Exchange bias effect and hard disk media studied by means of quantitative magnetic force microscopy
More than 15 years after its invention, the MFM has become a widespread tool to characterize \nmagnetic materials and structures. However, the impact of the technique in academic and \nindustrial research has declined in recent years due to the lack of lateral magnetic resolution \nand the possible influence of the tip stray field on the magnetization structure of the sample. \nMoreover, a profound understanding of the contrast formation in an MFM experiment and \nhence the quantification of measured MFM data remains elusive. \nIn the first chapter of this thesis, the contrast formation in an MFM image is described by a \ntransfer function theory. The theory in combination with experimental methods introduced \nby van Schendel [9] allows the calibration of the imaging properties of an MFM tip and \ntherefore the quantitative interpretation of an MFM measurement (→ chapter 1). \nAll data presented in this thesis were acquired by either a low temperature ultra high vacuum \nmicroscope (LTSFM) or a high resolution magnetic force microscope (hr-MFM) designed and \nbuild for the investigation of hard disk media in an industrial environment (→ chapter 2). \nIn order to improve the lateral resolution down to about 10 nm, ultrasharp tips with a high \naspect ratio and a magnetic coating thickness of only a few nanometers have to be used. In \naddition, the tip–sample distance during data acquisition has to be decreased to below 10 nm. \nIn this distance regime, non-magnetic forces, e.g. the van derWaals force, become comparable \nto the magnetic force. As a consequence, new methods to separate the magnetic from the \nnon-magnetic contributions to an MFM measurement (and therefore to an MFM image) were \ndeveloped (→ chapter 3.1). Further, the influence of the tip geometry and its coating on the \nmagnetic imaging properties were studied. In high resolution MFM ultra sharp, fragile tips are \nused, so that any tip–sample contact destroys the tip. However, achieving this is not trivial because the tips are typically used for the acquisition of micron-sized images, which takes \nplace at tip–sample distances of only a few nanometers. In the last part of the chapter, the \ndegradation of the imaging properties of tips of various geometry, after tip–sample contacts \nof increasing intensity were studied (→ chapter 3.3). \nThe high resolution images on hard disk media obtained with the hr-MFM has sparked the \ninterest in the magnetic recording industry in this technique. The knowledge gained in the \nprocess of the work presented in the thesis and the wish of the industrial researchers to \nperform high resolution MFM measurements on their hard disk media has led to an intense \ninteraction between the research group in Basel and various industrial labs. In several visits \nto these labs during this thesis, the importance of a quantitative interpretation of hr-MFM \nimages of recording media became apparent. Up to now, the performance of hard disk media, \nread and write heads was mostly characterized in spin stand experiments. Although, the \nMFM images obtained with the hr-MFM resolved the micromagnetic structure of written \ntracks and bits in the media in great detail, the interpretation of these images in relation \nto typical quantities determined in spin stand experiments remained unclear. Hence, new \nimage analysis procedures were developed which allow the extraction of head and media \nproperties from the measured MFM data (→ chapter 4). In the process of the analysis, it \nbecame apparent, that the recording performance of todays hard disk drives is determined \nby nano-scale physics of the media and the write head stray field. \nThe continuous demand for increased storage density throughout the last decade could only \nbe met with an astonishingly rapid transfer of newly discovered physical phenomena into \nproducts of the computer industry. One example of this kind is the use of the GMR effect [15, \n16] in read heads of magnetic storage devices such as hard drives. The basis of such a read \nhead is a so-called spin-valve [87]. One of its components is a ferromagnetic (FM) layer with a \nmagnetization direction stabilized by the exchange coupling of an adjacent antiferromagnetic \n(AF) layer. Although the effect of the exchange coupling is used in todays products, a detailed \nqualitative and quantitative understanding of the mechanism is still lacking. It is commonly \naccepted, that the interface of the AF and FM layer plays a key role. So far, the magnetic \nstructure of the interface has only been studied by XMCD methods. These methods have \nthe advantage of chemical sensitivity, thus in principle, the magnetic state of one specific \nelement can be accessed. However, the disadvantages are the limited spacial resolution of \nXMCD microscopy methods, the complexity of the instrumentation,which involves a beam \nline at a synchrotron and the long data acquisition times. The latter makes experiments that \nrequire the variation of external parameters such as the temperature or the magnetic field, \ntedious. In chapter 5, an MFM based method is discussed that allows the determination of the \nuncompensated spin density with a superior lateral resolution and the study of the evolution \nof the domain structure in the ferromagnetic layers as a function of an applied external field.
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