L. Lechevallier

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.

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.

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.

Structural and Mössbauer analyses of ultrafine Sr1−xLaxFe12−xZnxO19 and Sr1−xLaxFe12−xCoxO19 hexagonal ferrites synthesized by chemical co-precipitation

Ultrafine M-type hexagonal ferrites of nominal composition Sr1−xLaxFe12−x ZnxO19 and Sr1−xLaxFe12−xCoxO19 with x = 0, 0.1, 0.2, 0.3 and 0.4 were produced by chemical co-precipitation. The phase make-up of the samples was investigated by x-ray diffraction. The analyses show that the hexagonal M-type phase is the main phase in all the samples. Secondary (La,Sr)FeO3, ZnFe2O4 and CoFe2O4 phases are also detected, indicating that the La, Zn and Co content in the M-type phase is lower than the nominal content. A complete Mossbauer analysis of the substitution effects in the M-type phase was made. The results show that the M-type phase contains La3+ in the same proportion as Zn2+ and La3+ in the same proportion as Co2+, in the La–Zn and La–Co samples, respectively. The evolution with x of the hyperfine parameters of the components used to fit the contribution of the M-type phase have been interpreted consistently in relation to the substitution effects by means of the comparison between the spectra of the La–Zn and La–Co substituted samples. The results show unambiguously that La3+ ions are located in the Sr2+ sites, Zn2+ ions are located in the 4f1 sites and Co2+ ions are located in both 4f2 and 2a sites.

Structural analysis of a (Pt/Co)3/IrMn multilayer: Investigation of sub-nanometric layers by tomographic atom probe

The structure of a Ta3 nm/[(Pt2 nm/Co0.4 nm)3/IrMn7 nm]7/Pt10 nm multilayer exhibiting perpendicular exchange bias has been investigated by x-ray reflectometry and laser-assisted tomographic atom probe (LATAP). A strong intermixing at the Co/IrMn interface is pointed out by x-ray reflectometry, this interface being more diffuse than the IrMn/Pt interface. A direct observation of this intermixing at the atomic scale is obtained thanks to the LATAP in real space. The three-dimensional reconstructions reveal the atomic planes in the Pt layers and the Pt–Co intermixing in the (Pt/Co)3 multilayer. The analysis of the concentration profiles allows to determine the chemical composition of the Co subnanometric layers; thus providing for the first time an accurate structural characterization of such layers leading to an estimation of their thickness, roughness, atomic concentration and width of their interfaces.

Temperature stability of (Pt/Co)3/IrMn multilayers

The effect of annealing on the structural stability of (Pt2nm/Co0.4 nm)3/IrMn7nm multilayers has been investigated using atom probe tomography. The composition of individual layers was measured after annealing at 300, 400, 500, and 700 °C. While results show that the (Pt/Co)3/IrMn stacking sequence is preserved up to 400 °C, there is an almost complete destruction of the multilayered structure when annealing at higher temperatures (500 and 700 °C). Co layers no more alternate with Pt-rich layers. The whole stack is transformed into an IrCo/PtMn bilayer. These results are interpreted on the basis of atomic mobilities and chemical affinities. Diffusion of Co and Mn is shown to become important when annealing temperature approaches 500 °C. Results are well accounted for by thermodynamics arguments considering the Co-Ir and Pt-Mn phase diagrams.

Characterization of (Pt2 nm/Co0.4 nm)3/Ptx/IrMn7 nm multilayers by tomographic atom probe: On the role of a Pt spacer

Structural investigation of Ta3 nm/[(Pt2 nm/Co0.4 nm)3/Ptx/IrMn7 nm]7/Pt10 nm multilayers without (x=0 nm) and with (x=0.4 nm) a Pt spacer has been performed by laser-assisted tomographic atom probe. Without a Pt spacer a strong intermixing is observed at the Co/IrMn interface. In the multilayer containing a Pt spacer the Co/Pt/IrMn interface is very weakly intermixed. It thus appears that the Pt spacer acts as a diffusion barrier that prevents the Ir and Mn atoms from diffusing into the Co layer. The consequences of this effect on the magnetic properties are discussed. The exchange bias field and the anisotropy direction of these two multilayers are analyzed and correlated with the structural investigation.

Effect of a Cu spacer between Co and Pt layers on the structural and magnetic properties in (Co/Cu/Pt)5/Pt type multilayers

We investigate the effect of inserting an ultrathin Cu layer at Co/Pt interfaces in a (Co/Pt) multilayer with perpendicular magnetic anisotropy. We observe a clear correlation between structural properties probed by laser-assisted atom probe tomography and magnetic properties such as effective anisotropy. While Co and Pt are strongly mixed in a standard (Co/Pt) multilayer, inserting a thin Cu layer at Co/Pt interfaces strongly reduces the Co–Pt interdiffusion, leading to a well-defined layered stack with improved effective anisotropy. Such elements are of great importance for spintronic devices that require materials with high perpendicular magnetic anisotropy.