Application of ultra slow muon beam for the surface and thin film

自旋极化正µ子(1+)作为极灵敏的磁性微探针，在凝聚态物理中有着独特的应用。注入到宿主材料中的介子可以通过自旋的旋转或弛豫来传递关于局部磁场分布和涨落的信息。通过测量介子衰变到的正电子的角分布，可以观察到介子自旋分布的变化。这项技术被称为1SR，并已被用于各种材料。到目前为止，已经用表面µ子进行了1SR实验，它们在水中的射程约为1.4 mm，在铁中的射程约为0.3 mm。为了在纳米尺度上研究具有深度敏感性的薄膜或多层结构的磁性，需要动能从亚电子伏安到几十电子伏特的介子束，其不确定度在几个百分点以内。我们一直在开发一种技术，这种技术允许我们产生动能为…的介子更低的低至0.2 eV的热Mu原子(1+Ei)的共振激光电离。自2001年以来，我们已经提高了电离激光的功率，优化了入射微子束的聚焦，优化了低能微子再加速后的输运效率。这使得样品位置的低能介子产额增加了几百倍，达到15.1+/秒。我们的测量表明，随着电离激光功率的增加，产额没有达到饱和，这表明随着激光系统的改进，产量可能会进一步增加。用二维位置灵敏MCP测量了低能介子的特性。在样品位置观察到的束流分布显示，这些µ子被紧密聚焦到3x4mm2(半高宽)的光斑上，这比常规表面µ子束的尺寸小100多倍。低能Muon的时间宽度小于Lonsec，比表面Muon束窄10倍。这些结果表明，我们的方法非常适合于脉冲Muon源，包括RIKEN-Ral Muon装置。在光束线上安装了一台紧凑的1SR光谱仪。螺线管类型的磁体可以在样品位置施加高达700高斯的磁场。塑料闪烁体覆盖了样品位置约80%的总立体角，最大限度地利用了来自低能量µ子的正电子信号。一台转基因制冷机安装在光谱仪上，使样品能够冷却到10K。为了证明低能Muon束的高时间分辨率，我们在SiO_2中注入了15keV的Muon，并在室温下观察到了50Gaus和100Gauss的横向磁场中三重态Muomon的自旋进动信号。两次测量之间没有观察到不对称性的减少，这意味着可以在不损失不对称性的情况下观察到高达30 MHz的更高频率。作为第一个利用低能Muon束进行的1SR实验，我们正在考虑测量一种生长在SrTiO3衬底上的钙钛矿型锰氧化物薄膜的磁性。钙钛矿材料表现出各种有趣的现象，如巨磁电阻效应，具有很大的应用潜力。然而，由于衬底的晶格应变，钙钛矿锰氧化物薄膜的相图与大块晶体的相图是不同的。它的测量将很好地展示低能介子作为薄膜和多层膜系统磁性的独特探针的能力。较少

Spin polarised positive muons (1+) have a unique application in condensed matter physics as extremely sensitive magnetic micro-probes. Muons implanted into a host material can convey information about the local magnetic field distribution and fluctuations through rotation or relaxation of its spin. The change of muon spin distribution can be observed by measuring angular distribution of positrons which muons decay to. This technique is called 1SR, and have been used for various materials. Until now, 1SR experiments have been carried out by using surface muons, whose range is approximately 1.4mm in water and 0.3mm in iron respectively. In order to study magnetic properties of thin films or multi-layered structure with the depth sensitivity on nano-meter scale, muon beams whose kinetic energy can be varied from sub-keV to a few tens keV, with its uncertainty within several per cent, are required. We have been developing a technique1), which allows us to produce muons with kinetic energy … More as low as 0.2eV by resonant laser ionization of thermal muonium atoms (1+e_i). Since our first generation of lowenergy muons in 2001_2), we have increased the power of ionizing lasers, optimised both the focusing of the incident muon beam and the transport efficiency of low-energy muons after re-acceleration. This has resulted the yield of low-energy muons being increased by a few hundreds fold to 15 1+/sec at the sample position. Our measurement shows no saturation of the yield with the power of ionizing laser, suggesting further increase can be expected as laser system being improved. The characteristics of the low-energy muons were measured by a 2-dimensional position sensitive MCP. Observed beam profile at the sample position showed the muons were tightly focused to a spot of 3x4mm2 (FWHM), which is more than 100 times smaller than the size of conventional surface muon beam. The temporal width of the low-energy muon was less than lOnsec, which is 10 times narrower than that of surface muon beam. These results demonstrate that our method is ideally suited to pulsed muon sources, including the RIKEN-RAL muon facility. A compact 1SR spectrometer has been installed in the beam line. A solenoid-type magnet can apply a magnetic field of up to 700 Gauss at the sample position. Plastic scintillators cover around 80% of total solid angle from the sample position, maximising the positron signal from low-energy muons. A GM refrigerator is mounted to the spectrometer, enabling to cool a sample down to 10K. As a demonstration of the high time resolution of the low-energy muon beam, We have implanted 15keV muons into Si0_2 and observed the spin precession signal from triplet muonium in transverse magnetic field of 50 Gauss and 100 Gauss at room temperature. There was no observed reduction of asymmetry between two measurements, which implies that higher frequencies of up to 30MHz could be observed without a loss of asymmetry. As the first 1SR experiment using low-energy muon beam, we are considering to measure magnetic properties of a thin perovskite manganite film grown on SrTiO_3 substrates. Perovskite show varieties of interesting phenomena, like colossal magnetoresistance (CMR) effect, which have large potencial for device application. It is understood, though, that thephase diagram of thin perovskite manganite films are different from that of bulk crystals due to lattice strainform the substrates. The measurement of such will be a good demonstration of capability of loweenergy muons as a unique probe of magnetic properties for thin films and multi-layer system. Less