L. Bessais

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 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.

Series expansions for the magnetisation of a solid superparamagnetic system of non-interacting particles with anisotropy

The calculation of the magnetisation curve of an assembly of non-interacting fine superparamagnetic particles, with uniaxial anisotropy and easy axes fixed in a solid non-magnetic matrix is considered. The presence of anisotropy complicates the calculation which otherwise would result in the Langevin function. The calculation for particles with anisotropy and easy axes fixed at arbitrary angles to the external field, requires the calculation of the partition function, which has previously been expressed exactly as a double integral or as a sum of single integrals. We have recently shown how the partition function can be reduced to a single integral and here we show how this can be expressed as a double infinite series containing known functions. Special cases are considered, some existing analytic formulae are reobtained, and some new analytic formulae are presented. For identical particles the deviation from the Langevin function is known to be considerable. The formulae presented should facilitate the incorporation of the effects of anisotropy.

Static and dynamic properties of small ferromagnetic particles: Comparison with spin-glass behaviour

Abstract The static and dynamic properties of a system of small iron particles dispersed in an alumina matrix are reported and compared with spin-glass properties. The non-linear susceptibility terms b 3 and b 5 in the expansion M = χ 0 H - b 3 (χ 0 H ) 3 + b 5 (χ 0 H ) 5 + … are temperature independent. Different laws for the dynamic scaling are tested.

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.

Structural and Mössbauer spectral study of the metastable phase Sm(Fe, Co, Ti)10

We have performed a Mossbauer spectral analysis of nanocrystalline metastable P 6/mmm SmTi(Fe1−xCox)9, correlated with structural transformation towards its equilibrium derivative I4/mmm SmTi(Fe1−xCox)11. The Rietveld analysis shows that the 3g site is fully occupied, while the 6l occupation is limited to hexagons surrounding the Fe–Fe dumb-bell pairs 2e. A specific programme for the Wigner–Seitz cell (WSC) calculation of the metastable disordered structure was used. The hyperfine parameter assignment based on the isomer shift correlation with the WSC volumes sequence leads to Co 3g preferential occupation, with Ti location in 6l sites. The mean hyperfine field increases with Co content in connection with the enhancement of the negative core electron polarization term upon additional Co electron filling. The same trend is observed for each individual site leading to the sequence HHF {2e }≥HHF {6l}≥HHF {3g}.

Microstructure and magnetic properties of uncoupled Sm2Co17-Cu nanocomposites

Nanocomposite Sm2Co17–Cu particles have been fabricated by low energy comilling of mechanically alloyed Sm2Co17 particles with Cu nanoparticles. The x-ray diffraction analyses show that the diffraction crystallite size (DCS) of Sm2Co17 decreases with increasing comilling time. Scanning and transmission electron microscopy observation demonstrates that the particle size is in the range of the DCS and the Sm2Co17 particles are separated by Cu particles. The coercivity as well as the remanence ratio decreases with increasing milling time due to the grain size reduction and grain separation. The nanocomposite Sm2Co17–Cu exhibits suitable magnetic and microstructure properties for high-density recording.

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.

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.

Enhancement of coercivity in the nanocomposite R40Fe30Co15Al10B5 (R = Nd, Pr)

The structure and magnetic properties of high-energy-milled R 40 Fe 30 Co 15 Al 10 B 5 (R = Nd, Pr) have compounds been investigated by means of x-ray diffraction, high-resolution transmission electron microscopy, magnetic measurements, and Mossbauer spectroscopy. The x-ray diffractograms of the as-milled alloys are typical for the amorphous state. Nanocrystalline R 2 (Fe, Co, Al) 14 B coexisting with R 6 (Fe, Co) 13-x Al 1+x is observed after recrystallization at 750°C. The mechanically milled amorphous samples exhibit a relatively moderate coercivity of 6.5 kOe at room temperature and a Curie temperature (T C ) 650 K. After subsequent annealing, both systems show hard magnetic behaviours such as a record-high coercivity of 29 kOe with approximately the same T C as for amorphous as-milled alloys.