In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Understanding the correlation between magnetic properties and nanostructure involves collaborative efforts between chemists, physicists, and materials scientists to study both fundamental properties and potential applications. This article introduces a classification of nanostructure morphology according to the mechanism responsible for the magnetic properties. The fundamental magnetic properties of interest and the theoretical frameworks developed to model these properties are summarized. Common chemical and physical techniques for the fabrication of magnetic nanostructures are surveyed, followed by some examples of recent investigations of magnetic systems with structure on the nanometer scale. The article concludes with a brief discussion of some promising experimental techniques in synthesis and measurements.

Magnetic Properties of Nanostructured Materials

Electrical and thermal conductivity of polymers filled with metal powders

Magnetorheological materials are a class of smart materials whose rheological properties may be rapidly varied by application of a magnetic field These materials typically consist of micron-sized ferrous particles dispersed in a fluid or an elastomer A quasi-static, one-dimensional model is developed that examines the mechanical and magnetic properties of magnetorheological materials This model attempts to account for magnetic non-linearities and saturation by establishing a mechanism by which magnetic flux density is distributed within the composite material Experimental evidence of the viscoelastic behaviour and magnetic properties of magnetorheological fluids and elastomers suggests that the assumptions made in the model development are reasonable It is shown that the model is semi-empirical in that it must be fit to the experimental data by adjusting a parameter that accounts for unmodelled magnetic interactions

http://iopscience.iop.org/article/10.1088/0964-1726/5/5/009/pdf

A model of the behaviour of magnetorheological materials

A new compounding method for exfoliated graphite–polypropylene nanocomposites with enhanced flexural properties and lower percolation threshold

Electrical and magnetic properties of composite materials prepared by incorporating various nickel-based fillers of different shapes into polyethylene were investigated. The fillers used were nickel powders, nickel filamentary powders, nickel flakes, and nickel-coated graphite fibers. The particle-size distributions of the fillers were determined both before and after the processing of the composite samples. A wide range of filler volume fractions were used. In some cases, the volume fraction approached the maximum packing fraction of the solid phase to significantly exceed the percolation threshold. The composite samples were characterized in terms of their volume resistivity, dielectric constant, and magnetic permeability values. Filler particles of asymmetric shapes were very effective in terms of altering the electrical properties of the composite samples. At the highest loading levels of the nickel fillers, the volume resistivity values of the composites decreased by more than 17 orders of magnitude. AT such high filler concentrations, the dielectric constant values of the composite samples increased considerably, to values that were greater than 1,000. The permeability values of the samples increased linearly with the volume fraction of the nickel filler and were insensitive to the shape of the fillers. The highest relative permeability value measured was 5.8 for compositesmore » with 67% by volume of nickel powder.« less

Effects of particle shape and size distributions on the electrical and magnetic properties of nickel/polyethylene composites

Electric and magnetic properties of composite materials consisting of low density polyethylene filled with powdered ferromagnetic materials were investigated. The volume fractions of the fillers were varied from 10% up to the theoretical maximum packing fractions, i.e. between 0.70 and 0.77, so that the percolation phenomenon could be investigated. The ferromagnetic fillers used were HyMu 800 (a nickel-iron-molybdenum alloy), MnZn ferrite and NiZn ferrite. The particle sizes and size distributions of the fillers were well characterized by image analysis techniques. Based on the particle size distribution the maximum loading levels of fillers as permitted by geometric considerations were calculated. The properties of the composites characterized included: volume and surface resistivities, dielectric constants, electrical loss factors and magnetic permeabilities.

Percolation in magnetic composites

The electric and magnetic properties of a composite material consisting of a thermoplastic elastomer incorporated with iron powder and nickel-iron alloy powder are investigated. The volume fraction of the powder filters ranges from 0.1 to 0.5. The composites are characterized in terms of their volume and surface resistivities, permittivities and electric loss factors and magnetic permeabilities 0 and magnetic loss factors. With the addition of the ferromagnetic powders, the volume and surface resistivity values of the composites decrease by more than seven orders of magnitude, the permittivity values increase tenfold and magnetic permeability values increase by a factor of about six. The ferromagnetic powders are more effective in contributing the electric and magnetic properties the composite at higher loading levels. The frequency variations of AC electric and magnetic properties are also measured in the range 20 Hz to 1 MHz. >

Electric and magnetic properties of a thermoplastic elastomer incorporated with ferromagnetic powders

Effects of segregation on the packing of spherical and nonspherical particles

Composites with high relative magnetic permeability values can be used in many industrial applications, especially if they can be shaped using conventional polymer-processing technologies. In this study, various hybrid composite systems (i.e., particles with differing aspect ratio, size, and magnetic permeability embedded into a polymeric binder) were prepared in an attempt to reach high relative permeability values without the use of high pressures or sintering. It was determined that an interaction effect between the different types of fillers exists and enhances the relative magnetic permeability value of the composite in relation to the use of single type of magnetic filler. Relative magnetic permeability values of over 100 were achieved. Such relative magnetic permeability values represent a significant increase in the magnetic permeability over available magnetic composites prepared using similar processing techniques. The significant gains in magnetic permeability were realized by altering the maximum packing fraction, and ultimately the percolation threshold of the composite, by using low and high aspect ratio particles simultaneously in the formulation of the magnetic composites. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1371–1377, 1997

Thomas J. Fiske

Papers

Effects of particle shape and size distributions on the electrical and magnetic properties of nickel/polyethylene composites

Percolation in magnetic composites

Electric and magnetic properties of a thermoplastic elastomer incorporated with ferromagnetic powders

Effects of segregation on the packing of spherical and nonspherical particles

Enhancement of the relative magnetic permeability of polymeric composites with hybrid particulate fillers