetic tunnel junctions with different (110)-textured Heusler alloy electrodes such as Co2MnSi, Co2FeSi or Co2Mn0.5Fe0.5Si, AlOx barrier, and Co–Fe counterelectrode are investigated. The bandstructure of Co2Mn1-xFexSi is predicted to show a systematic shift in the position of the Fermi energy EF through the gap in the minority density of states while the composition changes from Co2MnSi toward Co2FeSi. Although this shift is indirectly observed by x-ray photoemission spectroscopy, all junctions show a large spin polarization of around 70% at the Heusler alloy/Al–O interface and are characterized by a very similar temperature and bias voltage dependence of the tunnel magnetoresistance. This suggests that these transport properties of these junctions are dominated by inelastic excitations and not by the electronic bandstructure. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (Mn[sub 83]Ir[sub 17]/Co[sub 70]Fe[sub 30]/AlO[sub x]/Ni[sub 80]Fe[sub 20]) were bombarded by 10–20 keV He[sup +] ions in an applied magnetic field to manipulate the exchange bias of the magnetically hard electrode. The tunneling magnetoresistance of the bombarded junctions is up to 37.8% for ion doses high enough to ensure a well defined exchange bias. This should allow the preparation of high quality tunnel junctions with magnetic micropatterned hard electrodes. For very high ion doses the tunneling magnetoresistance starts to decrease, whereas the area resistance product increases. Possible explanations based on structural alterations of the tunneling barrier are discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The temperature stability of magnetic tunnel junctions is an important requirement for the fabrication of magnetic memory devices and the integration in the semiconductor process technology. We have investigated the temperature evolution of the tunneling magnetoresistance (TMR) and the structural properties by isochronal annealing experiments up to 750 K. The magnetically hard electrode of the junction consists of an artificial antiferromagnet Co/Cu/Co, the soft electrode of a Co/Fe bilayer. The tunnel barriers are formed by plasma oxidized Al. The tunnel junctions have TMR signals up to 22% at room temperature. Besides a small increase of the TMR signal after annealing up to 480 K, a first decrease at 530 K and a breakdown beyond 600 K are found. This behavior can be attributed to structural changes of the junctions. Auger depth profiles show an unaltered Al[sub 2]O[sub 3] barrier up to 600 K, but the beginning of interdiffusion processes within the magnetic electrodes already at 540 K and above. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two kinds of magnetic tunnel junctions with Co2FeSi electrodes are compared. Using Co2FeSi single layers a tunnel magnetoresistance of 52% is reached, whereas the magnetization of the Co2FeSi is only 75% of the expected value. By using [Co2MnSi/Co2FeSi]x10 multilayer electrodes the magnetoresistance can be increased to 114% and the full bulk magnetization is reached. All junctions show an inverse tunnel magnetoresistance, when the electrons are tunneling from the Co–Fe into the Heusler compound electrode. This results from a special band structure feature of full Heusler compounds, which is robust even for atomic disorder in the films. [ABSTRACT FROM AUTHOR]

Spintronics needs half-metallic materials implemented in technologically relevant devices. We prepare Co2MnSi/AlOx/Co7Fe3 junctions showing a tunneling magnetoresistance of 94.6% at 1 mV and 20 K. Their inelastic electron tunneling spectra at 20 K show typical magnon and phonon excitations in the electrode and the barrier and an additional shoulder around -22 mV not observed in Co7Fe3/AlOx/Ni81Fe19 reference junctions. Furthermore, the bias voltage and temperature dependence of the tunneling magnetoresistance is considerably larger than for the reference junctions. The transport properties are discussed with respect to a variety of current contributions associated with the structural and magnetic properties of the Co2MnSi/AlOx interface. [ABSTRACT FROM AUTHOR]

As a consequence of the growing theoretical predictions of 100% spin-polarized half- and full-Heusler compounds over the past six years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated Co2MnSi, as a representative of the full-Heusler compound family, as one magnetic electrode into magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co2MnSi at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer, subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co2MnSi, and (3) homogenizes the AlOx barrier. It is shown that the magnetic switching of the ferromagnetic electrodes is well controlled from room temperature down to 10 K. The resulting tunnel magnetoresistance-effect amplitude of the Co2MnSi containing magnetic tunnel junctions has been determined as a function of temperature and the spin polarization of the Co2MnSi Heusler compound has been estimated to be 61% at 10 K. Thus, the spin polarization of the Co2MnSi layer at 10K exceeds that of conventional transition metals. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions usually consist of different layer stacks at the two sides of the tunneling barrier. The exposure of these junctions to high temperatures thus can induce large asymmetries in the electronic potential with respect to the barrier. Using the example of Co/Cu/Co/Al[sub 2]O[sub 3]/Co tunnel junctions, we show that the measured current/voltage characteristics develop a large asymmetry upon annealing at a temperature larger than 230 °C. This is accompanied by a Cu enrichment at one side of the barrier and cannot be explained by simply using the work function differences between the electrode materials. Thus, a Cu-Al[sub 2]O[sub 3] intermixing zone at the barrier interface must be taken into account, which leads to an asymmetric step-like barrier shape. The interpretation is supported by numerical evaluation of model barriers which reproduce the experimental asymmetries if an intermixing zone of only 0.2 nm thickness is assumed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The local topographic and conducting properties of ultra-thin MgO films with polycrystalline Ru and amorphous CoFeB as lower electrodes were investigated. The local conductance and topography of the ultra-thin MgO films (from 0.5 to 1.0nm) were simultaneously measured with a modified conducting atomic force microscope (CAFM). The imaging force was carefully chosen in order to avoid structural damages in the insulating layers. The promising results include the decrease of the density of hotspots with large conductance with the thickness of MgO and show that an insulating barrier is obtained at 1nm thickness. [Copyright &y& Elsevier]

Abstract: It is demonstrated that the atomic ordering of Co2FeSi can significantly be improved in terms of magnetization and tunnel magnetoresistance using multilayered {Co2MnSi5nm/Co2FeSi5nm}10 full-Heusler magnetic electrodes in magnetic tunnel junctions. [Copyright &y& Elsevier]