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We have investigated perpendicular transport properties in three types of metallic and metal/insulator magnetic multilayered systems : i) Antiferromagnetically coupled metallic multilayers, ii) Magnetic tunnel junctions, and iii) A new magnetic field sensor named "spin-valve transistor". The feasability of using perpendicular transport in making field sensors has been assessed. Among the various systems investigated, the most promising for applications are the magnetic tunnel junctions and the spin-valve transistor. At the end of the project, macroscopic and microscopic magnetic tunnel junctions showing MR amplitude of more than 15% can be reproducibly prepared. Concerning the spin-valve transistor, changes in collector current of more than 300% by application of a magnetic field has been observed. A technique of direct bonding under ultrahigh vacuum has been developed. This allows to prepare spin-valve transistors with high gain. These transistors operate at room temperature. Regarding perpendicular transport in metallic multilayers, very interesting results have been obtained from a fundamental point of view, especially concerning the angular variation of the CPP-GMR. The CPP transport properties of pillar multilayered structures have been investigated to assess the potentiality of these systems for making magnetic field sensors. Unfortunately, a persistent difficulty has been encountered with the too large resistance of the interconnecting leads. This in-series lead-resistance significantly reduces the output magnetoresistance of these devices. As a result, the SNR of these pillar structures is lower than for conventional CIP giant magnetoresistance devices.
In 1988, the discovery of giant magnetoresistance (GMR) in (Fe 10Angstroms/Cr 9Angstroms)20 multilayers launched a considerable activity on the in-plane transport properties of magnetic multilayers. In contrast, the transport properties in the direction perpendicular to the interfaces which constitute the aim of the present project, have remained almost unexplored because of great technological difficulties. The motivation of this work is that much more dramatic magnetic effect on the transport properties are expected in the perpendicular versus in-plane geometry such as for instance GMR one to two orders of magnitude larger than observed so far with in-plane current.

This project associates five group (3 from research centres, 1 from university, 1 from industry) with complementary experience in different aspects of structural magnetic and transport properties of multilayers. The interest in this project is both for fundamental research and applications in magnetic flux sensors.

The project focuses on two types of systems : metallic multilayers consisting of an alternation of magnetic and non-magnetic metallic layers and multilayers comprising of ferromagnetic layers with insulating spacers.

For metallic multilayers, we plan to characterise the perpendicular GMR of various (Fe/Cr) type multilayers and (NiFe/Cu/NiFe/FeMn) type spin-valve sandwiches and search for extremely large GMR in these systems. In a technology for elaboration of magnetoresistive devices used as magnetic flux sensors based on the perpendicular GMR. For multilayers with insulating barriers we plan to acquire the ability to grow good insulating junctions between ferromagnetic films (R = +/- 1k * for area * 0.1mm2) and explore the possibility of GMR in these type of multilayers.

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Commissariat à l'Energie Atomique (CEA)
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Centre d'Études de Grenoble 17 avenue des Martyrs
38041 Grenoble

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