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

二氧化铬（CrO_2）是一种半金属材料，实验证明其具有近乎完美的自旋极化，因此在未来的自旋电子学器件中有望成为一种强自旋极化剂。冷压CrO_2粉末压块在低温下观察到大的磁电阻效应，这是由晶间自旋相关隧穿引起的。但CrO_2的居里温度较低（T_c ≤ 400 K），使其在器件中的应用受到限制。提高CrO_2的居里温度是实现室温磁电阻（PMR）传感器不可缺少的因素，本文采用机械化学法成功地合成了CrO_2粉体，并以掺杂剂为原料进行了掺杂。因此，在本研究中，我们研究了掺杂元素及其浓度对CrO_2粉末压块的磁性和磁输运性质的影响。用机械化学法合成了（Cr-M）O_2（M=V，Mn，Fe，Co）粉末，研究了掺杂剂种类和含量对粉末压坯磁电阻效应的影响。调查结果概述如下。在CrO_2中，M对V、Mn和Co的取代极限为30at%，对Fe的取代极限为5at%。V掺杂有效地降低了T_c，而Fe掺杂显著地提高了T_c（420 K，5at%Fe）。此外，在钴和锰的情况下观察到的变化是微不足道的。在4.2 K时，掺杂5at%的Fe，磁电阻率显著提高，最高可达22%，而掺杂V和Co，磁电阻率反而下降。（Cr-Fe）O_2粉末压坯中磁电阻增强的原因是势垒特性的改变。

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.