X. Portier

The formation of 360° domain walls in magnetic tunnel junction elements

The formation of 360° domain walls has been observed in exchange-biased magnetic tunnel junction elements using Lorentz transmission electron microscopy. These domain walls occur under certain circumstances and remain stable up to a high external field (∼150 Oe) compared to the value observed to achieve the antiparallel state of the free- and pinned-layer magnetizations (∼20 Oe). They have been found to play an important role during reversal to the parallel magnetization state, inducing a much lower switching field and a very different reversal mechanism.

Magnetization reversal processes in exchange-biased spin-valve structures

In this paper we provide an initial, qualitative description of the basic magnetization reversal processes that occur when a soft ferromagnetic layer is coupled to an antiferromagnet. We find that the magnetization reversal in the ferromagnetic layer adjacent to the antiferromagnet, i.e., the pinned layer, is dominated by thermal activation processes, We have developed, and report here, a model that accounts for seven distinct features of the magnetization curve of the pinned layer. The model is based upon the formation of domains in the antiferromagnetic layer whose growth is dominated by thermal activation processes. The thermal activation of these domain processes can result in shifts of the hysteresis loop in either direction, depending on the magnetic history of the sample and the rate of field sweep, In consequence, we call into question the current definition of the exchange field H/sub ex/ typically used to characterize such systems.

Magnetization reversal processes in exchange-biased systems

The magnetization reversal mechanism in sputtered bilayer films of NiFe coupled to a range of antiferromagnets has been studied using Lorentz microscopy and magnetic measurements. The reversal mechanism on the forward and recoil loops appears different and the results have been interpreted in terms of a recently published seven-point model. Reversal is controlled by the magnetic domain structure in the antiferromagnet. Time-dependent studies show that the reversal field for the NiFe layer decreases for both the forward and recoil loops, as the time for which the film is held above the saturation field of the NiFe layer increases. This can be explained by viscous rotation of the magnetization in thermally activated domains in the antiferromagnetic layer.

In-situ Magnetoresistance Measurements On Spin Valve Elements Combined With Lorentz Transmission Electron Microscopy

In order to correlate giant magnetoresistance with changes in magnetic domain structure, we have studied lithographically-defined spin-valve elements in-situ by Lorentz transmission electron microscopy, whilst simultaneously passing a controlled current through the element and applying a magnetic field. For a given current, the changes in magnetic domain structure can thus be correlated with changes in the resistance of the film. Spin valve structures with MnFe and MnNi pinning layers were examined land the technique proved particularly useful in identifying regions of the spin valve corrupted by corrosion of the pinning layer.

HREM Study Of Spin Valves With MnNi Pinning Layer

Four NiFe/Co/Cu/Co/NiFe/MnNi spin valves (SVs) with two different growth configurations have been investigated by conventional and high resolution electron microscopy (CTEM and HREM). Structures with the MnNi pinning layer above and below the ferromagnetic sandwich in both as-grown and annealed states have been studied. A correlation between their magnetic properties and their microstructure has been found. For each of these configurations, the structure of the MnNi pinning film is different and as a result, the exchange field (H/sub ex/) value changes. In addition, the coercivity (H/sub c/) of the sense layer seems to be related to its grain size and crystallographic texture.

HREM study of the `orange-peel’ effect in spin valves

Abstract Spin-valve microstructure has been studied by means of high-resolution electron microscopy. The investigation has focused on the waviness of the interfaces between the nonmagnetic and adjacent ferromagnetic layers in order to estimate the magnetostatic coupling termed the `orange peel’ effect. Numerical analysis of the high-resolution electron micrographs has enabled quantitative data on the interface waviness and thus on the magnetostatic coupling to be obtained.

Lorentz transmission electron microscopy on NiFe/Cu/Co/NiFe/MnNi active spin valve elements

In situ magnetoresistance measurements on lithographically defined spin-valve elements were performed by means of Lorentz transmission electron microscopy. The observation of a magnetic domain structure and the simultaneous magnetoresistance measurement by applying controlled field and controlled current have led to a clear correlation between giant magnetoresistance and changes in the magnetic domain structure. A study of the spin-valve behavior with the increase of the applied current value is also shown.

Microstructure and magnetic domain structure of Co/NiFe/MnNi systems

A Co/NiFe ferromagnetic bilayer has been exchange-coupled to MnNi and the resulting system has been annealed at different temperatures for several hours in a static magnetic field. The magnetic and microstructural properties have been studied as well as the effect of a Ta seed layer. The study has focused on the evolution of the exchange-bias field with that of MnNi structure which transforms from a nonmagnetic as-grown cubic phase to the antiferromagnetic tetragonal phase after annealing.

In-situ Lorentz microscopy studies of spin-valve structures

Lorentz electron microscopy, including a quantitative magnetisation-mapping technique, has been used to study the local magnetisation reversal mechanism in active spin-valve devices for correlation with the giant magnetoresistance and domain structure. Varying various parameters such as device size, applied current value and direction of easy-axis have been found to have an effect on the magnetisation reversal mechanism and on the GMR curve.

Temperature dependence of the reversal mechanism in spin-valve films

The thermal behavior of spin valves (SVs) and exchange-coupled films has been investigated by in situ experiments in a Lorentz microscope. In situ magnetizing combined with in situ heating experiments have been performed during observation of the magnetization reversal of SV materials. A clear demonstration of the thermal effect on the reversal of the free layer is shown as well as a decrease of the exchange-bias pinning with increasing temperature. This latter effect results in the switching of the pinned layer near the blocking temperature.