| dc.description.abstract | In the present thesis, we have explored multiple aspects concerning the stochasticity
of magnetic domain wall motion during magnetization reversal, all of which
originated from our initial study of magnetic field sensing using planar Hall effect.
Magnetic field sensors occupy a very important and indispensable position in modern technology. They can be found everywhere, from cellphones to automobiles,
electric motors to computer hard disks. At present there are several emerging areas
of technology, including biotechnology, which require magnetic field sensors which
are at the same time simple to use, highly sensitive, robust under environmental
conditions and sufficiently low cost to be deployed on a large scale. Magnetic field
sensing using planar Hall effect is one such feasible technology, which we have explored
in the course of the thesis. The work was subsequently expanded to cover some fundamental aspects of the stochasticity of domain wall motion, studied with planar Hall effect, which forms the main body of work in the present study.
In Chapter 1, we give an introduction to the phenomenology of planar Hall effect, which is the most important measurement technique used for all the subsequent studies. Some early calculations, which had first led to the understanding of anisotropic magnetoresistance and planar Hall effect as being caused by spin-orbit
interaction are discussed.
In Chapter 2, we discuss briefly the experimental techniques used in the present
study for sample growth and fabrication, structural and magnetic characterization,
and measurement. We discuss pulsed laser ablation, which is the main technique
used for our sample growth. Particular emphasis is given to the instrumentation
that was carried out in-house for MOKE and low field magnetotransport (AMR
and PHE) measurement. This includes an attempt at domain wall imaging through
MOKE microscopy. Some of the standard equipments used for this work, such as
the SQUID magnetometer and the acsusceptometer are also discussed in detail.
In Chapter 3 we discuss our work on planar Hall sensors that led to the fabrication
of a device with a very simple architecture, having transfer characteristics of
650V/A.T in a range of _2Oe. The sensing material was permalloy (Ni81Fe19), and
the value had been obtained without using an exchange biased pinning layer. Field
trials showed that the devices were capable of geomagnetic field sensing, as well as
vehicle detection by sensing the anomaly in Earth's magnetic field caused by their
motion. Its estimated detection threshold of 2.5nT made it well suited for several
other applications needing high sensitivity in a small area, the most prominent of
them being the detection of macromolecules of bio-medical significance.
Chapter 4: The work on Barkhausen noise was prompted by reproducibility problems faced during the sensor construction, both between devices as well as within the same device. Study of the stochastic properties led us to the conclusion that the devices could be grouped into two classes: one where the magnetization reversal occurred in a single step, and the other where it took a 0staircase0 like path with multiple steps. This led us to simulations of Barkhausen noise using nucleation models like the RFIM whence it became apparent that the two different groups of samples could be mapped into two regimes of the RFIM distinguished by their magnetization reversal mode. In the RFIM, the nature of the hysteresis loop depends on the degree of disorder, with a crossover happening from single-step switching to multi-step switching at a critical disorder level. Appropriate changes also appear in the Barkhausen noise statistics due to this disorder-induced crossover. By studying the Barkhausen noise statistics for our permalloy samples and comparing them with simulations of the RFIM, we found nearly exact correspondence between the two experimental groups with the two classes resulting from crossing the critical disorder.
What remained was to quantify the 0disorder0 level of our samples, which was done
through XRD, residual resistivity and a study of electron-electron interaction effects
in the resistivity. All these studies led to the conclusion that the samples reversing
in multiple steps were more 0defective0 than the other group, at par with the model
predictions. This completed the picture with respect to the modeling of the noise. In
experiments, it was found that a high rate of film deposition yielded less 0defective0
samples, which severed as an important input for the sensor construction.
These results can be viewed from a somewhat broader perspective if we consider the present scenario in the experimental study of Barkhausen noise, or crackling noise in general. Two classes of models exist for such phenomena: front propagation models and nucleation models. Both appear to be very successful when it comes to experiments with bulk materials, while the comparison with experiments on thin films is rather disappointing. It is still not clear whether the models are at fault or the experiments themselves. Through our study, we could demonstrate that there can be considerable variation in the Barkhausen noise character of the same material deposited in the same way, and what was important was the degree of order at the microscopic level. This may be a relevant factor when experimental papers report non-universality of Barkhausen noise in thin films, which can now be interpreted as either insufficient defects or a sample area too small for the study.
Chapter 5: Defects in a sample are not the only cause for stochastic behavior
during magnetization. In most cases, random thermal 0events0 are also an important
factor determining the path to magnetization reversal, which was also true for our
permalloy samples. We studied the distribution of the external fields at which
magnetization reversal took place in our samples, and tried to explain it in terms
of the popular Neel-Brown model of thermal excitation over the anisotropy barrier.
The analysis showed that even though the coercivity behaved 0correctly0 in terms
of the model predictions, the behavior of the distribution width was anomalous.
Such anomalies were common in the literature on switching field distributions, but
there seemed to be no unified explanation, with different authors coming up with
their own 0exotic0 explanations. We decided to investigate the simplest situations
that could result in such a behavior, and through some model-based calculations,
came to the conclusion that one of the causes of the anomalies could be the different
magnitudes of barrier heights/anisotropy fields experienced by the magnetic domain
wall when the reversal occurs along different paths. Though an exact match for the
behavior of the distribution width could not be obtained, the extended Neel-Brown
model was able to produce qualitative agreement.
Chapter 6 contains a study of some interesting 0geometrical0 effects on Barkhausen noise of iron thin films. By rotating the applied magnetic field out-of plane, we could observe the same single-step to multi-step crossover in hysteresis loop nature that was brought about by varying disorder in Chapter 4. We could explain this through simulations of a random anisotropy Ising model, which, apart from exhibiting the
usual disorder induced crossover, showed a transition from sub-critical to critical
hysteresis loops when the external field direction was rotated away form the average
anisotropy direction. Once again, simulation and experiment showed very good agreement in terms of the qualitative behavior.
In the second part of this chapter, a study of exchange biased Fe-FeMn system was carried out, where it was observed that the reversal mode has been changed from domain wall motion to coherent rotation. Barkhausen noise was also suppressed.
Though many single-domain models existed for this type of reversal, our system was not found to be strictly compatible with them. The disagreement was with regard to the nature of the hysteresis, which, if present, had to be a single step process for a single domain model. The disagreement was naturally attributed to interaction with the nearby magnetic moments, to verify which, simulations were done with a simplified micromagnetic code, which produced excellent agreement with experiment.
In Chapter 7, we have studied the temporal properties of Barkhausen avalanches, to compare the duration distributions with simulation. We had used a permalloy
sample that was sub-critical according to avalanche size distributions, and our measurement was based on magneto-optic Kerr effect. We measured duration distributions
which showed a similar manifestation of finite-size effects as were shown by the size distributions. The power law exponent was calculated, which was deemed 0reasonable0 upon comparison simulations of the sub-critical RFIM.
Appendix A contains a study of high-field magnetoresistance of permalloy, which shows that the dominant contribution to magnetoresistance is the suppression
of electron-magnon scattering. An interesting correlation is observed between the
magnetization of samples and an exchange stiffness parameter d1, that was extracted
from magnetoresistance measurements. Here we also re-visit our earlier observation
of permalloy thin films possessing a resistance minimum at low temperature. The
origin of this minimum is attributed to electron-electron interaction.
Appendix B contains the source codes for most of the important programs used for simulation and data analysis. The programs are written in MATLAB and FORTRAN 95. LabView programs used for data acquisition and analysis are not included due to space requirements to display their graphical source codes.
Appendix C discusses the studies on a disordered rare-earth oxide LaMnO3.
The re-entrant glassy phase is characterized with ac susceptibility and magnetization
measurements to extract information about the nature of interactions between the
magnetic 0macrospins0 in the system.
Appendix D deals with electron scattering experiments performed with spinpolarized
electrons (SPLEED) from clean metal surfaces in UHV. A study of the scattering cross sections as a function of energy and scattering angle provides information
about spin-orbit and exchange interactions of the electrons with the surface atoms, and can answer important questions pertaining to the electronic and magnetic structure of surfaces.
In the course of this study, planar Hall effect is seen to emerge as a powerful tool
to study the magnetic state of a thin film, so that it is interesting to apply it to thin
films of other materials such as oxides, where magnetization noise studies are next
to nonexistent. What also emerged is that there is still a lot of richness present in
the details of supposedly well-understood magnetization phenomena, some of which
we have explored in this thesis in the context of stochastic magnetization processes. | en_US |
