Manfred Rührig

Highly sensitive strain sensors based on magnetic tunneling junctions

Micrometer-sized highly sensitive strain sensors are presented. The sensors are based on magnetic tunneling junctions (MTJs) incorporating magnetostrictive free layers. The influence of mechanical strain upon the free layer is explained by a model taking into account the total free energy of the sensing layer. Those MTJ devices prepared in situ with magnetostrictive Fe50Co50 layers exhibit a tunneling magnetoresistance (TMR) ratio of 48%. The changes in strain Δe on the order of 0.4 parts per thousand (‰) result in resistance changes of 24%, which in turn leads to gauge factors [(ΔR/R)/Δe] on the order of 600, whereas gauge factors of 2–4 are typical for metal based, and 40–180 for piezoresistive semiconductor strain gauges.

Antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers

This work reports on the magnetic interlayer coupling between two amorphous CoFeB layers, separated by a thin Ru spacer. We observe an antiferromagnetic coupling which oscillates as a function of the Ru thickness x, with the second antiferromagnetic maximum found for x=1.0–1.1nm. We have studied the switching of a CoFeB∕Ru∕CoFeB trilayer for a Ru thickness of 1.1nm and found that the coercivity depends on the net magnetic moment, i.e., the thickness difference of the two CoFeB layers. The antiferromagnetic coupling is almost independent of the annealing temperatures up to 300°C while an annealing at 350°C reduces the coupling and increases the coercivity, indicating the onset of crystallization. Used as a soft electrode in a magnetic tunnel junction, a high tunneling magnetoresistance of about 50%, a well-defined plateau and a rectangular switching behavior is achieved.

Pressure sensor based on magnetic tunnel junctions

Magnetic tunnel junctions consisting of a magnetically stable reference layer plus a magnetostrictive sense layer separated by an insulating MgO tunnel barrier have been fabricated and characterized particularly with regard to their application as strain and pressure sensors. Using a four-point-bending apparatus it has been proven that the application of mechanical stress causes the magnetization of the sense layer to rotate, while simultaneously the resistance of the strained magnetic tunnel junction changes. So far gauge factors up to 840 have been reached in resistance versus strain measurements. It has been demonstrated that these magnetic tunnel junctions also work as highly sensitive pressure gauges when placed on bulk or surface micromachined membranes.

Characterization of embedded MgO/ferromagnet contacts for spin injection in silicon

In this work we present the structural and electrical characterization of sputter-deposited CoFe(B)/MgO/Si metal-insulator-semiconductor tunneling junctions for injection and detection of spin polarized current in silicon. The multilayers have been deposited in 700 nm deep trenches, patterned in thick SiO2 dielectric, on n- and p-doped wafers. The films inside the trenches are continuous with a correlated and low roughness. The MgO barrier grows amorphous without indication of pinholes. The dc and ac transport properties of the junctions were studied as a function of temperature and frequency. A relatively high interface trap density at the MgO/Si-interface is extracted from admittance spectra measurements. Transport is dominated by majority carriers in the case of n-doped and by minority carriers for the p-doped wafers. This leads to distinct rectification characteristics for the two wafer types, which would significantly influence the spin injection efficiency of the tunneling junctions.

Novel strain sensors based on magnetostrictive GMR/TMR structures

Summary form only given. While there has been considerable research devoted to the use of GMR (giant magnetoresistance) and TMR (tunnel magnetoresistance) layered structures-these studies have focused mainly on data storage applications (e.g., MRAM, Read-write heads) and magnetic field sensors. Comparatively few studies have been devoted to the exploitation of these materials in broader industrial applications for use such as stress, strain or pressure sensors. This study presents an investigation into various magnetic layer structures suitable for devices sensing mechanical responses such as stress, strain and pressure.

Interface sharpening in CoFeB magnetic tunnel junctions

We report grazing incidence x-ray scattering evidence for sharpening of the interface between amorphous Co60Fe20B20 and AlOx during in situ annealing below the Co60Fe20B20 crystallization temperature. Enhancement of the interference fringe amplitude in the specular scatter and the absence of changes in the diffuse scatter indicate that the sharpening is not a reduction in topological roughness but a reduction in the width of the chemical composition profile across the interface. The temperature at which the sharpening occurs corresponds to that at which a maximum is found in the tunneling magnetoresistance of magnetic tunnel junctions.

Exchange coupling between an amorphous ferromagnet and a crystalline antiferromagnet

We have investigated the exchange bias effect in bilayers of an amorphous ferromagnet (CoFeB) and a crystalline antiferromagnet (IrMn) in a top-pinned configuration. When the crystalline IrMn layer was deposited on top of the amorphous CoFeB layer, no exchange bias was observed. On insertion of a thin crystalline ferromagnetic layer of NiFe between the amorphous CoFeB and the crystalline IrMn, exchange coupling appeared and it was dependent on the thickness of the NiFe layer. An enhancement in the blocking temperature of the CoFeB/NiFe/IrMn layers was observed on increasing the thickness of the NiFe layer. These effects were directly correlated with the (111) texture in the antiferromagnetic phase of the IrMn layer, which developed progressively with increasing thickness of the NiFe layer. The blocking temperature was found to vary linearly with the intensity under the (111) IrMn X-ray diffraction peak. A NiFe interlayer can be used to introduce an additional source of anisotropy in a giant magnetoresista...

Micromagnetic analysis of thin‐film elements (invited)

The magnetization behavior of soft magnetic thin‐film elements is investigated in detail, based on a nanocrystalline iron‐Permalloy multilayer system. Advantages of this material for domain analysis are a smooth surface and a good magneto‐optical contrast due to a high saturation. Deviations from the mathematical model of an ideally soft magnetic two‐dimensional film are due to the presence of anisotropy, to the actual structure of domain walls, and to edge anisotropies due to stress relaxation. These phenomena are observed for elements of various shapes. The effects of different anisotropies are studied by applying an external stress onto the thick ceramic substrate. Particular features connected with hysteresis, such as regular and anomalous corner configurations and the transitions between asymmetric Bloch and Neel walls are investigated.

Magnetization reversal of sub-micron ferromagnetic tunnel junctions in external magnetic fields

Sub-micron sized magnetic tunnel junctions are fabricated by electron beam lithography. Magnetoresistance measurements were done at crossed easy- and hard-axis fields and the critical switching curves for 3 different sub-μm junctions are discussed. Single domain like switching according to the Stoner and Wohlfarth model can be achieved, but Neel coupling effects and AAF stray field effects have to be controlled.

Magnetic properties of antiferromagnetically coupled CoFeB/Ru/CoFeB

This work reports on the thermal stability of two amorphous CoFeB layers coupled antiferromagnetically via a thin Ru interlayer. The saturation field of the artificial ferrimagnet which is determined by the coupling, J, is almost independent on the annealing temperature up to more than 300 D C. An annealing at more than 325 D C significantly increases the coercivity, H-c, indicating the onset of crystallization. © 2004 Elsevier B.V. All rights reserved.