Onset of size independent cationic exchange in nano-sized CoFe2O4 induced by electronic excitation (original) (raw)
Exchange bias and vertical shift in CoFe2O4 nanoparticles
Journal of Magnetism and Magnetic Materials, 2007
Magnetic properties of core-shell cobalt ferrite nanoparticles 15 to 48nm prepared by a sol-gel route have been studied. It is shown that the coercivity follows non-monotonic size dependence varying as 1/d above the maximum (d is the particle size). Field cooled magnetization exhibited both horizontal (exchange bias) and vertical shifts. The exchange bias is understood as originating at the interface between a surface region with structural and spin disorder and a core ferrimagnetic region. The dependence of the exchange bias and vertical shifts on the particle sizes and cooling fields are found to have significant differences and the differences are explained in the light of recent results which suggest that both weakly and strongly pinned spins are present at the interface. It is suggested that the exchange bias is dominated by the weakly pinned spins while the vertical shift is affected by the strongly pinned ones. PACS Number(s): 75.75.+a, 75.50.Tt, 75.70.Cn Corresponding author: arif@qau.edu.pk (A. Mumtaz)
Preparation of exchange coupled CoFe2O4/CoFe2 nanopowders
Journal of Magnetism and Magnetic Materials, 2020
The CoFe 2 O 4 cobalt ferrite nanoparticles were synthesized by the hydrothermal method. At the reaction temperature of 200°C, the particle sizes were controlled in the range of 22-41 nm by varying the reaction time t r from 2.0 to 15 h and the spontaneous magnetization M s of samples was increased from 68.6 to 78.2 Am 2 /kg. The X-ray diffraction diagrams of obtained samples showed their pure ferrite spinel structure. The highest coercivity H c value of 184.5 kA/m was achieved by t r = 8 h (denoted by CFO). The sample was synthesized with a time reaction of 2 h which was reduced to form CoFe 2 (denoted by CF) nanoparticles by H 2 flow 300 mL/min at 380°C. The CFO/CF powders were milled by the high energy mechanical ball milling in a SPEX 8000. After milling for 2 h, the M s and H c values of the CFO/CF nanocomposite powders with 10 wt% CoFe 2 nanoparticles fraction were 81.5 Am 2 /kg and 192.4 kA/m, respectively. The microstructure and magnetic properties of all samples are discussed in detail.
Cationic distribution and spin canting in CoFe2O4 nanoparticles
2011
Abstract CoFe 2 O 4 nanoparticles (< D NPD>~ 6 nm), prepared by a thermal decomposition technique, have been investigated through the combined use of dc magnetization measurements, neutron diffraction, and 57 Fe Mössbauer spectrometry under high applied magnetic field.
Exchange coupling behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite
Journal of Magnetism and Magnetic Materials, 2012
In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe 2 O 4 and ferrimagnetic oxide/ferromagnetic metal CoFe 2 O 4 /CoFe 2 nanocomposites. The latter compound is a good system to study hard ferrimagnet/soft ferromagnet exchange coupling. Two steps were used to synthesize the bimagnetic CoFe 2 O 4 /CoFe 2 nanocomposites: (i) first preparation of CoFe 2 O 4 nanoparticles using the a simple hydrothermal method and (ii) second reduction reaction of cobalt ferrite nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures, particle sizes, morphology, and magnetic properties of CoFe 2 O 4 nanoparticles have been investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) with applied field up to 3.0 kOe at room temperature and 50K. The mean diameter of CoFe 2 O 4 particles is about 16 nm. Mossbauer spectra reveal two sites for Fe3+. One site is related to Fe in an octahedral coordination and the other one to the Fe3+ in a tetrahedral coordination, as expected for a spinel crystal structure of CoFe 2 O 4. TEM measurements of nanocomposite show the formation of a thin shell of CoFe 2 on the cobalt ferrite and indicate that the nanoparticles increase to about 100 nm. The magnetization of nanocomposite showed hysteresis loop that is characteristic of the exchange spring systems. A maximum energy product (BH) max of 1.22 MGOe was achieved at room temperature for CoFe 2 O 4 /CoFe 2 nanocomposites, which is about 115% higher than the value obtained for CoFe 2 O 4 precursor. The exchange-spring interaction and the enhancement of product (BH) max in nanocomposite CoFe 2 O 4 /CoFe 2 have been discussed.
RSC Advances, 2020
Herein, the size dependent behavior of cobalt ferrite nanoparticles was investigated using synchrotron radiation based techniques. Scanning electron micrographs revealed the enhancement of particle/ crystallite size with increase of annealing temperature. Moreover, the shape of these particles also changed with increase of crystallite size. Saturation magnetization increased with increase of crystallite size. The higher saturation magnetization for larger crystallite size nanoparticles was attributed to a cation distribution similar to that of bulk CoFe 2 O 4. The optical band-gap of these nanoparticles decreased from 1.9 eV to 1.7 eV with increase of crystallite size. The enhancement of the optical bandgap for smaller crystallites was due to phenomena of optical confinement occurring in the nanoparticles. Fe L Co Ledge near edge extended X-ray absorption fine structure (NEXAFS) measurements showed that Fe and Co ions remain in the 3+ and 2+ state in these nanoparticles. The results obtained from Fe & Co K-edge X-ray absorption near edge structure (XANES)-imaging experiments further revealed that this oxidation state was possessed by even the crystallites. Extended X-ray absorption fine structure (EXAFS) measurements revealed distribution of Fe and Co ions among tetrahedral (A) and octahedral (B) sites of the spinel structure which corroborates the results obtained from Rietveld refinement of X-ray diffraction patterns (XRD). X-ray magnetic circular di-chroism (XMCD) measurements revealed negative exchange interaction among the ions situated in tetrahedral (A) and octahedral (B) sites. Theoretical and experimental calculated magnetic moments revealed the dominancy of size effects rather than the cation redistribution in the spinel lattice of CoFe 2 O 4 nanoparticles.
Exchange-spring behavior in nanopowders of CoFe2O4/CoFe2
Tuning electronic and magnetic properties of zigzag graphene nanoribbons by large-scale bending Appl. Phys. Lett. 100, 263115 (2012) Magnetic and electronic properties of α-graphyne nanoribbons J. Chem. Phys. 136, 244702 (2012) Reduced spin transfer torque switching current density with non-collinear polarizer layer magnetization in magnetic multilayer systems Appl. Phys. Lett. 100, 252413 (2012)
Consequences of electronic excitations in CoFe1.90Dy0.10O4
Current Applied Physics, 2015
Present work reports the irradiation induced effects in Dy 3þ doped cobalt ferrite nanoparticles in the regime of dominant electronic excitation processes induced by 100 MeV O 7þ ion irradiation. Irradiation leads to the deterioration of crystalline phase as envisaged by X-ray diffraction. Crystallite size decreases with the increase of irradiation fluence. Disappearance of certain bands in Raman spectra at higher fluence of irradiation confirms the crystalline disorder induced by electronic excitations. Fourier transform infrared spectra show onset of cation migration from tetrahedral site to octahedral site and vice versa. X-ray absorption fine structure measurements depict the preservation of valence state of metal ions after irradiation. These measurements further infer bond breaking process in irradiated materials. Magnetic measurements carried out on these materials indicate slight increase of saturation magnetization at room temperature followed by the decrease of coercive field. Obtained results are discussed on the basis of appropriate mechanism.
Electron paramagnetic resonance, magnetic and electrical properties of CoFe2O4 nanoparticles
Journal of Magnetism and Magnetic Materials, 2013
CoFe 2 O 4 nanoparticles were prepared by solution combustion method. The nanoparticle are characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy and scanning electron microscopy (SEM). PXRD reveals single phase, cubic spinel structure with Fd3m (227) space group. SEM micrograph shows the particles are agglomerated and porous in nature. Electron paramagnetic resonance spectrum exhibits a broad resonance signal g ¼ 2.150 and is attributed to super exchange between Fe 3 þ and Co 2 þ. Magnetization values of CoFe 2 O 4 nanoparticle are lower when compared to the literature values of bulk samples. This can be attributed to the surface spin canting due to large surface-to-volume ratio for a nanoscale system. The variation of dielectric constant, dielectric loss, loss tangent and AC conductivity of as-synthesized nano CoFe 2 O 4 particles at room temperature as a function of frequency has been studied. The magnetic and dielectric properties of the samples show that they are suitable for electronic and biomedical applications.
Exchange-spring behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite
2011
In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe 2 O 4 and ferrimagnetic oxide/ferromagnetic metal CoFe 2 O 4 /CoFe 2 nanocomposites. The latter compound is a good system to study hard ferrimagnet/soft ferromagnet exchange coupling. Two steps were used to synthesize the bimagnetic CoFe 2 O 4 /CoFe 2 nanocomposites: (i) first preparation of CoFe 2 O 4 nanoparticles using the a simple hydrothermal method and (ii) second reduction reaction of cobalt ferrite nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures, particle sizes, morphology, and magnetic properties of CoFe 2 O 4 nanoparticles have been investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) with applied field up to 3.0 kOe at room temperature and 50K. The mean diameter of CoFe 2 O 4 particles is about 16 nm.
Study on Synthesis and Characterization of CoFe 2 O 4 Nanoparticles
This work reports the use of micro emulsion method to prepare CoFe 2 O 4 nano-particles with sodium hydroxide as an alkaline solution and ferric salt. More vigilance is required while obtaining consistent products as the ferrous salt is unstable in nature. Nano particles developed were characterized for size and morphology by X-ray diffraction (XRD) & Scanning electron microscope (SEM). SEM examined the particle size distribution of 40-70nm. It is difficult to obtain uniform-sized nano-particles as their preparations are mostly done in bulk aqueous media in which it is hard to control nucleation and grain growth.
A facile method to control the size and magnetic properties of CoFe2O4 nanoparticles
Materials Chemistry and Physics, 2009
We discuss a facile approach to control the size and the magnetic properties of CoFe 2 O 4 nanoparticles by varying the solvent dielectric constant. X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Thermo-Gravimetric Analysis (TGA) and Vibrating Sample Magnetometer (VSM) are used to study the structural, thermal and magnetic properties of CoFe 2 O 4 nanoparticles. XRD analysis confirms the cubic spinel phase of CoFe 2 O 4 nanoparticles. The average particle size increases from 10 ± 1 to 16 ± 1 nm as the dielectric constant of the solvent is increased from 47 to 80. The room temperature magnetization measurement confirms that the saturation magnetization increases with particle size. The linear dependence of the dielectric constant of the medium on particle size is in good agreement with nucleation theory. Our experimental findings unambiguously confirm that the change in supersaturation has a significant influence on the nucleation rate and the particle size.
SN Applied Sciences, 2019
CoFe 2 O 4 nanostructures were prepared by a facile co-precipitation route using polyvinyl alcohol (PVA) and sodium dodecyl sulphate (SDS) as capping agents. The effects of PVA and SDS concentration on the structural, optical behaviour and photocatalytic response of the prepared CoFe 2 O 4 nanostructures were studied. XRD studies showed cubic spinel structure of CoFe 2 O 4 nanostructures. The crystallite size of CoFe 2 O 4 nanostructures was found to vary from 13 to 19.2 nm with change in PVA and SDS concentration, while the optical band gap varied from 2.2 to 2.31 eV. CoFe 2 O 4 nanostructures prepared using optimal SDS concentration exhibited enhanced photocatalytic performance and efficiently degraded methylene blue in only 20 min of solar illumination. In situ scavenger studies confirmed the photoexcited electrons to be the major active species responsible for the strong photocatalytic performance of CoFe 2 O 4 nanostructures. The observed strong photocatalytic response of nanostructured CoFe 2 O 4 photocatalysts and their simpler magnetic separation capability make them promising for photocatalytic water purification applications.
Exchange Bias in CoFe 2O 4/NiO nanocomposites
Superlattices and Microstructures, 2009
Structural and magnetic properties of nanocomposites of ferrimagnetic (CoFe2O4) and antiferromagnetic (NiO) nanoparticles with different percentages of cobalt ferrite (50% and 15% wt) have been studied. The observed Exchange Bias properties and their dependence on interparticle interactions are discussed.
Magnetic CoFe2O4 nanoparticles doped with metal ions: A review
Ceramics International, 2020
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Microstructural, morphological and electrical properties of sol-gel derived CoFe 2 O 4 nanomaterials
Journal of Physics: Conference Series , 2018
CoFe 2 O 4 nanomaterials have been synthesized through conventional sol-gel process by using CoCl 2 .6H 2 O and FeCl 3 as precursors and citric acid is used as a capping agent. The as synthesized nanomaterials have been investigated thoroughly using x-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and an impedance analyzer. The XRD patterns indicate the formation of body centered cubic spinel CoFe 2 O 4 having cell constant 8.403 Å and the corresponding space group is Fd-3m, in which the Fe 3+ and Co 2+ ions occupied in octahedral and tetrahedral sites in the crystallographic orientations. The average crystallite size was found to be 30 nm. The SEM micrographs reveal that the synthesized nanomaterials (NMs) formed as octahedron and tetrahedron and the particles are well dispersed and contain some pores. We found 17.5% porosity of the nanomaterials that plays an important role in technological applications, e.g. water and chemical filtration related to bio-medical applications. Measurement of the AC electrical properties shows that dielectric constant increases up to 5.13 for 0.3 MHz and then it decreases with increasing applied frequency. Lower dielectric constant at high frequency region appears due to decreasing interfacial polarization and limiting grain boundary effect.
Correlation between magnetic and electrical properties of Co0.6Sn0.4Fe2O4 nanoparticles
Journal of Nanoparticle Research, 2014
The Co 0.6 Sn 0.4 Fe 2 O 4 nanoparticles with 20-nm size were prepared by chemical coprecipitation method. The material exhibits wasp-waist type M(H) loop at lower temperatures (below 360 K) that has been attributed to the simultaneous existence of double-exchange and superexchange interactions in these nanoparticles. It has been investigated that at 360 K and above, the double-exchange interactions (Fe 3? -O 2--Fe 2? and Co 2? -O 2--Fe 3? ) are stronger than the superexchange (Fe 3? -O 2--Fe 3? ) and vice versa. The temperature-dependent ac conductivity reveals the semiconductor-metal transition around 360 K. It is suggested that around 360 K, the metal-like behavior of nanoparticles is due to the dominant ferromagnetic order in the system, while below this temperature, the charge carriers are thermally activated and the material becomes antiferromagnetic in nature.
ChemElectroChem, 2020
The structural properties of CoFe composites fabricated from inexpensive Co(II) and Fe(III) precursors using a Prussian blue analogue (PBA) strategy without additional reductants were investigated. Microporous CoFe-200 and microporous/mesoporous CoFe-550 structures of the CoFe catalysts (CoFe-PBA) were produced by calcination in N 2 at 200 ℃ or 550 ℃ for 1 h, respectively. The electrocatalytic activities of the CoFe catalysts produced for the oxygen evolution and reduction reactions (OER/ORR) were studied in alkaline media. The OER measurements revealed the CoFe-200 catalyst to be superior to CoFe-PBA and CoFe-550, and even surpass the activity of commercial Ir/C in terms of the overpotential at 10 mA cm-2 and onset potential (). On the other hand, the ORR activity of CoFe-550 exhibited a more positive half-wave potential (0.837 V vs. RHE) and (0.942 V vs. RHE) than CoFe-200. The Tafel slope (-55.9 mV dec-1) of CoFe-550 was lower than that of Pt/C (-77.8 mV dec-1). A comparison of CoFe-200 and CoFe-550 suggested that the microporosity of CoFe-200 (average pore diameter (d) ≤ 2 nm) was beneficial in terms of the OER. In contrast the mesoporous (d ≈ 35.6 nm) structure of the CoFe-550 promoted the mass-transport kinetics of oxygen through the electrode surface. CoFe nanocubes with tunable porosity are potential catalysts that can be utilized selectively for the OER and ORR.
The magnetic and colloidal properties of CoFe2O4 nanoparticles synthesized by co-precipitation
Acta chimica Slovenica, 2014
Magnetic CoFe(2)O(4) nanoparticles were synthesized by co-precipitation at 80 °C. This co-precipitation was achieved by the rapid addition of a strong base to an aqueous solution of cations. The investigation of the samples that were quenched at different times after the addition of the base, using transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDXS) and X-ray powder diffractometry, revealed the formation of a Co-deficient amorphous phase and Co(OH)(2), which rapidly reacted to form small CoFe(2)O(4) nanoparticles. The nanoparticles grew with the time of aging at elevated temperature. The colloidal suspensions of the nanoparticles were prepared in both an aqueous medium and in a non-polar organic medium, with the adsorption of citric acid and ricinoleic acid on the nanoparticles, respectively. The measurements of the room-temperature magnetization revealed the ferrimagnetic state of the CoFe(2)O(4) nanoparticles, while their suspensions dis...