Onset of size independent cationic exchange in nano-sized CoFe2O4 induced by electronic excitation (original) (raw)
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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-spring behavior in nanopowders of CoFe2O4/CoFe2
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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.
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.
Formation Mechanism and Shape Control of Monodisperse Magnetic CoFe2O4 Nanocrystals
Chemistry of Materials, 2009
The formation mechanism and shape control of monodisperse magnetic cobalt ferrite (CoFe 2 O 4) nanocrystals produced by thermolysis of a stoichiometric Co 2+ Fe 2 3+-oleate complex in organic solution has been investigated. Synthesis of the pure ternary CoFe 2 O 4 inverse spinel phase, without formation of any intermediate binary cobalt and iron oxides, is favored by the close thermal decomposition temperature of the Co 2+-oleate and Fe 3+-oleate precursors. For reaction temperatures between 250 and 320°C, the nucleation and growth dynamics dictate the size and shape evolution of the nanocrystals. Prenucleation of CoFe 2 O 4 occurs at 250-300°C but without any growth of nanocrystals, because the monomer concentration is lower than the critical nucleation concentration. For temperatures in the range of 300-320°C, which is above the thermolysis temperature of the mixed Co 2+ Fe 2 3+-oleate complex, the monomer concentration increases rapidly resulting in homogeneous nucleation. Atomic clusters of CoFe 2 O 4 with size <2 nm are initially formed at 314°C that then grow rapidly when the temperature is raised to 320°C in less than a minute. The shape of the CoFe 2 O 4 nanocrystals can be reproducibly controlled by prolonging the aging time at 320°C, evolving from initial spherical, to spherical-to-cubic, cubic, corner-grown cubic, or starlike shapes. Thus, with careful choice of reaction parameters, such as the precursor concentration and the heating rate, it is possible to achieve large-scale synthesis of shape-controlled monodisperse CoFe 2 O 4 nanocrystals with high yield.