As and Epitaxial-Growth MnSi Thin Films - OPUS Würzburg

Summary
The study of magnetic phases in spintronic materials is crucial to both our fundamental
understanding of magnetic interactions and for finding new effects for future applications.
In this thesis, we study the basic electrical and magnetic transport properties of both
epitaxially-grown MnSi thin films, a helimagnetic metal only starting to be developed
within our group, and parabolic-doped ultra-thin (Ga,Mn)As layers for future studies
and applications.
In the first part of this thesis we focus on the study of the magnetic anisotropies in ultrathin
(Ga,Mn)As with a novel parabolic doping profile and the effects of an applied electric
field. The main goal is to observe the magnetic properties of the material as it is driven
to the metal-insulator transition by charge depletion. The first section introduces the
ferromagnetic semiconductor (Ga,Mn)As and its properties, focusing particularly on the
concepts important for this work. Aside from basic information regarding its structural
and magnetic properties (e.g. magnetic anisotropies), we describe the effects of ultrathin
growth of this material on the magnetocrystalline anisotropy. We also look at the
behavior of its transport properties as it is driven through the metal-insulator transition
and also give some detail on previous work on conductance fluctuations in (Ga,Mn)As.
Forour workonelectrical gatingofultra-thin(Ga,Mn)As, we usean all-semiconductor
p-n tunnel barrier.[Owen 09] We used both standard macroscopic Hall bar and fourterminal
Corbino structures for our measurements with DC back-gate. Our experiments
on the Hall bar device have shown increased contributions by the hard uniaxial
anisotropies in magnetoresistance measurements. This is consistent with transport
properties of samples near the hopping regime and in the vicinity of the metal-insulator
transition.[Rush 06, Jung 06] Monotonic change is observed in the uniaxial contributions
for the large Hall bar structure, showing decreasing asymmetry between the uniaxial hard
axes. For Hall measurements, we also see monotonic decrease in the Hall signal as the
gate voltage is increased, again consistent with the sample being driven closer to the
non-magnetic insulating state.[Ohno 00]
We also make use of a four-terminal Corbino structure to extract pure crystalline
contributions to the magnetoresistance. Reducing the dimension of the device is shown to
drive thesystem into a conduction regime wherein the fluctuations become prominent and
finally affect the magnetic behavior of the material. The magnetoresistance (in-plane and
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