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

二氧化铬（CrO_2）一方面理论上被预测为半金属材料，另一方面在实验上也证明了其近乎完美的自旋极化，因此有望在未来的自旋电子器件中成为一种强自旋极化剂。在低温条件下，在冷压制的CrO_2粉末中，观察到由于晶间自旋相关的隧道效应而产生的大磁电阻。然而，相对较低的居里温度（T_c ~ 400k）阻碍了它在器件应用中的应用。提高CrO_2的居里温度是实现粉末磁阻（PMR）室温传感器不可缺少的因素。本文作者采用机械化学方法成功地合成了CrO_2粉末，并且以掺杂剂为原料，易于掺杂。因此，在本研究中，我们研究了掺杂元素及其浓度对CrO_2粉末压片磁性和磁输运性能的影响。采用机械化学方法合成了（Cr-M）O_2 （M=V,Mn,Fe,Co）粉末，并研究了其粉末压实体的磁阻（MR）效应与掺杂类型和含量的关系。调查结果总结如下。在CrO_2中，用M代替Cr的极限为V、Mn、Co的极限为30 at%， Fe的极限为5 at%。V能有效降低T_c，而Fe能显著提高T_c （420k, 5 at% Fe）。此外，在Co和Mn的情况下观察到的变化是边际的。在4.2 K时，当Fe含量为5%时，磁流变率显著提高，达到22%，而当V和Co掺杂时，磁流变率下降。在（Cr-Fe）O_2粉末压块中MR增强的原因是由于势垒特性的改变。

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.