Mössbauer effect study of fine atomic structure of Fe50Al40Ni10 powders

In the present paper, we discuss the local atomic environment of Fe atoms in the mechanically alloyed Fe₅₀Al₄₀Ni₁₀ powders on the basis of hyperfine data estimated from ⁵⁷Fe Mössbauer spectra. B_hf decreases with increasing milling time due to the diffusion of Al and/or Ni into Fe grains. Nickel atoms did not diffuse inside the first coordination sphere of Fe and if the diffusion takes place the number is not more than one atom. Analyses of P(B_hf), indicate that the high hyperfine field values ranging from 30 to 33 T have to be partially attributed to Fe crystalline nanograins and the presence of the defects in them, the hyperfine field values ranging from 15 to 30 T can be associated to the nanocrystalline bcc Fe(Al, Ni) solid solution while the low hyperfine field values (<15 T) result from Fe atoms located in the disordered grain boundaries.     

Growth,structure and thermal reduction of MOCVD-deposited Fe films on Al2O3 (0 0 0 1) substrates

Fe films with strong preferred orientation were prepared on Al₂O₃ (0 0 0 1) substrates by a new two-step method using low-pressure metal-organic chemical vapor deposition (LP-MOCVD) method. X-ray diffraction (XRD) and a vibrating sample magnetometer were employed to characterize the structure and magnetic properties of the Fe films before and after thermal reduction, which was performed in hydrogen flow at 723–1023 K. XRD patterns indicate that the films changed into α-Fe (bcc) mono-phase from a mixture of α-Fe₂O₃ and/or Fe (bcc).     

Structural transformations of Fe81B13Si4C2 amorphous alloy induced by heating

Thermal stability and crystallization of the Fe₈₁B₁₂Si₄C₂ alloy were investigated in the temperature range 25-700 °C by the XRD and Mössbauer analysis. It was shown that on heating the as-prepared amorphous Fe₈₁B₁₂Si₄C₂ alloy undergoes thermal stabilization through a series of structural transformations involving the process of stress-relieving (temperature range 200-400 °C), followed by a loss of ferromagnetic properties (Curie temperature at 420 °C) and finally crystallization (temperature range 450-530 °C). The process of crystallization begins by formation of two crystal phases: Fe₃B and subsequently Fe₂B, as well as a solid solution α-Fe(Si). With increase in annealing temperature, the completely crystallized alloy involved only two phases, Fe₂B and solid solution α-Fe(Si).XRD patterns established a difference in phase composition and size of the formed crystallites during crystallization depending on the side (fishy or shiny) of the ribbon. The first nuclei of the phase α-Fe₃Si were found on the shiny side by XRD after heat treatment even at 200 °C but the same phase on the fishy side of ribbon was noticed after heat treatment at 450 °C. The largest difference between the contact and free surface was found for the Fe₂B phase crystallized by heating at 700 °C, showing the largest size of crystallites of about 130 nm at 700 °C on the free (shiny) surface.     

Mössbauer spectroscopy studies of spin reorientations in amorphous and crystalline (Co0.2Fe0.8)72.5Si12.5B15 glass coated micro-wires

Thermo-gravimetric, differential scanning calorimetry and comprehensive ⁵⁷Fe Mössbauer spectroscopy studies of amorphous and crystalline ferromagnetic glass coated (Co_0.2Fe_0.8)_72.5Si_12.5B₁₅ micro-wires have been recorded. The Curie temperature of the amorphous phase is T_C(amorp) ∼730 K. The analysis of the Mössbauer spectra reveals that below 623 K the easy axis of the magnetization is axial-along the wires, and that a tangential or/and radial orientation occurs at higher temperatures. At 770 K, in the first 4 hours the Mössbauer spectrum exhibits a pure paramagnetic doublet. Crystallization and decomposition to predominantly α-Fe(Si) and Fe₂B occurs either by raising the temperature above 835 K or isothermally in time at lower temperatures. Annealing for a day at 770 K, leads to crystallization. In the crystalline material the magnetic moments have a complete random orientation. After cooling back to ambient temperature, both α-Fe(Si) and Fe₂B in the glass coated wire show pure axial magnetic orientation like in the original amorphous state. The observed spin reorientations are associated with changes in the stress induced by the glass coating.     

Characterization of nanocrystalline FeSiCr powders prepared by ball milling

FeSi₁₀Cr₁₀ powder was mechanically alloyed by high energy planetary ball milling, starting from elemental powders. The microstructural and magnetic properties of the milled powders were characterized by scanning electron microscopy, X-ray diffraction, ⁵⁷Fe Mössbauer spectrometry and a vibratory sample magnetometer.After 3 h of milling, the formation of two bcc solid solutions α-Fe₁ (Si, Cr) and α-Fe₂ (Si, Cr) is observed. Their grain sizes decrease with increase in milling time attaining, at 15 h of milling, 23 and 11 nm, respectively. Mössbauer spectra of the milled powder show the presence of two components. One is a ferromagnetic type with a broad sextuplet. Its distribution of hyperfine field is characterized by high and low hyperfine field’s peaks and a mean value of 26.5 T. The other is a single paramagnetic peak. Its low concentration increases to ∼4% at 15 h of milling. These results can be explained by different atomic environments affected by Si or/and Cr elements, as well as the increased disordered grain boundaries.Magnetic measurements of the milled FeSi₁₀Cr₁₀ alloy powder exhibit a soft ferromagnetic character with a decrease of both magnetization at saturation (Ms) and coercive force (Hc) with milling time attaining values of Ms=151 emu/g and Hc=2500 A/m at 30 h of milling time.     

Synthesis and characterization of room-temperature ferromagnetism in Fe- and Ni-co-doped In2O3

Observation of room-temperature ferromagnetism in Fe- and Ni-co-doped In₂O₃ samples (In_0.9Fe_0.1−xNi_x)₂O₃ (0?x?0.1) prepared by citric acid sol-gel auto-igniting method is reported. All of the samples with intermediate x values are ferromagnetic at room-temperature. The highest saturation magnetization (0.453 μ_B/Fe+Ni ions) moment is reached in the sample with x=0.04. The highest solubility of Fe and Ni ions in the In₂O₃ lattice is around 10 and 4 at%, respectively. The 10 at% Fe-doped sample is found to be weakly ferromagnetic, while the 10 at% Ni-doped sample is paramagnetic. Extensive structure including Extended X-ray absorption fine structure (EXAFS), magnetic and magneto-transport including Hall effects studies on the samples indicate the observed ferromagnetism is intrinsic rather than from the secondary impurity phases.     

Research on preparing compact bulk nanocomposite Nd2Fe14B/α-Fe magnetic materials by hot extrusion

In this paper, compact bulk nanocomposite Nd₂Fe₁₄B/α-Fe magnetic materials were prepared by hot extrusion of amorphous and nanocrystalline powders, which were prepared by high-energy ball-milling (HEBM) of the Nd₂Fe₁₄ B-type hard magnetic phase with 20 vol% of α-Fe as soft magnetic phase. The extrusion temperature has important influence on magnetic properties and microstructure of magnetic materials. The results show that the grain size of Nd₂Fe₁₄B and α-Fe phase increases steadily with increasing extrusion temperature. Furthermore, optimal extrusion temperature of 1223 K occurs, at which the highest magnetic properties and relative density can be obtained.     

Ferromagnetism and spin reorientation in Sm12Fe14Al5

The single crystal of the new ternary compound Sm₁₂Fe₁₄Al₅ was grown and its crystallographic and magnetic properties were investigated. Sm₁₂Fe₁₄Al₅ has a hexagonal structure of the space group p-3m1 and shows ferromagnetism with a Curie temperature of 245 K. The easy direction of magnetization is parallel to the c-axis at temperatures between 245 and 85 K; however, it changes to the c-plane below 85 K through a first-order-like phase transition. No saturation is observed in the magnetization curve even under the applied field of 55 kOe at 5 K. Sm₁₂Fe₁₄Al₅ seems to have a large coercive field at very low temperatures. The anisotropy field was estimated at 5 and 120 K and the saturation magnetization of low temperature phase is explained assuming a ferromagnetic coupling between Fe and Sm sublattices.     

Effect of quenching wheel speed on the structure, magnetic properties and magnetoimpedance effect in Co64Fe4Ni2B19−xSi8Cr3Alx (x=0, 1 and 2) melt-spun ribbons

Melt-spun ribbons of Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al_x (x=0, 1 and 2) alloy at wheel speed of 25 m/s and Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al₁ alloy at different wheel speeds (25, 30, 35 and 40 m/s) have been prepared and investigated for structural and magnetic properties and magnetoimpedance effect. Based on the results obtained, it was shown that replacement of B by Al can improve the magnetoimpedance ratio (MIR) and the highest value of MIR (191%) was observed for the sample with X_Al=1 at wheel speed of 25 m/s. Further, it was seen that the higher quenching wheel speed gives rise to a higher coercivity and lower magnetic permeability/MIR value.     

Microstructure evolution in the rapidly quenched Fe78Si9B13 ribbons

We report microstructure evolution in as-spun Fe₇₈Si₉B₁₃ ribbons under various wheel speeds (s), which was investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). With decreasing s, the volume fraction of the residual amorphous phase (V_a) in the as-spun ribbons decreases gradually, and the total exothermic heat of the crystallization in the DSC curves also decreases, but the ratio of the exothermic heat of the second crystallization to the first one is on the contrary. α-Fe is found in the ribbon with s of 32.9 m/s, while α-Fe, eutectic α-Fe+Fe₂B, and Fe₃Si phases are found in ribbons with s of 25.6 and 18.3 m/s. The phase precipitating behavior in cooling processes is well consistent with the annealing process in the literatures.     

The effect of different temperature annealing on phase relation of LaFe11.5Si1.5 and the magnetocaloric effects of La0.8Ce0.2Fe11.5−xCoxSi1.5 alloys

The phase relation of LaFe_11.5Si_1.5 alloys annealed at different high-temperature from 1223 K (5 h) to 1673 K (0.5 h) has been studied. The powder X-ray diffraction (XRD) patterns show that large amount of 1:13 phase begins to form in the matrix alloy consisting of α-Fe and LaFeSi phases when the annealing temperature is 1423 K. In the temperature range from 1423  to 1523 K, α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase, and LaFeSi phase is rarely observed in the XRD pattern of LaFe_11.5Si_1.5 alloy annealed at 1523 K. With annealing temperature increasing from 1573  to 1673 K, the LaFeSi phase is detected again in the LaFe_11.5Si_1.5 alloy, and there is La₅Si₃ phase when the annealing temperature reaches 1673 K. There almost is no change in the XRD patterns of LaFe_11.5Si_1.5 alloys annealed at 1523 K for 3-5 h. According to this result, the La_0.8Ce_0.2Fe_11.5−xCo_xSi_1.5 (0≤×≤0.7) alloys are annealed at 1523 K (3 h). The analysis of XRD patterns shows that La_0.8Ce_0.2Fe_11.5xCo_xSi_1.5 alloys consist of the NaZn₁₃-type main phase and α-Fe impurity phase. With the increase of Co content from x=0 to 0.7, the Curie temperature T_C increases from 180 to 266 K. Because the increase of Co content can weaken the itinerant electron metamagnetic transition, the order of the magnetic transition at T_C changes from first to second-order between x=0.3 and 0.5. Although the magnetic entropy change decreases from 34.9 to 6.8 J/kg K with increasing Co concentration at a low magnetic field of 0-2 T, the thermal and magnetic hysteresis loss reduces remarkably, which is very important for the magnetic refrigerant near room temperature.     

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.     

Effect of Cu and Ga additions on the anisotropy of R2Fe14B/α-Fe nanocomposite die-upset magnets (R=Pr,Nd)

Fully dense nanocomposite magnets containing hard R₂Fe₁₄B and soft α-Fe phases were produced from both melt-spun and mechanically milled alloys by hot pressing and subsequent die upsetting. Although R-lean R–Fe–B alloys that do not contain the grain-boundary R-rich phase are known not to be susceptible to texture development by means of die upsetting, we found that small additions of Cu make the texturing possible. The resulting microstructure of oriented platelet grains is similar to that of the R-rich die-upset magnets. Properties of the Cu-containing R₂Fe₁₄B/α-Fe die-upset magnets can be further improved by adding Ga. The anisotropic Pr₁₂Fe₈₀Cu₁Ga₁B₆ magnet made from mechanically milled alloy and containing 17.2 wt% α-Fe had a remanence of 13 kG and a maximum energy product of 23.4 MG Oe. The Pr_11.25Fe_80.75Cu₁Ga₁B₆ magnet made from melt-spun alloy and containing 16.2 wt% α-Fe had a maximum energy product of 19.9 MG Oe. The low coercivity of 3–4 kOe typical for the Cu-containing R₂Fe₁₄B/α-Fe die-upset magnets is due to the relatively coarse α-Fe grains. The latter grains are too large for intergranular exchange interaction, but, nevertheless, they are well coupled with the R₂Fe₁₄B grains by a long-range magnetostatic interaction.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Fe65B22Nd9Mo4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The Fe₆₅B₂₂Nd₉Mo₄ nanocomposite permanent magnets in the form of a rectangular cross sectioned rod have been prepared by annealing the amorphous precursors. The thermal behavior, structure and magnetic properties of the magnets have been investigated by differential scanning calorimetry, X-ray diffractometry, electron microscopy and magnetometry techniques. The as-cast Fe₆₅B₂₂Nd₉Mo₄ alloy showed soft magnetic properties, which changed into magnetically hard after annealing. Results provoke that the magnetic properties of the alloy are sensitive to thermal processing conditions. The optimum hard magnetic properties with a remanence (B_r) of 0.56 T, coercivity (_iH_c) of 920.7 kA/m and maximum energy product (BH)_max of 50.15 kJ/m³ were achieved after annealing the alloy at 983 K for 10 min. The good magnetic properties of Fe₆₅B₂₂Nd₉Mo₄ magnets are ascribed to the exchange coupling between the nano-scaled soft α-Fe, Fe₃B and hard Nd₂Fe₁₄B magnetic grains.     

Synthesis, structural and magnetic properties of the nanocomposite Fe63B23Nd7Y3Nb3Cr1 magnets

The Fe₆₃B₂₃Nd₇Y₃Nb₃Cr₁ nanocomposite magnets in the form of sheets have been prepared by copper mold casting technique. The phase evolution, crystal structure, microstructural and magnetic properties have been investigated in the as-cast and annealed states. The as-cast sheets show magnetically soft behaviors which become magnetically hard by thermal annealing. The optimal annealed microstructure was composed of nanosize soft magnetic α-Fe (19-29 nm) and hard magnetic Nd₂Fe₁₄B (45-55 nm) grains. The best hard magnetic properties such as intrinsic coercivity, _jH_c of 1119 kA/m, remanence, B_r of 0.44 T, magnetic induction to saturation magnetization ratio, M_r/M_s=0.61 and maximum energy product, (BH)_max of 55 kJ/m³ was obtained after annealing at 680 °C for 15 min. The annealing treatment above 680 °C results in non-ideal phase grains growth, which degrade the magnetic properties.     

Structural and magnetic properties of Sc1.1Fe3.9Al8 alloy

The crystallochemical and magnetic nature of ternary Sc_1+δFe_4−δAl₈ intermetallic with a small Sc excess δ=0.1 was investigated by scanning electron microscope, X-ray powder diffraction, neutron diffraction, Mössbauer effect and superconducting quantum interference device techniques. The sample crystallizes in a tetragonal ThMn₁₂ type structure. The excess of Sc atoms substitute Fe at the (8f) positions and have a pronounced effect on the magnetic properties. The experiments carried out in temperature range 4-320 K show that below 120 K the magnetic structure of the alloy forms a double cycloid with magnetic moments rotating according to the incommensurate in-plane wave vector, which is temperature independent up to 160 K. The value of Fe magnetic moment is close to 0.9 μ_B atom⁻¹ at 4 K. Temperature dependence of unit cell dimensions can be explained within the Debye-Grüneisen approximation.     

Microstructural behavior on particle surfaces and interfaces in Sm2Fe17N3 powder compacts during low-temperature sintering

Sm₂Fe₁₇N₃ sintered compacts were prepared below 450 °C by a high-pressure current sintering technique. The coercivity of the sintered compacts decreased linearly as the sintering temperature increased. Transmission electron microscopic analyses indicated that thin Fe-rich layers containing α-Fe phases were formed just inside the initial oxide layer on the particle surfaces and interfaces in the sintered samples. The generation of α-Fe phases was supposed to cause the coercivity decrease. In addition, X-ray photoelectron spectroscopy analysis revealed that Fe₂O₃ and FeO contained in the oxide layer of the raw powder disappeared subsequent to heat treatment. These results suggested that the α-Fe phases were generated by the oxidation–reduction reaction between the initial iron oxides and the primary Sm₂Fe₁₇N₃ phase but not by thermal decomposition or exogenous oxidation during sintering. This mechanism was supported by the fact that extending the sintering time did not result in any further decrease in the coercivity.     

Effect of Mn on the magnetic properties of Fe3B/Nd2Fe14B nanocomposites

The magnetic properties of Nd_4.5Fe_77−xMn_xB_18.5 (x=0, 1 and 2) nanocomposites prepared by the crystallization of amorphous precursors were investigated. Addition of Mn is found to decrease the crystallization temperature of the amorphous ribbons. The intrinsic coercivity _iH_c and maximum energy product (BH)_max increase from 2.6 kOe and 9.1 MGOe for x=0 to 3.1 kOe and 10.3 MGOe for x=1, respectively, and the remanence ratio M_r/M_s increases from 0.70 to 0.72. The effect of Mn on Curie temperature T_C and the thermal stability of M_r and _iH_c were also studied. ⁵⁷Fe Mössbauer spectra have been recorded for x=0, 1 and 2 ribbons at room temperature and site preference of the Mn atoms in Fe₃B and Nd₂Fe₁₄B phases is discussed using the Mössbauer spectroscopy.