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

自旋极化的正介子（1+）在凝聚态物理中作为极其敏感的磁性微探针具有独特的应用。植入宿主材料中的μ子可以通过其自旋的旋转或弛豫传递有关局部磁场分布和波动的信息。通过测量介子衰变成的正电子的角分布，可以观察到介子自旋分布的变化。这种技术被称为1SR，并已用于各种材料。到目前为止，使用表面μ子进行了1SR实验，其范围在水中约为1.4mm，在铁中约为0.3mm。为了在纳米尺度上研究具有深度灵敏度的薄膜或多层结构的磁性，需要动能从亚keV到几十keV不等，不确定度在百分之几以内的介子光束。我们一直在开发一种技术，使我们能够通过共振激光电离热μ子原子（1+e_i）来产生动能低至0.2eV的μ子。自2001年我们的第一代低能μ子以来，我们增加了电离激光的功率，优化了入射μ子束的聚焦和再加速后低能μ子的输运效率。这导致低能μ子的产率增加了几百倍，在样品位置达到15 1+/秒。我们的测量表明，产率随电离激光功率的增加没有饱和，表明随着激光系统的改进，产率有望进一步提高。利用二维位置敏感MCP测量了低能μ子的特性。在样品位置观察到的光束轮廓显示，μ子紧密聚焦在一个3x4mm2 （FWHM）的点上，比传统表面μ子束的尺寸小100倍以上。低能μ子的时间宽度小于lOnsec，比表面μ子束的时间宽度窄10倍。这些结果表明，我们的方法非常适合于脉冲μ子源，包括RIKEN-RAL μ子设施。在束流线上安装了一个紧凑的1SR光谱仪。螺线管式磁铁可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的总立体角的80%左右，最大限度地提高了低能μ子的正电子信号。一台通用冰箱安装在光谱仪上，可以将样品冷却到10K。为了证明低能介子束的高时间分辨率，我们在Si0_2中注入了15keV的介子，并在室温下观察了介子在50高斯和100高斯横磁场下的自旋进动信号。没有观察到两次测量之间不对称性的减少，这意味着可以观察到高达30MHz的更高频率而不会失去不对称性。作为第一个使用低能介子束的1SR实验，我们正在考虑测量生长在SrTiO_3衬底上的钙钛矿型锰薄膜的磁性能。钙钛矿表现出各种有趣的现象，如巨磁阻效应，具有很大的器件应用潜力。然而，可以理解的是，由于衬底的晶格应变，薄钙钛矿锰矿薄膜的相图与体晶体的相图不同。这将很好地证明低能μ子作为一种独特的薄膜和多层体系磁性探针的能力。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