Structural peculiarities of ε\varepsilon-Fe2_2O3_3 / GaN epitaxial layers unveiled by high-resolution transmission electron microscopy and neutron reflectometry

The present paper is dedicated to the structural study of crystallographic peculiarities appearing in epitaxial films of metastable epsilon iron oxide ($\varepsilon$-Fe$_2$O$_3$) grown by pulsed laser deposition onto a semiconductor GaN (0001) substrate. The columnar structure of the nanoscale $\varepsilon$-Fe$_2$O$_3$ films has been for the first time investigated using high resolution electron microscopy (HRTEM) direct space technique complemented by reciprocal space methods of high-energy electron diffraction and color-enhanced HRTEM image Fourier filtering. The study of $\varepsilon$-Fe$_2$O$_3$ / GaN interface formation has been further expanded by carrying out a depth resolved analysis of density and chemical composition by neutron reflectometry and energy-dispersive X-ray spectroscopy. The obtained results shed light onto the properties and the origin of the enigmatic few-nanometer thick low density transition layer residing at the $\varepsilon$-Fe$_2$O$_3$ / GaN interface. A detailed knowledge of the properties of this layer is believed to be highly important for the development of $\varepsilon$-Fe$_2$O$_3$ / GaN heterostructures that can potentially become part of the iron-oxide based ferroic-on-semiconductor devices with room temperature magneto-electric coupling.Comment: 14 pages, 9 figure

Technological Peculiarities of Epsilon Ferrite Epitaxial Stabilization by PLD

The present paper describes the technological peculiarities relevant to the nucleation and further epitaxial growth of the metastable epsilon phase of iron oxide by means of pulsed laser deposition (PLD). The orthorhombic epsilon ferrite ε-Fe2O3 is an exotic member of a large family of iron oxide polymorphs, which attracts extensive attention nowadays due to its ultra-high magneto-crystalline anisotropy and room temperature multiferroic properties. Continuing the series of previous publications dedicated to the fabrication of ε-Fe2O3 films on GaN, this present work addresses a number of important requirements for the growing conditions of these films. Among the most sensitive technological parameters, the growth temperature must be high enough to aid the nucleation of the orthorhombic phase and, at the same time, low enough to prevent the thermal degradation of an overheated ε-Fe2O3/GaN interface. Overcoming the contradicting growth temperature requirements, an alternative substrate-independent technique to stabilize the orthorhombic phase by mild aluminum substitution is proposed. The advantages of this technique are demonstrated by the example of ε-Fe2O3 films PLD growth carried out on sapphire—the substrate that possesses a trigonal lattice structure and would normally drive the nucleation of the isostructural and energetically more favorable trigonal α-Fe2O3 phase. The real-time profiling of high-energy electron diffraction patterns has been extensively utilized throughout this work to keep track of the orthorhombic-to-trigonal balance being the most important feed-back parameter at the growth optimization stage

Magnetization reversal in YIG/GGG(111) nanoheterostructures grown by laser molecular beam epitaxy

Thin (4–20 nm) yttrium iron garnet (Y3Fe5O12, YIG) layers have been grown on gadolinium gallium garnet (Gd3Ga5O12, GGG) 111-oriented substrates by laser molecular beam epitaxy in 700–1000 °C growth temperature range. The layers were found to have atomically flat step-and-terrace surface morphology with step height of 1.8 Å characteristic for YIG(111) surface. As the growth temperature is increased from 700 to 1000 °C the terraces become wider and the growth gradually changes from layer by layer to step-flow regime. Crystal structure studied by electron and X-ray diffraction showed that YIG lattice is co-oriented and laterally pseudomorphic to GGG with small rhombohedral distortion present perpendicular to the surface. Measurements of magnetic moment, magneto-optical polar and longitudinal Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD) were used for study of magnetization reversal for different orientations of magnetic field. These methods and ferromagnetic resonance studies have shown that in zero magnetic field magnetization lies in the film plane due to both shape and induced anisotropies. Vectorial MOKE studies have revealed the presence of an in-plane easy magnetization axis. In-plane magnetization reversal was shown to occur through combination of reversible rotation and abrupt irreversible magnetization jump, the latter caused by domain wall nucleation and propagation. The field at which the flip takes place depends on the angle between the applied magnetic field and the easy magnetization axis and can be described by the modified Stoner–Wohlfarth model taking into account magnetic field dependence of the domain wall energy. Magnetization curves of individual tetrahedral and octahedral magnetic Fe3+ sublattices were studied by XMCD