Development of room-temperature halfmetallic layered oxides

半金属材料预计将在新兴的下一代电子产品（称为自旋电子学）中发挥关键作用。尽管如此，迄今为止已知的此类材料数量仍屈指可数。本论文主要研究了新型层状氧化物半金属材料的开发，包括：（1）寻找新型半金属材料，（2）开发高质量半金属块体材料，（3）寻找室温半金属材料。（1）新型半金属材料（1-1）BaRECo_2 O_（5+δ）采用我们独创的高精度氧含量控制技术，合成了高质量的缺氧层状双钙钛矿氧化物BaRECo_2 O_（5 +8）（RE=Nd，Sin），并研究了其磁输运等相关性质。特别是<5.503>δ = 0.503±0.003的BaNdCo_（20）在230 - 260 K温度区间内表现出较大的隧道型磁阻效应。虽然这个温度的下限 ...更多信息 结果表明，由于反铁磁性的出现，在230 K（相对较高的温度）的MR值在7 T时高达12%。因此，人们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.（1-2）（LaMn）_O_3<1-x>用高压技术成功地合成了一个新的氧化物家族（LaMn）<1-x>_O_3。结果表明，（LaMn）<0.901>O_3在250 K以下表现出巨磁电阻效应，这可能是由于双交换机制引起的。我们最近详细的研究中子衍射的磁性结构的这种材料支持这种材料作为半金属的识别。（2）由双钙钛矿结构Sr_2FeMoO_6（SFMO）组成的均质复合材料是一种半金属（T_C = 420 K）。为了改善半金属块材的隧穿磁电阻特性，提出了“同质复合”的概念，作为半金属多晶块材的一种形式。两种或两种以上不同尺寸的相同材料的粉末在烧结成均质复合材料块体之前混合。在这样的块体中，预期晶粒接触比普通陶瓷块体中的晶粒接触更高。首次成功制备了SFMO用半金属同质复合体材料，获得了良好的低场隧穿磁电阻（LF-TMR）特性，室温下0.5T时，LF-TMR值约为5%。发现对于最佳LF-TMR，较小尺寸的粉末与较高尺寸的粉末的比率为约20%。认为纳米尺度的晶界可能处于超顺磁性状态，而晶粒内部可能处于半金属状态。（3）室温下的半金属层状氧化物在前面的（1）和（2）部分中，我们报道了新的半金属的发现和在室温下显示出优良的LF-TMR的半金属块体的发展。尽管如此，本研究尚未获得新颖的室温半金属层状氧化物。我们相信这种性质的半金属是自旋电子学的必需品，并希望在本项目结果的基础上继续对这种半金属的进一步研究。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