High resolution magnetic force microscopy: instrumentation and application for recording media

Steffen Porthun

    Research output: ThesisPhD Thesis - Research UT, graduation UT

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    This thesis describes aspects of the use of magnetic force microscopy for the study of magnetic recording media. The maximum achievable storage density in magnetic recording is limited by the magnetic reversal behaviour of the medium and by the stability of the written information. The shape and size of the small magnetic units (bits as wen as domaiDs) in the media is strongly correlated to the microstructure of the layers. Surface roughness, colurnnar structure of the layer, segregation of the elements inside the crystallites as wen as surface and volume defects influence the size and shape of the magnetic structures in the layer. The physicallimit for the bit size is estimated to be about 50 x 50 Dm. Such an increase in bit writing density requires a magnetic microscopy technique which can image magnetic details of about 10 Dm size. The advantage of magnetic force microscopy for this purpose lies in the easy sample preparation, combined with a high achievable resolution. In Chapter 2 we have discussed the optimisation of the tip shape, the tip magnetic properties as weIl as the detection mode in order to reach the required resolution. It is shown there that an elongated magnetic needIe can combine a sufficient volume with a small tip front end and is therefore superior to a soft tip which should have a spherical shape. Por the bar type tip we have calculated a one-dimensional transfer function which relates the spatial frequency components of the force ( or force deri vati ve ) to the spatial frequency components of the sample magnetisation distribution. This transfer function depends on the sample thickness, the tip magnetic layer thickness, the tip length and the tip sample separation and shows a behaviour very similar to the readback frequency characteristics of a magnetic recording channel. With a given detector sensitivity a minimum detectable wavelength Ac can be determined from the transfer function which serves as a measure for the comparison of different detection modes. Prom this comparison, for maximum resolution an MFM should be operated in a constant distance dynamic mode with a low oscillation amplitude. The tip has to have a high Ms and Hc. The tip coercivity has to be a result of the shape anisotropy due to the high aspect ratio of the needIe like magnetic tip layer . In Chapter 3 the instrumentation of an S FM scan head is described. This scan head win be applied for MFM measurements in extemal magnetic fields of arbitrary direction with respect to the sample. Por this purpose it has to be operated between the poles of a resisti ve electromagnet in a vacuum in order to increase the Q factor of the resonating cantilever . in dynamic detection mode. The properties of the scan head are: -Extemal magnetic field: continuously between -1.8 T and 1.8 T, with a 1 p pm long term stability -Scan range: about 40 ~m -Detection limit: 0.1 Dm in a 1 kHz bandwidth -Scan head size: 26 x 30 x 42 mm -Complete software control of the whole instrument The instrument is controlled by two digital signal processor (DSP) boards. One of them generates the scan signals, the second board is responsible for the detection and feedback. Chapter 4 describes the fabrication and magnetic properties of magnetic tips made from electron beam assisted deposited carbon needIes. The carbon needIes are grown in a SEM where the hydrocarbon gas contamination originating from the diffusion pump is locaIly cracked by the electron beam. These carbon needIes are very weIl suited for evaporating magnetic materia! as Co or CogoNi2o onto their side. This way a tip shape very similar to the bar type tip as described in Chapter 2 can be created. Lorentz electron microscopy images prove a single domain behaviour of the tip in extemal fields up to about 0.2 T . For higher fields the tip is split up into two antiparallel magnetic domains. This contiguration remained stabIe even in remanent states. At no time did the tip front end contain more than one domain. On a Co-Ni/Pt multilayer sample we could observe magnetic structures smaller than 30 nm dimension with EBill magnetic tips in a constant force derivative detection mode. Chapter 5 describes the application of magnetic force microscopy for the characterisation of magnetic recording media. First of a!l the techniques used for the quantification of data from MFM images are presented. We used a thresholding a!gorithm to estimate the underlying magnetisation pattem from the constant signa! MFM images. We have used Fourier descriptors to characterise the jaggedness of the boundary of magneto- optica! bits as weIl as of conventional bits. This a!lows a comparison of bits in different materials, written under the same settings as weIl as different writing conditions on the same media. The principle of this technique is shown in an example of magneto- optical bits. Furthermore, Chapter 5 gives examples of results on magnetic recording media which were gained by MFM. The influence of the sputtering bias voltage on the magnetic structures of Co-Ni/Pt multilayers bas been shown. On ME tape the edge overwrite behaviour has been studied as weIl as the difference in bit structure when the bits were written in the direction of the colurnnar inclination or antiparallel to it. Furthermore the the writing behaviour of a CoCrTa perpendicular materia! bas been investigated. 400 nm wide bit tracks of 200 kfrpi (127 nm bit length ) have been studied in this materia!. AFM observations of the sample surface topography show that the smallest observed bits still contain about 30 crysta1lites. The maximum linear bit density we could observe in this materia! was 300 kfrpi (85nm bit length). Here the bit period is smallerthan the domain size ofthe surrounding remanent domain pattem. Chapter 6 contains conclusions of the work a!ong with some suggestions for continuing work.
    Original languageEnglish
    Awarding Institution
    • University of Twente
    • Popma, Th.J.A., Supervisor
    • Lodder, J.C., Supervisor
    Thesis sponsors
    Award date1 Jun 1996
    Place of PublicationEnschede
    Print ISBNs90-900-9664-7
    Publication statusPublished - Jun 1996


    • SMI-TST: From 2006 in EWI-TST

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