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Magnetic force microscopy (MFM) has been widely used in the study of magnetic recording media. It requires high resolution MFM tips as well as better understanding of the obtained magnetic images. In this study, we compared the images obtained by superparamagnetic and permanent MFM tips, that allows us to explain the issues related to the frequency double in some of domain images of the recording media. We also investigated the domain structures of high density recording bits (up to 1100 kfci) written on ac and dc-erased CoPtCrO perpendicular magnetic recording (PMR) media with permanent magnet MFM tips under ambient conditions.
As shown in Fig. 1, we compared the magnetic (phase) image of the 200 kfci track in an ac-erased area taking with a permanent and a superparamagnetic tip. Fig. 1a is the image obtained by the permanent magnet MFM tip, that shows a dominate spectral frequency of about 200 kfci. Fig. 1b is the image taken by a superparamagnetic tip, that shows a dominate spectral frequency of about 400 kfci. This clearly demostrated that the frequency doubling were observed by using a superparamagnetic tip. This may explain the observation of the frequency doubling by Zhifeng Deng et. al  in their phase images of a PMR medium using metal-coated carbon nanotube tips. We show that the magnetic images obtained by different types of MFM tips can provide valuable magnetic information about the sample.