Development of room-temperature halfmetallic layered oxides

半金属材料预计将在被称为自旋电子学的新兴下一代电子学中发挥关键作用。尽管如此，迄今为止已知的此类材料仍然屈指可数。本研究致力于新型层状氧化物半金属材料的开发，包括(1)寻找新的半金属材料，(2)开发高质量的半金属块体材料，(3)探索室温半金属材料。(1)利用我们的独创技术合成了新型的缺氧层状双钙钛矿氧化物halfmetals(1-1)BaRECo_2O_&lt；5+δ&gt；High-quality样品BaRECo2O5+8(RE=NdBaRECo2O5+8)和Sin，并从磁输运和其他相关性质方面进行了研究。特别是，δ=0.503±0.003的BaNdCo20样品在230-260K的温度区间内表现出较大的隧道型磁阻，尽管该温度的下限为…由于反铁磁性的出现，在230K(一个相对较高的温度)下的磁阻在7T时高达12%，从而发现这种层状氧化物材料在接近冰点的温度下是半金属的。利用高压技术成功地合成了(1-2)(LaMN)_&lt；1-x&gt；O_3这一新氧化物系列的成员。通过(LSDA+U)能带计算发现，(LaMN)0.901&GT；O_3在250K以下表现出巨大的磁阻，可能是由于双交换机制。我们最近用中子衍射法对该材料的磁结构进行了详细的研究，支持该材料为半金属。(2)由双钙钛矿、B位有序双钙钛矿组成的同质复合材料，Sr2FeMoO_6(SFMO)是一种半金属(T_C=420K)。为了改善块体半金属的隧道磁阻特性，提出了半金属多晶块体的“同质复合材料”的概念。将同一材料的两种或两种以上不同大小的粉末混合在一起，然后烧结成均匀复合的块体。在这样的块体中，颗粒接触预计比普通陶瓷块中的定义更高。成功地制备了第一个用于SFMO的半金属同质复合材料块材，在0.5T的室温下获得了良好的低场隧道磁阻(LF-TMR)特性，例如~5%。结果表明，在最佳LF-TMR工艺条件下，细粉与高粉的比例约为20%。(3)半金属层状氧化物。在前面的章节(1)和(2)中，我们报道了新的半金属的发现和半金属块体的发展，它们在室温下表现出良好的LF-TMR。然而，在这项研究中并没有得到新奇的室温半金属层状氧化物。我们认为，这种性质的半金属是自旋电子学所必需的，并希望根据本项目的结果，继续进一步寻找这类半金属。较少

Halfmetallic materials are anticipated to play a key role for emerging next-generational electronics called spintronics. Nonetheless only a handful number of such materials have been known to date. In the present research, development of new layered-oxide halfmetallic materials was conducted, consisting of the works on (1) search for new halfmetals, (2) development of high quality halfmetallic bulk materials and (3) quest for room-temperature halfmetals.(1)Novel halfmetals(1-1)BaRECo_2O_<5+δ>High-quality samples of oxygen-deficient layered double-perovskite oxides, BaRECo2O5+8 with RE=Nd, and Sin were synthesized by means of our original techniques which allow us to control the oxygen content with high accuracy, and studied in terms of magneto-transport and other related properties. In particular the sample, BaNdCo_20_<5.503> with δ = 0.503±0.003 exhibited large magneto-resistance of a tunneling-type for a temperature interval of 230 -260 K. Although the lower limit of this temperature … More interval shows due to the appearance of anti-ferromagnetism, the magnitude of MR at 230 K (a relatively high temperature) is as high as 12 % at 7 T. Thus this layered- oxide material was found to be halfmetallic at temperatures close to the ice point.(1-2)(LaMn)_<1-x>O_3Members of a new oxide family, (LaMn)_<1-x>O_3, were successfully synthesized employing a high-pressure technique. It was found that as had been predicted from a band calculation (by a (LSDA + U) method), (LaMn)_<0.901>O_3 was proven to show colossal magneto-resistance (CMR) below 250 K probably due to a double exchange mechanism. Our recent detailed study by neutron diffraction on the magnetic structure of this material supports the identification of this material as a halfmetal.(2)Homocomposites consisting of double-perovskite, Sr_2FeMoO_6A B-site ordered double-perovskite, Sr_2FeMoO_6 (SFMO), is known as a halfmetal (with T_C = 420 K). In order to improve the tunneling magneto-resistance characteristics of bulk halfmetals, an original concept of "homocomposite" was developed as a fashion of halfmetallic polycrystalline bulks. Two or more powders of different sizes of the same material are mixed before sintered into a homocomposite bulk. In such a bulk the grain contact is anticipated to be higher defined than in an ordinary ceramic bulk. The first halfmetallic homocomposite bulk was successfully fabricated for SFMO to yield excellent low-field tunneling magnetoresistance (LF-TMR) characteristics, e.g. ~5% at 0.5T at room temperature. It was found that the ratio of the smaller sized powder to the higher for the optimal LF-TMR is ~ 20%. It was considered that the nano-scale grain boundary is probably in a super-paramagnetic state yet while the grain inside is in a halfmetallic state.(3)Halfmetallic layered oxides at room temperatureIn previous sections (1) and (2) we reported discoveries of new halfmetals and development of halfmetallic bulks which show excellent LF-TMR at room temperature. None the less novel room-temperature halfmetallic layered oxides have not been obtained in this research. We believe that halfmetals of such nature are musts for spintronics and hope that further search for such halfmetals shall be continued, based on the results of the present project. Less