Exploration of Novel Transition Metal Oxyarsenides

自旋电子材料在施加一种称为磁阻的磁场后，可以表现出很大的电阻率降低。这种材料目前用于磁传感器和磁存储设备，例如计算机硬盘。巨磁电阻(GMR)在由磁性和非磁性薄膜组成的多层膜中的发现是一项重大突破，使硬盘的存储容量从1 Gb增加到20 Gb。在施加磁场的情况下，GMR器件的电阻率最高可降低50%。近年来，由于观察到了巨磁电阻效应，La_(1-x)x_xMnO_3(A=Ca，Sr，Ba)等锰氧化物类钙钛矿材料的磁性和电学性质得到了广泛的研究。这些锰氧化物是值得注意的，因为它们在施加磁场时可以显示出高达99.9%的电阻率下降。到目前为止，生产CMR所需的大磁场限制了其商业实施。因此，一个重要的研究目标是发现在290K时在低场(&lt；&lt；1T)表现出CMR的新材料，以用于更小、更快、更便宜和更高效的自旋电子器件。因此，合成和研究新型的CMR材料是至关重要的，以便更好地了解不同的CMR机制，然后可以在未来的CMR设备中利用这些机制。我们最近合成了一种新的CMR材料NdMnAsO1-xFx(x=0.05-0.08)，它在低温下施加磁场后，其电阻率惊人地下降了95%，与在CMR锰氧化物中观察到的结果相当。这是一种新的CMR机制。未掺杂的化合物LnMnAsO(Ln=Nd，La)已经表现出相当大的室温负磁阻(-MR；在7T磁场下，Ln=ND和La的MR分别为-8%和-11%)。此外，我们还将合成和研究新型Mn2+氧化物如Sr2Mn2MAs2O2-xFX(M=Mn，Ni，Fe，Cu)的磁输运性质，通过增强Mn2+和M2+离子之间的磁耦合作用来操纵CMR。到目前为止，报道的3d过渡金属氧砷化物的磁性和电学性质都是出众的。除了观察到LnFeAsO1-xFx和NdMnAsO1-xFx的CMR高温超导电性外，LaNiAsO在2.75K以下也有超导电性的报道，而LnCoAsO在66K和85K以下分别是铁磁体，Ln=La和Nd。我们将研究4d/5d过渡金属氧砷化物中是否也存在新的超导和磁流变路径，这一工作不仅具有重要的基础意义，而且如果有可能优化其磁阻性能，也将具有实际应用价值。

Spintronic materials can exhibit a large reduction in electronic resistivity upon application of a magnetic field, called magnetoresistance. Such materials are currently employed in magnetic sensors and magnetic memory devices such as computer hard disks. The discovery of giant magnetoresistance (GMR) in multilayers consisting of magnetic and nonmagnetic thin films was a major breakthrough, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabits. GMR devices can exhibit a reduction in electronic resistivity of up to 50% upon application of a magnetic field. In recent years there has been intense study into the magnetic and electronic properties of manganite perovskites such as La1-xAxMnO3 (A = Ca, Sr, Ba) due to the observation of colossal magnetoresistance (CMR). The manganites are remarkable as they can exhibit a reduction in electronic resistivity of up to 99.9 % upon application of a magnetic field. As yet the large magnetic fields required to produce the CMR has limited its commercial implementation. An important research objective therefore is to discover new materials that exhibit CMR in low fields (<<1 T) at 290 K to be employed in spintronic devices for smaller, faster, cheaper and more efficient computing applications. It is therefore vital to synthesise and investigate novel CMR materials in order to gain greater understanding of different CMR mechanisms which can then be exploited in future CMR devices. We have recently synthesised a new CMR material NdMnAsO1-xFx (x = 0.05 - 0.08) which surprisingly exhibits a reduction in electronic resistivity by 95% upon applying a magnetic field at low temperature, which is comparable to that observed in the CMR manganites. This is a new mechanism of CMR. The undoped compounds LnMnAsO (Ln = Nd, La) already exhibit a sizeable room temperature negative magnetoresistance (-MR; MR = -8% and -11% for Ln = Nd and La respectively in a 7 T magnetic field). We propose to improve the magnetoresistant properties of these fascinating Mn2+ oxyarsenides by exploring the effects of chemical substituents.We will also synthesise and study the magnetotransport properties of novel Mn2+ oxyarsenides such as Sr2Mn2MAs2O2-xFx (M = Mn, Ni, Fe, Cu) in order to manipulate the CMR by enhancing magnetic coupling between Mn2+ and M2+ cations. The magnetic and electronic properties of 3d transition metal oxyarsenides reported so far are exceptional. Alongside the observation of high temperature superconductivity in LnFeAsO1-xFx and CMR in NdMnAsO1-xFx, superconductivity has also been reported in LaNiAsO below 2.75 K whereas LnCoAsO is a ferromagnet below 66 K and 85 K for Ln = La and Nd respectively. We will investigate if novel superconducting and MR pathways are also present in 4d/5d transition metal oxyarsenide which have been relatively unexplored until now.This work will not only be of great fundamental importance but may also have practical applications if it is possible to optimise the magnetoresistive properties.