A.M. Konn

Electromagnetic and magnetic properties of multicomponent metal oxides heterostructures: Nanometer versus micrometer-sized particles

We have measured the composition and frequency-dependent complex effective permittivities and permeabilities in zero applied field of a series of ZnO and ferrimagnetic γ-Fe2 O3 composites prepared by powder pressing. The overall features of the room temperature electromagnetic properties of these diluted magnetic semiconductor composites exhibit a strong dependence on the powder size of the starting materials. For instance, electromagnetic spectroscopy over the frequency range (300 MHz–10 GHz) shows that composites made of nanoparticles (N-type samples) display a strong increase of the real and imaginary parts of the permeability compared to composites made of micron-sized particles (M-type samples). The observed dielectric behavior as a function of composition is manifestly at odds with the predictions from the simple property-averaging continuum model of Bruggeman. Additionally, a gyromagnetic resonance in the gigahertz region of frequency has been established for N-type samples which is not observable ...

Electromagnetic characterization of fine-scale particulate composite materials

Abstract We report the results of the composition and frequency-dependent complex permittivity and permeability of ZnO and γ-Fe 2 O 3 composites prepared by powder pressing. The electromagnetic properties of these materials exhibit a strong dependence on the powder size of the starting materials. In the microwave frequency range, the permittivity and permeability show nonlinear variations with volume fraction of Fe 2 O 3 . As the particle size decreases from a few micrometers to a few tens of nanometers, the data indicate that local mesostructural factors such as shape anisotropy, porosity and possible effect of the binder are likely to be intertwined in the understanding of electromagnetic properties of fine-scale particulate composite materials.

A physical model for heterogeneous magnetic materials

Abstract A macroscopic transposition of the composite state (a mixture of magnetic and non-magnetic millimetre-sized balls) is proposed to interpret and anticipate the properties of the heterogeneous magnetic materials. The model is supported by a simple theory leading to a useful relationship connecting the effective susceptibility to the basic properties of the material.

Effect of the magnetic fraction on the complex susceptibility of soft magnetic composite materials

Abstract A model for the description of soft magnetic composite materials has been developed and tested in frequency through two basic behaviours: the relaxation-type mechanism and the resonance process described by the Gilbert modification of the Landau-Lifshitz equation. Prediction and comparison with experimental data have been found to be very good.

Magnetic behavior of heterogeneous magnetic materials

The magnetic susceptibility change in heterogeneous magnetic materials as a function of the magnetic matter characteristics (grain size, chemical composition, concentration) and also of the sample shape is studied. Techniques for measuring weak susceptibilities are described. They are used to show the existence of macroscopic demagnetizing fields in rod specimens beyond a certain volume of magnetic matter. >

High-sensitivity and broad-band techniques applied to magnetic susceptibility measurements in ferrofluids

Abstract Two sensitive and useful techniques for susceptibility measurements in magnetic liquids are proposed. Both cover the 10 Hz–3 GHz range and are based on the accurate determination of a reference level. The importance of the field line distribution is shown. Examples are given that clearly show the two classical frequencies of dispersion.

Experimental demonstration of the nonreciprocity of magnetic composite materials for microwave applications

The purpose of this article is to bring out the nonreciprocity of ferrimagnetic powder loaded composites. We first describe their technology of preparation. Then we briefly recall the principle of the broadband measurement method used to determine the permeability tensor components of magnetized materials. Our experimental results performed at X-band frequencies (8–12 GHz) on two different ferrimagnetic loaded composite samples are presented and discussed. We finally show that for each material under test, the off-diagonal component κ of the permeability tensor, which is at the origin of the nonreciprocal effect, is of a magnitude comparable to the magnitude observed in bulk ferrites. This result proves that powders technology can be used to realize composite materials for nonreciprocal microwave applications.

Study of relaxation and spin resonance in magnetic liquids

Abstract The complex susceptibility of magnetic liquids is investigated in a wide frequency range (10 Hz-2 GHz) with a microwave-type technique. Three loss mechanisms can be identified: Brownian relaxation, Neel relaxation and natural spin resonance. The effect of the temperature on Neel relaxation and natural spin resonance is confirmed theoretically.

Susceptibility measurement in diluted magnetic media

Abstract Two sensitive methods for susceptibility measurements in diluted magnetic media are proposed. Both are based on the accurate determination of the reference level given by the sensing coil when the magnetic substance is removed. One of the methods uses ferro-to-paramagnetic transition ( T c -transition).

A common expression for both direct and inverse magnetization curves in soft ferrites

Abstract The model proposed by Globus in 1963 to explain the magnetic behavior of polycrystalline soft ferrites is not truly consistent with the hysteresis properties. A new calculation is proposed which shows that the hysteresis loop can be derived from a single analytical representation. An example is given which confirms the validity of the improved model.