How Metallic Fe Controls the Composition of its Native Oxide

Abstract

We have studied in situ the oxidation of ultrathin iron layers and monitored the chemical changes induced by subsequent deposition of Fe metal using hard x-ray absorption spectroscopy. The site sensitivity of the technique allows us to quantify the composition of the layer throughout the oxidation or deposition process. It is found that the thin native oxide incorporates a significant fraction of Fe atoms remaining in a metallic configuration even in the saturated state. Subsequent deposition of Fe leads to a complete reduction of the oxide that adopts an FeO-like structure containing Fe 2 sites only. DOI: 10.1103/PhysRevLett.101.056101 PACS numbers: 81.15.ÿz, 61.05.cj, 81.16.Pr In recent years, intensive research has been conducted on transition metal oxide compounds, thin films, and multilayers due to their potential use as building blocks in magnetic structures like spin valves and magnetic tunnel junctions (for a review, see [1] and references therein). Recently, metal -native-oxide multilayers (MNOM, produced by metal deposition and subsequent oxygen exposure) have been proposed as new compounds combining high magnetization and low conductivity [2]. In these systems, the buried oxide possesses magnetic properties that strongly deviate from any bulk iron oxide phase [2,3]. However, the influence of the surrounding metal on the structure of the oxide is not yet identified and prevents clear understanding of the observed magnetic properties of MNOM systems.The oxidation of iron itself has been extensively studied in the past decades. Studies performed on polycrystalline [4] as well as on single crystalline [5] surfaces have shown that the oxidation of iron can be understood in the framework of the coupled current mechanism introduced by Fromhold and Cook more than 50 years ago [6]. While those experiments explain very well the observed growth kinetics, the actual composition of the native oxide is still a matter of debate and is obviously highly dependent on preparation parameters like temperature and surface orientation ([7-11] and references therein). X-ray absorption spectroscopy at the L edges was recently used to characterize the extent of oxidation or reduction processes arising at the interface between Fe and other transition metal oxides (CoO and NiO) [12]. Remarkably, the interaction of Fe with its own oxide phases was never quantitatively studied before.In this Letter, we present a quantitative x-ray absorption spectroscopy (XAS) study of the oxidation of ultrathin iron layers and the changes induced by subsequent coverage with Fe. The sensitivity of the XAS signal to the oxidation state and the local environment of the absorbing atom is used to follow the evolution of the oxide's structure in a quantitative and site specific manner. The use of ultrathin layers (in contrast to other studies which used thick substrates [4,5,[7][8][9]) allows one to directly resolve the intrinsic composition of the growing oxide and disentangle the influence of the substrate.The experiment was performed in situ in...

X-ray magnetic circular dichroism study of the blocking process in nanostructured iron-iron oxide core-shell systems

“…Also, considering the oxidation process in transition metals, 5,25 it should be expected that the oxidation is progressive, and thus the higher oxidation states (γ -Fe 2 O 3 ) are present in the outer regions of the core-shell entity, whereas the lower oxidation state (FeO) is expected to be present at the interface metal-oxide core-shell regions. This model is illustrated in the inset of Fig.…”

“…1,2 This effort has recently been aided by developments in preparation methods, 3,4 resulting in, e.g., the precise control of the composition of the metal oxide 5 and the production of compacted metal-oxide granules, in which metal nanoparticles are uniformly embedded in an oxide matrix. Such an enhanced magnetic stability, in terms of increasing coercivity and remanence, as well as the effective decoupling of the magnetic moments of the particles (superspins) with respect to other particles, are provided by the exchange anisotropy, which is due to strong exchange interactions between the core and shell spins.…”

Decoupling mechanisms and magnetic stability of nanostructured iron chains prepared by sputtering

“…Such a collective behavior is commonly prevented by covering the particle surfaces with native-oxide shells. 6 Metal/native-oxide systems obtained by metal deposition and subsequent oxygen exposure 7,8 have been proposed as promising materials for their combination of high magnetization and low conductivity, also providing a key response in the enhancement of magnetic stability. Precise manipulation of the particle surface oxidation allows tuning the magnetic and transport properties of core-shell systems through modifications in the spin configuration of metal and oxide phases, 9,10 with obvious interest for implementation nanoscale magnetoresistive sensors.…”

Nuclear resonant scattering of synchrotron radiation: Applications in magnetism of layered structures

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