The role of uncompensated spins in exchange biased systems

In the last century, the role of magnetic materials has changed drastically. Of all the
electronic properties of solids, magnetism maybe became of interest to the widest range of
scientists and technologists. In addition to fundamental interests in magnetic properties
there is a large and growing technology based interest of the properties of magnetic materials.
Quite small improvements in permeability, coercivity or saturation magnetisation
can be of great economic significance.
On the other hand, the magnetic properties of antiferromagnetic materials were not of
technological interest until 1956 where Meiklejohn and Bean reported: “A new type of
magnetic anisotropy has been discovered which is best described as an exchange anisotropy.
This anisotropy is the result of an interaction between an antiferromagnetic material and
a ferromagnetic material”. Meiklejohn and Beans discovery was initiated by the observation
that the hysteresis loop of a sample of nominal cobalt nanoparticles was shifted along
the field axis after cooling in an applied field. It was subsequently established that the
particles had been partially oxidised to CoO which is an antiferromagnet.
A biased magnetisation direction in a ferromagnet (FM) provided by an antiferromagnet
(AFM), the so-called exchange bias (EB) effect, is nowadays essential to state-of-the-art
magnetic read-head technology, highly sensitive magnetic field sensors and MRAM (magnetoresistive
random access memory) devices. For the above mentioned technologies, the
EB-effect plays a key role even though a complete description of the microscopic coupling
mechanism is still missing. It is widely accepted that the origin of the EB-effect can be
traced back to the existence of pinned uncompensated spins (UCS) in the antiferromagnet
(AFM) or at its interface. Such UCS have been observed by various experimental
techniques. In a simple extension of the model originally proposed by Meicklejohn and
Bean, the observed small size of the exchange bias field could be related to pinned UCS.
The compensated interfacial spins and the rotating (non-pinned) UCS were found not
to contribute to the exchange bias effect. However, the understanding of the underlaying
mechanism is still clouded by contradictory reports: For example, both a parallel
as well as an antiparallel orientation of the UCS relative to the magnetization direction
of the ferromagnet were reported for systems containing the same AFM and FM materials.
In this thesis, two different EB-systems were investigated by low-temperature magnetic
force microscopy (MFM) and vibrating sample magnetometry (VSM). These complementary
experimental techniques allow us to image the spatial distribution, orientation and
density of the UCS at nanometer scale (MFM) and to determine their orientation and
density in various externally applied magnetic fields (VSM). Different magnetisation histories in magnetometry and MFM measurements are used advantageously to demonstrate
the co-existence of pinned UCS that are parallel and antiparallel to the cooling field in
metallic (IrMn) and oxidic(CoO) EB-systems. We further conclude that the EB-effect is
a result of pinned interfacial UCS, which are antiparallel to the FM spins. The often observed
positive vertical shift of the magnetisation loop after field cooling is due to pinned
UCS that align parallel to the cooling field, but are of little importance for the EB-effect
itself.
Furthermore we present a MFM study of an AFM/FM bilayer which, for the first
time, reveals that the UCS-density undergoes strong variations on single grain scale. The
large variations of the UCS-density observed on single grain scale are explained within a
simple statistical approach. The transmission electron microscopy (TEM) images reveal
that our sample satisfies the conditions of the model proposed by Takano et al.: sharp
grain interfaces with only few crystalline, atomic steps. The small number of steps per
grain generates a limited distribution of terrace sizes which leads to poor statistics and as
a consequence to a strong local variation of the UCSD, as indeed measured.
Quantitatively, three different areas can be distinguished: (1) regions with UCS aligned
antiparallel to the FM. (2) regions where no UCS exist. (3) regions with UCS aligned
parallel to the FM. Note that the regions (1) dominate such that on average the UCS are
aligned antiparallel to the FM spins. It is interesting to see that in an applied field, the FM
domains always “retract” to the regions (1), containing the UCS aligned antiparallel to
the FM, avoiding the regions (3). The fact that the FM domains retract from these areas
suggests that the locations with parallel coupling of the UCS exhibit a reduced exchange
coupling strength compared to the antipalallel coupled UCS. In addition, they seem to
weaken the overall exchange bias field and are thus defined as anti-biasing regions.
Microscopically, the observed anti-biasing regions are explained by a direct coupling
between neighbouring AFM grains. TEM images show a wide range of grain boundery
tilt-angles. We thus expect in some cases a strong direct coupling between AFM grains
(small tilt angles), in others we expect decoupled or weakly coupled grains (large tilt
angles). We suggest that a strong direct coupling between neighbouring AFM grains
may lead to the observed anti-biasing regions. From this simple picture we conclude that
sophisticated grain boundary engineering leading to decoupled AFM grains, is one possible
way to increase the EB effect. For instance we propose the co-deposition of Cr and Co for
the AFM layer. The segregation of Cr along the boundaries would then lead to decoupled
grains.
This work may provide guidelines for the design of experiments which correctly determine
the densities of those UCS that do contribute to the EB-effect. A considerably
improved microscopic understanding of the exchange coupling in polycristalline thin films
raises the possibility of an enhancement of the EB-effect by an order of magnitude.