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 . 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 .
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.