Development of room-temperature halfmetallic layered oxides

预计半金属材料将在新一代电子产品——自旋电子学中发挥关键作用。尽管如此，到目前为止，人们所知道的这类材料只有少数。在目前的研究中，开发新的层状氧化物半金属材料，包括(1)寻找新的半金属，(2)开发高质量的半金属块状材料，(3)寻找室温半金属。(1)新型半金属（1-1）BaRECo_2O_<5+δ>采用我们独创的技术合成了高质量的贫氧层状双钙钛矿氧化物BaRECo2O5+8 （RE=Nd）和Sin样品，使我们能够高精度地控制氧含量，并研究了其磁输运等相关性质。特别是BaNdCo_20_<5.503>, δ = 0.503±0.003的样品，在230 ~ 260 K的温度区间表现出隧道式的大磁阻。由于反铁磁性的出现，在230 K（一个相对较高的温度）的MR量级在7 t时高达12%，因此这种层状氧化物材料在接近冰点的温度下被发现是半金属。（1-2）（LaMn）_<1-x>O_3采用高压技术成功合成了一个新的氧化物族（LaMn）_<1-x>O_3。结果表明，与（LSDA + U）能带计算结果一致，（LaMn）_<0.901>O_3在250k以下表现出巨大的磁阻（CMR），这可能是由于双重交换机制。我们最近用中子衍射对这种材料的磁性结构进行了详细的研究，支持了这种材料为半金属的鉴定。(2)由双钙钛矿Sr_2FeMoO_6A (b位有序双钙钛矿Sr_2FeMoO_6 （SFMO）组成的同质复合材料被称为半金属（T_C = 420 K）。为了改善块状半金属的隧道磁阻特性，提出了“同质复合材料”的概念，作为半金属多晶块状材料的一种方式。同一材料的两种或两种以上不同尺寸的粉末在烧结前混合成均质复合体。在这样的块体中，晶粒接触预期比在普通陶瓷块体中具有更高的定义。成功制备了首个半金属均质复合体，具有优异的低场隧穿磁阻（LF-TMR）特性，室温0.5T下为~5%。结果表明，较细粉体与较高粉体的比例为~ 20%，可获得最佳的LF-TMR。认为纳米级晶界可能处于超顺磁状态，而内部晶界则处于半金属状态。(3)室温下的半金属层状氧化物在前面(1)和(2)节中，我们报道了新的半金属的发现和半金属块体的开发，这些半金属块体在室温下表现出优异的LF-TMR。然而，在本研究中尚未获得新颖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，并希望在本项目的结果基础上继续进一步寻找这种半金属。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