Joachim Wecker

Remanence enhancement due to exchange coupling in multilayers of hard- and softmagnetic phases

Nd-Fe-B/Fe/Nd-Fe-B trilayers with individual layer thicknesses between 10 and 100 nm have been grown by sputter deposition. The aim of our study is to investigate remanence enhancement due to exchange coupling between the soft- and hard-magnetic layers as a function of the Fe-interlayer thickness d. It is shown that for decreasing d the remanence of the trilayer system is significantly increased due to exchange coupling. The coercivity of the trilayer is also increased for d<40 nm which can be explained with a higher nucleation field for magnetization reversal.

Magnetoresistance in bulk Cu-Co based alloys

An enhanced magnetoresistance of up to 11% at room temperature is found in granular Cu/sub 1-x/Co/sub x/ (0.1 >

Oxide thickness mapping of ultrathin Al2O3 at nanometer scale with conducting atomic force microscopy

In this work, we introduce conducting atomic force microscopy (C-AFM) for the quantitative electrical characterization of ultrathin Al2O3 films on a nanometer scale length. By applying a voltage between the AFM tip and the conductive Co substrate direct tunneling currents in the sub pA range are measured simultaneously to the oxide surface topography. From the microscopic I–V characteristics the local oxide thickness can be obtained with an accuracy of 0.03 nm. A conversion scheme was developed, which allows the calculation of three-dimensional maps of the local electrical oxide thickness with sub-angstrom thickness resolution and nanometer lateral resolution from the tunneling current images. Local tunneling current variations of up to three decades are correlated with the topography and local variations of the electrical oxide thickness of only a few angstroms.In this work, we introduce conducting atomic force microscopy (C-AFM) for the quantitative electrical characterization of ultrathin Al2O3 films on a nanometer scale length. By applying a voltage between the AFM tip and the conductive Co substrate direct tunneling currents in the sub pA range are measured simultaneously to the oxide surface topography. From the microscopic I–V characteristics the local oxide thickness can be obtained with an accuracy of 0.03 nm. A conversion scheme was developed, which allows the calculation of three-dimensional maps of the local electrical oxide thickness with sub-angstrom thickness resolution and nanometer lateral resolution from the tunneling current images. Local tunneling current variations of up to three decades are correlated with the topography and local variations of the electrical oxide thickness of only a few angstroms.

Contactless current detection with GMR sensors based on an artificial antiferromagnet (AAF) subsystem

The advantages of the newly developed giant magneto-resistive (GMR) sensor scheme based on artificial antiferromagnetic (AAF) subsystem which was already demonstrated in angle detection, also promises a successful application in contactless current sensing. Wheatstone bridge structures were formed by combining sensors with opposite response characteristics. The sensor linearity was improved by an additional bias field influencing the slope and the maximum range of the signal curve. In our contactless sensing device, currents up to 40 A were measured reliably. Depending on the measuring range, excess currents up to 10 times of the nominal range do not change the sensitivity and bridge offset of the sensor.

Sandwiches mit riesigem Magnetwiderstand: Magnetoelektronische Bauelemente

Magnetoresistive Bauelemente verandern in einem auseren Magnetfeld ihren elektrischen Widerstand. Zwei Effekte ermoglichen in solchen Dunnschicht-Sandwiches sehr hohe Widerstandsanderungen und damit grose Empfindlichkeiten: der Riesenmagnetwiderstand (GMR)und der Tunnelmagnetwiderstand (TMR). In beiden Fallen schaltet das Magnetfeld die Magnetisierung einer weichmagnetischen Sensorschicht relativ zu der einer hartmagnetischen Referenzschicht um. Es andert so den elektrischen Widerstand des Systems. GMR-Systeme variieren ihren Magnetwiderstand um bis zu 20 %, TMR-Systeme bis zu 50 %. Das liegt gut eine Grosenordnung uber den alteren, einschichtigen Permalloy-Systemen. GMR-Systeme sind bereits in Lesekopfen von Festplatten etabliert. Beide Effekte konnten Speicherchips und Sensortechnik revolutionieren.

Magnetic phase transition and magnetocrystalline anisotropy of Sm2Fe17CxNy

Alternating current (ac) susceptibility and high magnetic field measurements were performed to investigate the magnetic phase transitions and the magnetic anisotropy fields in Sm2Fe17CxNy with x=0, 0.4, 0.7, and 0.9. An unidentified magnetic phase transition is observed in all Sm2Fe17CxNy compounds, which is neither due to a spin reorientation nor due to a first‐order magnetization process (FOMP) transition. Samples of Sm2Fe17CxNy (x≳0) have even higher anisotropy fields than are found in Sm2Fe17Ny.

Polymer light-emitting devices for large-area lighting

OLEDs gain more and more interest in the field of lighting applications. The OLED technology provides striking advantages and covers completely new application fields offering a new freedom in design for next generation lighting. Large area OLEDs might act as a 2-dimensional light source which is thin, flat and lightweight generating diffuse, nonglaring illumination. In the first part of our report we investigate small scale inhomogeneities of polymer based OLEDs. Devices were monitored during operation by taking pictures of the active area at constant periods of time. These pictures were analyzed by a software tool with respect to the occurrence and evolution of defects. Initially induced inhomogeneities are growing and dominate the performance with increasing operation time. Within the error margin of the setup no additional spots are generated during operation. The voltage drop inside the ITO anode due to a high resistivity plays an important role for the brightness homogeneity of large area devices. The voltage drop causes a brightness fall-off towards the center of the device. It is maintaining with increasing average current density and luminance, respectively. At a brightness of 1000cd/m2 the deviation at the center exceeds 30%. The homogeneity of luminance is improved by incorporation of additional metal lines on the anode layer. The best results were achieved with 200nm thick aluminum structures with a pitch of 1mm and a width of 60μm of each line. At an average current density of 45mA/cm2 the decay towards the center of the device is only half of the decrease without any additional metallization.

Angular sensor using tunneling magnetoresistive junctions with an artificial antiferromagnet reference electrode and improved thermal stability

Magnetic tunneling junctions (MTJs) are fabricated using CoFe-Ru-CoFe artificial antiferromagnet (AAF) sandwiches as a hard-magnetic reference layer and plasma-oxidized aluminum as a tunnel barrier. Tailoring the magnetic properties of the artificial antiferromagnet reference layer allows an on-chip magnetization (initialization) of individual junctions, which makes it possible to build monolithic bridges in a Wheatstone arrangement without multiple mask process steps or on-chip heating elements. The functionality of an angular field sensor based on this concept is demonstrated in detail. The thermal stability of such a sensor is investigated and the limitations of the concept are discussed.

Hysteresis modeling of tunneling magnetoresistance strain sensor elements

Utilizing the inverse magnetostriction effect, magnetic tunneling junction (MTJ) elements have been demonstrated to also be useful as highly sensitive stress or strain transducers. The prediction of the stress dependent hysteresis of the tunneling magnetoresistance R is done by the energetic model (EM), minimizing the total magnetization work which consists of the Zeeman energy density, of magnetocrystalline and stress anisotropy of the magnetization rotation processes, and of reversible and irreversible contributions of domain wall displacements. Furthermore, the law of approach to saturation and the initial susceptibility relate the parameters of the EM to the effective anisotropy energy which has to be overcome for a magnetization reversal. The calculated R(H) curves are in good aggreement with the measurements, allowing several predictions of the MTJ parameters as magnetostriction constant, stress (strain) sensitivity and hysteresis, and misalignment of the easy axis.

Influence of a magnetic seed line on the switching behaviour of submicrometre sized magnetic tunnel junctions

Magnetic tunnel junctions in the sub-micrometre range have been patterned by electron beam lithography. The hard reference electrode is composed of an artificial antiferromagnetic subsystem while the soft magnetic detection layer is Permalloy (NiFe). The magnetic switching characteristic is studied for various seed layers. For a polycrystalline iron seed line the magnetoresistance switching curve is irregular and not well behaved, while for a nonmagnetic seed layer (Ru) the reproducibility is good and the switching can be related to the shape of the element. We discuss possible coupling mechanisms and show that the magnetization reversal of the soft layer is mainly driven by the large stray fields of moving Neel-type domain walls in the Fe seed layer.