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

超慢μ子束在表面和薄膜中的应用

基本信息

批准号：
16206005
负责人：
MIYAKE Yasuyuki
金额：
$ 29.29万
依托单位：
High Energy Accelerator Research Organization
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
2004
资助国家：
日本
起止时间：
2004 至 2006
项目状态：
已结题

项目摘要

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

自旋极化正 μ 子 (1+) 在凝聚态物理中具有独特的应用，因为植入到基质材料中的极其灵敏的磁性微探针可以通过其自旋变化的旋转或弛豫来传递有关局部磁场分布和波动的信息。可以通过测量μ子衰变的正电子的角分布来观察μ子自旋分布，这种技术被称为1SR，并且已经用于各种材料，到目前为止，1SR实验都是利用表面进行的。 μ 子束，其射程在水中约为 1.4mm，在铁中分别约为 0.3mm 为了研究具有纳米尺度深度敏感性的薄膜或多层结构的磁特性，μ子束的动能可以变化为需要亚 keV 到几十 keV，其不确定性在百分之几内，我们一直在开发一种技术1)，它允许我们产生动能低至 0.2eV 的 μ 子。热介子原子的共振激光电离（1+e_i）自2001_2我们第一代低能介子以来，我们增加了电离激光器的功率，优化了入射介子束的聚焦和低能的传输效率。重新加速后，这导致样品位置的低能 μ 子产量增加了数百倍，达到 15 1+/秒。产率随着电离激光功率的增加而饱和，这表明随着激光系统的改进，低能 μ 子的特性是通过在样品位置观察到的光束轮廓来测量的。 μ子被紧密聚焦在3x4mm2（FWHM）的光斑上，比传统表面μ子束的尺寸小100多倍，低能μ子的时间宽度小于1Onsec。比表面 μ 子束窄 10 倍。这些结果表明，我们的方法非常适合脉冲 μ 子源，包括 RIKEN-RAL μ 子装置。光束线中安装了紧凑型 1SR 光谱仪。在样品位置施加高达 700 高斯的磁场，塑料闪烁体覆盖样品位置约 80% 的总立体角，从而最大限度地提高低能量的正电子信号。光谱仪上安装了 GM 制冷机，可将样品冷却至 10K 作为低能 μ 子束的高时间分辨率的演示，我们将 15keV μ 子植入到 SiO_2 中并观察了来自的自旋进动信号。室温下 50 高斯和 100 高斯的横向磁场中的三线态锷两次测量之间没有观察到不对称性的减少，这意味着高达 100 高斯的更高频率。可以在不损失不对称性的情况下观察到 30MHz。作为第一个使用低能 μ 子束的 1SR 实验，我们正在考虑测量在 SrTiO_3 基底上生长的钙钛矿锰氧化物薄膜的磁性，显示出各种有趣的现象，例如巨大的磁阻。（CMR）效应，在器件应用方面具有巨大潜力，但据了解，钙钛矿亚锰酸盐薄膜的相图与此不同。由于基底的晶格应变而产生的块状晶体的测量将很好地证明低能μ介子作为薄膜和多层系统磁特性的独特探针的能力。