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
magnetic materials and structures. However, the impact of the technique in academic and
industrial research has declined in recent years due to the lack of lateral magnetic resolution
and the possible influence of the tip stray field on the magnetization structure of the sample.
Moreover, a profound understanding of the contrast formation in an MFM experiment and
hence the quantification of measured MFM data remains elusive.
In the first chapter of this thesis, the contrast formation in an MFM image is described by a
transfer function theory. The theory in combination with experimental methods introduced
by van Schendel [9] allows the calibration of the imaging properties of an MFM tip and
therefore the quantitative interpretation of an MFM measurement (→ chapter 1).
All data presented in this thesis were acquired by either a low temperature ultra high vacuum
microscope (LTSFM) or a high resolution magnetic force microscope (hr-MFM) designed and
build for the investigation of hard disk media in an industrial environment (→ chapter 2).
In order to improve the lateral resolution down to about 10 nm, ultrasharp tips with a high
aspect ratio and a magnetic coating thickness of only a few nanometers have to be used. In
addition, the tip–sample distance during data acquisition has to be decreased to below 10 nm.
In this distance regime, non-magnetic forces, e.g. the van derWaals force, become comparable
to the magnetic force. As a consequence, new methods to separate the magnetic from the
non-magnetic contributions to an MFM measurement (and therefore to an MFM image) were
developed (→ chapter 3.1). Further, the influence of the tip geometry and its coating on the
magnetic imaging properties were studied. In high resolution MFM ultra sharp, fragile tips are
used, 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
place at tip–sample distances of only a few nanometers. In the last part of the chapter, the
degradation of the imaging properties of tips of various geometry, after tip–sample contacts
of increasing intensity were studied (→ chapter 3.3).
The high resolution images on hard disk media obtained with the hr-MFM has sparked the
interest in the magnetic recording industry in this technique. The knowledge gained in the
process of the work presented in the thesis and the wish of the industrial researchers to
perform high resolution MFM measurements on their hard disk media has led to an intense
interaction between the research group in Basel and various industrial labs. In several visits
to these labs during this thesis, the importance of a quantitative interpretation of hr-MFM
images of recording media became apparent. Up to now, the performance of hard disk media,
read and write heads was mostly characterized in spin stand experiments. Although, the
MFM images obtained with the hr-MFM resolved the micromagnetic structure of written
tracks and bits in the media in great detail, the interpretation of these images in relation
to typical quantities determined in spin stand experiments remained unclear. Hence, new
image analysis procedures were developed which allow the extraction of head and media
properties from the measured MFM data (→ chapter 4). In the process of the analysis, it
became apparent, that the recording performance of todays hard disk drives is determined
by nano-scale physics of the media and the write head stray field.
The continuous demand for increased storage density throughout the last decade could only
be met with an astonishingly rapid transfer of newly discovered physical phenomena into
products of the computer industry. One example of this kind is the use of the GMR effect [15,
16] in read heads of magnetic storage devices such as hard drives. The basis of such a read
head is a so-called spin-valve [87]. One of its components is a ferromagnetic (FM) layer with a
magnetization direction stabilized by the exchange coupling of an adjacent antiferromagnetic
(AF) layer. Although the effect of the exchange coupling is used in todays products, a detailed
qualitative and quantitative understanding of the mechanism is still lacking. It is commonly
accepted, that the interface of the AF and FM layer plays a key role. So far, the magnetic
structure of the interface has only been studied by XMCD methods. These methods have
the advantage of chemical sensitivity, thus in principle, the magnetic state of one specific
element can be accessed. However, the disadvantages are the limited spacial resolution of
XMCD microscopy methods, the complexity of the instrumentation,which involves a beam
line at a synchrotron and the long data acquisition times. The latter makes experiments that
require the variation of external parameters such as the temperature or the magnetic field,
tedious. In chapter 5, an MFM based method is discussed that allows the determination of the
uncompensated spin density with a superior lateral resolution and the study of the evolution
of the domain structure in the ferromagnetic layers as a function of an applied external field.