Magnetic properties of bio-synthesized zinc ferrite nanoparticles

The magnetic properties of zinc ferrite (Zn-substituted magnetite, Zn_yFe_1-yFe₂O₄) formed by a microbial process compared favorably with chemically synthesized materials. A metal reducing bacterium, Thermoanaerobacter, strain TOR-39 was incubated with Zn_xFe_1−xOOH (x=0.01, 0.1, and 0.15) precursors and produced nanoparticulate zinc ferrites. Composition and crystalline structure of the resulting zinc ferrites were verified using X-ray fluorescence, X-ray diffraction, transmission electron microscopy, and neutron diffraction. The average composition from triplicates gave a value for y of 0.02, 0.23, and 0.30 with the greatest standard deviation of 0.02. Average crystallite sizes were determined to be 67, 49, and 25 nm, respectively. While crystallite size decreased with more Zn substitution, the lattice parameter and the unit cell volume showed a gradual increase in agreement with previous literature values. The magnetic properties were characterized using a superconducting quantum interference device magnetometer and were compared with values for the saturation magnetization (M_s) reported in the literature. The averaged M_s values for the triplicates with the largest amount of zinc (y=0.30) gave values of 100.1, 96.5, and 69.7 emu/g at temperatures of 5, 80, and 300 K, respectively indicating increased magnetic properties of the bacterially synthesized zinc ferrites.     

Ferromagnetic resonance and magnetic crystallographic anisotropy of monocrystalline lithium ferrite films

The crystallographic anisotropy constant K₁ of monociystalline lithium ferrite films was measured by the methods of ferromagnetic resonance and rotational moments. The presence of uniaxial anisotropy in the plane of a film with the constant K_u 10₃ erg · cm^–3 is established experimentally. The nature of the uniaxial anisotropy is explained by the anisotropy of the stresses in the plane of the film, a formula is obtained to compute the angle of deflection of the easy magnetization axis from the crystallographic direction. An estimate is made of the difference in the stresses along the axes (_x–_z) 10¹⁰ dyne · cm^–2.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp. 86–89, September, 1973.     

Ferromagnetic resonance in a thin ferrite spheroid surrounded by dielectric

Second-order electromagnetic corrections to FMR resonance field and linewidth have been calculated for a flattened spheroid surrounded by infinite and finite (spheroidal shell) dielectric. This configuration can be considered as a model for the epitaxial thin films with the dielectric substrate.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.     

Ferromagnetic resonance in arrays of highly anisotropic nanoparticles

We present in this study computational simulations of the ferromagnetic resonance response of magnetic nanoparticles with a uniaxial anisotropy considerably larger than the microwave excitation frequency (in field units). The particles are assumed to be randomly oriented in a two dimensional lattice, and are coupled by dipolar interactions through an effective demagnetization field, which is proportional to the packing fraction. We have included in the model fluctuations in the anisotropy field (H_K) and allowed variations in the demagnetizing field. We then analyzed the line shape and line intensity as a function of both fields. We have found that when H_K is increased the line shape changes drastically, with a structure of two lines appearing at high fields. The line intensity has a maximum when H_K equals the frequency gap and decreases considerably for larger values of the anisotropy. The effects of fluctuations in H_K and variations in the packing fraction have been also studied. Comparison with experimental data shows that the overall observed behavior is dominated by the particles with lower anisotropy.     

Ferromagnetic resonance of cobalt nanoparticles in the polymer shell

The magnetic properties of cobalt spherical nanoparticles (～ 5–9 nm in size) in a polymer shell are investigated using ferromagnetic resonance (FMR) spectroscopy. The metal-polymer complex is prepared through the frontal polymerization of the cobalt acrylamide (CoAAm) complex, followed by the thermolysis at a temperature of 643 K. Analysis of the ferromagnetic resonance spectra demonstrates that the material has a high blocking temperature of ～700 K. The anisotropy constant equal to 0.5 erg/cm³ is somewhat larger than the anisotropy constants characteristic of cobalt macrostructures. This difference is associated with the predominance of the surface anisotropy of nanoparticles. The surface anisotropy constant is calculated to be 0.17 erg/cm², and the anisotropy field is determined to be ～350 Oe. It is revealed that the polymer shell affects the magnetic properties of nanoparticles.     

Anomalous magnetic antiresonance and resonance in ferrite nanoparticles embedded in opal matrix

Observation of magnetic antiresonance phenomenon is reported in 3D opal nanocomposite with embedded ferrite particles. Antiresonance at microwave frequencies of millimeter waveband was observed. It results in a sharp maximum of the reflection coefficient of an electromagnetic wave. Measurements were carried out in the frequency range from 26 to 38 GHz for two compositions of embedded ferrite particles, namely, the Co_0.5Zn_0.5Fe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄. The physical nature of antiresonance is discussed.     

Magnetization of sol-gel prepared zinc ferrite nanoparticles: Effects of inversion and particle size

Nanoparticles of ZnFe₂O₄ have been prepared by using sol-gel method in two different mediums (acidic and basic) in order to observe the influence of the medium on the magnetic properties of the obtained nanoparticles. X-ray diffraction and Mössbauer studies of these samples show the presence of single-phase spinel structure. The average size of the particles as determined by X-ray diffraction increases with the annealing temperature from 18 to 52 nm. With the increase in particle size, magnetization decreases while the magnetization blocking temperature increases. Magnetization studies show that the samples prepared in basic medium have more ferrimagnetic nature as compared to those prepared in acidic medium. We understand this increase in magnetization as reflective of the increased degree of inversion (transfer of Fe³⁺ ions from octahedral to tetrahedral sites) in the particles of smaller size unit cells. From lattice parameter calculations on different particles it is determined that inversion is more favorable in the particles prepared in a basic medium than in the acidic medium due to the smaller cell size in the former.     

Electron spin resonance studies on quantum tunneling in spinel ferrite nanoparticles

The electron spin resonance (ESR) spectrometer, a very sensitive instrument with fast detecting window to explore quantum phase transitions for magnetic nanoparticles, was exploited to study the fascinating interplay between thermal and quantum fluctuations in the vicinity of a quantum critical point. We have measured ESR in ferrofluid samples containing nanosize particles of Fe₂O₃. The evolution of the ESR spectrum with temperature suggests that quantum tunneling of spins occurs in single domain magnetic particles in the low temperature regime. The effects of various microwave fields, particle sizes, and temperatures on the magnetic states of single domain spinel ferrite nanoparticles are investigated. We can consistently explain experimental data assuming that, as the temperature decreases, the spectrum changes from superparamagnetic (SPR) to blocked SPR and finally evolves quantum superparamagnetic behaviour as the temperature lowers down further. A nanoparticle system of a highly anisotropic magnetic material can be qualitatively specified by a simple quantum spin model, or by the Heisenberg model with strong easy-plane anisotropy.Received: 29 August 2003, Published online: 15 October 2003PACS: 76.30.-v Electron paramagnetic resonance and relaxation - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.) - 05.30.-d Quantum statistical mechanics - 75.50.Dd Nonmetallic ferromagnetic materials     

Structure of manganese zinc ferrite spinel nanoparticles prepared with co-precipitation in reversed microemulsions

The structure of Mn_0.5Zn_0.5Fe₂O₄ spinel ferrite nanoparticles is studied as a function of their size and the experimental conditions of their synthesis using X-ray absorption spectroscopy. The nanoparticles of different sizes down to approximately 2 nm and with a narrow size distribution were synthesized using co-precipitation in reverse microemulsions. Simultaneous refinement of the X-ray absorption fine structure (EXAFS) of three constituting metals shows a migration of Mn and Zn ions to the octahedral site of the spinel lattice compensated by the corresponding migration of the Fe ions. To a smaller extent, Mn ions switch the occupation site already in bulk and in larger nanoparticles, while a sporadic migration of Zn is detected only in the nanoparticles with sizes below approximately 5 nm. X-ray absorption near edge structure (XANES) reveals considerable variations in the position of the Mn K edge, suggesting the average Mn valence in the nanoparticles to be higher than 3+. Annealing at 500 °C relaxes the structure of as-synthesized nanoparticles toward the structure of the ceramic bulk standard. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Structural characterization and magnetic properties of superparamagnetic zinc ferrite nanoparticles synthesized by the coprecipitation method

Single phase zinc ferrite (ZnFe₂O₄) nanoparticles have been prepared by the coprecipitation method without any subsequent calcination. The effects of precipitation temperature in the range 20–80 °C on the structural and the magnetic properties of zinc ferrite nanoparticles were investigated. The crystallite size, microstructure and magnetic properties of the prepared nanoparticles were studied using X-ray diffraction (XRD), Fourier transmission infrared spectrum, transmission electron microscope (TEM), energy dispersive X-ray spectrometer and vibrating sample magnetometer. The XRD results showed that the coprecipitated nanoparticles were single phase zinc ferrite with mixture of normal and inverse spinel structures. Furthermore, ZnFe₂O₄ nanoparticles have the crystallite size in the range 5–10 nm, as confirmed by TEM. The magnetic measurements exhibited that the zinc ferrite nanoparticles synthesized at 40 °C were superparamagnetic with the maximum magnetization of 7.3 emu/g at 10 kOe.     

Ferromagnetic resonance of cobalt nanoparticles used as a catalyst for the carbon nanotubes synthesis

Catalyst is considered to be the most crucial parameter for the growth of carbon nanotubes. In this work we study the ferromagnetic resonance (FMR) spectra of the catalyst nanoclusters. Moreover we report for the first time the angle FMR studies of catalyst particles with and without CNT layer. The dependencies of the FMR spectra, X-ray diffraction (XRD) patterns, Raman spectra and morphology of the CNT layers on the growth conditions are discussed.     

Study of the nonstoichiometric composition op zinc oxide

The x values in the nonstoichiometrie chemical formula Zn_1+xO have been measured in the temperature range from 400 to 1200°C under oxygen pressures from 1 to 10^?6 atm, and also 1 atm of air. The x values varied between 0.00824 and 0.06370 under the various oxygen pressures. The enthalpies of formation of excess zinc in zinc oxide were in general less than 7.40 kcal/mole under the above conditions, all positive values representing an endothennic process. The plots of log x vs log P_o2 (or log x = 1/n log P_o2) are linear, and the 1/n values from the slopes of the plots are $\text{?1}\text{14.0}\text{to}\text{?1}\text{11.1}$ in the temperature range 400–900°C. Many physical properties of zinc oxide such as electrical conductivity, catalytic effects and defects can be explained on the basis of the x values and the mechanism of formation of the nonstoichiometric compositions of the oxide.     

Ferromagnetic FeCo nanoparticles for biotechnology

Calculated magnetophoretic mobility of a variety of magnetic compounds has identified FeCo to be an alternative for magnetite in in vitro biological cell separations. The synthesis of FeCo nanoparticles and the resulting microstructure is discussed as a function of the particle size. Their synthesis kinetics is modeled using a consecutive decomposition and growth model and is compared to experimental data.     

Ferromagnetic behavior of high-purity ZnO nanoparticles

ZnO nanoparticles with the wurtzite structure were prepared by chemical methods at low temperature in aqueous solution. The size of the nanoparticles is in the range from about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room temperature and above. Magnetization versus temperature, M(T), and isothermal M(H) measurements were obtained. The coercive field clearly shows ferromagnetism above room temperature. An exchange bias was observed, and we related this behavior to the core-shell structure present in the samples. The chemical synthesis, structure, and defects in the bulk related to oxygen vacancies are the main factors for the observed magnetic behavior.     

Ferromagnetic resonance in amorphous ferromagnet

The peculiarities of the ferromagnetic resonance of an amorphous ferromagnet in which a certain number of atoms forms the two-level systems are studied. The logarithmic dependence upon the temperature and the applied magnetic field of the frequency shift of the ferromagnetic resonance is obtained. The comparison of this theoretical dependence with experimental results gives a chance of the direct examination of the idea of the existence of the two-level systems. The dependence upon the temperature and the magnetic field of the frequency linewidth is also calculated.