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Effects of native defects on the structural and magnetic properties of hematite alpha-Fe2O3
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.ORCID iD: 0000-0002-7315-9966
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.ORCID iD: 0000-0002-9050-5445
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 4, 045136- p.Article in journal (Refereed) Published
Abstract [en]

Antiferromagnetic and ferromagnetic configurations of hematite alpha-Fe2O3 structures have been investigated by first-principles methods, which have been used to theoretically analyze the local structural and magnetic effects due to the presence of interstitial atoms such as hydrogen and oxygen. This study is based on the projector-augmented wave method within the local spin density approximation (LSDA) in addition to an on-site Coulomb correction of the Fe d orbitals (i.e., the LSDA + U-SIC method). The results demonstrate that this correction potential is important in achieving an accurate description of both the structural and the magnetic properties of alpha-Fe2O3. The ground state of alpha-Fe2O3 is the antiferromagnetic phase. The presence of oxygen vacancies, interstitial oxygen, and an interstitial hydrogen decreases the local Fe magnetic moment vertical bar M-s(Fe)vertical bar in both the ferro- and antiferromagnetic phases, although the hydrogen has a rather modest effect. The density of states calculations demonstrate that the presence of interstitial atoms and defects yields a small reduction in the material's electronic gap.

Place, publisher, year, edition, pages
2013. Vol. 88, no 4, 045136- p.
Keyword [en]
Lithium-Ion Battery, Augmented-Wave Method, Electrical-Properties, Oxide, Nanoparticles, Temperature, Insulators, Nanotubes, Hydrogen, Oxygen
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-127498DOI: 10.1103/PhysRevB.88.045136ISI: 000322575500007ScopusID: 2-s2.0-84883145674OAI: diva2:644799
Swedish Energy AgencySwedish Research Council

QC 20130902

Available from: 2013-09-02 Created: 2013-08-30 Last updated: 2013-11-07Bibliographically approved
In thesis
1. Memory Effects on Iron Oxide Filled Carbon Nanotubes
Open this publication in new window or tab >>Memory Effects on Iron Oxide Filled Carbon Nanotubes
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this Licentiate Thesis, the properties and effects of iron and iron oxide filled carbon nanotube (Fe-CNT) memories are investigated using experimental characterization and quantum physical theoretical models. Memory devices based on the simple assembly of Fe-CNTs between two metallic contacts are presented as a possible application involving the resistive switching phenomena of this material.

It is known that the electrical conductivity of these nanotubes changes significantly when the materials are exposed to different atmospheric conditions. In this work, the electrical properties of Fe-CNTs and potential applications as a composite material with a semiconducting polymer matrix are investigated. The current voltage characteristics are directly related to the iron oxide that fills the nanotubes, and the effects are strongly dependent on the applied voltage history. Devices made of Fe-CNTs can thereby be designed fo gas sensors and electric memory technologies.

The electrical characterization of the Fe-CNT devices shows that the devices work with an operation ratio (ON/OFF) of 5 μA. The applied operating voltage sequence is -10 V (to write), +8 V (to read ON), +10 V (to erase) and +8 V (to read OFF) monitoring the electrical current. This operation voltage (reading ON/OFF) must be sufficiently higher than the voltage at which the current peak appears; in most cases the peak position is close to 5 V. The memory effect is based on the switching behavior of the material, and this new feature for technological applications such as resistance random access memory (ReRAM).

In order to better understand the memory effect in the Fe-CNTs, thesis also presents a study of the surface charge configuration during the operation of the memory devices. Here, Raman scattering analysis is combined with electrical measurements. To identify the material electronic state over a wide range of applied voltage, the Raman spectra are recorded during the device operation and the main Raman active modes of the carbon nanotubes are studied. The applied voltage on the carbon nanotube G-band indicates the presence of Kohn anomalies, which are strongly related to the material’s electronic state. As expected, the same behavior was shown by the other carbon nanotube main modes. The ratio between the D- and G-band intensities (ID/IG) is proposed to be an indicative of the operation’s reproducibility regarding a carbon nanotube memory cell. Moreover, the thermal/electrical characterization indicates the existence of two main hopping charge transports, one between the carbon nanotube walls and the other between the filling and the carbon nanotube. The combination of the hopping processes with the possible iron oxide oxygen migration is suggested as the mechanism for a bipolar resistive switching in this material.

Based on these studies, it is found that the iron oxide which fills the carbon nanotube, is a major contribution to the memory effect in the material. Therefore, a theoretical study of hematite (i.e., α-Fe2O3) is performed. Here, the antiferromagnetic (AFM) and ferromagnetic (FM) configurations of α-Fe2O3 are analyzed by means of an atomistic first-principles method within the density functional theory. The interaction potential is described by the local spin density approximation (LSDA) with an on-site Coulomb correction of the Fe d-orbitals according to the LSDA+U method. Several calculations on hematite compounds with high and low concentrations of native defects such as oxygen vacancies, oxygen interstitials, and hydrogen interstitials are studied. The crystalline structure, the atomic-resolved density-of-states (DOS), as well as the magnetic properties of these structures are determined.

The theoretical results are compared to earlier published LSDA studies and show that the Coulomb correction within the LSDA+U method improves both the calculated energy gaps and the local magnetic moment. Compared to the regular LSDA calculations, the LSDA+U method yields a slightly smaller unit-cell volume and a 25% increase of the local magnetic for the most stable AFM phase. This is important to consider when investigating the native defects in the compound. The effect is explained by better localization of the energetically lower Fe d-states in the LSDA+U calculations. Interestingly, due to the localization of the d-states the intrinsic α-Fe2O3 is demonstrated to become an AFM insulator when the LSDA+U method is considered.

Using the LSDA+U approach, native defects are analyzed. The oxygen vacancies are observed to have a local effect on the DOS due to the electron doping. The oxygen and hydrogen interstitials influence the band-gap energies of the AFM structures. Significant changes are observed in the ground-state energy and also in the magnetization around the defects; this is correlated to Hund’s rules. The presence of the native defects (i.e., vacancies, interstitial oxygen and interstitial hydrogen) in the α-Fe2O3 structures changes the Fe–O and Fe–Fe bonds close to the defects, implying a reduction of the energy gap as well as the local magnetization. The interstitial oxygen strongly stabilizes the AFM phase, also decreases the band-gap energy without forming any defect states in the band-gap region.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. x, 48 p.
Memory devices, Carbon Nanotubes, energy, nanotechnology, Iron Oxide
National Category
Nano Technology
urn:nbn:se:kth:diva-133566 (URN)978-91-7501-885-0 (ISBN)
2013-11-20, Kuben N111, Brinellvägen 23, Kungliga tekniska högskolan,, Stockholm, 12:52 (English)

QC 20131107

Available from: 2013-11-07 Created: 2013-11-06 Last updated: 2013-11-07Bibliographically approved

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