M. Pardavi-Horvath

Microwave applications of soft ferrites

Signal processing requires broadband, low-loss, low-cost microwave devices (circulators, isolators, phase shifters, absorbers). Soft ferrites (garnets, spinels, hexaferrites), applied in planar microwave devices, are reviewed from the point of view of device requirements. Magnetic properties, specific to operation in high-frequency electromagnetic fields, are discussed. Recent developments in thick film ferrite technology and device design are reviewed. Magnetic losses related to planar shape and inhomogeneous internal fields are analyzed.

Iron-alumina nanocomposites prepared by ball milling

Small magnetic particles of iron embedded in an insulating alumina matrix have been prepared by ball milling, either by direct milling of a mixture of iron and alumina powders or indirectly by ball-milling enhanced displacement reaction between magnetite and aluminum metal. The average particle size could be reduced to the 10-nm range as indicated by X-ray diffraction linewidths and SEM (scanning electron microscopy). The change in the saturation magnetization and the coercivity relates to the change of the phase composition, decrease of the particle size, and accumulation of internal stresses. >

A variable variance Preisach model (garnet film)

The analysis of an artificial medium has led to a new type of Preisach model where the variance of the interaction field is a function of the magnetization. The ideal particles of this medium have no reversible magnetization and only interact magnetostatically with their neighbors. In this model, the line separating the positively and the negatively magnetized regions of the Preisach plane is not a staircase. >

Nanocomposite formation in the Fe3O4‐Zn system by reaction milling

Magnetic nanocomposites of small iron particles embedded in nonmagnetic zinc oxide matrix have been prepared by ball milling, with an in situ displacement reaction between a metal oxide (Fe3O4) and a more reactive metal (Zn). The phase composition of the samples has been analyzed by x‐ray diffraction. Metallic zinc disappears during the first 100 min of milling and the magnetization decreases to almost zero, indicating the formation of a nonmagnetic intermediate iron‐zinc oxide phase. This intermediate phase decomposes into iron and ZnO upon further milling. The change in magnetic properties also reflects the decreasing size of the iron particles. The final particle size is about 9 nm, as estimated from x‐ray diffraction linewidth measurements. The final product of the process is a semihard magnetic material with a room‐temperature saturation magnetization of 40 emu/g and coercivity of 400 Oe. A significant fraction of the final Fe particles is superparamagnetic.

Magnetic properties of sputtered Bi/sub 3/Fe/sub 5/O/sub 12/

A series of Bi-substituted yttrium-iron-garnet films were grown including its end members, YIG and BiIG, by an RF-diode sputtering method. The low-temperature magnetization of highly Bi-substituted garnet is much lower than expected from the linear expansion of the lattice. The temperature dependence of the magnetization of these materials is described by a molecular field model in which some of the moment on the tetrahedral site Fe/sup 3+/ ion is assumed to be transferred to the Bi/sup 3+/ ion on the dodecahedral site. The proposed model contains three sublattices: Fe/sup 3+/ ion at the octahedral site, Fe/sup 3+/ with reduced spin at the tetrahedral site, and partially spin polarized Bi/sup 3+/ ions at the dodecahedral sites. Reasonable agreement between the model and data is obtained for BiIG if it is assumed that the spin on the tetrahedral iron sublattice is reduced by 0.5 mu /sub B/ and the Bi ion has a moment of 0.2 mu /sub B/ which is strongly coupled to the iron sublattice. The large values of the uniaxial anisotropy energy, the crystalline anisotropy energy, and the linewidths measured by ferromagnetic resonance are complementary to this result. >

Magnetic properties of copper‐magnetite nanocomposites prepared by ball milling

Small particles of Fe3O4, embedded in a Cu matrix, have been prepared both by direct milling of a mixture of Cu and Fe3O4 and by reaction milling of CuO and Fe. It is shown that the most significant changes in phase composition, magnetization, coercivity, remanence, and switching field distribution take place during the first 2 h of milling. Both processes provide small (about 25 nm) semihard magnetite particles after 1 h, with Hc up to 350 Oe and squareness of 0.3. Prolonged milling causes partial oxidation of magnetite to α‐Fe2O3 and the presence of superparamagnetic magnetite particles, both reducing the magnetization of the composites. Conclusions based on magnetic measurements about the details of the technological process and microstructural changes are in very good agreement with results of x‐ray diffraction measurements.

Magnetic nanocomposites by reaction milling

Abstract Systems of small magnetic particles embedded in a nonmagnetic matrix were prepared by high energy ball milling. Besides carefully chosen milling conditions, in situ chemical reactions were used to control the properties of the product. Nanocomposites of iron particles in metal oxides (Al 2 O 3 and ZnO), and magnetite particles in copper metal were prepared by reaction milling. The samples were characterized by X-ray diffraction and magnetic methods. A few hours of ball milling resulted in the completion of most chemical changes. Iron nanoparticles were formed with lattice strains of about 0.005; coercivities up to 400 Oe were achieved. The magnetization of the iron particles is 25–40% less than that expected for bulk iron.

Coercivity of epitaxial magnetic garnet crystals

Epitaxial magnetic garnet films of Ga and Ca-Ge substituted YIG grown for bubble memory applications are thought to be the most perfect magnetic materials. However, in spite of their very low dislocation and surface defect densities (≤5 cm2) the coercivity H c is of the order of 10-1Oe. This paper reviews the role of various mechanisms in determining static H c of stripe domains in epitaxial garnet crystals. Statistical fluctuations of material parameters, dislocations and localized surface defects are shown to give a negligible contribution to H c . The surface roughness of the free surface and stresses inside the transient layer at the substrate/epitaxy interface may cause H c from 0.1 up to some Oe. From the temperature dependence of H c a high density (1014to 1016/cm3) of micro-defects was deduced. Transmission electron microscopy revealed the existence of such precipitates responsible for the observed H c of device quality epitaxial garnets.

Interaction effects in switching of a two dimensional array of small particles

The magnetostatic interaction between elements of a regular two-dimensional array of rectangular garnet pixels has been investigated. The up- and down switching fields of an individual pixels, have been measured magnetooptically. The switching characteristics of a given pixel depends on the state of the neighbors. In this work the effect of the magnetization state of the first 5 coordination shells on a central, test pixel was investigated experimentally and numerically. The interaction field, i.e. the demagnetizing/magnetizing effects, is calculated exactly and in the dipole approximation for 24 pixels surrounding the central pixel. A good agreement between experimental and numerical results has been obtained.

Local demagnetizing tensor calculation for arbitrary non-ellipsoidal bodies

The distribution of the magnetization in planar ferrite devices is nonuniform due to the non-uniformity of the demagnetizing field. A method for calculating the local demagnetizing tensor for arbitrary 3-dimensional shapes, using the Finite Difference Method (FDM), is developed. The sample is discretized into rectangular elements and the magnetostatic interaction between each pair of elements is computed. The local demagnetizing tensor is obtained as the superposition of all interactions at the given element. With proper boundary conditions the method can be applied to the computation of the local demagnetizing tensor for arbitrarily shaped bodies. The local demagnetizing tensor can be used to calculate the non-uniform distribution in the ferrite, and to optimize the shape and size of the ferrite in the CAD of microwave devices.