Temperature dependence of phase shift in ferrite phase shifters

Reciprocal ferrite phase shifters exhibit a pronounced dependence of phase shift upon temperature. In a sample of magnesium-copper-chromium ferrite it is shown that the temperature dependence of the phase shift is basically determined by the temperature dependence of the microwave magnetic permeability.In conclusion the authors wish to thank G. I. Yudin for making the measurements of phase shift and its dependence on the field for various temperatures.     

Dynamic magnetoelectric effects in bulk and layered composites of cobalt zinc ferrite and lead zirconate titanate

Low-frequency magnetoelectric (ME) coupling is investigated in bulk samples and multilayers of cobalt zinc ferrite, Co_1-xZn_xFe₂O₄ (x=0–0.6), and lead zirconate titanate. In bulk samples, the transverse and longitudinal couplings are weak and of equal magnitude. A substantial strengthening of ME interactions is evident in layered structures, with the ME voltage coefficient a factor of 10–30 higher than in bulk samples. Important findings of our studies of layered composites are as follows. (i) The transverse coupling is stronger than the longitudinal coupling. (ii) The strength of ME interactions is dependent on Zn substitution, with a maximum for x=0.4. (iii) A weak coupling exists at the ferromagnetic-piezoelectric interface, as revealed by an analysis of the volume and static magnetic field dependence of ME voltage coefficients. (iv) The interface coupling k increases with Zn substitution and the k versus x profile shows a maximum centered at x=0.4. (v) The Zn-assisted enhancement can be attributed to efficient magneto-mechanical coupling in the ferrite. PACS 75.80.+q; 75.50.Gg;75.60.-d;77.65.-j;77.65.Ly;77.84.Dy     

Combined effect of demagnetizing field and induced magnetic anisotropy on the magnetic properties of manganese-zinc ferrite composites

This work is devoted to the analysis of factors responsible for the high-frequency shift of the complex permeability (μ^?) dispersion region in polymer composites of manganese-zinc (MnZn) ferrite, as well as to the increase in their thermomagnetic stability. The magnetic spectra of the ferrite and its composites with polyurethane (MnZn-PU) and polyaniline (MnZn-PANI) are measured in the frequency range from 1 MHz to 3 GHz in a longitudinal magnetization field of up to 700 Ое and in the temperature interval from −20 °С to +150 °С. The approximation of the magnetic spectra by a model, which takes into account the role of domain wall motion and magnetization rotation, allows one to determine the specific contribution of resonance processes associated with domain wall motion and the natural ferromagnetic resonance to the μ^?. It is established that, at high frequencies, the μ^? of the MnZn ferrite is determined solely by magnetization rotation, which occurs in the region of natural ferromagnetic resonance when the ferrite is in the “single domain” state. In the polymer composites of the MnZn ferrite, the high-frequency permeability is also determined mainly by the magnetization rotation; however, up to high values of magnetizing fields, there is a contribution of domain wall motion, thus the “single domain” state in ferrite is not reached. The frequency and temperature dependence of μ^? in polymer composites are governed by demagnetizing field and the induced magnetic anisotropy. The contribution of the induced magnetic anisotropy is crucial for MnZn-PANI. It is attributed to the elastic stresses that arise due to the domain wall pinning by a polyaniline film adsorbed on the surface of the ferrite during in-situ polymerization.     

Magnetocaloric effect in ferrite nanoparticles

A comparative study of the magnetocaloric effect (MCE) is reported in two different types of chemically synthesized magnetic nanoparticle systems—cobalt ferrite and manganese zinc ferrite with mean size around 5 and 15 nm, respectively. While CoFe₂O₄ nanoparticles were synthesized using co-precipitation, the Mn_0.68Zn_0.25Fe_2.07O₄ (MZFO) nanoparticles were prepared by reverse micelle technique using AOT as surfactant. Our results indicate that the change in entropy with the change in applied magnetic field (dS/dH) is reasonably large for this class of nanoparticles and has a wide distribution over a broad temperature range covering the region above and below the blocking temperature. The maximum entropy change is influenced by the particle size, overall distribution in anisotropy and magnetic moments.     

Anisotropy and phase states of garnet ferrite films with misoriented surfaces

Anisotropy of garnet ferrite films is investigated in the framework of the two-parametric model. It is shown that a garnet ferrite film with an arbitrarily oriented surface is characterized by biaxial anisotropy. The directions of the easy, intermediate, and hard magnetization axes are determined as functions of the misorientation angle and weak cubic anisotropy. It is demonstrated that the region of existence of homogeneous states in a magnetic field is bounded by a slant astroid. The magnetic susceptibility tensor and the ferromagnetic resonance frequency are calculated, and the dispersion law of spin waves is determined.     

A contribution to the study of ferromagnetic resonance in manganese zinc ferrites

A study is made of the temperature dependence of ferromagnetic resonance in the 9300 Mc band on three samples of manganese zinc ferrite. The results are discussed from the point of view of the Tsuya model for the production of an additional internal field during ferromagnetic resonance.     

Temperature characteristics of magnetic crystallographic anisotropy constants in single crystals of lithium and lithiumcobalt ferrites

The results of measurements of the temperature dependence of the first and second magnetic anisotropy constants In single-crystal lithium ferrite, and of the first magnetic anisotropy constant and the induced uniaxial anisotropy constant in single-crystal lithium-cobalt ferrite are reported. A ferromagnetic resonance method at 9375 Me was used. An explanation, based on the single-ion theory, is put forward for the temperature dependence of the first magnetic anisotropy constant in single-crystal lithium-cobalt ferrites.The authors are grateful to S. M. Zhilyakov and A. N. Elsukov for their help in the static measurements of anisotropy constants and to G. E. Pashneva for carrying out the chemical analysis of the ferrites.     

Principal mode of the nonlinear spin-wave resonance in perpendicular magnetized ferrite films

The paper reports a theoretical and experimental study of the nonlinear spin-wave resonance (SWR) modes in normally magnetized ferrite films. Particular attention is focused on the principal, lowest frequency, SWR mode. It is shown theoretically that, as the precession amplitude increases, the profile of the principal mode changes to make the excitation distribution across the film thickness more uniform. The nonlinear shift of the resonance field depends on the surface-spin pinning parameters. An experimental study has been made of YIG films with a strong uniaxial anisotropy field gradient over the film thickness, as well as of YIG films of submicron thickness. As the microwave power was increased, the principal-mode resonance field was observed undergoing a sublinear shift accompanied by a superlinear growth of absorbed power. This behavior is attributed to a change in the profile of the spatial distribution of ac magnetization.     

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.     

FMR Investigation of the Magnetic Anisotropy in Films Synthesized by Co<Superscript>+</Superscript> Implantation into Si

Thin ferromagnetic films with the uniaxial magnetic anisotropy were synthesized by Co⁺ implantation into single-crystal silicon in the magnetic field. It was concluded that the formation of the induced magnetic anisotropy is due to the directional atomic pair ordering (Neel–Taniguchi model). The synthesized films were studied by the ferromagnetic resonance (FMR) method in the temperature range from 100 to 300 K. The FMR linewidth is almost independent of temperature, which is in agreement with the Raikher model describing the magnetic resonance of uniaxial magnetic particles. It is found that the temperature dependence of the anisotropy constant is linear. This dependence can be associated with the difference in the coefficients of thermal expansion of the Si (111) substrate and the ion-beam-synthesized cobalt silicide films.     

Classical dependence of the domain wall velocity on the magnetic field in garnet ferrite films with orthorhombic magnetic anisotropy

The dependence of the domain wall velocity V on the acting magnetic field H is investigated for bismuth-containing single-crystal garnet ferrite films with orthorhombic magnetic anisotropy. It is shown that this dependence includes both the initial linear portion and a saturation portion and exhibits a complex behavior. This behavior is explained within the model of domain wall motion with spin wave radiation.     

Electromagnetic oscillations in a rectangular waveguide with a transversely magnetized ferrite plate

The complex propagation constant in a rectangular waveguide with a transversely magnetized ferrite plate has been found by a numerical calculation. The magnetic properties of the ferrite medium are described by the standard permeability tensor for a polycrystalline ferrite, having real and imaginary parts. The solutions are given as curves on the complex propagation-constant plane. The parameter is the relative magnetizing field, which varies over a wide range including a ferromagnetic resonance. From the family of curves obtained for various relative magnetizations and damping parameters, one can quantitatively evaluate the relation between the types of oscillations. This relation depends on the position of the ferrite plate in the waveguide and on the dielectric constant of the ferrite as well as on the magnetization and damping constant.Translated from Izvestiya VUZ. Fizika, No. 4, pp. 74–81, April, 1970.     

Magnetic aftereffects and the barkhausen effect in a magnesium manganese ferrite single crystal

The temperature dependence of the critical magnetic field [H₀(T)] of a magnesium manganese ferrite single crystal is analyzed, together with the changes taking place in the critical magnetic field with time, H₀(t), the temperature dependence of the domain-boundary stabilization field H_st(T), and the time required to establish the equilibrium state of the domain boundaries; the domain structure is examined, and the induced anisotropy constant is calculated; so is the activation energy of the process leading to the stabilization of the domain boundaries. The magnetic aftereffects and the Barkhausen jumps accompanying them are of a diffusion nature.     

Domain structure of (011) crystalline garnet ferrite plates

The domain structure in (011) crystalline garnet ferrite plates is studied with allowance for induced uniaxial anisotropy and two-constant cubic anisotropy. It is shown that the inclusion of the second constant of cubic anisotropy greatly affects the orientational phase diagram and also the topology of magnetic inhomogeneities in a given magnet. It is found, in particular, that 180°non-Bloch domain walls may appear in a certain range of combined anisotropy constants, causing a continuous change in the wall orientation.     

Dependence of the magnetic and magnetoelastic properties of cobalt ferrite on processing parameters

The dependence of the magnetic and magnetoelastic properties of highly magnetostrictive cobalt ferrite on processing parameters has been investigated. The cobalt ferrite samples used in this study were prepared via conventional ceramic processing methods. The processing parameters of interest were sintering temperature, holding time at the sintering temperature and powder compaction pressure. It was observed that the crystal structure, composition and saturation magnetization of the samples studied did not vary with changes in processing parameters but coercive field decreased with increasing sintering temperature. The amplitude of peak to peak magnetostriction was dependent on the holding time and powder compaction pressure. The strain derivative on the other hand was found to depend on powder compaction pressure at any given sintering temperature or holding time. The results show how the magnetoelastic properties of cobalt ferrite can be varied by changing the processing parameters.     

Magnetic and ultrasonic studies on stable cobalt ferrite magnetic nanofluid

Stable cobalt ferrite nanofluids of various concentrations have been prepared through co-precipitation method. Structural and morphological studies of nanoparticles are made with the help of X-ray diffraction technique and Transmission Electron Microscope respectively and it is found that the particles exhibit face centered cubic structure with an average size of 14 nm. The magnetic properties of the nanofluids have been analyzed at room temperature which revealed ferromagnetic behavior and also the very low value of coupling constant which ensures the negligible interparticle interaction in the absence of magnetic field. Ultrasonic investigations have been made for the nanofluids at different temperatures and magnetic fields. The temperature effects are explained with the help of open and close-packed water structure. The inter particle interactions of surface modified CoFe₂O₄ particles and the cluster formation at higher concentrations are realized through the variations in ultrasonic parameters.     

Preparation and characterization of cobalt ferrite nanoparticles coated with fucan and oleic acid

Cobalt ferrite has attracted considerable attention in recent years due to its unique physical properties such as high Curie temperature, large magnetocrystalline anisotropy, moderate saturation magnetization, large magnetostrictive coefficient, excellent chemical stability and mechanical hardness. In this work we present the preparation, of fucan coated cobalt ferrite nanoparticles by a modified co-precipitation method and the study of their structural, microstructural and magnetic characteristics for their application as a solid support for enzymes immobilization and other biotechnology applications. Aqueous suspensions of magnetic particles were prepared by coprecipitation of Fe(III) and Co(II) in the presence of NaOH, acid oleic and fucan polymer. The X-ray diffraction indicates that the funtionalization does not degrade the core cobalt ferrite. The infrared (FTIR) bands, indicate the functional characteristics of the coating on the cobalt ferrite. Mössbauer spectra at room temperature indicate the presence of a broadened sextet plus a doublet which is typical of superparamagnetic relaxation. For the Co-ferrite uncoated and coated with fucan the doublets have areas of 36.1 % and 40.3 % respectively, indicating the presence of non-interacting particles and faster relaxation time. The Co-ferrite coated with oleic acid and oleic acid plus fucan have areas around 17.5 % and 17.1 % respectively which indicate a weak superparamagnetic relaxation due to a slow relaxation time. The magnetization measurements of the cobalt ferrite nanoparticles with and without coating confirm that they are superparamagnetic and this behavior is produced by the core nanoparticles rather than the coatings. The cobalt ferrite nanoparticles coated with oleic acid presented the highest magnetization than when coating with fucan.     

Physical facets of ultrasonic cavitational synthesis of zinc ferrite particles

This paper addresses the physical features of the ultrasonic cavitational synthesis of zinc ferrite particles and tries to establish the relationship between cavitation physics and sonochemistry of the zinc ferrite synthesis. A dual approach of coupling experimental results with simulations of radial motion of cavitation bubbles has been adopted. The precursors for the zinc ferrite, viz. ZnO and Fe₃O₄ are produced in situ by the hydrolysis of Zn and Fe(II) acetates stimulated by OH radicals produced from the transient collapse of the cavitation bubbles. Experiments performed under different conditions create significant variation in the production of OH radicals, and hence, the rate of acetate hydrolysis. Correlation of the results of experiments and simulations sheds light on the important facets of the physical mechanism of ultrasonic cavitational zinc ferrite synthesis. It is revealed that too much or too little rate of acetate hydrolysis results in smaller particle size of zinc ferrite. The first effect of a higher rate of hydrolysis leads to excessively large growth of particles, due to which they become susceptible to the disruptive action of cavitation bubbles. Whereas, the second effect of too small rate of hydrolysis of Zn and Fe(II) acetates restricts the growth of particles. It has been observed that the initial reactant concentration does not influence the mean particle size or the size distribution of zinc ferrite particles. The present investigation clearly confirms that the rate-controlling step of zinc ferrite synthesis through ultrasonic cavitational route is the rate of formation of OH radicals from cavitation bubbles.     

Magnons in Co dot

Eigen spin wave frequencies and profiles of a cobalt hexagonal dot with exchange and anisotropy energies are derived. The lowest mode frequency is shown to be a linear function of edge anisotropy, so edge anisotropy controls the whole dot magnetization reversal and can be measured from spin wave resonance. The low-temperature dependence of cobalt dot magnetization is shown to be driven by edge anisotropy as well.     

Surface anisotropy energy of thin amorphous magnetic films of (Gd1−xCox)1−yMoy alloys; experimental results

Surface excitations in thin amorphous (Gd_1?xCo_x)_1?yMo_y films obtained by the rf sputtering technique were studied. A microwave spectrometer at X-band was used for magnetic resonance investigation with external magnetic field rotating from perpendicular to parallel resonance orientations. The critical angle and angular dependence of the position of the surface mode and the uniform mode were determined. The Surface Inhomogeneity (SI) model was applied with symmetrical boundary conditions. The surface anisotropy energy term was assumed as a superposition of the uniaxial anisotropy term and a biaxial anisotropy term. The origin of the latter term is not known yet. We also performed the resonance experiment for different temperatures ranging from 180 to 300 K. From the experiment, the uniaxial surface anisotropy constant K′_s1 and the biaxial surface anisotropy constant K′_s2 were found as functions of the temperature; the uniaxial anisotropy energy against temperature changes the sign for y=0.02 from easy axis to easy plane while the biaxial surface anisotropy does not change its character.