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Magnetic thin films and devices

Ballistic Magnetoresistance in Nanoconstrictions

The investigation of magnetic nanostructures is one of the major current research topics in magnetism. This interest relies on different reasons. On the one hand, nanofabrication techniques as well as magnetic characterization methods have greatly progressed. On the other hand, the fundamental research is stimulated by the perspective of technological applications for high density magnetic storage within a short period of time.

Currently the reading heads for high-density magnetic data storage devices are based on the giant magnetoresistance effect (GMR). GMR is a quantum mechanical effect observed in structures composed of alternating layers of ferromagnetic and nonmagnetic layers [1]. When the magnetic moments of the ferromagnetic layers are parallel, the spin-dependent scattering of the carriers is minimized and the material has a low resistance. When the ferromagnetic layers are antiparallel, the spin-dependent scattering of the carriers is maximized and the material has a high resistance. An other effect, the tunnelling magneto-resistance effect (TMR), is used in magnetic random access memory elements. In this case the tunnelling current between two ferromagnetic layers separated by an insulating barrier depends on the relative orientation of the magnetic moments in the ferromagnetic films.

While the GMR and TMR devices show a magnetoresistance up to a few ten percents, so-called ballistic magnetoresistance effect (BMR) in point contacts has been shown to reach larger values [2].

BMR is attributed to spin-polarized electrons travelling through a extremely narrow magnetic domain wall that forms at nano-sized constriction. If the spin-flip mean free path is long compared to the magnetic domain-wall width, as in the case of thin walls, spin conservation occurs and the magnetoresistance increases because of the strong backscattering for electrons with antiparallel spins.

Fig. 1: Ballistic transport of conduction electrons through a nanocontact. The domain wall is pinned in the nanoconstriction.

Our project aims at the reliable preparation and characterization of  one or several nano-constrictions in sputter deposited magnetic films and multilayers in order to better understand the ballistic magnetoresistance effect and to explore possible applications for device fabrication. The magnetic properties of the films are analyzed with classical methods as well as with new quantitative high resolution magnetic force microscopy.

References

[1] M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988)
[2] N. Garcia et al., Phys. Rev. Lett. 82, 2923 (1999)

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