Structural phase diagram for ultra-thin epitaxial Fe₃O₄ / MgO(0 0 1) films: thickness and oxygen pressure dependence

Abstract

A systematic investigation of the thickness and oxygen pressure dependence for the structural properties of ultra-thin epitaxial magnetite (Fe3O4) films has been carried out; for such films, the structural properties generally differ from those for the bulk when the thickness ⩽10 nm. Iron oxide ultra-thin films with thicknesses varying from 3 nm to 20 nm were grown on MgO (0 0 1) substrates using molecular beam epitaxy under different oxygen pressures ranging from 1 × 10(-7) torr to 1 × 10(-5) torr. The crystallographic and electronic structures of the films were characterized using low energy electron diffraction (LEED) and x-ray photoemission spectroscopy (XPS), respectively. The quality of the epitaxial Fe3O4 ultra-thin films was judged by magnetic measurements of the Verwey transition, along with complementary XPS spectra. It was observed that under the same growth conditions the stoichiometry of ultra-thin films under 10 nm transforms from the Fe3O4 phase to the FeO phase. In this work, a phase diagram based on thickness and oxygen pressure has been constructed to explain the structural phase transformation. It was found that high-quality magnetite films with thicknesses ⩽20 nm formed within a narrow range of oxygen pressure. An optimal and controlled growth process is a crucial requirement for the accurate study of the magnetic and electronic properties for ultra-thin Fe3O4 films. Furthermore, these results are significant because they may indicate a general trend in the growth of other oxide films, which has not been previously observed or considered.

Evolution of magnetic properties in the vicinity of the Verwey transition in Fe3O4 thin films

“…In previous work, however, the Verwey transition in Fe3O4 thin films was very broad and the TV was quite low; the quality of these thin films should not be high enough (the presence of chemical off stoichiometry or many APBs), which would greatly influence the magnetic properties. In our previous work, we have grown quite high quality epitaxial Fe3O4 thin films; the Verwey transition is very sharp and the TV reaches to 122 K for 200-nm-thick Fe3O4 film on MgO 3 (001) [37], which is close to that of the single-crystal bulk and much higher than in previous work [15][16][17][41][42][43][44][45][46][47][48][49][50]. Therefore, to more comprehensively understand the magnetic behaviors in Fe3O4 thin films, it is very necessary to reinvestigate the magnetic properties in high-quality Fe3O4 thin films.…”

“…With lowering temperature across the Verwey transition, the transformation from cubic to monoclinic structure leads to a sharp jump for some magnetic parameters at TV , such as the rapid drop of magnetization and the sharp enhancement of coercivity. On the other hand, although a tremendous amount of work has been devoted to investigating the magnetic properties of Fe3O4 thin films [14][15][16][17][18]20,33,34,[39][40][41][42][44][45][46][47][48][49][50], the systematic study of magnetic properties of the thin films in the vicinity of the Verwey transition has been seldom reported [44,45]. Bollero et al [44] investigated the influence of thickness on microstructural and magnetic properties in Fe3O4 thin films.…”

Epitaxial Fe3O4 Films Grown on R-Plane Sapphire by Pulsed Laser Deposition

“…Magnetite has a cubic lattice (inverse spinel structure), so the (001) crystallographic orientation of Fe 3 O 4 films is of most interest for practical applications. Epitaxial magnetite films were previously grown on various single-crystal substrates, most often on MgO (001), because the lattice mismatch with such substrates is 0.34% [3,4]. Besides, magnetite is epitaxially compatible with iron (Fe) (001), which can be grown epitaxially on MgO substrates [5].…”

Nondeteriorating Verwey Transition in 50 nm Thick Fe3O4 Films by Virtue of Atomically Flattened MgO Substrates: Implications for Magnetoresistive Devices

“…Magnetite (Fe 3 O 4 ) is a transition metal oxide with excellent physical properties, such as a metal–insulator phase transition (MIT) called the Verwey transition at 120–125 K, − wherein the resistivity abruptly increases by 2 orders of magnitude. This attractive property of magnetite, i.e., the abrupt increase in resistivity, around the Verwey transition temperature ( T V ) is being explored for possible technological applications.…”

“…Numerous deposition methods, which include sputtering, molecular beam epitaxy, and pulsed laser deposition (PLD), have been explored for the fabrication of high-quality Fe 3 O 4 thin films on various substrates. However, achieving the Verwey transition on thin films with a thickness of less than 100 nm is still challenging − because of the defects induced, especially at the film/substrate interface, such as antiphase boundaries (APBs), oxygen vacancies, and misfit dislocations. The crystallinity and stoichiometry of the Fe 3 O 4 film are closely related to the properties of the Verwey transition: the appearance, temperature, and sharpness of the Verwey transition. − It was reported that a poor Verwey transition or its absence for Fe 3 O 4 nanostructured samples was caused by a high density of growth defects. − Fe 3 O 4 films grown on specific regulated substrates having low lattice mismatch with Fe 3 O 4 show a higher transition temperature than that of bulk Fe 3 O 4 , yet the sharpness of the Verwey transition, i.e., the magnitude of the resistance change and the broadness of the temperature change, is still deteriorated. , Thus, the ongoing demand for high device performance requires prevention of the degradation of the intrinsic functionalities of Fe 3 O 4 through the film growth process.…”