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Magnetic Oxide Heterostructures: EuO on Cubic Oxides ... - JuSER
Magnetic Oxide Heterostructures: EuO on Cubic Oxides ... - JuSER
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4. Results I: Single-crystalline epitaxial EuO thin films on cubic<br />
oxides<br />
Researchers engineering oxide heterostructures and<br />
layer-by-layer film growers [...] candramatically accelerate the<br />
rate at which new functional oxides are discovered, enabling<br />
innovative materials for spintronics or new energy technologies.<br />
(after J. Chakhalian, 2012 146 )<br />
A key prerequisite of functionalizing magnetic oxides as spin filter materials is a high-quality<br />
stoichiometric synthesis, which ensures a bulk-like ferromagnetism.<br />
In this chapter, we discuss the synthesis of ultrathin EuO and investigate the impact of different<br />
cubic substrates on the electronic properties of epitaxial EuO thin films. First, we<br />
investigate a system with perfect lattice match, EuO on yttria-stabilized zirconia (YSZ). This<br />
system is perfectly suited to study and optimize the growth of single-crystalline EuO thin<br />
films, since it allows for coherent growth. Second, we characterize electron beam-treated<br />
YSZ with unaltered crystal structure and a certain electrical conductivity. This “conductive<br />
YSZ” substrate opens up the possibility to perform electron diffraction and photoelectron<br />
emission experiments on otherwise insulating oxides.<br />
Aiming towards ultrathin EuO films in the nanometer regime for application as tunnel barriers,<br />
we investigate the magnetic behavior of single-crystalline epitaxial EuO from bulk thickness<br />
down to one nanometer. We confirm the stoichiometry and chemical cleanliness of EuO<br />
on conductive YSZ, and investigate the depth-dependent chemical homogeneity by high resolution<br />
HAXPES core-level spectra. Finally, taking advantage of the MCD in photoemission,<br />
we investigate the spin-dependent electronic structure of ultrathin magnetic EuO films.<br />
Tuning the lateral lattice parameter of thin EuO films is not only of interest from the synthesis<br />
point of view, but also fundamentally changes the magnetic behavior of the EuO film under<br />
the tensile or compressive force of the underlying lattice. We discuss epitaxially grown EuO<br />
on LaAlO 3 (100) which provides a moderate tensile strain, and investigate magnetic properties<br />
by SQUID and hard X-ray magnetic circular dichroism. Finally, we investigate a large<br />
compressive strain in the heteroepitaxial system EuO on MgO (100).<br />
Key parameters for stoichiometric single-crystalline EuO growth on oxides<br />
First, we discuss a route how to stabilize thin films of the metastable magnetic oxide EuO<br />
by reactive MBE. Stoichiometric EuO is divalent and formed with comparable thermodynamic<br />
probability as the Eu oxides with higher valency, as recognized from the Ellingham<br />
diagram depicted in Fig. 4.1a. The Gibbs free energies of all possible Eu oxide phases are located<br />
left to an intersection (out of range) with the gas line of p(O 2 )=1× 10 −9 mbar (dashed<br />
line) which reveals that the oxidation reaction of Eu + O 2 is favored and stable for nearly<br />
all temperatures and oxygen pressures. Since the mixed-valent Eu 3 O 4 and the trivalent native<br />
Eu 2 O 3 compounds have complex non-cubic structures and are not ferromagnetic, they<br />
The thermodynamic potentials and the Ellingham diagram are introduced in Ch. 2.1.<br />
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