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Different natures of the magnetization reversal are studied by means of magnetization and magnetostriction measurements for magnetostrictive “spring-magnet” multilayers of TbFeCo/Fe with a fixed Fe layer thickness of 10 nm and variable TbFeCo layer thickness of 12, 16 and 20 nm. In such multilayered systems, magnetization reversal occurs at different coercive fields for each layer related to formation of the domain wall at interfaces. The results show that at low temperatures, transition from the ferromagnetic saturation state (with domain walls) to the ferrimagnetic transient saturation state (without domain walls) takes place by the reversal of the magnetic moments in the high-magnetization Fe layers. This process is governed by large magnetic anisotropy of the TbFeCo layer. Increasing temperature, this anisotropy decreases and transition results from the reversal of magnetic moments in the small magnetization TbFeCo layers. Formation of the domain wall at interfaces is clearly evidenced by the negative contribution to the parallel magnetostriction.

Longitudinal extraordinary Hall Effect (LEHE) of magnetic Tb(Fe 0.55 Co 0.45 ) 1.5 (known as Terfecohan) thin films with perpendicular magnetic anisotropy has been investigated as a function of both the intensity of applied magnetic fields and the angle α between the applied field and film normal directions. The Hall voltage loops exhibit a parallelogram shape, which is almost similar to those of the perpendicular magnetization. The high-field Hall voltage susceptibility is positive at α =0. Its value decreases with increasing α and changes in sign at α m =20°, which is considered as the easy magnetizable direction of the film. This finding is comparable with those obtained from the magnetization, magnetic force microscopy (MFM) and conversion electron Mossbauer spectra (CEMS) measurements. The obtained LEHE behaviors are rather promising for applications such as magnetic recording heads and magnetic field detectors, where a large output signal is required at low magnetic fields.

International audience; The modification of the magnetic and magnetostrictive properties of TbFe2/Co exchange-coupled multilayers irradiated with 700 MeV Pb ions was investigated as a function of ion fluence. After irradiation, the magnetostrictive properties of the sample are first strongly improved up to a fluence of 1 × 1012 ions cm−2 where the magnetoelastic susceptibility is six times higher than for the as-deposited sample. This effect is ascribed to stress relaxation and Fe–Co intermixing at the interfaces. For higher fluences, the magnetostrictive properties do not vary further, indicating a stationary state of mixing.

Exchange-spring TbFeCo/YFeCo multilayers exhibit interesting magnetic and magnetostrictive properties that are rather promising for application in microsystems. In this paper, we present a study of the effect of the exchange coupling on the magnetic properties of these magnetostrictive multilayers. An exchange bias phenomenon, revealed by a shift of the minor hysteresis loop along the applied field axis, is found as the result of the creation of interfacial domain walls. This effect strongly depends on the magnetic properties of the soft YFeCo layer, and becomes more pronounced at low temperatures due to the enhancement of the magnitude of the exchange coupling between the layers.

The SmCo5 hard magnetic phase was added to magnetoresistive granular Cu–Fe alloys in order to investigate the influence of the presence of a hard magnetic phase on the magnetoresistive properties of a granular alloy that contains a soft magnetic phase. Cu80(Sm0.17Co0.83)xFe20−x ribbons, with x = 20,15,10,5, obtained by melt spinning, were investigated. The ribbons are composed of magnetic Fe, SmCo5, and Co precipitates embedded in a Cu matrix. In the as-quenched state, the magnetic interactions between magnetic precipitates lead to the formation of magnetic coherent regions and the magnetoresistance effect is only observed at high field (>1 T). After annealing, the strength of interactions decreases and a magnetoresistance effect is observed at low field (

Machined surfaces of two steels (0.38 and 0.8 wt.% C) were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Mossbauer spectrometry. Because of the shear and tool wear during the chip formation, a white etching layer appears on these surfaces. X-ray diffraction and Mossbauer spectrometry data show the presence of retained austenite and suggest the presence of martensite in the white etching layer. The formation of these phases is discussed and from crystallographic data, the temperature reached during turning is estimated.

Sr1−xSmxFe12O19 M-type hexagonal ferrites with x=0, 0.125 and 0.25 were produced according to a conventional ceramic process (calcination, milling and sintering). Magnetic measurements show that, as x increases, the coercivity of the calcinated material increases while its remanence decreases. However, the sintering process has a detrimental effect on the magnet properties. X-ray diffraction analysis of the calcinated material reveals that, first, the hexagonal M-type phase is the main phase and, second, secondary α-Fe2O3 and SmFeO3 phases are present, whose proportions increase with x. The presence of secondary phases is responsible for the remanence decrease. Mossbauer investigation of the calcinated material confirms the X-ray diffraction analysis and the evolution of the hyperfine parameters of the M phase with x suggests that some Sm atoms enter the M phase. The increase of the coercivity of the calcinated material is attributed both to microstructural changes in relation to the presence of secondary phases, and to the presence of Sm in the M phase.

The spin reorientation transition in DyFe11Mo around the spin reorientation temperature (220 K) is investigated by Mossbauer spectrometry. The temperature dependence of the hyperfine parameters for each Fe site reveals an obvious discontinuity of the hyperfine field. The magnitude of the discontinuity is more important for the 8f site than for the 8i and 8j sites, indicating that the most prominent contribution to the overall anisotropy in the Fe sublattice should be from the Fe ion at the 8f site. This is attributed to the 3d(Fe(8f))–3d(Mo(8i)) hybridization, which may play a quite important role in R(Fe,Mo)12 compounds.

Mechanical alloying (MA) of iron and chromium powder mixtures was performed from 0 to 190 h. SEM, transmission electron microscopy with energy-dispersive X-ray analyses and Mossbauer spectrometry were used to study the mixing at micrometric and nanometric scale. At the stationary mixing state (≈20 h of milling), MA of Fe60Cr40 powder mixtures gives particles of several micrometers consisting of magnetic nanograin cores characteristic of crystalline Fe60Cr40 alloy surrounded by a disordered paramagnetic grain surface (at 300 K), which increases during milling. At 85 h, the nanograins are totally disordered. For a longer time of milling, a de-mixing occurs by partial recrystallization of this disordered structure.

Sr 1− x La x Fe 12− y Co y O 19 powders with y / x =0.75 and x =0.1, 0.2, 0.3, 0.4 were prepared according to a ceramic process. X-ray diffraction analysis shows that all the powders are of single, hexagonal M-type, phase. The Mossbauer investigation confirms that Co 2+ substitutes for Fe 3+ in both 4f 2 (mainly) and 2a sites. The most important hyperfine parameters changes concern the 12k, 4f 2 and 2b sites and are related to both Co 2+ /Fe 3+ and La 3+ /Sr 2+ substitution effects.

X-ray diffraction, magnetization and Mossbauer effect investigations have been performed for the sputtered Fe0.2Cu0.8 thin films. A segregation forming isolated body-centred cubic Fe grains in the face-centred cubic Cu matrix with an average diameter between 1 and 40 nm took place upon annealing in the temperature range from 100 to 500oC. In addition, the magnetic coercive field is found to be enhanced and reaches a value of 37 mT for the sample annealed at 400oC. A perpendicular surface anisotropy constant KS = 0.04 mJ m−2 is deduced. Surface spin configurations are discussed for the granular films investigated.

Magnetite Fe3O4 films were grown on single crystal MgO (001) substrates using facing target sputtering technique. Conversion Electron Mossbauer Spectroscopy and magneto optical polar Kerr spectra have confirmed the stoichiometric repartition of Fe cations corresponding to the inverse spinel structure and the electronic structure characteristic of bulk Fe3O4. Hysteresis loops carried out at room temperature show that, in a 1 T applied magnetic field, only 60% of the saturation magnetization is detected. This behavior is discussed in correlation to the antiphase boundaries (APBs) observed by electron microscopy. Magnetic force microscopy studies show that magnetic domains are larger than the mean distance between APBs.

A detailed magnetic and Mossbauer study focusing on intrinsic magnetic and anisotropy properties of the DyFe11Mo compound is reported. The compound shows a spin reorientation phase transition at Tsr=220 K. Anomalies in physical properties such as saturation magnetization, AC-susceptibility and hyperfine field at Tsr were identified, analysed and are discussed in terms of the individual site anisotropy model.

Both conventional thermal treatments (CTTs) and nitriding thermochemical treatments (NTTs) are used to crystallize Fe74Si14B12 and Fe73.5Si13.5B9Cu1Nb3 melt spun ribbons. For the FeSiB sample, nitrogen is detected throughout the whole thickness, while nitrogen is observed only in the periphery of the (Cu–Nb) containing sample. Nitrogen diffusion is related to the crystallization. The remaining phase in the nanocrystallized FeSiBCuNb sample slows down the nitrogen diffusion. This kind of treatment applicated to FeSiBCuNb sample enables the formation of nanocrystallites with a grain size that is slightly smaller than that obtained following a CTT.

The magnetic interactions between the nanograins in heterogeneous granular Cu–(SmCo 5 )–Fe ribbons were evaluated with a classical mean field model to obtain the exchange coupling as well as the magnetic moment per nanoparticle. The samples were subjected to various thermal treatments and a significant magnetoresistive effect was obtained when the optimal annealing conditions had been achieved.

Films with a nominal composition of Tb(Fe0.55Co0.45)1.5 were fabricated by rf-magnetron sputtering from a fixed target configuration at various Ar gas pressures. Samples were investigated by means of x-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), conversion electron Mossbauer spectra (CEMS) and magnetostriction measurements. As the Ar pressure increases, the Tb and Fe content increases slightly, whereas the Co content decreases. In addition, a small amount of Ar is introduced into the films. The as-deposited films are amorphous alloys, but their magnetic behaviour depends strongly on the deposition conditions: a perpendicular magnetic anisotropy is obtained only in film deposited at lowest Ar pressure and a parallel magnetic anisotropy exhibits in remaining films. These samples show an intrinsic magnetostriction (λ≈10-3) in an applied field of 0.7 T. In this state, it was determined that the hyperfine field reaches the value Bhf = 24.5 T. Effects of the heat treatment on the magnetostrictive softness are also reported. The parallel magnetostriction with a huge magnetostrictive susceptibility (χλ = 1.8×10-2 T-1) obtained at (µ0H = 10 mT) makes these materials useful for applications.

The local order in TM80-xRExMe20 (TM = transition metal, RE = rare earth, Me = metalloid) amorphous alloys is investigated by considering two limit cases: either (TM80-xMe20)REx, where the alloy would be a dilute solution of RE atoms in a (TM100-y Mey) distorted prismatic network, or TM80-xREx) Me20, where the alloy would be a dilute solution of RE atoms in a polytetrahedral compact network of TM atoms containing interstitial Me atoms. The comparison of the calculated atomic pairs with the experimental results provides evidence for a critical RE concentration range close to 8-9 at.%. This is in agreement with a previous Mossbauer study of Fe80-x RExB20 amorphous alloys, which pointed out a structural modification of the iron environments in this rare earth concentration range.

Mechanical milling under reactive hydrogen atmosphere of a Nd–Fe–B powder, followed by vacuum annealing up to 600°C, was achieved. Both as-milled and annealed powders were investigated by X-ray diffraction and Mossbauer spectrometry. During reactive milling, the Nd2Fe14B phase disproportionates into neodymium hydrides (NdH2±x) poorly crystallized α-Fe and an amorphous Fe–B phase. Upon annealing, hydrogen desorbs from the neodymium hydride and this is followed by the recombination of the Nd2Fe14B phase. The recombination temperature was found to be 520°C. The recombination reaction is incomplete, likely due to oxygen contamination, as neodymium oxides, α-Fe and Nd1.1Fe4B4 phases are detected in the powder annealed at 600°C.

The Tb(Fe0.55Co0.45)1.5 films were fabricated by rf magnetron sputtering from a composite target. Samples were investigated by means of x-ray diffraction, vibrating sample magnetometer, conversion electron Mossbauer spectra, and magnetostriction measurements. The as-deposited film is an amorphous alloy with a perpendicular magnetic anisotropy and an intrinsic magnetostriction λ=1080×10−6 in an applied field of 0.7 T. In this state, it was determined that the hyperfine field Bhf=23.5 T and the cone-angle between the Fe moment direction and the film-normal direction β=12°. After annealing in the temperature range of TA=250–450 °C the amorphous structure still remained, however the anisotropy was changed to a parallel one. The soft magnetostrictive behavior has also been improved by these heat treatments: the parallel magnetostriction λ∥=465×10−6 was almost developed in low applied fields of less than 0.1 T and, especially, a huge magnetostrictive susceptibility χλ=dλ∥/d(μ0H)=1.8×10−2 T−1 was obtained at μ0H...

The crystallization behaviour of Fe74 Dy6 B20 and Fe70 Dy10 B20 amorphous alloys was carefully investigated by differential scanning calorimetry, Mossbauer spectrometry and x-ray diffraction up to 800 °C. Calorimetric studies were performed in limited temperature ranges that were progressively extended. For Fe74 Dy6 B20 , after partial crystallization into the tetragonal Fe3 B compound, the remaining amorphous part segregates into two amorphous `phases', respectively enriched and impoverished in dysprosium. Tetragonal Fe3 B further transforms into orthorhombic Fe3 B. Metastable Dy3 Fe62 B14 compound then forms from the Dy-impoverished amorphous fraction, and subsequent crystallization of the Dy1 + Fe4 B4 phase occurs in the Dy-enriched fraction. Finally, Dy3 Fe62 B14 decomposes into bcc iron, Dy1 + Fe4 B4 and iron borides. The nature of the first crystallization product suggests the existence of local environments of t-Fe3 B type for this Dy concentration. The crystallization process of Fe70 Dy10 B20 strongly differs from that of Fe74 Dy6 B20 . Segregation phenomena occur in the amorphous state prior to any crystallization. If the nature of the first crystallization product is assumed to be correlated with short-range order in the amorphous state, our results suggest that the local environments differ from those of Fe74 Dy6 B20 , as they probably involve dysprosium atoms. This behaviour would agree with a previous Mossbauer study performed on the as-quenched amorphous alloys, providing evidence for a structural modification of the iron environments in the rare earth concentration range 8-9 at.%.

Sputtered Fe/Tb multilayered systems have been successfully deposited on needle-shaped substrates. These specimens have been observed by field ion microscopy and the layered sequence is evidenced together with the preferential field evaporation of terbium versus iron. The first direct atomic scale concentration data of Fe/Tb multilayers by means of the tomographic atom probe are given. The three-dimensional reconstructions of the layers were obtained and the compositional modulation was observed across the specimens.

The Fe spin structure in ultrahigh-vacuum-deposited hcp Tb(t Tb )/bcc Fe(t Fe ) multilayers (t Tb and t Fe are the Tb and Fe layer thicknesses respectively) with perpendicular magnetic anisotropy (PMA) and large coercivity at low temperatures was investigated by 57 Fe Mossbauer spectroscopy, the polar magneto-optical Kerr effect, and superconducting quantum interference device magnetometry. A spontaneous reversible temperature-driven Fe spin reorientation transition from orientation parallel to the film plane to an out-of plane orientation (with Fe spins tilted by an angle relative to the film normal direction) was observed upon cooling below the reorientation temperature T R . At a fixed value of t Tb , T R was found to follow a t -1 Fe behaviour, indicating that the PMA originates from the interfaces. Upon cooling, the perpendicular remanent magnetization shows a step-like increase near the magnetic ordering temperature of Tb, contrary to (T), which exhibits no such anomaly. Apparently, strong coupling of Fe layers via magnetic Tb layers affects the perpendicular magnetic domain structure at remanence. Further, 57 Fe probe layer Mossbauer results concerning structure, composition and Fe spin orientation within Fe-on-Tb and Tb-on-Fe interfaces are reported.

Nitriding thermochemical treatment under suitable parameters is used to nano-crystallize Fe73.5Cu1Nb3Si13.5B9 ribbon. This new kind of treatment leads to finer nano-structure and modifies the structural parameters of the α-Fe(Si) phase, obtained during the treatment. The magnetic properties are also improved; specific magnetization at room temperature and Curie temperature of the crystalline phase increase, and the magnetostriction constant becomes smaller. The nitrogenation seems to offer a new way to obtain nanostructured ribbons. PACS No.: 75.70

A Monte Carlo investigation of the A thickness (A≡Fe) dependence of the transition and compensation temperatures of a simple cubic Heisenberg A/B bilayer is carried out. Our model, which includes a few mixed planes of the A1−xBx type that represent the disordered interfaces, basically consists of several coupled magnetic parts with different bulk transition temperatures. Numerical results are compared with the experimental data for amorphous Fe/Tb multilayers. The simulated Fe thickness (tFe) dependence of TC, which reproduces the decrease of TC as tFe increases, is consistent with the experimental one. Since our simulations indicate that the magnetic ordering is driven by the disordered interfaces, the decrease of TC is explained by the fact that the interfaces are more and more decoupled as tFe increases. The experimental thermal behavior of each sublattice magnetization can be reproduced with a Tb-magnetic moment of 6μB which seems to be reasonable for amorphous layers, whereas a value of 9μB is too la...

In recent years, nanocrystalline magnetic materials have been widely investigated in order to manufacture a material with high saturation magnetic flux density, and good soft magnetic properties compatible with thermal stability. These materials have been achieved by the granular structure crystallized from an amorphous phase as is the case for Fe{sub 73{center_dot}5}Cu{sub 1}Nb{sub 3}Si{sub 13{center_dot}5}B{sub 9} melt spun ribbon. This material exhibits excellent soft magnetic properties due to the formation of bcc Fe(Si) nanocrystallites immersed in a remaining amorphous phase. Such a material has a magnetostriction close to zero. The factors contributing to such nanophased structures are the additive elements to the basic FeSiB system, and the method of treatment. Copper addition is known to increase the nucleation centers in crystals while niobium is known to hinder their growth process. Conventional thermal treatment (CTT) is commonly used as annealing method. In this study, starting from the FeSiBCu system, the authors have added Nb. For the annealing, they have used a nitriding thermochemical treatment (NTT). The authors show the nitriding treatment effect on the structural behavior of FeSiBCu({+-}Nb) ribbons and they propose a nitriding mechanism.

Structural and magnetic properties of melt-spun Cu–Co–(Fe–Si) ribbons, as-quenched and after appropriate annealing, are investigated and correlated with their magnetoresistive properties. The effect of structural refinement of the magnetic phases and of the coexistence of multiple magnetic phases on the magnetoresistance is discussed. The existence of an optimum in the annealing conditions for obtaining a significant magnetoresistive effect, is inferred.

Fe73.5Cu1Si13.5B12 and Fe73.5Cu1Nb3Si13.5B9 nanocrystallized amorphous ribbons are investigated. To obtain the nanocrystalline state, a new kind of treatment is performed: nitriding thermochemical treatment (NTT). We show that this treatment can slightly improve the nanocrystalline state compared to the conventional thermal treatment (CTT). The new treatment leads to smaller grain size and higher crystalline volume fraction. It increases the Curie temperature of the crystalline α-FeSi phase and leads to smaller values of the magnetostriction constant.

The magnetic structure of the iron in melt-spun amorphous ribbons with the compositions and was investigated using high-field Mossbauer spectrometry. The low-temperature variation of the hyperfine parameters under fields ranging from 0 to 8 T is reported. A geometrical model was developed that accounts quantitatively for the experimental results. It describes the evolution of the spin structure in the framework of angular deformation of the cone formed by the iron magnetic moments. It is shown that exhibits a collinear spin structure of the iron from 2 T, while in , the iron spins remain distributed at random within a cone up to 8 T. At 8 T, the semi-angle of the cone apex amounts to about .

The positive influence of addition elements (V and Nb) on the oxidation behaviour of the Nd 2 Fe 14 B phase in sintered Nd–Fe–B magnets was quantified by Mossbauer spectrometry. The dissociation rate of the phase decreases in magnets containing corrosion-resistant intragranular precipitates (V–Fe–B and Nb–Fe–B). The activation energy is comparable for each sample, indicating that the pre-exponential factor of the diffusivity has a greater influence on the dissociation rate.

The oxidation behaviour of the phase in Nd-Fe-B sintered magnets containing additional elements (Al, Co, V, Nb, Mo) was investigated by Mossbauer spectrometry. The microstructure of the different samples was first characterized. Added elements were detected in both intergranular and intragranular precipitates. The presence of X-Fe-B (, V, Mo) precipitates was evidenced by high-resolution scanning electron microscopy, x-ray diffraction and Mossbauer spectrometry. The presence of Al and Co substituted to Fe in the phase was evidenced, and quantified using Mossbauer spectrometry and Curie temperature measurements. Powdered magnets sieved to a particle size less than m were oxidized in an ambient air furnace in the 200-C temperature range. These conditions are known to allow the oxidation process of the hard matrix (namely the intragranular diffusion process) to be followed accurately. The experimental oxidation kinetics were determined using Mossbauer spectrometry and fitted according to a single-particle analysis model. The results show a decrease of the dissociation rate of the hard matrix compared with the rate obtained for a sintered magnet containing no additives. As the activation energy was found to be comparable for each sample (106-112 kJ ), the pre-exponential factor of the diffusivity has a greater influence on the dissociation rate ( for the magnets containing V or Mo and for the magnet containing Nb, compared with for a magnet without additives). The slowing down of the dissociation rate is attributed to the presence of corrosion-resistant X-Fe-B (, V, Mo) intragranular precipitates in the oxidized layer and appears to be strongly dependent on the density of the intragranular precipitates.

In FeSiB-based systems, nanocrystalline state needed for good soft magnetic properties is obtained by controlling nucleation and growth rates of crystallization. These rates are known to depend on the additive elements in the FeSiB alloys but also on the annealing treatment. Generally, Cu and Nb are chosen as additive elements and conventional thermal treatment (C.T.T.) is used to produce the nanocrystalline phase. In this work, we studied Fe 73.5 Cu 1 Si 13.5 B 12 , Fe 74 Nb 3 Si 14 B 9 and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 ribbons. To obtain the nanocrystalline state, in addition to C.T.T., we used a nitriding thermochemical treatment (N.T.T.) at 520°C and we show that this treatment improves the nanocrystalline state (smaller grain size D and higher crystalline volume fraction). This treatment also increases the Curie temperature of the crystalline α-FeSi phase.

Using a Monte Carlo method, the magnetic properties of a Heisenberg multilayer system consisting of four alternating ferromagnetic layers (A and B) with different spins and interaction constants are examined. The effects of sampling size, antiferromagnetic interlayer coupling ( J AB ) and disordered interfaces on the specific heat and magnetisation thermal variation are investigated. For multilayers with steep interfaces, it is shown that the thermal variation of the specific heat exhibits two peaks, but only a single-phase transition occurs. The transition temperature is independent on J AB in the investigated range. On the other hand, this temperature depends on the kind of the interfaces.

A Monte Carlo method has been used to simulate Heisenberg multilayer systems ( L × L × 4 P ) consisting of alternating P ferromagnetic layers A and B with antiferromagnetic interface coupling J AB . Finite-size effects on the specific heat and magnetisation thermal variation for two kinds of boundary conditions at the top and bottom planes are investigated. In particular, our Monte Carlo data evidence that the specific heat exhibits two peaks and a single phase transition occurs at the temperature which corresponds to the location of the high temperature peak (as L → ∞).

The effect of niobium addition on the microstructure of Nd–Fe–Nb–B alloys (with 0, 2, 4 and 5 at.% Nb) has been investigated by transmission Mossbauer spectrometry and X-ray diffraction analyses. In the as-cast alloys, the Nb 26 Fe 32 B 42 (at.%) phase was characterised by a paramagnetic doublet, with an isomer shift IS=−0.04 mm s −1 relative to α-Fe, and a quadrupolar splitting QS=0.22 mm s −1 . The contribution of the Nd 2 Fe 17 phase was observed in the Mossbauer spectra of the alloys with 4 and 5 at.% Nb. Niobium additions allow a drastic reduction of the homogenisation times during annealing in the 800–1100°C temperature range, in agreement with previous studies. The kinetics of the disappearance of α-Fe in alloys with 2 at.% Nb were determined by Mossbauer spectrometry. The experimental curves were fitted with a simple theoretical model of growth of a spherical Nb–Fe–B precipitate in a spherical α-Fe matrix. The activation energy related to the α-Fe disappearance was estimated to be 112±16 kJ mol −1 .

The influence of Gd addition on the structural properties of FeCuNbSiB nanocrystallized and amorphous alloys is studied. The crystallization temperature increases and the microstructure of the annealed samples changes. Gd addition induces the formation of GdFeB phases. In fully crystallized FeGdCuNbSiB alloys the α-Fe(Si), FeNbB, Gd3Fe62B14 and Gd1.1Fe4B4 phases are observed. The evolution of the microstructure is followed as a function of the cumulative effects of annealing time and temperature. The results suggest the transformation of the metastable Gd3Fe62B14 phase into Gd1.1Fe4B4 and α-Fe. The hyperfine parameters of the Gd3Fe62B14 Mossbauer contribution are reported.

The long term high temperature oxidation properties of a Nd 16.4 Fe 75.7 B 7.9 commercial sintered magnet were investigated in pure oxygen atmosphere up to 500°C and in air between 350 and 600°C. In pure oxygen atmosphere, three exothermic reactions occur, corresponding to the oxidation of the Nd-rich intergranular regions, the Nd 2 Fe 14 B matrix phase, and the α -Fe phase that forms during the oxidation of Nd 2 Fe 14 B. In air, the oxidation of Nd 2 Fe 14 B was investigated further. Instead of the oxidation proceeding along the grain boundaries, the Nd 2 Fe 14 B matrix dissociates to form an adherent grey surface layer which grows transgranularly into the magnet. This is probably due to a reaction occurring in the Nd-rich regions which prevents fast path oxygen diffusion along the grain boundaries. The main reaction is the dissociation of the Nd 2 Fe 14 B matrix into α -Fe nanocrystals which contain small precipitates of oxides of Nd. The products of this reaction form the adherent grey layer which grows transgranularly into the magnet. The activation energy and the diffusivity pre-exponential factor for this reaction were found to be 114 kJ mol- −1 and 0.7 mm 2 s −1 , respectively. After further oxidation of the dissociated grey layer, α -Fe is oxidised to form α Fe 2 O 3 and finally, from about 600°C, some of the α -Fe 2 O 3 reacts with the small precipitates of oxides of Nd to form FeNdO 3 .

Powdered Nd - Fe - B permanent magnets were sieved to different particle sizes and oxidized in an ambient air furnace in the temperature range for different times. Transmission Mossbauer spectrometry was used to determine the dissociation kinetics of the phase. The experimental curves were fitted according to the Ginstling - Brounshtein model, taking into account the real distribution of particle size. The kinetic parameters, namely the activation energy and the diffusivity pre-exponential factor were determined to be and respectively, It appeared that the kinetics parameters could be determined as well by considering a simple theoretical single-particle analysis model. The influence of the particle size was studied in the case of this simple model.

The critical temperature of some non-frustrated ferromagnetic Ising spin systems on the two-dimensional octagonal tiling is calculated by Monte Carlo simulations using the simulated annealing method. The ferromagnetic interactions are limited to the third neighbours ( J 1 , J 2 , J 3 ) where only J 2 is a percolating interaction. The infinite-tiling critical temperature is estimated at kT c / J = 2.39 ± 0.02 ( J 2 = J and J 1 = J 3 = 0), kT c / J = 3.05 ± 0.03 ( J 2 = J 1 = J and J 3 = 0), kT c / J = 3.60 ± 0.04 ( J 2 = J 3 = J and J 1 = 0) and kT c /〈 z 〉 J is generally slightly higher in the octagonal tiling than in the square and triangular lattices indicating that the tendency to ferromagnetic ordering is higher in quasiperiodic tilings. It is found that the critical temperature, T c , varies linearly with the different exchange integrals.

Nd Fe Al alloys containing either δ or μ phase (Al content in the 2.5–11 at.% range) were studied by X-ray diffraction analysis and Mossbauer spectrometry. A lattice expansion of the δ phase (composition Nd 6 Fe 14− x Al x ) is observed, due to the substitution of Al for Fe in the 16 l 2 site of space group I 4/mcm. The room-temperature Mossbauer spectrum is paramagnetic for x = 2.8 and magnetic for x = 2. For x = 2, the mean iron moment is estimated to be 2.0 ± 0.1 μ B . Room-temperature Mossbauer spectra of the μ phase (Nd 33.3 Fe 66.7− x Al x with x = 5.3 and 3.7) are magnetic, and the results support a slight distribution of environments around the Fe atoms due to the substitution of Al for Fe.

Powdered Nd-Fe-B-type permanent magnets were heated at 200 ‡C for times up to 32 days. The evolution of the Nd2Fe14B magnetic phase, followed by57Fe Mossbauer spectroscopy, evidenced the positive influence of both the hydrogen decrepitation process and Co, Nb and V additives on the corrosion resistance.

(Ce0.2Fe0.8)1-xAlx(0 ≤x≤ 0.9) ribbons have been prepared by planar flow casting under an He atmosphere with a linear velocity of 29 m s-1. Analyses of the ribbons by X-ray diffraction and57Fe Mossbauer spectrometry in the temperature range 4–300 K show that all the ribbons are crystalline. With increasingx, the observed phases are Ce(Fe, Al)2, Ce2(Fe, Al)17, CeFe4+y,Al8-y and the single fcc aluminium phase. For the different phases, the line intensities of the Mossbauer spectra agree with previous results on the preferential substitution sites for aluminium. A coherent hyperfine parameters set was deduced from fitting spectra in the temperature range 4–300 K.

Ce20Fe80 ribbons have been produced by planar flow casting under an He atmosphere at linear wheel velocities between 19 and 29 m s−1. Analysis of ribbons by X-ray diffraction and57Fe Mossbauer spectrometry in the temperature range 77–300 K shows that the ribbons are crystallized. For higher velocities, the ribbon is constituted of the two equilibrium phases CeFe2 and Ce2Fe17, but, for lower velocities, there appears a third iron metallic phase, which can be explained by the quenching rate of the melt. A coherent hyperfine parameter set was deduced from fitting Mossbauer spectra in the whole temperature range.

Mossbauer and X-ray studies of the corrosion products formed on 42CD4 steel plates immersed in natural seawater under atmospheric pressure showed that small particles of γ-FeOOH and α-FeOOH are the main constituents of the rust layer. Their Neel temperatures are estimated to be 39 and 280 K, respectively.

The {Tb(Fe 0.55 Co 0.45 ) 1.5 /Y 0.2 Fe 0.8 } 40 multilayers are investigated by X-ray diffraction and SQUID measurements. The results show that in the as-deposited multilayer, the TbFeCo and YFe layers are amorphous. An excellent magnetic softness with a coercivity of 0.3 mT was achieved after annealing at 250°C. These novel magnetic properties are associated to the evolution of a nanocrystalline structure in the YFe layers. Different temperature dependences of the coercive field are described for the amorphous and nanocrystalline multilayers.

Amorphous ribbons of Fe 80.3 Ho 1.7 B 18 have been studied by differential scanning calorimetry and by magnetization measurements. The Curie temperature, determined from both methods, is T c = 628 K and the primary crystallization appears up to T m = 800 K . Comparison with results where the rare earth is replaced by Er shows that Ho leads to better thermal and magnetic stabilities than Er. The crystallization kinetics, analyzed from the Kissinger method, provides an apparent activation energy of 6.4 eV, a value attesting to the good stability of the amorphous state of this system.