|dc.description.abstract||Phase transitions play an important role in many fields of physics and engineering, and their study in bulk materials has a long tradition.
Many of the experimental techniques involve measurements of thermodynamically extensive parameters.
With the increasing technological importance of thin-film technology there is a pressing need to find new ways to study phase transitions at smaller length-scales,
where the traditional methods are insufficient.
In this regard, the phase transitions observed in thin-films of MnAs present interesting challenges.
As a ferromagnetic material that can be grown epitaxially on a variety of technologically important substrates,
MnAs is an interesting material for spintronics applications.
In the bulk, the first order transition from the low temperature ferromagnetic $\alpha$-phase to the
$\beta$-phase occurs at 313~K. The magnetic state of the $\beta$-phase has remained controversial.
A second order transition to the paramagnetic $\gamma$-phase takes place at 398~K.
In thin-films, the anisotropic strain imposed by the substrate leads to the interesting phenomenon of
coexistence of $\alpha$- and $\beta$-phases in a regular array of stripes over an extended temperature range.
In this dissertation these phase transitions are studied in films grown by molecular beam epitaxy on GaAs (001).
The films are confirmed to be of high structural quality and almost purely in the $A_0$ orientation.
A diverse set of experimental techniques, germane to thin-film technology, is used to probe the properties of the film:
Temperature-dependent X-ray diffraction and atomic-force microscopy (AFM), as well as magnetotransport give insights into the structural properties,
while the anomalous Hall effect is used as a probe of magnetization during the phase transition.
In addition, reflectance difference spectroscopy (RDS) is used as a sensitive probe of electronic structure.
Inductively coupled plasma etching with BCl$_3$ is demonstrated to be effective for patterning MnAs.
We show that the evolution of electrical resistivity in the coexistence regime of $\alpha$- and $\beta$-phase can be understood in terms of a simple model.
These measurements allow accurate extraction of the order-parameter "phase fraction" and thus permit us to study the hysteresis of the phase transition in detail.
Major features in the hysteresis can be correlated to the ordering observed in the array of $\alpha$- and $\beta$-stripes.
As the continuous ferromagnetic film breaks up into isolated stripes of $\alpha$-phase,
a hysteresis in the out-of-plane magnetization is detected from measurements of the anomalous Hall effect.
The appearance of out-of-plane domains can be understood from simple shape-anisotropy arguments.
Remarkably, an anomaly of the Hall effect at low fields persists far into the $\beta$-phase.
Signatures of the more elusive $\beta$- to $\gamma$-transition are found in the temperature-dependence of resistivity, the out-of-plane lattice constant, and
reflectance difference spectra.
The transition temperature is significantly lowered compared to the bulk, consistent with the strained state of the material.
The negative temperature coefficient of resistivity, as well as its anisotropic changes, lend support to the idea of an antiferromagnetic order within the $\beta$-phase.||en_US