Fe3O4 Phase Research Articles

Enhanced magnetic properties of Sr0.7Ce0.3Fe11.7Zn0.3O19 by tuning pre-sintering temperature

In this work, pre-sintering temperature was tuned to investigate its effect on the structural and magnetic properties of oriented M-type hexagonal strontium ferrite Sr0.7Ce0.3Fe11.7Zn0.3O19. X-ray diffraction pattern shows that Fe2O3 and CeO2 impurity phases exist in all samples. As pre-sintering temperature increased, both saturation magnetization (Ms) and remanence ratio (Mr/Ms) first increased and then decreased. In contrast, coercivity (Hc) and magnetocrystalline anisotropy field (Ha) first decreased and then increased as pre-sintering temperature went up. The average grain size (D) increases monotonously with pre-sintering temperature. At pre-sintering temperature of 1200 ℃, Sr0.7Ce0.3Fe11.7Zn0.3O19 achieved optimal magnetic properties of Ms = 58.24 emu/g, Mr/Ms = 0.95, Hc= 1816 Oe, Ha= 10433 Oe and K1 = 1.51 × 105 J/m3. This work shows that the magnetic properties of M-type hexagonal strontium ferrite could be significantly improved by optimizing the pre-sintering temperature, which helps the design of high-performance and self-biased circulator.

Chemical looping gasification of biomass char for hydrogen-rich syngas production via Mn-doped Fe2O3 oxygen carrier

Because of its low cost, an iron-based oxygen carrier is a promising candidate for hydrogen-rich syngas production from the chemical looping gasification of biomass. However, it needs modification from a reactivity point of view. In this study effect of Mn doping on Fe2O3 has been investigated for hydrogen-rich syngas production from biomass char at different temperatures (700–900 °C) and steam flow rates (60–100 μL/min). Several techniques (XRD, XPS, BET, and TPR-H2) have been utilized to characterize fresh and spent oxygen carriers. The result demonstrated Mn-doing boosted the redox activity and the amount of oxygen vacancies, which increased hydrogen gas generation. Hydrogen production displayed different behavior across temperatures due to detecting Fe2O3 and MnFeO3 phases for spent oxygen carriers. For the Fe2O3 oxygen carrier hydrogen gas yield is 1.67 Nm3/kg which is due to reduction of Fe2O3 phase to Fe3O4. However, the MnFe2O4 spinel phase detected in the spent MnFeO3 oxygen carrier is a reason for improving hydrogen gas yield to 1.84 Nm3/kg. Change reaction temperature from 900 °C to 850 °C reduced hydrogen gas yield from 1.84 Nm3/kg to 1.83 Nm3/kg for with MnFeO3 oxygen carrier. Regarding different steam flows, the proper flow rates that can maintain the formed phases and obtained best hydrogen gas yield are 80 and 90 μL/min, respectively. Meanwhile, the best hydrogen gas yield (2.21Nm3/kg) are obtained with MnFeO3 oxygen carrier at optimum conditions (850 °C and 90 μL/min).

Magneto-transport and magneto-dielectric anomalies in Co doped iron oxide thin films – Microwave assisted sol-gel route

Use of microwave (MW) energy as an alternate to high temperature treatment during material synthesis is gaining much attention as it offers the advantages of being faster and economical than other heating methods. A comprehensive study of structure, magnetization, and polarization behaviour of Cobalt (Co) doped Fe3O4 thin films, prepared by means of microwave assisted sol–gel method is performed. Co doped (0-10 wt%) sols are prepared after irradiating them at MW power of 1000 W. Phase analysis is confirmed using XRD where Fe3O4 phase persisted with Co concentrations of 0-6 wt%. Higher concentration results in the appearance of cobalt oxide phase. Magnetic and transport properties are strongly affected by the presence of Co ions. Co doped Fe3O4 films show anisotropic behaviour with high saturation magnetization obtained for 6 wt% Co doped Fe3O4 thin films, i.e. ∼ 125 emu/g. Verwey transition is observed for thin films with low dopant concentration while no sharp transition in magnetization behaviour is observed for higher dopant concentration. High frequency anomaly in dielectric analysis is observed at high frequencies for Co doped Fe3O4 thin films. Negative values of magneto-dielectric coefficient show coupling of magnetic and dielectric behaviour at room temperature.

A SIMPLE FABRICATION OF POLYVINYL PYRROLIDONE (PVP)-COATED Fe3O4 NANOPARTICLES TOWARDS IMPROVED CONGO RED DEGRADATION PHOTOCATALYTIC ACTIVITY

PVP-coated nanoparticles are increasingly being developed for photocatalytic applications. The fabrication of PVPcoated Fe2O3 nanoparticles is demonstrated in this work using a novel method. The existence of PVP and Fe2O3 phases is identified by powder XRD analysis. SEM, FTIR, and UV techniques were used to examine the morphology and structural examination of the materials. UV–Visible DRS studies reveal optoelectronic transfer and redshifts in PVP-coated composites. Besides, the reduction of average particle size was also identified by SEM and XRD. The potential application of PVP-coated Fe2O3 as a photocatalyst was investigated with the use of an aqueous solution of degraded Congo Red dye. Data from photocatalytic research show that Fe3O4-coated PVP (6 wt%) exhibits considerably better photocatalytic activity towards the destruction of CR dyes. The improved photocatalytic activity of the hybrid composite is caused by the combination of phases of PVP and Fe2O3 with differing band gaps. Detailed evidence of the photocatalytic activity's mechanism is also provided.

A Study of Enhancement in Beta Phase and Dielectric Properties of Fe2O3 Reinforced in PVDF Nanocomposite Thin Films

A Study of Enhancement in Beta Phase and Dielectric Properties of Fe2O3 Reinforced in PVDF Nanocomposite Thin Films

Characterization of Iron Oxide Samples Using Mössbauer Spectroscopy and Hysteresis Loops

In this work, we used Mössbauer spectroscopy and magnetometry at room temperature to investigate the magnetic properties of four different iron oxide samples (α-FeOOH, γ-Fe2O3, or Fe3O4 and α-Fe2O3) before and after annealing at 1000°C for one hour. We found that after annealing, all four samples exhibited the same properties as hematite (α-Fe2O3). These results show that the iron oxides in this study underwent phase transformations from an initial phase of either α-FeOOH, γ-Fe2O3, or Fe3O4 to a single final phase of α-Fe2O3.

Microstructure, ion adsorption and magnetic behavior of mesoporous γ-Fe2O3 ferrite nanoparticles.

Magnetic nanoparticles with capacity for surface functionalisation have potential applications in water purification and biomedicine. Here, a simple co-precipitation technique was used to synthesize mesoporous ferrite nanoparticles in the presence of cetyltrimethylammonium bromide (CTAB) micellular surfactant. The as-synthesized ferrite nanoparticles were calcined at 250 °C for 5, 10, 15, and 24 h to remove the surfactant and create a mesoporous structure. The prepared samples were characterised using a wide range of analytical techniques. Microscopical images showed that all uncalcined particles have cauliflower shape without porosity. However, after calcination, surface and deep pores were created on the synthesized nanoparticles. In addition, transmission electron microscope (TEM) images of calcined nanoparticles revealed a wormhole-like structure, which is typical for the mesoporous architectures. Based on X-ray diffraction (XRD), the uncalcined and calcined samples exhibit pure Fe3O4 (magnetite) and γ-Fe2O3 (maghemite) ferrite phases, respectively. The γ-Fe2O3 nanoparticles demonstrated a high Brunauer-Emmett-Teller (BET) surface area with pore diameters smaller than 10 nm and a type IV isotherm similar to the mesopores. Hysteresis loops measured by vibrating sample magnetometry (VSM) showed the superparamagnetic nature for mesoporous γ-Fe2O3 nanoparticles. The first-order reversal curve (FORC) diagram revealed the formation of a mesoporous structure in calcined materials which reduces coercive distribution (Hc) and magnetostatic interaction (Hu) once compared to non-calcined samples. Mesoporous γ-Fe2O3 nanoparticles were successfully employed as an adsorbent for the removal of heavy metal ions of Pb(ii) from an aqueous solution. The highest lead ion adsorption was observed in mesoporous γ-Fe2O3 nanoparticles prepared with 3% CTAB.

Aerosol-assisted CVD method for the synthesis of solid particles of t-YSZ-Fe3O4

This work details the production of solid composite particles by the aerosol-assisted chemical vapor deposition method. With this method it is feasible to produce at low temperature (450 °C) the tetragonal phase of zirconium oxide stabilizing it with yttrium oxide (YSZ) and cubic iron oxide (Fe3O4) at the same time. The particles have a solid morphology in which both metal oxides coexist without mixing. The average size of the obtained particles is 329 ± 81 nm, moreover, each particle is formed by thousands of crystallites of size 2 ± 0.5 nm. The formation of solid structures is due to the amount of Zr and Y found in each particle. These particles can be applied as reinforcements of metallic structures.•A simple and low-cost method for producing composite particles to be applied as reinforcing agents for metal structures.•The particles are formed by two phases of tetragonal yttria-stabilized zirconia (t-YSZ) and cubic Fe3O4, which was synthesized following a one-step process via the aerosol-assisted chemical vapor deposition method (AACVD).•The tetragonal phase of ZrO2 is obtained at 450 °C stabilizing it with ∼3.8% of yttrium oxide.

Studies on structural, dielectrics and magnetics properties of two stage sintered (1-x) Mg0.3Zn0.7Fe2O3–xBa0.7Sr0.3TiO3 composite

The (1-x)Mg0.7Zn0.3Fe2O4 – (x)Ba0.7Sr0.3TiO3 composites with x = 0.40, 0.50 and 0.60 were prepared by solid state reaction under two step sintering. Two step sintering conditions were determined by the first step (T1) at 1350 °C for holding time of 60 min and cooled down to the second sintering temperature (T2) with different temperatures between 900 and 1100 °C for holding time of 5 h. The sample with high percentage of shrinkage and dense ceramic was obtained from 0.4Mg0.7Zn0.3Fe2O4-0.6Ba0.7Sr0.3TiO3 with sintering of T2 at 1100 °C. XRD pattern showed combination of ferrite phase (Mg0.7Zn0.3Fe2O4) and ferroelectric phase (Ba0.7Sr0.3TiO3) and secondary phase of ZnO. Rietveld analysis confirmed main phases of Mg0.7Zn0.3Fe2O4 and Ba0.7Sr0.3TiO3, including MgFe2O4 and ZnO as secondary phase. It was found that the unit cell volume of Ba0.7Sr0.3TiO3 was smaller than Mg0.7Zn0.3Fe2O4 unit cell volume. Therefore, the short distance between ions of Ba0.7Sr0.3TiO3 phase produced the dense composites with increasing ferroelectric phase contents. The average grain size of ceramic was in the range of 0.74–1.07 µm with grain square shape in Mg0.7Zn0.3Fe2O4 phase and the average grain size of 0.59–0.70 µm with round oval shape in Ba0.7Sr0.3TiO3 phase. The dielectric constant was between 0.8 × 103 and 5.4 × 103 in the temperature range of 275 °C − 360 °C. The highest dielectric constant was obtained from 0.6Mg0.7Zn0.3Fe2O4 −0.4Ba0.7Sr0.3TiO3 with T1 at 1350 °C with holding time for 60 min and T2 at 900 °C with holding time for 5 h. The hysteresis measurement was carried out to determine saturation magnetization, remanent magnetization and coercivity of the composites. The results revealed that higher concentration of ferrite led to optimal magnetic properties.

Highly active and magnetically recoverable heterogeneous catalyst for hydrothermal liquefaction of biomass into high quality bio-oil

This article reports a safe, low-cost, and industrially applicable magnetite supported on activated carbon catalyst that can be magnetically retrieved from the solid and reused multiple times without the need of a regeneration step. The FeOx/C catalyst improved the bio-oil yield by 19.7 ± 0.96 % when compared to the uncatalysed reaction at 320 °C for the HTL of draff (brewer’s spent grains). The use of homogeneous Na2CO3 base as a catalyst and co-catalyst, improved carbon extraction into the aqueous phase. The exceptional catalytic activity can be attributed to the Fe3O4 phase which can produce in-situ H2 that improves the biomass decomposition and oil property with an energy recovery of ∼84 %. The FeOx/C catalyst was separated using magnetic retrieval and maintained its catalytic activity even up to 5 reaction cycles showing potential as a cheap catalyst for HTL reactions and can be scaled-up for industrial applications.

Innovative back-contact for [formula omitted]-based thin film solar cells

Among the most promising material for thin-film photovoltaics, Sb2Se3 (ASe) plays a pivotal role, being constituted of Earth-abundant elements and owing to its optical and electrical properties. Recently, ASe solar cells exhibited relatively good photovoltaic performances, reaching a considerable efficiency close to 9 %. Despite these remarkable results, much is still to be investigated, particularly regarding the active materials, interfaces, and electrical contacts. In this study we propose an alternative back contact for ASe solar cells. It is based on Fe, S and O elements, and was deposited at room temperature by radiofrequency magnetron sputtering. XRD and Raman measurements revealed that it is actually made of two materials: Fe3O4, in orthorhombic and cubic phases and FeS in troilite phase. This material provided an ohmic contact on top of the ASe film; its contact resistivity, extrapolated from current vs voltage characteristics of complete ASe-based solar cells, was estimated to be 0.8Ω·cm2. Monitoring the photovoltaic parameters, we observed a negligible average change after three months.

A Comprehensive Study of Synthesis and Analysis of Anisotropic Iron Oxide and Oxyhydroxide Nanoparticles.

One-dimensional anisotropic nanoparticles are of great research interest across a wide range of biomedical applications due to their specific physicochemical and magnetic properties in comparison with isotropic magnetic nanoparticles. In this work, the formation of iron oxides and oxyhydroxide anisotropic nanoparticles (ANPs) obtained by the co-precipitation method in the presence of urea was studied. Reaction pathways of iron oxide and oxyhydroxide ANPs formation are described based on of X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and pulse magnetometry studies. It is shown that a nonmonotonic change in the Fe3O4 content occurs during synthesis. The maximum content of the Fe3O4 phase of 47.4% was obtained at 12 h of the synthesis. At the same time, the reaction products contain ANPs of α-FeOOH and submicron isotropic particles of Fe3O4, the latter formation can occur due to the oxidation of Fe2+ ions by air-oxygen and Ostwald ripening processes. A subsequent increase in the synthesis time leads to the predominant formation of an α-FeOOH phase due to the oxidation of Fe3O4. As a result of the work, a methodological scheme for the analysis of iron oxide and oxyhydroxide ANPs was developed.

Preparation and Characterization of Superparamagnetic Iron Oxide Nanoparticles (Fe3O4) for Biological Applications

Superparamagnetic iron oxide nanoparticles (SPIONs) are developed considering the importance of this class of in different fields of biochemical and biomedical applications owing to their distinctive chemical and physical properties. In this work, the preparation of iron oxide nanoparticles (SPIONs (using Co-precipitation method has been done as the most commonly used wet chemical method of magnetic nanoparticles preparation for biological applications. The SPIONs synthesis was based on sodium hydroxide (NaOH) mediated precipitation of Fe3+ and Fe2+ salts in an aqueous solution using trisodium citrate as surfactant within a closed system. The size and stability of the magnetite nanoparticles were carefully controlled using different chemical and physical parameters in order to obtain the SPIONs with small particle size and distribution that is needed for biomedical applications. The synthesized Fe3O4 nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), vibrating sample magnetometer (VSM), and Zeta potential analysis (Zp). XRD pattern showed the presence of peaks corresponding to the phase of magnetite Fe3O4. Moreover, SEM and TEM results revealed spherical particles with a mean diameter of ≥ 5 nm. The monodispersed SPIONs were successfully prepared with a mean hydrodynamic size of 209.32 nm at a stirring speed of 900 rpm and NaOH concentration of 1.2 gm. The results showed that the particle size is considerably dependent on the stirring rate and NaOH concentration. Fe3O4 nanoparticles exhibited superparamagnetic behavior and the saturation magnetization was around 50 emu/g.

Thermo-physical properties of paraffin wax with iron oxide nanoparticles as phase change material for heat storage applications

Phase change materials (PCMs) are growing in importance in many thermal applications as heat storage or to smooth the energy peak demand in many technological fields in industrial as well as in civil applications. Conductive nanoparticles can be added to phase change material to improve their thermo-physical properties. In this work, Iron oxide nanoparticles (IOx-NPs) were synthesized using a simple and green synthesis method, free of toxic and harmful solvents, using the extract of a plant as a reducer and stabilizer at two different temperatures of calcination 500°C and 750°C. The metallic oxide was used as an additive with 2% wt. compositions to paraffin wax to prepare a nanocomposite. The variation in thermal properties of paraffin wax in the composite was experimentally investigated. The biosynthesized IOx-NPs were characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscope (SEM) and Thermal Gravimetric Analysis (TGA) techniques. The thermal properties of the synthesized nanocomposites were characterized by a thermal conductivity analyzer and differential scanning calorimetry (DSC). The FTIR spectra showed a bond at 535 cm-1, which confirms the Fe-O vibration. The XRD powder analysis revealed the formation of the cubic phase of Fe3O4 with an average particle size of 11 nm at 500°C and the presence of the phase α-Fe2O3 with Fe3O4 at 750°C. Scanning Electron Microscopy (SEM) showed that the obtained oxide was made up of particles of nanoscale size. Experimental measurements showed that the presence of nanoparticles can improve the latent heat capacity by a maximum of 16.16 % and the thermal conductivity of the nanocomposites by a maximum of 16.99%.

Enhancement of the Magnetic Properties in Si4+-Li+-Substituted M-Type Hexaferrites for Permanent Magnets

A series of charge-balanced Si4+-M1+,2+ (M = Mg2+, K+, Li+) substitution compositions with the chemical formulae of SrFe12−2xSixMgxO19 (x = 0, 0.05, 0.1, 0.2), Sr1−yFe12−ySiyKyO19 (y = 0.05, 0.1, 0.2), and SrFe12−z(Si0.6Li0.6)zO19 (z = 0.05, 0.1, 0.2, 0.4, 0.6) were prepared using conventional ceramic processes. While the sole doping of Si with x = 0.1 (SrFe12−xSixO19 (x = 0.1)) causes a noticeable Fe2O3 phase formation, the co-doping of Si-Mg and Si-Li allow a single M-type phase formation with x up to x = 0.1 and z = 0.4, respectively. Notably, a 2.6% increase in the saturation magnetization was obtained for Si-Li-substituted SrM with z = 0.05—that is, SrFe11.95Si0.03Li0.03O19. Enhancement of the magnet performance can also be achieved when anisotropic permanent magnets are fabricated based on the substitution composition SrFe11.95Si0.03Li0.03O19. The remnant magnetic flux density was improved by 2.5% compared to that of the unsubstituted SrM (from 4207 to 4314 G). The maximum energy product (BHmax) value also increased from 4.24 to 4.46 M·G·Oe. The enhancement in permanent magnet performance is attributed to the increase in the MS of SrM by the optimal Si-Li substitution. This is a promising result because enhanced permanent magnet performance is achieved with intrinsic magnetic property improvement.

Optimizing electrical and thermal transport properties of Ca3Co4O9 based thermoelectric materials by Ag and Fe co-addition

Ca 3-x Ag x Co 4-y Fe y O 9 (x=0, 0.05; y=0, 0.1, 0.2, 0.4) polycrystalline materials were prepared by sol-gel method with spark plasma sintering. The phase composition, microstructure and high temperature thermoelectric property of the bulk samples were systematically studied. The sintered Ca 3-x Ag x Co 4-y Fe y O 9 ceramics exhibit fine lamellar grain and dense morphology. Compared with single addition of Ag or Fe, appropriate co-additive amount of Ag and Fe can further increase electrical conductivity due to higher carrier concentration and mobility, while decreasing thermal conductivity by stronger phonon scattering. The coordinated regulation of electron and phonon transport is realized in Ca 2.95 Ag 0.05 Co 3.8 Fe 0.2 O 9 with the maximum ZT value of 0.25 at 1000 K, which is 34% larger than that of pure Ca 3 Co 4 O 9 . • Appropriate co-doping content of Ag and Fe in Ca 3 Co 4 O 9 can bring higher carrier concentration and mobility, while enhancing phonon scattering. • Simultaneously enhanced power factor and suppressed thermal conductivity can be achieved by Ag and Fe co-doping with appropriate doping contents. • Excessive doping of Fe would generate secondary phase FeO and deteriorate thermoelectric property.

Co-precipitation synthesis of LaFe1-xAlxO3 (x = 0–0.2) on structure and electromagnetic properties

The synthesis of LaFe1-xAlxO3 (x = 0–0.2) on the structure and electromagnetic properties using the co-precipitation method were evaluated. LaFe1-xAlxO3 (x = 0–0.2) materials were synthesized using lanthanum chloride and natural iron sand powders. The mixing process of precursors was held with a hotplate temperature of 60 °C. The calcination process was done at 400 °C for 2 h. Then samples were analyzed using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Vibrating Sample Magnetometer (VSM), and CV meter. The XRD patterns result in two phases of LaFeO3 and Fe2O3. SEM images were performed that the powders are porous agglomerates. The particle size distribution is shown in the histogram that the samples are in the range of 50–500 nm, respectively. Then, the magnetic properties measurement of VSM analysis studied the substitution of Fe to Al ions decreases magnetic properties and BHmax in LaFe1-xAlxO3 (x = 0–0.2). The I-V measurements conducted the substitution of Fe to Al ions increase with a nonlinear reversal current in LaFeO3. These results show that LaFe1-xAlxO3 (x = 0–0.2) was recommended as the magnetic permanent material and perovskite material based on electromagnetic properties.

Sensitive colorimetric sensing of glutathione and H2O2 based on enhanced peroxidase mimetic activity of MXene@Fe3O4.

Theperoxidase-like activity of MXene@Fe3O4 nanocomposites and the relevant colorimetric application have been investigatedfor the first time. According to a kinetic study, MXene@Fe3O4 nanocomposites displayed higher peroxidase activity than their individual components, and the catalytic process followed a ping-pong mechanism. The improved peroxidase-like activity of the MXene@Fe3O4 nanocomposites was obtained due to the synergetic impact of the Fe3O4 and MXene phases, the huge ion-accessible interface, and quick electron transfer channels in the nanocomposites. In the presence of hydrogen peroxide (H2O2), the MXene@Fe3O4 nanocomposites can catalyze the reaction of the colorless substrate 3, 3, 5, 5-tetramethylbenzidine (TMB) into oxidized TMB which revealed a blue color at 652nm. After the introduction of glutathione (GSH), the blue color was gradually fading owing to the hydroxyl radical scavenging effect of glutathione. A colorimetric detection platform based on the peroxidase-like activity of the MXene@Fe3O4 nanocomposites was developed for H2O2 and GSH with a detection limit of 0.4μM and 0.5μM, respectively. Thecalibration plot for glutathione detection is calculated by absorbance difference at 652nm, which ranged from 0.5 to 10μM with a detection limit of 0.2μM. The recoveries of GSH with different concentrations ranged from 93.76 to 108.50% and the relative standard deviation from 0.30 to 5.96%. This work significantly extends the application of MXene@Fe3O4 nanocomposites in the construction of colorimetric sensors and reveals a satisfactory result in real sample analysis.

INFLUENCE OF IONIZING RADIATION ON THE ELEMENTAL CONTENT AND STRUCTURE OF NATURAL MINERALS

The granites of the Ukrainian crystalline shield and the Donetsk-Dnieper hollow of the Pre-Cambrian basement, discovered at a depth of 840…900 to 3420 m were irradiated on a KUT-1 linear electron accelerator with the following parameters: electron energy Е ≈ 7 MeV, average beam current I = 500 µA, temperature of the irradiated samples Т~ 40°С, absorbed dose Dabs = 107 Gy. Gamma-activation analysis was used to determine the content of elements in the samples. The samples were irradiated by bremsstrahlung from the linear accelerator electron NSC KIPT with energy 23 MeV and current 500 µA. The structure and phase composition of samples were investigation by infrared spectroscopy in the frequency 400…4000 cm-1. Crystal optical studies were carried out on polarizing microscope using immersion liquides. It is shown that high-energy electron irradiation of granites causes radiationstimulated transformations in the structure and phase homogeneity of mineral. The most pronounced changes in biotites after high-energy electron irradiation of granites are manifested in a change in the color of biotite, apparently due to oxide phases Fe2O3 and Fe3O4. Considering the state of granites under radiation conditions it can be used for the immobilization and disposal of radioactive waste.

Emergent magnetism and exchange bias effect in iron oxide nanocubes with tunable phase and size

We report a systematic investigation of the magnetic properties including the exchange bias (EB) effect in an iron oxide nanocube system with tunable phase and average size (10, 15, 24, 34, and 43 nm). X-ray diffraction and Raman spectroscopy reveal the presence of Fe3O4, FeO, and α-Fe2O3 phases in the nanocubes, in which the volume fraction of each phase varies depending upon particle size. While the Fe3O4 phase is dominant in all and tends to grow with increasing particle size, the FeO phase appears to coexist with the Fe3O4 phase in 10, 15, and 24 nm nanocubes but disappears in 34 and 43 nm nanocubes. The nanocubes exposed to air resulted in an α-Fe2O3 oxidized surface layer whose thickness scaled with particle size resulting in a shell made of α-Fe2O3 phase and a core containing Fe3O4 or a mixture of both Fe3O4 and FeO phases. Magnetometry indicates that the nanocubes undergo Morin (of the α-Fe2O3 phase) and Verwey (of the Fe3O4 phase) transitions at ∼250 K and ∼120 K, respectively. For smaller nanocubes (10, 15, and 24 nm), the EB effect is observed below 200 K, of which the 15 nm nanocubes showed the most prominent EB with optimal antiferromagnetic (AFM) FeO phase. No EB is reported for larger nanocubes (34 and 43 nm). The observed EB effect is ascribed to the strong interfacial coupling between the ferrimagnetic (FiM) Fe3O4 phase and AFM FeO phase, while its absence is related to the disappearance of the FeO phase. The Fe3O4/α-Fe2O3 (FiM/AFM) interfaces are found to have negligible influence on the EB. Our findings shed light on the complexity of the EB effect in mixed-phase iron oxide nanosystems and pave the way to design exchange-coupled nanomaterials with desirable magnetic properties for biomedical and spintronic applications.