C. Djega-Mariadassou

Correlation between Sm2(Fe,Ga)17 and its precursor Sm(Fe,Ga)9

We have shown that the out-of-equilibrium hexagonal P6∕mmm phase adopts the stoichiometry Sm(Fe,Ga)9 in the Sm–Fe–Ga system. X-ray diffraction analysis has been combined with Mossbauer spectrometry. A specific program for the Wigner–Seitz cell (WSC) volume calculation of the equilibrium and of the metastable structure, deduced from the results given by the Rietveld analysis, was used. The Mossbauer spectra have been simulated with a model based on: (i) the multinomial law related to the statistical distribution of Ga and Sm vacancies obtained by the crystallographic approach, and (ii) the correlation between the WSC volume and the isomer shift of each inequivalent crystallographic site. The specific behavior of the 3g site isomer shift evolution corroborates the structure model. The Curie temperatures of the Sm(Fe,Ga)9 series are higher than those of the Sm2(Fe,Ga)17 alloys. Sm(Fe,Ga)9 represents an interesting host lattice for interstitial C or N atoms, with possible permanent magnet applications.

High field magnetic study of small Fe particles dispersed in an alumina matrix

A high-field magnetic study has been performed on small Fe particles dispersed in an alumina matrix. The absolute value of the magnetic moment and its thermal variation are obtained. A reduction of its value is observed depending on the particle size. This behavior is caused mainly by particle surface magnetic disorders; the relative fraction of the perturbed atomic layers increases when the particle size decreases. It is concluded that the magnetization behavior of small particles in a high field can be explained on the basis of three contributions: two static terms corresponding to the magnetic moment of the nonrelaxing system and the effect of anisotropy and one fluctuating term related to the finite volume effect. >

High coercivity in nanocrystalline carbides Sm(Fe,Ga)9C

The structural and magnetic properties of metastable hexagonal SmFe9−yGayC compounds, with y=0.25, 0.5, 0.75, and 1, have been investigated by means of powder x-ray diffraction, magnetic measurements, and high-resolution transmission electron microscopy. After carbonation, the Rietveld analysis points out an anisotropic lattice expansion more important for the carbon site. The Curie temperatures are systematically 20to70K higher than those of the homologous Sm2(Fe,Ga)17 carbides. A record coercivity of 27kOe is obtained for y=0.25 with annealing temperature of the noncarbonated powder at 710°C which corresponds to an optimal grain size around 28nm. The MR∕M(90kOe) ratio of 0.57 combined with high coercivity and high TC makes this alloy suitable for permanent magnet applications.

Combined effect of gallium and carbon on the structure and magnetic properties of nanocrystalline SmFe9

SmFe9−yGaxC carbides (0≤x≤1) were prepared by a two-step method of powder high-energy milling with annealing between 873 and 1200 K and a subsequent carbonation with heavy hydrocarbon at 693 K. The x-ray diffraction analysis with the Rietveld technique indicates an anisotropic volume expansion under carbonation slackened by the gallium substitution. The average saturation magnetic moment per Fe atom, measured with an applied field up to 200 kOe, increases slightly to 2.00 µB with Ga content. The Curie temperature, always 15 K above that of the homologous 2/17 carbides, is reduced by a dilution effect with Ga substitution and a concomitant increase of the negative Fe–Fe interactions. The Mossbauer spectra have been analysed on the basis of the binomial law related to the different Fe environments. The hyperfine parameter assignment to each individual crystallographic site was performed according to the correlation between isomer shift and Wigner–Seitz cell volumes calculated with a specific code in the P 1 space group allowing the splitting of the partially occupied atomic positions. The role of the C insertion is shown, on the one hand, by the increase of isomer shift due to the volume expansion with an enhancement of the charge transfer from the rare-earth atoms and, on the other hand, by the hyperfine field increase, pointing out the predominance of the negative core electron polarization term. The coercive field of 27 kOe, combined with a Curie temperature of 680 K, makes the alloy SmFe8.75Ga0.25C promising for further applications in the field of high-performance permanent magnets.

Effect of nanocrystallization on the structure and the magnetic properties of Nd–Fe–Co–Al–B glassy alloy

Nd40Fe30Co15Al10B5 bulk amorphous prepared by high energy milling shows a coercivity of 650kA∕m with a Curie temperature of 645K. The controlled nanocrystallization enhances the coercivity to 1600kA∕m and the remanence ratio is equal to 0.59. The coexistence of two crystalline magnetic phases, ferromagnetic Nd2(Fe,Co,Al)14B, and antiferromagnetic Nd6(Fe,Co,Al)14 are revealed by x-ray diffraction, high-resolution transmission electron microscopy, magnetization measurements, and Mossbauer spectrometry. The grain size for optimal magnetic properties is around 30nm. The nucleation process may play a leading role in the high magnetic behavior.

Small angle neutron scattering in a superparamagnet

Abstract Small angle neutron scattering (SANS) measurements and neutron depolarisation have been performed in superparamagnetic Fe particles. The particles size and interparticle distance are close to the values obtained from electron microscopy and magnetization. As a new result, the neutron probe show that the particle becomes ferromagnetically correlated with decreasing temperature. In applied field, we observe ferromagnetic correlations between spin components transverse to the mean magnetization.

Magnetic structure of fine Fe particles included in an alumina matrix

Abstract In-field Mossbauer spectroscopy on small iron particles embedded in an alumina matrix reveal the existence of several iron species. One of the shows classical hyperfine field values, the others are related to iron families with weak spin canting angle corroborating the reduction of the magnetic moment per iron atom observed from magnetization measurements under high field.

Magnetic properties of GdFe11−xSixTi

Abstract Induction melted GdFe 11− x Si x Ti alloys ( x ≤2) with ThMn 12 -type structure have been investigated by X-ray diffraction, weak field thermomagnetometry from 300 to 850 K and isotherm magnetization measurements in magnetic fields up to 5.5 T from 4.5 to 300 K. The unit-cell volume decreases with increasing Si content. The reduction of the Curie temperature is correlated to the decrease of the Fe sublattice ordering temperature deduced from the Curie temperature of the homologous YFe 11− x Si x Ti series. The exchange coupling parameters were derived in the approximation of the molecular field. Their increase with x gives evidence for an enhancement of the indirect 3 d –4 f interaction via 5 d which results from the 3 d –5 d hybridization favored by the Si substitution. The magnetocrystalline anisotropy of aligned and fixed polycrystalline samples was deduced from the fits of the M ⊥ ( H ) isotherms in the assumption of a Gaussian distribution of easy axis around the alignment direction. The anisotropy constants K 1 , K 2 and the anisotropy field H A decrease with increasing temperature. The Si substitution is responsible for a reduction of K 1 but K 2 , which increases slightly at low temperature, appears more sensitive to the exchange interaction Fe–Fe.

Small particle susceptibility: Validity of the volume determination and meaning of the superparamagnetic constant ϑsp

Abstract Volume determination of cosputtered Fe-Al 2 O 3 small particles have been performed from magnetization measurements in low and medium fields ( H ≤ 18 kOe). The low field approach seems less reliable than the medium field one because of difficulties inherent to the value assigned to the magnetic moment of the non-relaxing particle and its thermal variation.

Influence of Si substitution on the structure and magnetic properties of YFe11−xSixTi (x⩽2)

Abstract The unit cell parameters of induction melted (I4/mmm) YFe 11− x Si x Ti ( x ⩽2) show a small reduction with x increasing. The decrease of the Curie temperature with x results from the competition between the reduction of the Fe magnetic moment and the increase of the Fe–Fe exchange interaction. Both effects are partially explained by the 3d-band filling with the 3p-Si electron transfer.