Development of New Magnetic Sensor Utilizing the PMR Effect of Half-Metallic Ferromagnetic Powders

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Chromium dioxide (CrO_2) is expected to be a strong spin polarizer in forthcoming spinelectronics devices, since it has been theoretically predicted as a half-metallic material on one hand and nearly perfect spin polarization has been experimentally demonstrated on the other. In cold-pressed CrO_2 powder compacts, large magnetoresistance (MR) that originates due to the intergranular spin dependent tunneling was observed at low temperatures. However, the relatively low Curie temperature of CrO_2 (T_c 〜 400 K) prevents it from being used in device applications. Elevating the Curie temperature of CrO_2 is indispensable factor to realize powder magnetoresistance (PMR) sensors for room temperature application.The present authors have succeeded in synthesizing CrO_2 powders through mechanochemical method, and doping was easily achieved by introducing the dopant as raw material. Thus, in the present study, we investigated the influence of doping elements and their concentration on the magnetic and magnetotransport properties of CrO_2 powder compacts. The (Cr-M)O_2 (M=V,Mn,Fe,Co) powders were synthesized by the mechanochemical method and the magnetoresistance (MR) effect of their powder compacts was investigated as a function of the type and content of the dopant. The findings are summarized below. Substituting limit of M for Cr in CrO_2 was 30 at% for V, Mn, and Co, and 5 at% for Fe. Doping with V effectively decreases the T_c, while Fe significantly elevates the same (420 K with 5 at% Fe). Furthermore, the changes observed in the cases of Co and Mn was marginal. MR ratio was significantly enhanced up to 22% at 4.2 K with the doping of 5 at% of Fe, however, decreased in the cases of V and Co doping. The reason for the MR enhancement in (Cr-Fe)O_2 powder compacts is due to the change in the barrier characteristics.