Epsilon Iron Oxide Research Articles

Formation of the hard-magnetic epsilon iron oxide phase from akaganéite nanoparticles

Elongated akaganéite (β-FeOOH) nanoparticles were prepared by a forced hydrolysis route using FeCl3·6H2O employing various urea concentrations.β-FeOOH nanoparticles stabilized within the SiO2matrix were annealed at different temperatures, ranging from 500 °C to 1300 °C. It was observed thatβ-FeOOH underwent a temperature-induced conversion toγ-Fe2O3and subsequently toϵ-Fe2O3. Due to theϵ-Fe2O3phase formation, the coercivity rapidly increased to 16 kOe for samples annealed at 900 °C and reached values up to 21.5 kOe when annealed at 1200 °C. At a higher temperature of 1300 °C, theϵ-Fe2O3phase transforms mainly into theα-Fe2O3phase, which causes the coercivity to rapidly drop to negligible values.

Impact of Hard Magnetic Nanocrystals on the Properties of Hardened Cement Paste

AbstractIn this work, nano‐sized hard magnetic gallium‐substituted iron oxide crystals, wherein gallium is used to stabilize the metastable epsilon iron oxide phase, were added to cement‐water suspensions at different ratios, which were subsequently hydrated for at least 28 days. It is shown that higher contents of such nanocrystals in the hardened cement paste introduce a magnetic moment, whereas the mechanical properties remain unchanged compared to non‐blended hardened cement paste for a wide concentration range.

Seed-mediated Generation of Epsilon Iron Oxide Nanocrystals at 300 °C

Epsilon iron oxide (ε-Fe2O3) is a material with a high coercive field. Herein we report seed-mediated generation of ε-Fe2O3. The generation of ε-Fe2O3 can be accelerated with the calcination of Prussian blue (PB) by a proliferative reaction, using ε-Fe2O3 nanoparticles as a seed. The present synthetic reaction occurs at a temperature below 300 °C, suggesting this is a promising method for low-temperature synthesis of ε-Fe2O3.

Sub-terahertz/terahertz electron resonances in hard ferrimagnets

Intensive development of ultrafast electronics requires materials with high-frequency spin dynamics. In this light, the insulators that possess the magnetization precession phenomenon due to magnetic anisotropy are dark horses. On the one hand, modern hard magnetic materials reveal relatively moderate resonance frequencies of the ferromagnetic mode (generally, dozens of GHz), which are lower than the frequencies of the antiferromagnetic resonances; on the other hand, the research in this area is quite scanty, which implies a room for a breakthrough. Here, an example of a hard ferrimagnetic insulator (cobalt ferrite CoFe2O4) was obtained in the form of nanoparticles and bulk ceramics via high-temperature methods. Due to high magnetic anisotropy fields, the samples in a single domain state show broad hysteresis loops. The materials also possess intensive resonance absorption at frequencies higher than 0.20 THz in zero external magnetic fields. For the first time, natural ferromagnetic resonance (NFMR) frequencies higher than 0.30 THz were registered. The ceramic sample demonstrates the highest-known NFMR frequency of 0.35 THz. The model based on the Landau-Lifshitz equation was developed to explain the demonstrated magnetodynamic properties and shed light on those of hard ferrimagnets in general. The practical application of the electron resonances in hard magnetic insulators, including cobalt ferrite, Al-doped M-type hexaferrite, and epsilon iron oxide, is discussed. Our findings reveal that these materials should provide several orders of magnitude more powerful spin pumping at sub-terahertz/terahertz frequencies compared to insulating antiferromagnets, even under unpolarized irradiation and even in the absence of external magnetic fields. This opens new horizons for the development of practical ultrafast electronics.

Nanoceramics of metastable ε-Fe2O3: effect of sintering on the magnetic properties and sub-terahertz electron resonance.

In this study, we demonstrate the sintering of metastable ε-Fe2O3 nanoparticles into nanoceramics containing 98 wt% of the epsilon iron oxide phase and with a specific density of 60%. At room temperature, the ceramics retain a giant coercivity of 20 kOe and a sub-terahertz absorption at 190 GHz inherent in the initial nanoparticles. The sintering leads to an increase in the frequencies of the natural ferromagnetic resonance at 200-300 K and larger coercivities at temperatures below 150 K. We propose a simple but working explanation of the low-temperature dynamics of the macroscopic magnetic parameters of the ε-Fe2O3 materials via the transition of the smallest nanoparticles into a superparamagnetic state. The results are confirmed by the temperature dependence of the magnetocrystalline anisotropy constant and micromagnetic modeling. In addition, based on the Landau-Lifshitz formalism, we discuss features of the spin dynamics in ε-Fe2O3 and the possibility of using nanoceramics as sub-terahertz spin-pumping media. Our observations will expand the applicability of ε-Fe2O3 materials and promote their integration into telecommunication devices of the next generation.

Structural peculiarities of ε-Fe2O3/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.

Studying the potential of rock magnetism to distinguish combustion structures of different type

Rock magnetism has a significant potential to elucidate archaeological issues related to firing process owing to the strong dependence of clay magnetic properties on firing conditions. The present study is an attempt to characterize some basic fuel installations (hearths, household ovens, pottery kilns, kilns for building ceramics and metallurgical ovens) by the most common magnetic parameters (magnetic remanence, initial magnetic susceptibility, Koenigsberger ratio, frequency-dependent magnetic susceptibility and viscosity coefficient), accumulating a set of relevant magnetic data for exemplary structures. Rasmussen method was used for firing temperature determinations.It is concluded that household ovens, used for cooking/heating activities only, were fired at temperatures below 500 °C. The acquired thermoremanence is in most cases partial TRM, generally less than 800 mA/m, the measured magnetic susceptibility does not exceed 300× 10−8m3/kg, and the corresponding Q ratios are mostly below 10. Experimentally baked clays show that already a firing at temperatures slightly above 500 °C yield much stronger and more stable TRM, and higher χ and Q ratio. Hearth and pottery kilns appear indistinguishable at this research stage, but systematically higher Q ratios were observed for the pottery kilns. Rising the temperature in the kilns for building ceramics and metallurgical ovens causes formation of a higher proportion of coarse-grained hematite and epsilon iron oxide, resulting in thermoremanence increase and magnetic susceptibility decrease.Firing temperatures determined for the household ovens corroborate well the observed magnetic parameters indicating a maximum operating temperature of not more than 460–470 °C. On the other hand, they appear to be underestimated when the highest firing intensity was expected (at T > 900 °C). This is indicative that further studies in this aspect are needed. The slags studied fail to produce any feasible result in terms of their firing temperatures, probably because of their strong inhomogeneity and/or complicated firing history. There is an excellent agreement between the estimated temperatures for clays baked in an experimental dual chamber kiln and the thermocouple readings. Differences in firing atmosphere between archaeological structures (or their experimental analogues) and laboratory furnaces probably have the least effect on low-temperature heated objects (i.e., household ovens) and the strongest on high-temperature ones where reducing conditions dominate that should be taken into account in the data interpretation. An overestimation of Tfiring can happen for low-temperature materials due to possible intense magnetic transformations above 600 °C.

Aluminum-titanium-cobalt substituted epsilon iron oxide nanosize hard magnetic ferrite for magnetic recording and millimeter wave absorption

We synthesized aluminum-titanium-cobalt substituted epsilon iron oxide (ATC-ε-Fe2O3), ε-Al0.11Ti0.04Co0.05Fe1.80O3 (1), ε-Al0.14Ti0.04Co0.05Fe1.77O3 (2), and ε-Al0.26Ti0.04Co0.05Fe1.65O3 (3). The magnetic coercive field (Hc) values are 7.1 kOe (1), 6.5 kOe (2), and 4.4 kOe (3). Zero-field ferromagnetic resonance frequencies (fr) were observed at 112 GHz (1), 109 GHz (2), and 100 GHz (3) using terahertz time-domain spectroscopy. These Hc and fr values are suitable for high-density magnetic recording and high-frequency millimeter wave absorbers. Additionally, the present series is non-toxic, low-cost, and could be used as practical magnetic fillers for magnetic recording and millimeter wave absorber applications.

Resonance Frequency Tuning of a 200 GHz Band Absorber by an External Magnetic Field

Resonance Frequency Tuning In article number 2100319, Shin-ichi Ohkoshi and co-workers investigate the effect of the external magnetic field on the millimeter-wave absorption of rhodium-substituted epsilon iron oxide, a 200-GHz band absorber, and achieve tuning of the resonance frequency.

A Microstructural and Compositional Study of ε-Fe2O3 Crystals in the Hare’s Fur Jian Ware

The Jian kilns in the present-day Jianyang County of Fujian Province are well known for their thick and lustrous black-glazed porcelain production. The hare’s fur (HF) glazed Jian wares characterized by radial fur-like strips, as one of the most typical representatives of black-glazed tea bowls, originate from phase separation of glaze melt and crystallization of iron oxides. In this study, various techniques were performed on the yellowish-brown HF samples, including portable energy-dispersive X-ray fluorescence (PXRF), synchrotron X-ray absorption near-edge spectroscopy (XANES), optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy (RS). The objective of this study was to well understand the microstructure characteristics and chemical compositions of glaze patterns. Results showed that the main constituents of the ceramic glaze were alumina (10.61–16.43 wt.%), silica (62.20–77.07 wt.%), calcium (3.85–6.97 wt.%), and iron oxide (4.10–8.35 wt.%). The studies provided evidence that metastable epsilon-iron oxide crystals (ε-Fe2O3) formed on the brownish-yellow glazed surface. Microstructural analysis revealed that there were three types of crystal structures in the glaze surface: One consisted of well-grown leaf-like or dendritic-like structure with highly ordered branches at micrometers scales; another comprised flower-like clusters accompanied by branches radiating from the center, petals growing along the branches, and needles on both sides of the petals; the last type involved a honeycomb structure tightly packed with plentiful spherical or irregular-shaped particles. In addition, ε-Fe2O3 crystals in the cross-section of the glaze showed a gradient distribution.

Optical Properties of Epsilon Iron Oxide Nanoparticles in the Millimeter- and Terahertz-Wave Regions

Abstract Epsilon iron oxide (ε-Fe2O3) is attracting global attention as a magnetic material with a large magnetic anisotropy. In this article, the optical properties of ε-Fe2O3 nanoparticles and the metal-substituted series of ε-MxFe2−xO3 (M = Ga, In, and Al) are studied over a wide frequency range from the millimeter-wave to terahertz-wave region, 30 GHz–30 THz, using terahertz time-domain, far-infrared, and Raman spectroscopies. To understand the spectroscopic data, first-principles calculations of the electronic structure and phonon modes are performed. First, an ε-Fe2O3 bar magnet is introduced and its atomic movements are calculated by phonon mode calculations. Second, the phonon modes of Ga-substituted ε-Fe2O3 are calculated. Far-IR, mid-IR, and Raman spectroscopies confirm that the calculated and observed spectra show good agreement. Third, the influences of In-substitution on the crystal structure, magnetic properties, and millimeter-wave absorption are described. In high-frequency millimeter-wave absorption due to magnon, the resonance frequency decreased with In-substitution. Finally, the millimeter-wave absorption property of ε-AlxFe2−xO3 is described. An absorption peak due to the natural resonance occurs at 100 GHz. The rotation data of the transmitted millimeter wave are determined by millimeter-wave–polarization-plane measurements.

Resonance Frequency Tuning of a 200 GHz Band Absorber by an External Magnetic Field

The effect of the external magnetic field on the millimeter‐wave absorption of rhodium‐substituted epsilon iron oxide, ε‐Rh0.13Fe1.87O3 (1) and ε‐Rh0.19Fe1.81O3 (2) nanomagnets, is investigated. Terahertz time‐domain spectroscopy (THz‐TDS) shows that 1 and 2 display a zero‐field ferromagnetic resonance (the so‐called natural resonance) at 201 GHz with a linewidth of 20 GHz and at 210 GHz with a linewidth of 30 GHz, respectively. The ferromagnetic resonance is measured under an external magnetic field (H ex). Applying +3.5 kOe parallel to the remnant magnetization and −3.5 kOe antiparallel to the remnant magnetization shifts the resonance frequency at 201 GHz in 1 by +6 and −4 GHz and that in 2 by +4 and −4 GHz, respectively. Simulations of the ferromagnetic resonance using the Landau−Lifshitz−Gilbert model reproduce the magnetic field‐induced shifts of the resonance frequencies. The 220 GHz band millimeter wave is the highest frequency with a high transparency for “atmospheric windows” and is expected to be a carrier frequency for sixth‐generation mobile communication systems (6 G) or 7 G. This study may help realize applications for millimeter‐wave isolators, circulators, or shutters.

Facile synthesis of epsilon iron oxides via spray drying for millimeter-wave absorption

A simple, scalable spray drying method was developed for high-yield epsilon iron oxide (ε-Fe2O3) synthesis. The ε-Fe2O3 particle size can be tailored by varying the annealing temperature and molar ratio of Fe/Si, producing a high-purity ε-phase. This strategy also enables ferromagnetic resonance tuning, making it potentially usable in millimeter-wave absorbers.

Particle size effect on millimeter-wave absorption, rotation, and ellipticity of gallium-substituted epsilon iron oxide.

Various applications employ millimeter waves. For example, the carrier frequencies of vehicle radar in advanced driver assistance systems are 76–81 GHz millimeter waves. Here, we investigate the particle size effect on millimeter-wave absorption of gallium-substituted epsilon iron oxide ε-GaxFe2−xO3 with x = 0.44 ± 0.01. Samples were composed of nanoparticles with sizes of 16.9(1) nm, 28.8(2) nm, and 41.4(1) nm. Millimeter wave absorption, Faraday rotation, and Faraday ellipticity were measured by terahertz time-domain spectroscopy. This series exhibits millimeter-wave absorption at 78.7, 78.2, and 77.7 GHz without an external magnetic field. The millimeter-wave absorption increases from 4.6 dB to 9.4 dB as the particle size increases. In the magnetized sample, the Faraday rotation angle increases from 9.1° to 18.4°, while the Faraday ellipticity increases from 0.27 to 0.52. The particle size effect can be explained by the change in the ratio of the surface and core of the nanoparticles. The present study should contribute to the realization of high-performance millimeter-wave absorbers.

A combined micro-Raman, X-ray absorption and magnetic study to follow the glycerol-assisted growth of epsilon-iron oxide sol-gel coatings

Epsilon iron oxide (ε-Fe2O3) coatings on Si(100) substrates are obtained by an easy one-pot sol-gel recipe assisted by glycerol in an acid medium. Glycerol, given its small dimensions, enables the formation of ε-Fe2O3 nanoparticles with a size of a few nanometers and the highest purity is reached in coatings after a densification treatment at 960 °C. The structural and compositional evolution up to 1200 °C is studied by confocal Raman microscopy and X-ray absorption spectroscopy techniques, correlating the existing magnetic properties. We report a novel characterization method, which allows monitoring the evolution of the precursor micelles as well as the intermediate and final phases formed. Furthermore, the inherent industrial technology transfer of the sol-gel process is also demonstrated with the ε-Fe2O3 polymorph, impelling its application in the coatings form.

Advances in magnetic films of epsilon-iron oxide toward next-generation high-density recording media.

Iron oxide magnets, which are composed of the common elements iron and oxygen, are called ferrite magnets. They have diverse applications because they are chemically stable and inexpensive. Epsilon-iron oxide (ε-Fe2O3) is a polymorph that shows an extremely large coercive field as a magnetic oxide. It maintains its ferromagnetic ordering even when downsized to a single nano-sized scale (i.e.,<10 nm). Due to these characteristics, ε-Fe2O3 is highly expected to be used for high-density magnetic recording media in the big data era. Here, we describe the recent developments of magnetic films composed of metal-substituted ε-iron oxide, ε-MxFe2-xO3 (M: substitution metal), toward the next-generation of magnetic media.

New archeomagnetic secular variation data from Central Europe, II: Intensities

Variations of the geomagnetic field during the past millennia can be obtained by investigating archeological material. The spatial distribution of the available data across Europe is rather uneven and only about 15% of the sites provide a full vector record of the geomagnetic field. Here we report 39 new reliable paleointensity data for sites from Austria, Germany and Switzerland, with ages ranging between 1500 BCE and 1900 CE. For 31 of them, directional information is available, too. Paleointensities were determined with Thellier type techniques, with the multiple-specimen domain-state corrected paleointensity protocol or with both techniques. Corrections for anisotropy of thermoremanent magnetization were carried out for the structures, showing small variations of only a few per cent in most cases. Cooling rate dependence was tested for 16 structures. Often strong alteration during the experiments was observed. Seven successful cooling rate corrections were applied, which lowered the paleointensity values by about 10%. Values obtained from the multiple-specimen technique are generally somewhat lower than those from the Thellier experiments. Rock magnetic measurements revealed magnetite-type minerals mostly in pseudo single domain range as main magnetic carrier, but also high coercive mineral components like hematite or epsilon iron oxide are present. The new data are mostly in good agreement with published paleointensities. They support the presence of strong intensity variations around 800 CE and in the millennium BCE. The new full vector data reveal that the deep paleointensity minimum observed from German data around 850 BCE is accompanied by large variations in direction with declinations up to 50°.

Magnetic-Pole Flip by Millimeter Wave.

In the era of Big Data and the Internet of Things, data archiving is a key technology. From this viewpoint, magnetic recordings are drawing attention because they guarantee long-term data storage. To archive an enormous amount of data, further increase of the recording density is necessary. Herein a new magnetic recording methodology, "focused-millimeter-wave-assisted magnetic recording (F-MIMR)," is proposed. To test this methodology, magnetic films based on epsilon iron oxide nanoparticles are prepared and a focused-millimeter-wave generator is constructed using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide instantly switches its magnetic pole direction. The spin dynamics of F-MIMR are also calculated using the stochastic Landau-Lifshitz-Gilbert model considering all of the spins in an epsilon iron oxide nanoparticle. In F-MIMR, the heat-up effect of the recording media is expected to be suppressed. Thus, F-MIMR can be applied to high-density magnetic recordings.

Further progress in the study of epsilon iron oxide in archaeological baked clays

The occurrence of ε-Fe2O3 in archaeological samples that have been subjected to high temperatures is gradually being detected by the use of micrometric structural characterization techniques. This work provides new information by revealing that the ε-Fe2O3 is formed as a response to temperature, the aggregation state and the position within the baked clay with respect to the nearest heat source. In addition, depending mainly on the atmospheric environment, the temperature reached by the combustion structure, the distance from the heating source and the particle aggregation, other iron oxide magnetic phases are produced. In the baked clay studied here, hematite is found over the whole range of samples studied but its magnetic contribution is negligible. Magnetite is observed at the sample surface, probably due to local atmospheric environment closest to the combustion source. Maghemite is found at all depths up to 6 cm below the sample surface. ε-Fe2O3 has a limited distribution, found within 2–3 cm of the sample surface. Furthermore, the viability of this compound as a palaeofield marker has been evaluated in both archaeological and synthetic samples. The results indicate that ε-Fe2O3 is able to register the direction of the magnetic field. Linear palaeointensity plots have been obtained in synthetic samples, although the value of the palaeofield could be, sometimes, overestimated.

Archaeomagnetic knowledge of the Neolithic in Bulgaria with emphasis on intensity changes

Abstract Several archaeological baked clay structures from Bulgarian Neolithic sites were archaeomagnetically studied. According to the ancient firing conditions, the collected materials are dwelling remains and ovens with different heating histories. The variability of magnetic properties is greater in the burnt dwelling remains compared to the ovens. The prevailing magnetic minerals are magnetite/titanomagnetite and epsilon iron oxide. Less often, the presence of hematite is suggested. For one of the ovens, a well-pronounced evolution in magnetic properties was observed between its successive levels. The collected materials possess stable magnetic mineralogy and the success rate of archaeointensity determination experiment is 84%. The experiment completely failed only for the structure where epsilon iron oxide is not identified and where the lowest median destructive fields are determined. The obtained archaeomagnetic results are summarized as 10 new reference points. The structures subjected to the same firing event are combined as one feature. Their dating is based on well-established relative Neolithic chronology and stratigraphic constraints. The new data coincide fairly well with the Bulgarian dataset and with the data available for the neighbouring countries, confirming an intensity maximum around 5550 BC and a minimum around 5400 BC. No indications of geomagnetic ‘spikes’ or ‘jerks’ are detected.