Growth and structure of polycrystalline Cr/Au multilayered thin films

Brückl H, Vancea J, Lecheler R, Reiss G, Hoffmann H. Growth and structure of polycrystalline Cr/Au multilayered thin films. Thin solid films. 1994;250(1-2):56-60.Metallic multilayered thin films have recently been investigated due to their new magnetic and transport properties. The interest here is focussed on the characterization of the interfaces between the layers. The analysis of growth and structure of polycrystalline Cr/Au multilayers is accomplished by two complementary techniques: in situ ultrahigh vacuum scanning tunnelling microscopy and ex situ transmission electron microscopy. The combination of these powerful methods provides detailed information about structural characteristics such as crystallite size, surface roughness and crystallographic orientation. Moreover, conclusions can be drawn on the atomic arrangement and growth mechanism at the Cr-Au interface. The results are supported by semiempirical and theoretical expectations

Simulation of magnetic active polymers for versatile microfluidic devices

We propose to use a compound of magnetic nanoparticles (20-100 nm) embedded in a flexible polymer (Polydimethylsiloxane PDMS) to filter circulating tumor cells (CTCs). The analysis of CTCs is an emerging tool for cancer biology research and clinical cancer management including the detection, diagnosis and monitoring of cancer. The combination of experiments and simulations lead to a versatile microfluidic lab-on-chip device. Simulations are essential to understand the influence of the embedded nanoparticles in the elastic PDMS when applying a magnetic gradient field. It combines finite element calculations of the polymer, magnetic simulations of the embedded nanoparticles and the fluid dynamic calculations of blood plasma and blood cells. With the use of magnetic active polymers a wide range of tunable microfluidic structures can be created. The method can help to increase the yield of needed isolated CTCs

FORC Diagram Features of Co Particles due to Reversal by Domain Nucleation

First Order Reversal Curve (FORC) diagrams are a popular tool in geophysics and materials science for the characterization of magnetic particles of natural and synthetic origin. However, there is still a lot of controversy about the rigorous interpretation of the origin of certain features in a FORC diagram. In this study, we analyze FORCs computed by micromagnetic simulations of Co cubes with dimensions of 50, 100 and 150 nm and uniaxial magnetocrystalline anisotropy. For the larger cubes we observe the formation of a stable two-domain state. The nucleation of a reversed domain and its subsequent annihilation are clearly visible as separate peaks in the FORC diagram. They spread out along the coordinate axis in the FORC diagram, which is associated with the bias field $H_U$ of a Preisach hysteron. Based on our findings, we state that a FORC diagram peak spreading along the $H_U$ axis can have its origin in the step-wise magnetization reversal driven by nucleation of domains in a single particle. This means that we have identified another mechanism apart from the well-known magnetostatic interaction between a set of particles that leads to features in the FORC diagram extending along the $H_U$-axis. Our study demonstrates that if FORCs shall be used as a quantitative tool to assess the microstructure of samples containing magnetic material, more information from other methods will be required to identify the correct physical mechanism by which a certain "fingerprint" in a FORC diagram is produced

The influence of surface roughness on electronic transport in thin films

Reiss G, Brückl H. The influence of surface roughness on electronic transport in thin films. Surface science. 1992;269-270:772-776.In thin films, the structure of the surfaces considerably influences the transport of conduction electrons. For mesoscopic roughnesses in the range of a few nm, this is due to the varying film thickness, which gives rise to a spatially fluctuating conductance. Moreover, microscopic roughnesses can contribute to the scattering of the electrons and therefore additionally enhance the thin-film resistivity. For a quantitative understanding of the transport in these systems, a detailed investigation of the surface roughness combined with measurements of the electronic properties are necessary. Here, we discuss STM imaging of various metal films and the application of these results to the interpretation of electronic thin-film properties. Provided reasonable resolution of STM in the nm range, a good correspondence of STM results with the electrical behaviour of growing metal films can be established. Furthermore, a detailed two-dimensional analysis allows for a calculation of the potential on current-carrying thin films. On the other hand, this method supplies reliable values for the electronic transport parameters

Riesenmagnetowiderstand - Transfer in die Anwendung

Brückl H, Hütten A, Reiss G. Riesenmagnetowiderstand - Transfer in die Anwendung. Phys. Blätter. 1998;54(4):339-341

Near-field magneto-optical microscopy in collection and illumination mode

Matthes F, Brückl H, Reiss G. Near-field magneto-optical microscopy in collection and illumination mode. Ultramicroscopy. 1998;71(1-4):243-248.We present a new design and the application of a near-field magneto-optical microscope, which combines the operation in the collection (PSTM) or illumination mode (NSOM) and the simultaneous application of an external magnetic field. Magnetic information is obtained via polarisation contrast showing the relative orientation of the magnetisation in magnetic thin films and microstructures due to the Faraday effect in transmission. Controlled by the shear-force technique the topography is recorded simultaneously with the magneto-optical signal. For the preparation of the aperture at the fibre tips a new, simple metal-coating technique is additionally introduced. For the first time near-field magneto-optical images obtained in the collection mode under static magnetic field conditions are presented. We have compared the illumination and collection mode with respect to near-field magneto-optical imaging of yttrium iron garnet films. (C) 1998 Elsevier Science B.V. All rights reserved

In situ monitoring of GMR and M(H) evolution during Co/Cu multilayers growth

Lucinski T, Reiss G, Brückl H. In situ monitoring of GMR and M(H) evolution during Co/Cu multilayers growth. In: Journal of Magnetism and Magnetic Materials. JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS. Vol 193. ELSEVIER SCIENCE BV; 1999: 484-487.The field dependencies of resistance and magnetic moment of the Co/Cu multilayers manifesting the GMR effect have been simultaneously examined in situ, during deposition process. The combination of these measurements with in situ thickness dependent resistance measurements allowed us to interpret the changes of the magnetic hysteresis loops. It has been shown that the all measured quantities can be presumably affected by the surface anisotropy. (C) 1999 Elsevier Science B.V. All rights reserved

Hard magnetic CoCr layer in ferromagnetic tunnel junctions

Justus M, Brückl H, Reiss G. Hard magnetic CoCr layer in ferromagnetic tunnel junctions. In: Journal of Magnetism and Magnetic Materials. JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS. Vol 240. ELSEVIER SCIENCE BV; 2002: 212-214.In magnetic tunnel junctions a highly spin-polarizing layer is usually exchange biased by an antiferromagnetic layer, an artificial antiferromagnetic layer system or a combination of both, while the magnetically soft layer is free to rotate. The use of a single layer of a hard magnetic material is rarely investigated up to now. In this paper, we present the electric and magnetic properties of tunnel junctions with a hard magnetic Co83Cr17 layer. The soft magnetic electrode consists of either a single Co layer or a Co/Ni80Fe20 bilayer. The magnetic anisotropy and coercive field H-subset of. of the CoCr layer depend on its thickness and the kind of the bottom layer (Cu or Ta) and can vary from H-subset of = 50-700 Oe. It is found that a thin Co cap layer also influences the hysteretic behavior. Furthermore, only small changes after annealing up to 450 C promise a high thermal stability for the application in magnetic tunnel junctions. Measurements of the tunnel magnetoresistance on large area junctions, however, show a strong magnetic coupling of the hard and soft electrodes. (C) 2002 Elsevier Science B.V. All rights reserved

Microstructure and texture of Ir-Mn based magnetic tunnel junctions

Kanak J, Stobiecki T, Reiss G, Brückl H. Microstructure and texture of Ir-Mn based magnetic tunnel junctions. ARCHIVES OF METALLURGY AND MATERIALS. 2005;50(2):295-301.Magnetic tunnel junctions (MTJs) with the structure: substrate Si(100)/SiOx 47nm/ system of seed-buffer layers /IrMn 12nm/CoFe 15nm/AlOx, 1.4nm/NiFe 3nm/Ta 5nm were prepared with four different buffers: (a) Cu 25nm, (b) Ta 5nm/Cu 25nm, (c) Ta 5nm/Cu 25nm/Ta 5nm/Cu 5nm and (d) Ta 5nm/Cu 25nm/Ta 5nm/NiFe 2nm/Cu 5nm in order to enhance crystal texture of the MTJs. The annealed in vacuum at 275 degrees C junctions were characterized by XRD theta-2 theta-scans, rocking curve (omega-scans) and pole figures, in order to establish the correlation between texture and magnetic exchange bias coupling of IrMn/CoFe. The texture degree in the stack of MTJ depends on material, which was used for the buffer layers, and sequence of the layers. The strongest texture has been obtained if the seed layer of Ta was used (buffer (b)). The multilayer stack is textured in columnar-like fashion, which produces roughness. It was found, from the analysis of magnetic hysteresis loops and rocking curves of the CoFe layer, that the exchange bias and coercivity fields of CoFe pinned layer increase about two times in the case of using strong textured seed-buffer system (b). The design of seed-buffer layers allows to optimize the exchange bias coupling in magnetoelectronics devices