Ramat Aviv, 69978 Tel Aviv Israel Thin Co/Pd multilayers, with room temperature perpendicular anisotropy and an enhanced surface scattering, were studied for the possible use in the extraordinary Hall effect (EHE) -based magnetic memory devices. Polarity of the EHE signal was found to change from negative in thick samples to positive in thin ones. Reversal of EHE sign was also observed in thick samples with aging. The effect is argued to be related to the dominance of surface scattering having the EHE polarity opposite to that of the bulk. PACS: 73.50.-h, 75.50.Ss, 75.70.Cn Extraordinary or anomalous Hall effect (EHE) was discovered about 130 years ago, however, discussion of its practical application only started recently when it was demonstrated [1] that a relatively small EHE coefficient of bulk ferromagnets can be boosted by stimulating proper scattering events. The addition of insulating impurities into bulk transition metals [2] or the increase of surface scattering by reducing the thickness of magnetic films below the mean free path [3] can enhance the effect by orders of magnitude. Thin films of ordinary transition metals (Ni, Co, Fe) with dominant surface scattering were shown to reach EHE resistivity of the order of 1 µΩcm [1], whereas ultrathin alloys of FePt [4] and CoFe/Pt multilayers [5] reach the values of about 5 µΩcm.Tuning the perpendicular anisotropy in CoFe/Pt multilayers, and thus reducing the magnetic saturation field below the typical demagnetizing value, was shown to provide the field sensitivity exceeding 10 3 Ω/T [5] which competes with the best semiconducting Hall sensors. Use of the EHE for magnetic memory devices with a direct data read-out is another potential application. Combination of an out-of-plane anisotropy with a strong remanent EHE signal has been demonstrated in thin Ni films [1,6] at low temperatures.The goal of this work was the exploration of thin Co/Pd multilayers with room temperature perpendicular anisotropy in the thickness limit in which surface scattering can boost the magnitude of EHE, similar to the chemically homogeneous thin films. We shall demonstrate that the generation of room temperature magnetic memory units, and respectively, the magnetic random access memory based on the extraordinary Hall effect, is feasible. However, probably the most interesting and unexpected result we report on is the thickness and aging dependence of the EHE polarity in thin Co/Pd multilayers.

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