Microscopic Study on the Surface Magnetism by Ultra-slow Muon

1989年初，政府批准了在东京大学介子科学实验室建造一个新的超慢介子通道的计划，该计划是由本研究项目的研究员K. Nagamine提出的。新通道有望在1991年产生比传统设备高几个数量级的高强度超慢光束，将成为研究表面磁性的有力工具。考虑到研究项目未来的可能性，我们改变了最后一年的研究计划，将重点放在高强度光束的开发上，而不是使用低强度光束来完成研究。研究项目的主要成果总结如下：1. 高强度超慢μ子光束的发展（一）基于热μ子的激光激发方法，确定了超慢μ子新通道的概念和基本设计。（ii）观察到硅（100）晶片上SiO_2薄层向真空中的热介子离子发射。SiO_2厚度对产率的依赖性在3nm处达到最大，这为热μ子发射机理提供了重要信息。2. 用μ子自旋弛豫法研究多层结构磁单分子层的磁性(i)在石墨的大背景下检测到吸附在石墨上的2%氧单分子层的uSR信号，为12K下二维反铁磁体的磁相变提供了证据。（ii）在人造金属超晶格MnSb上，初步实验仔细估计信号/背景比大于20%。这个结果鼓励对这个主题进行详细的实验。

In the beginning of 1989, the government approved a plan to construct a new ultra-slow muon channel in Meson Science Laboratory, University of Tokyo proposed by K. Nagamine, a investigator of the present research project. The new channel is expected to produce high-intensity ultra-slow beam in 1991 with several orders of the magnitude higher than that produced by the conventional facility and will be powerful tool to study the surface magnetism. Considering the future possibilities of the research project, we changed the re- search plan of the final year to concentrate on the development of the high-intensity beam instead to complete the study by using the low intensity beam.The primary results of the research project are summarized as follows; 1. Development of the High-intensity Ultra-slow Muon Beam (i) The concept and the basic design of the new channel of ultra- slow muon is decided based on the laser-excitation method of the thermal muonium. (ii)Thermal muonium emission from thin SiO_2 layer on a Si(100) wafer into vacuum is observed. The SiO_2 thickness dependence of the yield, which becomes maximum at 3 nm, gives the important in formation about the mechanism of the thermal muonium emission. 2. Magnetism of the Magnetic Monolayers with Multilayered Structure by Muon Spin Relaxation (i) The uSR signal of 2% of Oxygen monolayers adsorbed on Graphite is detected from a huge background of Graphite and it gives the evidence for the magnetic phase transition of 2-D antiferromagnet at 12K. (ii) On artificial metallic superlattice MnSb, the signal/background ratio has been carefully estimated to be more than 20% by the preliminary experiment. The result encourages to perform detail experiments on the subject.