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This work focuses on the study of structural and magnetic properties of Mg2+-substituted cobalt nanoferrite (Co1-xMgxFe2O4, 0.0 ≤ x ≤ 1.0). The samples were synthesized by a coprecipitation route and then annealed at 850 °C for 2 h under atmospheric air. The X-ray diffraction patterns show that the samples crystallize in a cubic spinel structure with space group F d 3 ‾ m , and 3% secondary phase was identified as hematite ( R 3 ‾ c ) for x = 1.0. The average crystallite size (D) and the lattice parameter were greatly influenced by Mg content (D decreased from 38 to 20 nm). Transmission electron microscopy investigations revealed that the grains have an almost spherical shape and furthermore that the samples are chemically homogeneous. Magnetic measurements at 5 K revealed a wasp-waist magnetic hysteresis behavior of Co–Mg ferrite. The coercivity of Co0.4Mg0.6Fe2O4 with D = 25 nm was enhanced, with a maximum value of 4000 Oe. The room temperature Mossbauer spectra were fitted with two Zeeman sextets, which reflects the incorporation of Fe3+ in the A and B sublattices of the spinel structure. The hyperfine field of these sites decreases similarly to the saturation magnetization with increasing Mg content. The spectrum corresponding to x = 1.0 was calculated with the help of the hyperfine field distribution. Moreover, Mossbauer spectra, fore x = 0.0 and 1.0 were recorded at 6 and 80 K and were adjusted by the superposition of three sextets: one sextet for the A site and two sextets for the B sites (B1 and B2). The hyperfine parameters such as the hyperfine field and the isomer shift were improved at 6 and 80 K.

( R 1 − x R ′ x ) F e 17 − y C o y powders were prepared by induction melting. Nitrogen (2.9 ± 0.1 atoms per formula unit) was inserted under a static nitrogen gas atmosphere. The nitrided powders have been investigated by 57 Fe Mossbauer spectrometry and X-ray diffraction. The analyses show that the samples are composed of two phases: the nitrided ( R 1 − x R ′ x ) F e 17 − y C o y N z phase and α-Fe. The variations of the Mossbauer hyperfine parameters of the nitrided phase are explained both by the lattice expansion and by the presence of nitrogen. The observed variations do not indicate an influence of the rare-earth on the isomer shift. Nitrogenation increases the hyperfine field at the Fe sites in agreement with the enhancement of the Curie temperature.

We investigated (R 1− x R′ x ) 2 Fe 17 and (R 1− x R′ x ) 2 Fe 16 Co powders by 57 Fe Mossbauer spectrometry at 295 K where R and R′ are Y, Nd, Sm, Gd. The spectra of the (R 1− x R′ x ) 2 Fe 17 compounds have been fitted with seven sextets: 18f 12 , 18f 6 , 18h 12 , 18h 6 , 9d 6 , 9d 3 and 6c. The isomer shifts of the different Fe sites are in agreement with the Wigner–Seitz cell volumes. They do not depend on the nature of the rare earth. The hyperfine fields are consistent with the evolution of the Curie temperature. To account for substitution effects due to the partial replacement of Fe by Co, the Mossbauer spectra of the (R 1− x R′ x ) 2 Fe 16 Co samples have been fitted with four sextets corresponding to the 18f, 18h, 9d, and 6c Fe sites, by considering the site occupancy of the Co atoms in the lattice. The evolution of the hyperfine field with the rare-earth concentration is discussed and interpreted in relation with the magnetic coupling between rare-earth and Fe magnetic sublattices, in agreement with previous magnetic measurements.

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 (

Sputtered [Tb 0.4 (Fe 0.55 Co 0.45 ) 0.6 /(Y 0.2 Fe 0.63 Co 0.17 )] 40 multilayers were investigated by means of energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscope and SQUID magnetometer measurements. The results show that the amorphous state exists in the whole as-deposited sample. For the sample annealed at 350°C, the amorphous state still remains in the TbFeCo layers, whereas a fine grain (7–10 nm size) structure was formed in the YFeCo ones. Magnetisation analysis indicates the existence of a non-collinear magnetic structure and a field-induced magnetic phase transition, in which the TbFeCo magnetisation tends to rotate along the YFeCo magnetisation direction. The magnetic coercivity is discussed in terms of the magnetoelastic interactions.

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.

A magnetisation reversal associated to the formation of the so-called extended domain wall is investigated by means of magnetisation and magnetostriction measurements for the sputtered Fe/Tb(Fe 0.55 Co 0.45 ) 1.5 /Fe sandwich films with an individual TbFeCo-layer thickness of about 600 nm and Fe-layer thickness t Fe =30 and 60 nm. The obtained results are attributed to the contribution of the TbFeCo core as well as the interfacial domain wall. For comparison, magnetostriction data of {19 nm Terfecohan/11 nm Fe} multilayer is discussed.

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.

Tb/Fe multilayers have been prepared by rf-pulverization on Si〈111〉 substrates. Conversion electron Mossbauer spectrometry was used to obtain information on the structure and the spin texture of the multilayers. A structural and magnetic diagram is proposed in the e Tb = 0.2−1.9 nm, e Fe = 0.2−3.3 nm thickness range. In the amorphous samples, the magnetic hyperfine field varies continously with the iron thickness and exhibits a maximum for e Fe ≅ 1.0 nm.

Sputtered Tb/Fe multilayers (ρ Tb =0.2–1.9 nm, ρ Fe = 0.2–3.3 nm) have been characterized both by CEMS and the Kerr effect. Saturated Kerr angles υ K vary linearly with the hyperfine field at the iron site, indicating the υ K comes mainly from the iron magnetic moment. All the structural and magnetic results can be interpreted in a (ρ Tb ,ρ Fe ) diagram.

Fe/Tb multilayers of fixed Tb thickness (1.9 nm) and various Fe thichnesses (in the range 0.2-3.3 nm) have been prepared by rf-pulverization on Si(111) substrates. Conversion Electron Mossbauer Spectrometry was used to get information on the structure and the spin texture of the multilayers. Polar Kerr hysteresis loops have been used to obtain the global magnetic properties. The iron layers are amorphous up to 2.1 nm and the magnetic anisotropy lies in the plane of the layers above e Tb /e Fe =0.9

The magnetic and structural transformations in Tb/Fe multilayers irradiated with high energy Pb ions have been studied by 57Fe conversion electron Mossbauer spectrometry. Compositionally modulated amorphous FeTb alloy regions are formed at the interfaces in the vicinity of the ion path, accompanied by a loss of the perpendicular magnetic anisotropy. A phenomenological model, proposed earlier, has been used to explain the evolution of the magnetic anisotropy.

Two bcc-Fe/Tb MLs with t Fe far from and close to the limit of crystallization of Fe layers were irradiated with Xe ions at various fluences and investigated by 57 Fe Mossbauer spectrometry at 300 K. Two structural effects were evidenced: the usual mixing of layers and a demixing of the Fe and Tb atoms at the interfaces. The first process, observed in the two samples, destroys progressively the layered structure and leads to the formation of the corresponding amorphous alloy at high fluence. The second process, observed only at low fluence before the step of mixing for the multilayer close to the limit of crystallization of Fe layers, features the thermal metastability of this multilayer which relaxes towards a minimal energy state. It increases the pure iron part in the center of the iron layers, thus sharpening the interfaces.

Amorphous sputtered Fe/Tb Multilayers were annealed at different temperatures in a high-vacuum furnace and investigated at 300 K by conversion electron Mossbauer spectrometry and by high-angle X-ray diffraction, in order to study the effects of annealing on the structural and magnetic properties. At low annealing temperatures, a demixing process can be observed, which sharpens the interfaces and increases the perpendicular anisotropy. At intermediate temperatures, the mixing of the layers progressively destroys the layered structure and the sample exhibits the magnetic properties of the corresponding amorphous alloy. At higher annealing temperatures, the crystallization of α-Fe is observed.

International audience; Fe layers, 3, 6, 10, and 25 nm thick, were epitaxially deposited by ion-beam sputtering on InGaAs∕InP(100) wafers. For the 3-nm-thick layer, the sample shows a strong in-plane uniaxial magnetic anisotropy along the [110] direction between 5 and 300 K. The 6-nm film exhibits competition between the uniaxial magnetic anisotropy and the magnetic anisotropy of the bulk bcc Fe. The fourfold magnetic anisotropy of the bulk Fe dominates for the 10-nm Fe film. A decrease of the magnetization is observed for the thinner sample as compared to the bulk. This decrease is discussed in terms of Fe thickness, interface effect, diffusion effect, and possible phases at the interface with the semiconductor.

Fe/Tb multilayers with fixed Tb thickness (1.0 nm) and various Fe thicknesses (0.7–2.4 nm) in the domain where both Fe and Tb layers are amorphous, have been investigated by CEMS and magnetic measurements in the 4–300 K range. The results on magnetic anisotropy are interpreted taking into account the composition modulation in the multilayers and the dominant magnetic subnetwork.

UHV evaporated (Fe/Y) n multilayers deposited into a MBE evaporator on MgO substrate were investigated by RHEED, GXRD, high-angle XRD, 57 Fe Mossbauer spectrometry at room temperature, and magnetization measurements at 300 and 5 K. The RHEED study was performed for (Fe 15 nm/Y) bilayers as a function of the Y thickness and for two (Fe/Y) n= 5 multilayers ( t Fe = t Y =0.5 nm and 1.0 nm) to determine the growth and the nature of the interfaces. The GXRD, high-angle XRD, 57 Fe Mossbauer spectrometry at room temperature, and magnetization measurements studies were performed as a function of the Fe layer thickness ( t Fe =0.5–6.0 nm) on (Fe t Fe /Y 1.0 nm) n> 5 multilayers deposited at 300 K. The Mossbauer paramagnetic doublet obtained at 300 K for t Fe =0.5 nm, is characteristic of the Fe–Y interfaces. The crystallization of BCC Fe in the center of the Fe layers occurs for t Fe between 0.5 and 1.0 nm indicating that the Fe–Y interfaces are very sharp compared to other deposited (Fe/Y) multilayers. For (BCC Fe/Y) multilayers, the Mossbauer and the magnetization results indicate that the Fe–Y interfaces thickness remains constant as t Fe increases. In average, only one monolayer of Fe is involved at each Fe–Y interface.

X-ray diffraction, Mossbauer effect and magnetisation investigations have been performed on sputtered Fe40(Al2O3)60 thin films. Ultrafine Fe particles of nanometer size in an amorphous Al2O3 matrix have been formed by annealing in the temperature range from 100°C to 500°C. Their particle sizes, however, show a rather wide distribution. Mossbauer spectra are constituted of both paramagnetic and ferromagnetic contributions. The paramagnetic contribution is associated with small grains of Fe, whereas the magnetic component is contributed by large iron grains. This assumption is supported by the ZFC- and FC-measurements, in which the “blocking” temperatures of 60 and 90 K were evidenced for as-deposited and 200°C-annealed films, respectively.

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Amorphous multilayers with 57 Fe probe layers were irradiated with swift heavy ions and investigated by 57 Fe Conversion Electron Mossbauer Spectroscopy. Partial recrystallization of iron has been evidenced in the amorphous Fe layers. Furthermore, a demixing effect between Fe and Tb atoms is observed at the interfaces. The electronic stopping power threshold value for the recrystallization and the demixing process has been estimated between 5 and 15 keV/nm.

Tb/bcc-Fe evaporated multilayers with 57Fe probe layers were irradiated with swift uranium ions. For the sharp Tb Fe interface, only continuous mixing is observed, but for the diffuse Fe Tb interface, an homogenization is first evidenced at low fluence. The mixing efficiency saturates at high fluence and it still remains a crystallized Fe part. The magnetic anisotropy was reported and interpreted versus the ion fluence.

Amorphous Tb/Fe multilayers were irradiated with Kr, Xe, Pb and U ions at various fluences and investigated by 57 Fe Mossbauer spectrometry at 300 K. Two processes are generally evidenced. The demixtion of Fe and Tb atoms at the interfaces produces an increase of the pure Fe thickness in the center of iron layer. The mixing of layers destroys progressively the layered structure and at high ions fluence the samples exhibit magnetic properties of amorphous alloys. The electronic stopping power threshold of mixing is estimated.

Sputtered {15 nm Fe/600 nm Tb(Fe0.55Co0.45)1.5/15 nm Fe} sandwich films have been investigated by X-ray diffraction, magnetostriction and Mossbauer measurements. A perpendicular anisotropy is evidenced from the magnetostriction data for both as-deposited and 450°C-annealed films. The Mossbauer spectra are fitted with two contributions. The magnetisation of the TbFeCo layer is orientated out of the film plane, whereas that of the Fe layers tends to be in the plane. The enhancement of the coercivity is attributed to the top TbFeCo/Fe interface.

Site selective 57 Fe Mossbauer spectrometry has been used to study the structural and magnetic properties of UHV evaporated Fe/Tb multilayers. Probe multilayers were prepared at different temperatures (150 and 300 K) with amorphous and crystalline Fe layers. The modulated composition is clearly evidenced for both structures of Fe layers. For amorphous layers, pure amorphous iron in the centre of Fe layers is evidenced. When Fe layers are crystalline, the Mossbauer results indicate different structural and magnetic properties of the different interfaces.