超薄对称失配锰氧化物庞磁阻(CMR)单晶膜中相分离结构、形成与调控机制的高灵敏局域研究

The next generation high density storage application of manganese oxide colossal magneto-resistive (CMR) material still does not come true due to its low magneto-resistance (MR) in low magnetic field. Recent studies tend to enhance the decision role of phase separation on the origin of CMR effect. So, the local (microscopic) behavior research of the microscopically structured phase separation becomes particularly important. Especially, very recent studies show that the anisotropic strains in the CMR film induced by the lattice symmetry mismatch between the CMR film and its substrate will greatly enhance the phase competition / separation in the film and result in relatively high MR in low field. This is a major breakthrough and the clarification of its local phase separation behavior and mechanism becomes the key to the understanding of low-field high MR effect. This research is rarely seen, implying that this project is important. But, such research is difficult because the optimal thickness of the symmetry mismatch CMR film is about 20-30nm, which is too thin to produce sufficiently strong local magnetic signal. I was the first author of a Science paper which indicated that the magnetic force microscope (MFM) was a power tool in the studies of local magnetic structures of CMR thin films and later further published the MFM images of local phase separations in CMR thin films (cited 140 times). Last year, we invented and implemented a new principle high sensitivity "high Q factor amplitude modulated MFM", from which we immediately obtained high quality "single domain" phase separation MFM images of a 60 nm thick symmetry mismatch CMR film. These single domains also showed rich behaviors. In this project, we will utilize this MFM (installed in a 20T ultra-strong Oxford superconducting magnet) to systematically study the local phase separation structures and behaviors of high quality "thinner than 40 nm ultra-thin" symmetry mismatch CMR films grown by pulsed laser deposition (PLD), including their responses to the changes of temperature, external magnetic field and light etc. The goal is to microscopically clarify the low field MR characteristics and the dramatic macroscopic transport behaviors of this type CMR films. Particular attention will be paid to the possible ultra-low field high MR locals and the reasons.

CMR材料的高密存储应用至今仍因低场MR弱而受阻。近期研究倾向拔高相分离对MR的决定作用，特别是CMR膜与衬底对称失配产生的各向异性应力可增强相分离竞争并导致低场MR、多重金属-绝缘转变和各向异性磁阻等重要效应，是重要突破口，故结构上微观的相分离的局域行为研究就成关键。该局域研究很少，体现本项目的重要。但有难度，因其最佳厚度仅25nm，磁信号太弱。我们最早在Science上揭示MFM是CMR膜局域研究有力手段，后又发表CMR膜中存在相分离的MFM图像。去年我们潜心研制出新原理的高Q调幅MFM，便在60nm对称失配CMR膜中获得高清晰"磁单畴"相分离图像且行为丰富。本项目以20T磁体中的该MFM研究相分离更强的<40nm超薄对称失配CMR高品质单晶膜中的微观磁结构，其形成和受磁场、温度、光照等调控的局域机制，从微观上弄清其低场MR特性和戏剧性输运行为，特别关注可能的超低场CMR局域及其成因。

本项目的原定研究内容是采用我们自主研制的20T强磁场磁力显微镜（20T-MFM）研究超薄对称失配锰氧化物单晶膜中的相分离微观磁结构，及其受磁场、温度等的调控机制。相关的研究成果还可期待反过来促进对MFM以及相关扫描探针测量装置的改进与提升，并获得自主知识产权。对此我们做了很好的执行，取得了一大批重要成果，包括：（1）相分离重现过程的首次直接观测：庞磁阻锰氧化物中处于竞争的热力学相同时存在，但由于需要较强磁场才能驱动这些相的转变，而此类仪器较少，因此相分离从电荷有序态从消亡到重现的完整过程一直没被观测过。我们利用20T-MFM在受各向异性外延应力调控的相分离锰氧化物薄膜中观测了这一过程，发现了丰富的相分离行为，特别是发现了重现时的维度效应：不同温度下分别以点、线、面的不同维度形态重现。相关工作发表于Nature Communications (2015)。（2）成功地在一维相分离单晶纳米线（包裹在纳米线上的LPCMO薄膜）中观测到本征隧道结，为该材料中出现的量子遂穿效应及新型稳定量子逾渗态提供了证据：低场下相邻铁磁畴被中间的较薄绝缘相隔开，形成隧穿结构；增大磁场，部分绝缘畴转为铁磁畴，形成形状更好的隧穿结。由于一维各向异性，在低温强磁场下绝缘畴仍能稳定存在，只是被压缩成很细条带，形成本征隧道结，是一种新型量子逾渗态。该研究成果发表于NANO LETTERS（2017）。这些研究都需对自主研制的20T-MFM进行不同的针对性改进，所以还催生了一批自主知识产权的高场高分辨成像技术。总体说来，在本项目支持下，我们发表了相关的SCI论文20篇，其中高影响因子的一区论文9篇，Nature Index论文3篇，包括Nature子刊1篇；授权了国家发明专利7项，还有3项在审，其中一项已经实现了转化，成立了“合肥中科微力科技有限公司”；相关技术通过了国家重大科技基础设施“稳态强磁场实验装置”的国家验收，验收意见中对我这部分工作的评价是：“建成了国际首创水冷磁体扫描隧道显微镜系统、扫描隧道-磁力-原子力显微镜系统，使得我国稳态强磁场相关实验条件达到国际领先水平”。为此，本人也获得了2017年度的中科院杰出科技成就奖（主要完成人）。