Magnetoelectronics

Ultra thin layers are the base for the development of magnetoelectronic sensors and memory chips with high potential for space applications. Such elements are based on magnetic resistance effects (e.g. Giant

44 see competence center „ultra thin functional layers" (www.nanotechnology.de)

with a high potential in space applications

Magneto Resistance, GMR), which occur in magnetic multilayer systems. For the production of such multilayers a controlled deposition of extremely thin metal and insulator layers (monolayer thickness approx. 1 nm) is necessary. The GMR effect, which has already been utilized for different commercial applications such as read heads in hard disc drives, occurs in the case of antiferromagnetic coupling of two (or several) magnetic layers seperated by a very thin (< nm) layer of non ferromagnetic material (e.g. Cu). If the antiparallel orientation of the magnetization in the ferromagnetic layers is disturbed by an exterior magnetic field, the electrical resistance is reduced along the layer system. Reasons for that are the spin dependent scattering of the electrons and changes in the electronic band structure. As a further magnetic resistance effect with a similarly broad application range, the magnetic tunnel resistance effect (TMR) should be mentioned. Here the spin dependent tunneling current between two magnetic layers, which are separated by a very thin insulator layer (< nm), is controllable by an exterior magnetic field.

Magnetoelectronic sen- Possible applications of magnetoelectronics in space are for example sors and data memories magnetic field sensors as position, acceleration or rotation sensors instead of conventional semiconducting magnetic field sensors (Hall effect sensors).45 For magnetoelectronic sensors, different resistance effects like GMR, TMR, CMR or EMR can in principle be utilized. Problematic for space applications is however the limited working temperature range of the sensors. Here magnetoresistive sensors on the basis of silver chal-cogenides could offer a solution. Thus researchers of the NEC Research Institute in Princeton developed a magnetoresistive sensor on the basis of AgSe2, which is applicable over a far temperature (1.5 to 290 K) and magnetic field range (up to 50 T) (Soh and Aeppli 2002).

A further important application range of magnetoelectronics are magnetic memories (MRAM) as replacements for conventional CMOS memory chips. The advantages of MRAM are the non volatileness (data remain preserved also in case of a power failure), a small energy consumption and the resistance against electromagnetic radiation. Some years ago the company Honeywell already developed MRAM chips, which were based on the anisotropic magneto resistence, with a size of 16 KB for special space applications. Meanwhile all main semiconductor manufacturers pursue the development of MRAM on a world-wide basis. In the USA for example IBM, Motorola and Honeywell, in Japan Fujitsu, Hitachi and Toshiba and in Europe Infineon and Philips. The general market readiness of MRAM memory, apart from special space applications, is expected for the year 2004. Also other types of memory chips, which are based on nanoscale structures, offer in principle application potentials in space (e.g. FRAM or phase change RAM, see section 5.5.2).

45 Kyle, Buckley 2000

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