Magnetic structure of zinc ferrite

A detailed analysis of the available experimental data has been made for the ordered phase of ZnFe₂O₄. The abnormally lowT _N and the observed spin arrangement have been explained on the basis of the hybridization of orbitals on the oxygen ions which gives rise to an anisotropic exchange interaction. The variation of hyperfine field and susceptibility with temperature along with the zero point reduction of the magnetic moment indicate that the ZnFe₂O₄ is a two-dimensional antiferromagnet.     

Magnetic and semi-conducting nano-composite films of spinel ferrite and cubic zinc oxide

Magnetic and semi-conducting nano-composite films have been prepared under bias polarization, by radio-frequency sputtering of a pure zinc ferrite target. These composite thin films are made of cubic Zn_1 − yFe_yO monoxide islands inside a spinel ferrite matrix. The relative proportion of each phase depends on the substrate polarization (i.e. bias power). When no bias is applied the films solely display the diffraction pattern of a spinel phase even if some islands inside the film can be observed by electron microscopy. When the bias power is increased, the spinel phase disappears progressively as enhanced formation of islands takes place in such a manner that the cubic Zn_1 − yFe_yO monoxide is solely revealed by X-ray diffraction for a bias power higher than 5 W. From bibliographical data and calculated phase diagrams, it can be inferred that these phases would require very low oxygen partial pressure, high temperature and mechanical pressure, to be obtained simultaneously by a conventional ceramic process. This underlines the strong potential of radio-frequency sputtering of oxide targets to prepare original oxides or composite materials.     

Magnetic ordering in Cu-Zn ferrite

The variation of magnetization with temperature of the Zn _x Cu_1–x Fe₂O₄ system has been obtained between 300 K and the Néel temperature at a constant magnetic field of 5.57×10⁵ A m^–1 for x=0 to 0.8. The observations indicate the existence of a Yafet-Kittel (Y-K) type of magnetic ordering in the mixed ferrites. A molecular field analysis of the Y-K spin-ordering using a three-sublattice model is shown to explain the experimental data satisfactorily. For the sake of verification, Néel temperatures of Cu-Zn ferrites were also determined from Mössbauer studies.     

The formation of zinc ferrite

The formation of zinc ferrite from zinc oxide and iron oxide is evaluated with high temperature X-ray diffraction experiments. Despite the problem of accurately determining the temperature of the part of the sample actually investigated, a kinetic analysis of the isothermal formation is shown to be possible. From these data the energy and entropy of activation are derived. It is further shown that neither corrections for grain growth during formation nor the presence of impurities have any influence on these results. On the other hand, the size of the reactant iron oxide particles is found to have a strong influence on both the energy and entropy of activation.     

Reactive sintering of zinc ferrite

The reactive sintering of zinc ferrite, i.e. firing without previously calcining, has been studied using three types of iron oxide. The conversion into ferrite as well as the simultaneous densification were investigated upon variations in morphology and impurity content. The formation reaction is shown to be accompanied by a strong increase of the median pore size. This phenomenon appears responsible for the generally smaller density of the final material as compared with single-phase sintering. It is shown that reactive sintering is inevitably accompanied by expansion, irrespective of molecular volume variations. A calcining step appears indispensable for the preparation of the highest quality of ferrites.     

Magnetic force microscopy of superparamagnetic nanoparticles

The use of magnetic force microscopy (MFM) to detect probe-sample interactions from superparamagnetic nanoparticles in vitro in ambient atmospheric conditions is reported here. By using both magnetic and nonmagnetic probes in dynamic lift-mode imaging and by controlling the direction and magnitude of the external magnetic field applied to the samples, it is possible to detect and identify the presence of superparamagnetic nanoparticles. The experimental results shown here are in agreement with the estimated sensitivity of the MFM technique. The potential and challenges for localizing nanoscale magnetic domains in biological samples is discussed.     

Weight losses from zinc ferrite and nickel ferrite during sintering

The sintering in air of fully formed zinc ferrite and nickel ferrite is always accompanied by a loss of weight from the ferrite, caused by removal of some oxygen. Although the rate of weight loss from the ferrite has the same time and temperature dependence as the rate of shrinkage of the specimens, it is found that altering the loss of weight by changing the oxygen pressure has little effect on the sintering of fully formed ferrites. Criteria are established for the preparation of high density and high resistivity sintered ferrites.     

Magnetic breakdown in zinc

The magnetic breakdown field between the second and third zones in zinc has been determined from measurements of the absolute amplitude of the de Haas-van Alphen oscillations of the needles by a torque method. By taking the measurements to lower magnetic field and by calculating the curvature of the needles we are able to determine the value of the breakdown field with much greater certainty than previously. With the external field along thec axis, the breakdown field is found to be 3.5±0.3 kG. As the external field is tipped away from thec axis, the breakdown field is found to vary with angle as though the energy gap between the zones were 6.75(1+2.28 cos ) meV for from 0 to 80°. The Dingle temperature was found to be isotropic within 0.1 K.This work was supported by the National Research Council of Canada under Grant A5948.     

Magnetic interaction between superparamagnetic particles in nanogranular cobalt films

A system of cobalt nanoparticles exhibits a transition from the superparamagnetic state into the state with cooperative magnetic ordering caused by the magnetic interaction between Co particles. It is shown that this transition can be used for obtaining nanogranular materials possessing soft magnetic properties at a large electric resistivity.     

锌铁氧体的合成及磁电性能

用化学共沉淀法制备了系列锌铁氧体微粉。借助于TG-DTA、XRD、SEM、VSM和PNA技术,对干凝胶的热分解过程、产物的物相、微观结构、磁性能和介电常数进行了研究。结果表明,随煅烧温度的提高,锌铁氧体微粉的粒径增大,磁性也增强;尖晶石结构的块状锌铁氧体具有负电阻率温度系数,而且随着煅烧温度的提高,介电常数减小。     

The magnetic behaviour of nanostructured zinc ferrite

We have investigated a series of nanostructured ZnFe₂O₄ samples produced by mechanical activation (mean particle sizes d ～50-8 nm) by variable temperature neutron diffraction measurements (2-535 K) supported by DC magnetisation measurements (4.2-300 K). The systematic increase in the mean inversion parameter (c ～0.04-0.43) with increasing milling time is accompanied by a gradual decrease in the occurrence of the long range antiferromagnetic ordering observed in the starting ZnFe₂O₄ material, as well as a gradual decrease in the related diffuse short range order peak. The neutron diffraction patterns of particles with d < ～15 nm and c> ～0.2 are consistent with the occurrence of ferrimagnetic order and exchange interactions of the type Fe³⁺A—O^2?—Fe³⁺ [B]. Diagrams summarising the magnetic regions of nanostructured ZnFe₂O₄ are presented. The magnetic behaviour overall agrees well with the enhanced magnetisation and ferromagnetic behaviour reported for nanostructured, ultrafine and thin films of ZnFe₂O₄ by other groups using mainly magnetisation and Mössbauer spectroscopy measurements.     

Modification of dielectric and mechanical properties of rubber ferrite composites containing manganese zinc ferrite

Spinel ferrites constitute an important class of magnetic materials. Polycrystalline ferrites are a complex system composed of crystallite grain boundaries and pores. Manganese zinc ferrites have resistivities between 0.01 and 10 Ω m. Making composite materials of ferrites with either natural rubber or plastics will modify the electrical properties of ferrites. Composite materials are ideally suited for many modern applications where ceramic materials have some drawbacks. The mouldability and flexibility of these composites find wide use in industrial and other scientific applications. Mixed ferrites belonging to the series Mn_(1−x)Zn_xFe₂O₄ (MZF) were synthesized for different ‘x’ values in steps of 0.2. These pre-characterized ceramic ferrites were then incorporated in a natural rubber matrix. The dielectric properties of the ceramic manganese zinc ferrite and RFC were also studied. A program based on G programming was developed with the aid of LabVIEW package to automate the dielectric measurements. The dielectric permittivity of the RFC were then correlated with that of the corresponding dielectric permittivity of the magnetic filler and matrix by a mixture equation, which helps to tailor properties of the composites.     

Magnetic properties of anisotropic Sr-La system ferrite magnets

An experiment was carried out to investigate the effect of La₂O₃ on the magnetic and physical properties of strontium ferrite magnets. It was found that the La₂O₃ addition to SrO-6Fe₂O₃ (stoichiometric composition) was very useful in stabilizing the magnetoplumbite structure and that these ferrites of the Sr-La system had excellent permanent-magnet properties. Various compositions were tested, and the optimum conditions for making magnets and some properties of typical specimens were determined. The semisintering condition was 1250°C for 1 h in oxygen, and the sintering condition 1300°C for 0.5 h in oxygen. Magnetic and physical properties are presented and discussed. The [(SrO)0.143(Fe₂O₃)_0.857]₉₈ (La₂O₃)₂ compound magnet exhibited the highest value of (BH)_max, and this value was equivalent to or higher than that of ordinary Ba and Sr ferrites     

Magnetic resonance in high-temperature superconductors

Since the discovery of high-temperature superconductors a new microwave absorption has been found. The microwaves are absorbed in the flux-quantized eigenstates so that the absorption is proportional to the Josephson current and hence it varies as the gap of the superconductor. This absorption is found in the electron-paramagnetic resonance configuration. The flux-quantized fields are found in small domains of the size of 10⁻⁶ cm. A giant moment is found to occur. The necessary theory as well as experiments in YBa₂Cu₃O_7−δ type compounds are described. The Cu²⁺ electron-paramagnetic resonance gives an anisotropic exchange narrowed line with anisotropicg-values withg-shift proportional to susceptibility. The symmetry of theg-value also reflects the symmetry of the superconducting gap.