Development of High Resolution Magnetic Force Microscopy with High Sensitivity and Low Noise and Its Application to Nano-scale Magnetic Imaging

磁力显微镜(MFM)被广泛应用于磁畴观察。然而，商用MFM的空间分辨率约为10 nm，不足以观察高密度记录介质的记录比特。本研究的目的是发展高分辨率磁力显微镜，并将其应用于纳米尺度的磁成像。取得了以下研究成果：1.采用针尖冷却的高分辨率MFM(1.1)低噪声MFM的研制低噪声MFM是在高真空MFM的基础上增加针尖冷却装置而发展起来的。通过冷却尖端，将MFM的噪声降低到MFM悬臂梁的热力学噪声的理论极限。MFM可以将尖端温度和样品温度分别降低到42K和32K。(1.2)高分辨率MFM针尖的制备通过在经O_2等离子体氧化处理的表面氧化的硅针尖上沉积FePT薄膜，并对针尖进行后退火，制备出高矫直磁化强度的针尖。针尖的矫顽力大于15kOe，MFM分辨率约为10 nm。为了进一步提高MFM的分辨率，提出了一种新的利用交换弹簧三层膜[Hard(低M_S)/Soft(高M_S)/Hard(低M_S)]截面的高分辨率MFM针尖，并通过计算机模拟验证了其理论分辨率小于5 nm。本论文的主要研究内容如下：1.制备了适用于MFM针尖的FePt/FeCo/FePt三层膜。高密度磁记录介质的纳米尺度磁成像利用磁力显微镜发展了中噪声-矫直力叠加法，并用它研究了400~600Gbit/in^2级垂直介质的介质噪声来源。介质噪声产生的原因是：(1)介质中磁化反转场的空间起伏，其中起伏长度延伸到约100 nm；(2)记录头的磁场梯度不足。

Magnetic force microscopy (MFM) is widely used for magnetic domain observation. However the spatial resolution of commercial MFM is about several 10nm and is not enough to observe the recording bits of high density recording media. The purpose of this study is development of high resolution MFM and its application to nano-scale magnetic imaging. The following results were obtained.1. Development of high resolution MFM(1.1) Low noise MFM using tip coolingLow noise MFM was developed by adding the tip cooling equipment to high vacuum MFM. The noise of the MFM was reduced to the theoretical limit of thermodynamic noise of MFM cantilever by cooling a tip. The MFM can decrease the tip temperature and sample temperature to 42K and 32K, respectively. Low noise measuring condition can be obtained within 1 hour.(1.2) Fabrication of high resolution MFM tipsHigh-coercivity tip was fabricated by depositing FePt film on a surface oxidized Si tip made by O_2 plasma oxidation treatment and post-annealing of the tip. The coercivity of the tip is above 15 kOe and MFM resolution is about 10 nm. For further improvement of MFM resolution, a new high-resolution MFM tip using the cross section of an exchange-spring trilayer film [hard (low M_s) /soft (high M_s)/hard (low M_s)] was proposed and clarified the theoretical resolution is less than 5 nm by computer simulation. FePt/FeCo/FePt trilayers was fabricated for the application of MFM tip.2. Nano-scale magnetic imaging of high density magnetic recording mediaMedium noise-coercivity overlapping method was developed by using magnetic force microscopy and was used to study the origin of medium noise of 400~600 Gbit/inch^2 class perpendicular media. It was made clear that medium noise arises from (1) a spatial fluctuation of magnetization reversal filed in the media, where fluctuation length extends to about 100nm, (2) an insufficient magnetic field gradient of recording head.