Magnetoresistance of Fe3O4/Au/Fe3O4 and Fe3O4/Au/Fe spin-valve structures

A detailed study of the in-plane magnetotransport properties of spin valves with one and two Fe₃O₄ electrodes is presented. Fe₃O₄/Au/Fe₃O₄ spin valves exhibit a clear anisotropic magnetoresistance in small magnetic fields but no giant magnetoresistance (GMR). The absence of GMR in these structures is due to simultaneous magnetization reversal in the two Fe₃O₄ layers. By contrast, a negative GMR effect is measured on Fe₃O₄/Au/Fe spin valves. The negative GMR is attributed to an electron spin scattering asymmetry at the Fe₃O₄/Au interface or an induced spin scattering asymmetry in the Au interfacial layers.     

The magnetic structure of Cs0.4 Mn0.4 Fe1.6 F6 and of Cs0.4 Ni0.4 Fe1.5 F6

The magnetic structure of the two compounds as determined by neutron diffraction is reported. Both compounds are antiferromagnetic with Néel-temperatures of 115 and 100 K respectively. Both compounds show a very pronounced diffuse scattering.     

Rotation des moments magnetiques dans BaLa2 Fe2 O7

In the compound BaLa₂Fe₂O₇, below 235 K, spins are oriented along the diagonal of the quadratic cell in an antiferromagnetic configuration. Above this temperature we observe by neutron diffraction a continuous rotation of the magnetic moments in the base plane xyO, the configuration remaining antiferromagnetic; the magnetic symmetry goes from I_pm′mm′ to P2′/m Shubnikov's group. At the transition temperature we observe a discontinuity of the thermal expansion coefficient. This phenomenon can be interpreted in agreement with Landau's theory of second order transitions.     

Effects of oblique-incidence evaporation on magnetic properties of Fe20Ni80/Fe70Co30 bilayer film

Evaporative deposition at oblique incidence is shown to enhance the magnetic anisotropy of an Fe₂₀Ni₈₀ magnetic film and induce magnetic anisotropy in an overlying, strongly isotropic Fe₇₀Co₃₀ film. This deposition method for the formation of an underlayer of several lattice parameters in thickness and semi-hard overlayer of a few thousands Angstroms in thickness achieves a significant change in the magnetization process and strong suppression of the coercive forces of Fe₇₀Co₃₀ in the hard magnetization direction. Soft magnetization of the Fe₇₀Co₃₀ overlayer is not achieved when one of the layers is deposited at oblique incidence. It is anticipated that shape magnetic anisotropy is responsible in part for the magnetic anisotropy induced in both in Fe₂₀Ni₈₀ under- and Fe₇₀Co₃₀ overlayer by oblique incidence evaporation.     

Fe80B20,Fe80Si6B14金属玻璃中近邻原子间的相互作用

用转靶X射线源及弯晶EXAFS谱仪测定了Fe₈₀B₂₀及Fe₈₀Si₆B₁₄金属玻璃中铁的X射线K吸收谱。由EXAFS谱确定了两种金属玻璃的近邻结构参量。此外,通过测定XANES谱并与纯铁及其有关的化合物Fe₂B,FeB中铁的XANES比较发现:吸收边能量、“白线”结构等均出现一些变化,表明两种金属玻璃的近邻原子Fe-B,Fe-Si以及Fe-Fe之间的相互作用比铁与硼所形成的稳定化合物中要强,这对于决定金属玻璃的短程序结构具有重要的作用。 关键词：     

Optical emissions of products sputtered from Fe, Fe2O3 and Fe3O4 powders

Powder iron has been bombarded by a 5 keV Kr⁺ ions in a vacuum better than 10^-7 torr and under few 10^-6 torr ultra pure oxygen partial pressure. The optical spectra of the sputtered particles were recorded between 340.0 nm and 410.0 nm. These spectra exhibit discrete lines, which are attributed to neutral excited atoms of iron. Two iron oxides, namely hematite (Fe₂O₃)_{3}) and magnetite (Fe₃O₄)_{4}), in powder form, were studied under the same experimental conditions and identical lines were observed in the obtained spectra. The absolute intensities of the spectral lines in all spectra were measured and the differences in the recorded yield photons were discussed in term of electron-transfer processes between the excited sputtered atom and the bombarded surface. In accordance with the proposed interpretation, we suggest values for the energy gaps and electronic affinities for the studied oxides and for the oxide layer that might be formed by the adsorption of oxygen atoms.     

Electron localization in Fe2PO5

Fe₂PO₅ is a mixed valence compound where Fe^II and Fe^III are in face sharing octahedra with a short Fe^II - Fe^III distance, so that electron delocalization along chains parallel to the $\text{b}$ axis of the orthorhombic cell was worth to investigate. From Mössbauer experiments, carried out up to 600°C, it appears that the 4 lines spectra evolution, as regards Isomer Shift, Quadrupolar Splitting, and Linewidth, is only consistent with a localized electron pattern. This result is interpreted from the unequivalence of the 2 sites. The strong Fe^II quadrupolar splitting decrease allows the determination of the energy gap between the d_xy and d_yz-d_zx orbitals.     

Magnetic excitations in invar alloys, Fe65Ni35 and Fe3Pt

Magnetic excitations in Fe₆₅Ni₃₅ and ordered Fe₃Pt have been studied by neutron scattering. It has been found that both alloys show common anomalous dynamical properties. At 5 K, well defined magnon groups could be detected up to 80 meV, but the temperature variations of the magnetization M(T) cannot be explained only by the magnon excitations. The integrated intensity measurements of the magnon spectra suggest that no other excitations take part in M(T). The magnons dampen significantly with the linewidth Γ(q, T) obeying a relation Γ(q, T) = (Γ₀+aT)q² with 1. The results suggest that the magnetic excitations with volume change occur in a slow relaxation process.     

Synthesis and properties of Au-Fe3O4 and Ag-Fe3O4 heterodimeric nanoparticles

Monodisperse Au-Fe 3 O 4 heterodimeric nanoparticles (NPs) were prepared by injecting precursors into a hot reaction solution.The size of Au and Fe 3 O 4 particles can be controlled by changing the injection temperature.UV-Vis spectra show that the surface plasma resonance band of Au-Fe 3 O 4 heterodimeric NPs was evidently red-shifted compared with the resonance band of Au NPs of similar size.The as-prepared heterodimeric Au-Fe 3 O 4 NPs exhibited superparamagnetic properties at room temperature.The Ag-Fe 3 O 4 heterodimeric NPs were also prepared by this synthetic method simply using AgNO 3 as precursor instead of HAuCl 4.It is indicated that the reported method can be readily extended to the synthesis of other noble metal conjugated heterodimeric NPs.     

Structural transformations and magnetic properties of Fe60Pt15B25 and Fe60Pt25B15 nanocomposite alloys

Structural and magnetic properties of two rapidly solidified and post-annealed Fe₆₀Pt₁₅B₂₅ and Fe₆₀Pt₂₅B₁₅ alloys are compared. The as-quenched Fe₆₀Pt₁₅B₂₅ ribbon was fully amorphous whereas in the Fe₆₀Pt₂₅B₁₅ alloy the amorphous phase coexists with an fcc FePt disordered solid solution. Differential scanning calorimetry curves of both alloys reveal a single exothermal peak with onset temperatures of 873 and 847 K for Fe₆₀Pt₁₅B₂₅ and Fe₆₀Pt₂₅B₁₅, respectively. Magnetically hard, tetragonal ordered L1₀ FePt and magnetically soft Fe₂B nanocrystalline phases were formed due to the annealing of the alloys, as indicated by X-ray diffraction and Mössbauer spectroscopy measurements. Two-phase behavior was detected in the temperature dependence of magnetization of the annealed samples. A magnetic hardening was observed for all annealed ribbons. Magnetic properties of the annealed alloys, studied by hysteresis loop measurements, were related to the differences in the relative fractions of the hard and soft magnetic phases calculated from Mössbauer spectra. The alloy with 25 at% Pt exhibits better hard magnetic properties (H_c=437 kA/m, M_r/M_s=0.74) than the alloy with smaller Pt content (H_c=270 kA/m, M_r/M_s=0.73) mainly due to the larger abundance of the ordered tetragonal FePt phase.     

Field-induced microwave absorption in Fe3O4 nanoparticles and Fe3O4/polyaniline composites synthesized by different methods

Three kinds of nanoscale powders containing Fe₃O₄ nanoparticles (NPs) have been studied by ferromagnetic resonance (FMR): (i) Fe₃O₄ NPs grown and then covered with polyaniline (PANI), (ii) unclad Fe₃O₄ NPs, and (iii) Fe₃O₄ NPs grown “in situ” with the PANI. In every case, there is no low field microwave absorption, rather a single FMR line is observed. However, the half-power widths are of order of 1 kOe presumably due to a distribution of internal fields. For type I particles with a low concentration (below 40%) of Fe₃O₄, the observed resonance fields (H_r) are close to those expected for spheres with negligible magnetocrystalline anisotropy. For all other cases, H_r values are significantly lower. Such shortfalls can be roughly understood by invoking dipolar interactions between the grains, stresses frozen in grains during manufacture (method III), as well as anisotropy fields when the specimens are prepared in an aligning field.     

Reactively sputtered Fe3 O4 -based films for spintronics

Half metallic polycrystalline, epitaxial Fe3O4 films and Fe3O4 -based heterostructures for spintronics were fabricated by DC reactive magnetron sputtering. Large tunneling magnetoresistance was found in the polycrystalline Fe3O4 films and attributed to the insulating grain boundaries. The pinning effect of the moments at the grain boundaries leads to a significant exchange bias. Frozen interfacial/surface moments induce weak saturation of the high-field magnetoresistance. The films show a moment rotation related butterfly-shaped magnetoresistance. It was found that in the films, natural growth defects, antiphase boundaries, and magnetocrystalline anisotropy play important roles in high-order anisotropic magnetoresistance. Spin injection from Fe3O4 films to semiconductive Si and ZnO was measured to be 45% and 28.5%, respectively. The positive magnetoresistance in the Fe3O4 -based heterostructures is considered to be caused by a shift of the Fe3O4 e g ↑ band near the interface. Enhanced magnetization was observed in Fe3O4 /BiFeO 3 heterostructures experimentally and further proved by first principle calculations. The enhanced magnetization can be explained by spin moments of the thin BiFeO 3 layer substantially reversing into a ferromagnetic arrangement under a strong coupling that is principally induced by electronic orbital reconstruction at the interface.     

Small angle neutron scattering in Fe3Pt and Fe65Ni35 invar alloys

Comparison of small angle neutron scattering in Fe₃Pt and Fe₆₅Ni₃₅ leads to the conclusion that Fe₆Pt is an instable ferromagnet showing strong magnetic interactions of both dynamic and quasistatic nature.     

Magnetostriction of Fe81Ga19 oriented crystals

The effect of the orientation on the magnetostriction in Fe 81 Ga 19 alloy has been investigated experimentally and theoretically.The Fe 81 Ga 19 [001] and [110] oriented crystals were prepared and the magnetostriction was measured under different pre-stress.The saturation magnetostriction of the [001] oriented crystal increases from 170×10-6 to 330×10-6 under the pre-stress from 0 to 50 MPa.The [110] oriented crystal has a saturation magnetostriction from 20×10-6 to 140×10-6 with the compressive pre-stress from 0 to 40 MPa.The magnetostriction of [001] and [110] oriented crystals has been simulated based on the phenomenological theory.The domain rotation path has been determined and the resultant magnetostriction calculated under different pre-stress.The experimental and simulated results both show that the [001] oriented crystal exhibits better magnetostriction than [110] oriented crystal.The enhancement of the saturation magnetostriction by the compressive pre-stress in the [110] oriented crystal is higher than that in the [001] oriented crystal.     

Electrical conduction in Fe2O3 and Cr2O3

The influence of some impurities on the conduction properties of Cr₂O₃ and Fe₂O₃ are examined and contrasted. A mechanism is proposed to account for the effect of Ti in Cr₂O₃.     

Galvanomagnetic effects in Fe40Ni40B20

Hall effect and magnetoresistance measurements on the amorphous ferromagnet Fe₄₀Ni₄₀B₂₀ are reported. Both properties are approximately independent of temperature (1.5–300 K) and related to magnetization. The extraordinary Hall coefficient is 3.5 × 10^?8 m³/As. The temperature dependence of the resistivity (1.5–30 K) is also reported.     

Pressure dependence of temperature variation of AC susceptibility for Fe50Ni37Mn13 and comparison to Fe68.1Ni31.9

Fe₅₀Ni₃₇Mn₁₃, which is a ferromagnetic alloy with FCC crystal structure, has been reported to show the Invar effect below the Curie temperature; however, this alloy shows a typical anti-Invar effect above the Curie temperature. In this paper, we discuss the pressure dependence of the temperature variation of the alternative current (AC) susceptibility at a frequency of 1 kHz for Fe₅₀Ni₃₇Mn₁₃ at various pressures up to 7.5 GPa above 77 K; we then compare the results with those for Fe_68.1Ni_31.9, which were obtained in a previous study. Fe₅₀Ni₃₇Mn₁₃ was in a ferromagnetic state throughout the entire pressure range measured. T_C decreases in inverse proportion to the increasing pressure; dT_C/dp and dln T_C/dp for Fe₅₀Ni₃₇Mn₁₃ are −26 K GPa⁻¹ and −0.07 GPa⁻¹, respectively. Further, the temperature variation of the shape of the χ′-T curve for different pressures indicates continuous combining of magnetic interactions occurs at high pressures. These results are similar to those obtained for Fe_68.1Ni_31.9.     

Electrical resistivity of amorphous metallic glasses Fe80B80 and Fe78Mo2B20 between 78 and 1000 K

Electrical resistivity (ρ) of amorphous Fe₈₀B₂₀ and Fe₇₈Mo₂B₂₀ have been studied as a function of temperature (T) between 78 and 1000 K. The ρ vs T curves, obtained with specified warming and cooling rates, show that such curves are sensitive probes of the crystallization process. Within the experimental error, no anomalies in the ρ behavior can be seen at the Curie temperature of each amorphous alloy.     

Magnetoelectric and Mössbauer studies of Fe2 TeO6

The magnetic symmetry of antiferromagnetic Fe₂TeO₆ indicates that this material should exhibit magnetoelectricity. This prediction has been confirmed by the observation of the electrically induced magnetoelectric (ME) effect in powder samples. Both parallel and perpendicular ME susceptibilities were measured as a function of increasing temperature. The ME effect vanishes at 209°K which is identified as the Néel point of the compound. The single crystal ME susceptibilities are derived from the powder results. The maximum value of the axial ME susceptibility α₃₃ = M₃/E₃ is (in Gaussian units) 3 × 10⁻⁵ at T = 175°K. In addition to magnetoelectricity the magnetic symmetry of Fe₂ TeO₆ indicates that no hyperfine field should exist at the Te⁶⁺ sites. This prediction was confirmed by Mössbauer studies on I¹²⁹ produced in Te⁶⁺ sites by irradiating samples of Fe₂ TeO₆ in a reactor.