Jean-Luc Mattei

Magnetic transition at 30–34 Kelvin in pyrrhotite: insight into a widespread occurrence of this mineral in rocks

Abstract A characteristic magnetic transition at 30–34 K is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low-temperature techniques as applied to magnetite and hematite, extended down to liquid helium temperature. A review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals. Unblocking temperature up to 350°C and extreme resistance against AF may be encountered in fine grained pyrrhotite.

Percolative behaviour and demagnetizing effects in disordered heterostructures

Abstract This study provides a behavioural analysis of the particular demagnetizing effects displayed by disordered heterostructures close to a particular value (Cp) of their magnetic load C. The emphasis is put on the convergence of evidences, got from several independent ways (experimental, analytical and numerical), of a percolative phenomenon in random distributions of magnetic particles that rules the sharp variations of the inner and outer demagnetizing effects at concentrations close to Cp. When C Cp, the delocalization of the magnetic poles leads the outer demagnetizing factor of the sample to switch from zero to its maximum value. The analytical approach of this phenomenon is based on an effective medium theory. It leads to a description of the interactions between magnetic inclusions that show a deep similarity with the main notions the reciprocity principle calls for.

Low-loss spinel nanoferrite with matching permeability and permittivity in the ultrahigh frequency range

Ferrite nanoparticles with a composition of Ni0.5Zn0.3Co0.2Fe2O4 were prepared by a coprecipitation method. The sample obtained after treating at appropriating conditions had almost equal real permeability and permittivity, of 4.8+j0.15 (loss tangent∼0.04) and 4.9+j0.10 (loss tangent∼0.018), respectively, over 100 to 500 MHz. This material had a refractive index n of close to 5 and a reduced impedance Z/Z0 of close to 1. These properties make the material useful to the design of miniaturized antennas at very high frequency-ultra high frequency (VHF-UHF) (100–500 MHz) bands.

Frequency Tunable Antenna Using a Magneto-Dielectric Material for DVB-H Application

This paper presents an ultracompact antenna design suited for digital video broadcasting-handheld (DVB-H) reception devices. The DVB-H frequency band is ranging from 4 70 to 862 MHz and divided in 49 channels of 8 MHz. Designed to be integrated in a tablet, it is not only heavily miniaturized (λ0 /49 × λ0/71 ×λ0/160 at 470 MHz), but also able to cover each channel thanks to the use of a magneto-dielectric material. The advantage of using such a material is studied and described in this paper. Moreover, the operating frequency is continuously tuned over the whole DVB-H band by the integration of a varactor diode. This varactor diode has been characterized and modeled to properly cosimulate its behavior within the antenna. Limitations in terms of accepted power by the diode are emphasizing. Finally, the antenna design, including both magneto-dielectric material and varactor diode is integrated in the DVB-H receiver device. Measurement performances are presented and discussed.

Suitability of Ni-Zn Ferrites Ceramics With Controlled Porosity as Granular Substrates for Mobile Handset Miniaturized Antennas

Low-loss Ni-Zn porous ferrite ceramics have been obtained (porosity close to 40%) after appropriate heat treatment and shaping of nanosized powders. The key point in this process is a moderate reaction sintering that promotes desired value of porosity. Measured complex permeabilities and permittivities in the [107 Hz -1010 Hz] frequency range are presented. The developed materials show almost constant refractive index value n close to 5 up to 900 MHz. The sum of the magnetic and dielectric losses shows moderate values up to a frequency value f, unusual for spinel ferrites: tanδμ+tanδε <; 0.083 at f=600 MHz. Their efficiency for miniaturization and performances improvement of several new antennas for Digital Video Broadcasting Handheld (DVB-H) devices (frequency ranging: 470-862 MHz) are presented. High miniaturizations (about 40% to 60% when compared to not loaded antenna) are obtained. Low loss, wide bandwidth and good radiation efficiency are measured. Antennas performances are enhanced. A new antenna is also presented. Its dimensions are very small and it covers the entire DVB-H band. Suitability of Ni-Zn porous ferrites ceramics for antenna miniaturization for DVB-H applications is demonstrated.

Influential parameters on electromagnetic properties of nickel–zinc ferrites for antenna miniaturization

Electromagnetic properties of nickel–zinc ferrites based materials make them potential candidates for applications linked to telecommunications. In the present study, nanosized particles of spinel ferrite Ni0.5Zn0.3Co0.2Fe2O4 were prepared by coprecipitation method. An optimized material is obtained after adequate heat treatment and partial filling of the porosity by epoxy resin. This material lies between ceramic and composite medium (with porosity close to 40%), and shows almost constant complex permeability and permittivity in the frequency range from 0.1–0.7 GHz, and equal to ∼3.5-j0.15 (loss tangent∼0.04) and ∼4-j0.2 (loss tangent∼0.02), respectively. The refractive index n is close to 3.75. These electromagnetic properties, in particular the low levels of losses, show that this material could be useful to the design of miniaturized antennas in the VHF-uhf (300–700 MHz) range of frequency.

Effects of the magnetic dilution on the ferrimagnetic resonance of disordered heterostructures

Abstract This study deals with the noteworthy effects of inner demagnetizing effects on the ferrimagnetic resonance (FMR) of disordered heterostructures, in particular, close to their percolation concentration ( C p ) in magnetic material. A recognized experimental fact is that the FMR increases with decreasing soft ferrite content in such composite materials. We have undertaken a systematic experimental study of Ni 1− x Zn x Fe 2 O 4 composite materials, at the low frequency range and at their gyroresonance. Our interpretation of the data, based on the Effective Medium Theory, uses a reciprocity principle developed for heterostrucures. At concentrations below C p magnetic gaps, which originate from the magnetic dilution, may cause a cut-off of the magnetic flux-paths in the composite medium, leading to inner demagnetizing effects that increase the magnetic anisotropy. Whereas above C p , the topology of the percolating cluster allows a magnetic flux path to be continuous throughout the material, here the inner demagnetizing effects vanish, leading the FMR to be that of the bulk itself in the concentration range going from C p to unity.

A New Magneto-Dielectric Material Loaded, Tunable UHF Antenna for Handheld Devices

In this letter, the impact of a magneto-dielectric resonator on a DVB-H antenna is investigated. A frequency reconfigurable antenna using a folded monopole antenna structure and varactor diodes is proposed. Two different designs are realized with and without the resonator. A measured instantaneous -6-dB bandwidth wider than 8 MHz is obtained continuously between 470 and 862 MHz for both prototypes. Design methodology is described. Measured return loss, gain, directivity, and radiation pattern are presented for different frequencies.

Gyroresonance in unsaturated composite bodies: Experiments and theory

The permeability spectra of composite magnetic materials (NiZn ferrite composites and YIG composites) has been experimentally studied on a wide range in magnetic load C. The resonance frequency decreases with the increasing magnetic load of the mixture. This is attributed to the fading of the magnetic pole on the surface of the magnetic inclusions. A theory is developed which is intended to give an expression for the gyroresonance frequency of magnetic composites in their unsaturated magnetic state. It fits the experimental data over its full range of volume fraction C without any corrective factors. The proposed relation, which is an extension of the Kittels expression for the resonance frequency of a saturated body, points out a noteworthy close link between the concept of reciprocity and the one of demagnetization.

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.