J.M. Le Breton

LaCo-substituted ferrite magnets, a new class of high-grade ceramic magnets; intrinsic and microstructural aspects

The science and technology of conventional ferrite magnets is reviewed, including their historical evolution and models to explain their properties. Next, a survey is given of the new LaCo-type ferrite magnets, representing a breakthrough in magnetic performance. The increased performance is explained with the classical models. Increased market share of M-ferrite magnets as well as re-intensification of background research is expected.

Structural analysis of hydrothermally synthesized Sr1−xSmxFe12O19 hexagonal ferrites

Abstract M-type hexagonal ferrites of composition Sr1−xSmxFe12O19 with x=0, 0.06, 0.11, 0.2 and 0.33 were produced by hydrothermal synthesis. The purity of the samples was investigated by X-ray diffraction and Mossbauer spectrometry. The analyses reveal for all the samples the presence of the hexagonal M-type phase as the main phase, but also the presence of Sm containing secondary phases of the (Sr,Sm)FeO3−δ type. The results of the Mossbauer analysis show that the hyperfine parameters of the M-type phase contribution do not vary with the Sm content, indicating that the Sm content in the M-type phase is very weak. This is in agreement with the fact that no Fe2+ is detected in the spectra. As both anisotropy field and saturation magnetization remain constant, the increase of the coercivity is thus attributed to microstructural changes, in relation with the presence of the Sm containing secondary phases.

Structural and magnetic properties of hydrothermally synthesised Sr1-xNdxFe12O19 hexagonal ferrites

Abstract M-type hexagonal ferrites of composition Sr 1− x Nd x Fe 12 O 19 with x =0, 0.06, 0.08, 0.11, and 0.20 were produced by hydrothermal synthesis. The phase composition of the samples was investigated by X-ray diffraction and Mossbauer spectrometry. The analyses reveal for all the samples the presence of the hexagonal M-type phase as the main phase. However, Nd containing secondary phases of the (Sr,Nd)FeO 3− δ type are also detected. Mossbauer spectrometry indicates that the Nd content in the M-type phase is very weak, as the hyperfine parameters of the M-type phase contribution do not vary with the Nd content. This is in agreement with the fact that both saturation magnetisation and remanence remain almost constant as the Nd content increases. The variations of the coercive field are thus attributed to microstructural changes, in relation with the presence of the Nd containing secondary phases.

Sublattice occupation in Sr1−xLaxFe12−xCoxO19 hexagonal ferrite analyzed by Mössbauer spectrometry and Raman spectroscopy

Abstract The cationic distribution of Co 2+ in Sr 1− x La x Fe 12− x Co x O 19 hexagonal ferrite is investigated by Mossbauer spectrometry and Raman spectroscopy. The two complementary techniques lead to the same conclusion: the majority of the Co 2+ is substituted for Fe 3+ in the octahedral 4f 2 and 2a sites, in agreement with the magnetization measurements. A valence change of some Fe 3+ to Fe 2+ in the 2a site is also evidenced.

Mössbauer investigation of Sr1−xLaxFe12−yCoyO19 ferrites

Abstract 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.

Homogeneous Cu–Fe supersaturated solid solutions prepared by severe plastic deformation

A Cu–Fe nanocomposite containing 50 nm thick iron filaments dispersed in a copper matrix was processed by torsion under high pressure at various strain rates and temperatures. The resulting nanostructures were characterized by transmission electron microscopy, atom probe tomography (APT) and Mössbauer spectrometry. It is shown that α-Fe filaments are dissolved during severe plastic deformation leading to the formation of a homogeneous supersaturated solid solution of about 12 at% Fe in fcc Cu. The dissolution rate is proportional to the total plastic strain but is not very sensitive to strain rate. Similar results were found for samples processed at liquid nitrogen temperature. APT data revealed asymmetric composition gradients resulting from deformation-induced intermixing. On the basis of these experimental data, the formation of the supersaturated solid solutions is discussed.

Characterisation of high temperature oxidation of NdFeB magnets

Abstract 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 .

Substituted Ferrites Studied by Nuclear Methods

We report on recent investigations on substituted hexaferrites of the type SrM, Sr 1-x La x Fe 12-x Co x O 19 (0 ≤ x ≤ 0.4). It was already proved that the simultaneous substitution of Sr and Fe by La and Co, respectively, significantly improves the permanent magnetic properties of the material. In order to get a deeper insight into the magetic order from an atomistic point of view, 57 Fe Mossbauer spectroscopy and NMR ( 57 Fe, 59 Co) studies were carried out. It is demonstrated that by applying local probe hyperfine techniques the knowledge about this complex material can be considerably improved. The main results concern the preferred Co 2+ occupation on the 4f 2 and the 2a sites and the occurrence of Fe 2+ ions in the substituted material.

Structural and magnetic properties of Sr1−xSmxFe12O19 hexagonal ferrites synthesised by a ceramic process

Abstract Sr 1− x Sm x Fe 12 O 19 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-Fe 2 O 3 and SmFeO 3 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.

On the solubility of rare earths in M-type SrFe12O19 hexaferrite compounds

Sr1−xRExFe12O19 and Sr1−xRExFe12−xCoxO19 (x = 0–0.4 and RE = Pr, Nd) M-type hexaferrite powders were produced by a conventional ceramic process. Structural investigations made by x-ray diffraction and Mossbauer spectrometry reveal that the solubility of the rare earth ion in the M-type phase depends on both the nature of the rare earth and the presence of Co. The solubility of Pr in the M-type phase is higher than the solubility of Nd, and the presence of Co increases the solubility of the rare earth ion. These results were interpreted in the frame of the published literature. It appears that only light rare earths can enter the M-type structure, with a solubility that is related to the shape of the 4f electronic charge distribution and to its surroundings in the crystal structure. Rare earth ions are located in the Sr2+ site, whose surroundings favour an oblate electronic distribution. Co2+ ions modify the surroundings of the Sr2+ site, improving the introduction of rare earth ions with oblate electronic distribution.