Surface microstructure and magnetic behavior in FeSiB amorphous ribbons from magneto-optical Kerr effect

The magneto-optical Kerr effect (MOKE) completed by other surface sensitive methods as integral low-energy and conversion electron Mössbauer spectroscopy, scanning and transmission electron microscopy and by X-ray diffraction have been used with the aim to trace the surface microstructure and magnetic properties of FeSiB amorphous ribbons prepared by planar flow casting. The general composition of studied samples is Fe₈₀Si_xB_20−x, where x=4, 6, 8, 10 at.%.It is shown that MOKE used for magnetization, hysteresis loop, and domain structure determination is highly beneficial in a detection of both surface crystallization and local ordering of atoms into magnetically different clusters of amorphous structure. Moreover, a combination of blue and red laser with different penetration depths yields unique results concerning the surface anisotropy and depth sensitivity. In the case of samples with 4, 6, and 8 at.% Si MOKE detects two magnetically different phases diverging in coercivity values H_c, however, not varying with the sample composition. These phases have been identified by Mössbauer measurements as FeSi and FeB clusters. Their relationship changes with Si concentration. On the other hand, a strong increase in the surface H_c found for the sample with 10 at.% Si has indicated a nanocrystallization. It was confirmed by electron microscopy, Mössbauer and X-ray diffraction results. The size of nanocrystals has varied between 200 nm and 500 nm.     

Mössbauer investigation of amorphous-to-crystalline transformation of Fe62Ni16B14Si8 by isothermal annealing

Room temperature transmission Mössbauer spectra of Fe₆₂Ni₁₆B₁₄Si₈ ribbons, annealed in vacuum for time periods ranging from 5 to 60 minutes at the crystallization temperatureT _x=720 K, have been used to investigate the crystallization mechanism by isothermal annealing. Reorientation of the magnetic anisotropy almost normal to the ribbon plane was observed and correlated to the annealing time dependence of the mean hyperfine field (HF) of the amorphous component. Crystallization started at the surface before the bulk and was found to occur in two steps: a metastable equilibrium of the amorphous phase with (FeNi), (FeNi)Si and t-(FeNi)₃B, followed by the decomposition of t-(FeNi)₃B into t-(FeNi)₂B and (FeNi). The increase of the mean magnetic moment at the Fe sites as compared to related iron-based alloys was attributed to compositional small Ni additions.     

Mössbauer spectroscopy and magnetic properties in thin films of FexNi100−x electroplated on silicon (1 0 0)

Fe_xNi_100−x thin films were produced by galvanostatic electrodeposition on Si (1 0 0), nominal thickness 2800 nm, and x ranging 7-20. The crystalline structure of the sample was determined by X-ray diffraction (XRD). The magnetic properties were investigated by vibration sample magnetometry (VSM) and room temperature ⁵⁷Fe Mössbauer spectroscopy. Conversion Electron Mössbauer spectroscopy (CEMS) in both film surfaces for the thick self-supported films showed that the magnetic moment direction is in the plane and conventional transmission (MS) that the directions are out of the plane films. The results were interpreted assuming a three-layer model where the external layer has in-plane magnetization and the internal one, out of plane magnetization.     

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.     

Effects of annealing temperature on structure and magnetic properties of amorphous Fe61Co27P12 nanowire arrays

Arrays of Fe₆₁Co₂₇P₁₂ nanowire with an aspect ratio about 70 were prepared in anodic aluminum oxide templates by electrodeposition. The influences of annealing temperature on structure and magnetic properties of Fe₆₁Co₂₇P₁₂ nanowires were studied. When the specimens were annealed below 400 °C, there are no obvious changes in structure except relaxation. With the annealing temperature increasing from 400 to 600 °C, the Fe-Co phase is detected by X-ray diffraction and Mössbauer spectra. The crystalline fraction and hyperfine field can be derived from Mössbauer spectra. The room temperature magnetic hysteresis loops show that the coercivity and squareness of the nanowire arrays in parallel to the wire axis increase with the increasing of annealing temperature, which mainly attributes to the strengthening of anisotropy.     

Properties of Fe–Ga based powders prepared by mechanical alloying

Alloys of Fe–Ga with starting compositions of 17, 19, 21, 23, and 25 at% Ga and Fe₈₁Ga₁₇Z₂ (Z=Si, Sn) have been prepared by mechanical alloying. Samples were milled in a SPEX Model 8000 mill with a ball to sample weight ratio of about 4:1. Phase formation as a function of milling time has been investigated for the 19 at% Ga sample and suggests that milling times of 12 h produce fully alloyed samples. Alloys have been studied by electron microprobe, X-ray diffraction, vibrating sample magnetometery and ⁵⁷Fe Mössbauer effect spectroscopy. Fully milled powders have measured compositions of Fe_100−xGa_x with x=15.7, 17.0, 19.0, 22.4, and 24.0 and Fe_83.1Ga_15.2Z_1.7 (for both Z=Si and Sn). X-ray diffraction showed the presence of a disordered bcc phase with no indication of an ordered D0₃ phase. However, the latter is difficult to observe with X-ray diffraction because of the low intensity of the fcc superlattice peaks. A bimodal Fe hyperfine field distribution as obtained from Mössbauer effect spectra indicated the presence of two discrete Fe environments. The results suggested a lower degree of Ga clustering than has been previously observed in Fe–Ga alloys, of similar composition, prepared by melt spinning. The microstructure is similar to that of Fe–Ga thin films prepared by combinatorial sputtering. Some samples have also been studied after annealing at 800 °C for 8 h. No changes were observed in X-ray diffraction patterns after annealing. However, Mössbauer effect studies show the formation of D0₃ and L1₂ order in annealed samples analogous to the phases observed in melt spun ribbons of similar composition.     

Effect of preparation and metal content on the introduction of Fe in BEA zeolite, studied by DR UV-vis, EPR and Mössbauer spectroscopy

Fe-containing SiBEA zeolites were prepared by a two-step postsynthesis method: creation of vacant T-sites by dealumination of tetraethylammonium BEA zeolite with nitric acid and then impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO₃)₃. X-ray diffraction shows that iron is incorporated in SiBEA at lattice sites. The presence of Fe in its oxidation state +3 and at isolated tetrahedral sites for low metal content, was demonstrated by diffuse reflectance UV-vis, EPR and Mössbauer spectroscopy. For high iron content, diffuse reflectance UV-vis and Mössbauer spectra revealed the additional presence of extra-lattice FeO_x oligomers and superparamagnetic Fe-oxyhydroxide. Mössbauer spectroscopy identified superparamagnetic Fe-oxyhydroxide as the main phase when basic conditions are used for the preparation.     

Surface and bulk magnetic properties of as-quenched FeNbB ribbons

The surface and bulk magnetic properties of amorphous FeNbB ribbons in as-quenched state are investigated using various non-destructive methods. The conversion electron Mössbauer spectroscopy has detected the presence of crystalline phase at both surfaces of ribbon sample while the bulk was amorphous. The coexistence of crystalline and amorphous phase was shown also in the X-ray diffraction pattern. Magnetic properties measured by bulk sensitive vibrating sample magnetometer (VSM) strongly differs from the surface characteristics investigated by magneto-optical Kerr effect (MOKE).     

Synthesis and characterization of the xZnO-(1−x)α-Fe2O3 nanoparticles system

The xZnO-(1−x)α-Fe₂O₃ nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x=0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x=0.3, while for x=0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mössbauer spectra corresponding to x=0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mössbauer spectrum for 12 h of milling had to be fitted with two sextets. For x=0.3 and 12 milling hours, the Mössbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe₂O₄. This doublet clearly dominates the Mössbauer spectrum for x=0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.     

Temperature dependence of the Mössbauer spectra of amorphous and nanocrystallized Fe86Zr7Cu1B6

Mössbauer measurements have been performed on amorphous and nanocrystalline alloy ribbons of nominal composition Fe₈₆Zr₇Cu₁B₆. The nanocrystalline samples were obtained by annealing the as-quenched alloy at different temperatures in the range between 650 and 870 K. Mössbauer spectra of the as-quenched amorphous sample have been recorded at 77 K, room temperature and above the Curie temperature (330 K) at 360 K. We have also performed Mössbauer measurements at room temperature in the nanocrystalline alloys to characterize the phases that appear after the annealing and their relative concentration. The as-quenched sample spectra reveal the existence of two inequivalent sites for Fe. Such a feature is also observed in the remaining amorphous phase of the annealed samples. In the first steps of crystallization, -Fe precipitates and its concentration increases with the annealing temperature. The experimental results suggest that the composition of the whole amorphous phase does not suffer large changes during crystallization.     

The existence of giant magnetocaloric effect and laminar structure in Fe73.5−xCrxSi13.5B9Nb3Cu1

Amorphous soft magnetic ribbons Fe_73.5−xCr_xSi_13.5B₉Nb₃Cu₁ (x=1–5) have been fabricated by rapid quenching on a single copper wheel. The differential scanning calorimetry (DSC) patterns showed that the crystallization temperature of α-Fe(Si) phase is ranging from 542 to 569 °C, a little higher than that of pure Finemet (x=0). With the same annealing regime, the crystallization volume fraction as well as the particle size of α-Fe(Si) crystallites decreased with increasing Cr amount substituted for Fe in studied samples. Especially, the interesting fact is that the laminar structure of heat-treated ribbons on the surface contacted to copper wheel in the fabricating process has been firstly discovered and explained to be related to the existence of Cr in studied samples. The hysteresis loop measurement indicated that there is the pinning of displacement of domain walls. The giant magnetocaloric effect (GMCE) has been found in amorphous state of the samples. After annealing, the soft magnetic properties of investigated nanocomposite materials are desirably improved.     

Rapid stress annealing dependence of structural and magnetic properties of Fe75 − x Co x Cu1Nb3Si15B6 alloys

Structural and magnetic properties of nanocrystalline Fe_75???x Co _x Cu₁ Nb₃Si₁₅B₆ (x?=?0, 2, 5) alloys are reported using magnetic measurements X-ray diffraction, Mössbauer spectroscopy. Results show that: (1) for the specimens with x?=?0 reveal that the volume fraction of the nanograins and their grain diameter ranges between 56% and 80% and 10 and 18 nm, (2) annealing above 700°C apart from Fe₃Si type nanocrystals, magnetically hard Fe₃B, Fe₂₃B phases also appear, leading to a sharp increase of the coercive field, (3) Co content and applied stress during annealing has considerable effect on relative permeability and stress induced anisotropy, which is perpendicular to the ribbon axis, Mössbauer spectroscopy also suggests changes in spin texture.     

X-ray and Mössbauer study of rapidly solidified Ce20Fe80 ribbons; effect of the quenching rate

Ce₂₀Fe₈₀ ribbons have been produced by planar flow casting under an He atmosphere at linear wheel velocities between 19 and 29 m s^–1. Analysis of ribbons by X-ray diffraction and⁵⁷Fe Mössbauer spectrometry in the temperature range 77–300 K shows that the ribbons are crystallized. For higher velocities, the ribbon is constituted of the two equilibrium phases CeFe₂ and Ce₂Fe₁₇, but, for lower velocities, there appears a third iron metallic phase, which can be explained by the quenching rate of the melt. A coherent hyperfine parameter set was deduced from fitting Mössbauer spectra in the whole temperature range.     

Structural and magnetic correlation of Finemet alloys with Ge addition

The correlation between saturation magnetization and the magnetic moment per Fe atom in the nanocrystalline state is studied for Finemet-type alloys. These studies were performed on nanocrystalline ribbons whose compositions were Fe_73.5Si_13.5−xGe_xNb₃B₉Cu₁ (x=8, 10 and 13.5 at%). We used a simple lineal model, X-ray diffraction and Mössbauer spectroscopy data to calculate the magnetic contribution of the nanocrystals and the results were contrasted with the measured saturation magnetization of the different alloys. The technique presented here provides a very simple and powerful tool to compute the magnetic contribution of the nanocrystalline phase to the alloy. This calculus could be used to determine the volume fraction of nanocrystalline and amorphous phases in the nanocrystallized alloy, without using a very sophisticated microscopy method.     

Mössbauer study of cobalt ferrite nanocrystals substituted with rare-earth Y ions

Mössbauer spectroscopy is used to characterize the crystallite size and structure of CoFe_2−xY_xO₄ (x=0, 0.1, 0.3, 0.5) ferrite nanocrystallites synthesized by the sol-gel auto-combustion method. The effect of the substitution of Fe³⁺ ions by Y³⁺ ions on the structure of cobalt ferrite nanocrystallites is investigated. The Mössbauer spectra showed two sets of six-line hyperfine patterns for all the samples, indicating the presence of Fe in both A and B-sites. On increasing the concentration of doped Y, the hyperfine field strength and the isomer shift first increase and then decrease, whereas the quadrupole splitting continuously increases. The superparamagnetism was observed for all the samples and the change of ratio of the superparamagnetism component reflects the size of crystal grain.     

Tailoring the magnetic properties of manganese ferrite with substitution of a small fraction dysprosium for iron

Manganese ferrite nanoparticles with dysprosium (Dy) ions substituted for iron ions have been prepared by using a sol-gel method. Substitution of a small fraction Dy for Fe results in the larger magnetocrystallite anisotropy of MnFe_2−xDy_xO₄ (x=0.2, 0.4) nanoparticles than that of MnFe₂O₄ nanoparticles. The magnetosrystallite anisotropy was enhanced with the increase in the substituted dysprosium content. Combining the result of Mössbauer spectra with ZFC and FC curves, we know clearly that the Dy substitution can modify the anisotropy of MnFe₂O₄ nanoparticles for its strong spin-orbital coupling. Through this simple substitution, we can control the magnetosrystallite anisotropy of the magnetic nanoparticles and make good use of the products according as we need.     

Structural relaxation of FeNiB amorphous alloy

Young's modulus was measured in rapidly quenched ribbons of Fe₄₄Ni₃₈B₁₈ annealed at a rate of 5 K/min up to different temperatures. An oscillating behavior was observed. Samples annealed up to temperatures that correspond to maxima and minima values of Young's modulus were studied with Mössbauer spectroscopy in order to correlate the macroscopic parameter with the hyperfine field distribution.     

Magnetic anisotropy of amorphous (Fe1−x Co x )78Si10B12 alloys

In order to better understand the problem of magnetic anisotropy in amorphous alloys produced by a rapidly quenching technique, in-plane magnetic anisotropy of amorphous (Fe_1−x Co _x (₇₈Si₁₀B₁₂ alloys was measured by means of a torque magnetometer using a disk specimen made from the amorphous alloy ribbon. The amorphous ribbons were prepared by a single roller type quenching apparatus. It was found that the anisotropy had mostly twofold symmetry in all the alloy cases, and that the concentrationx dependence of the anisotropy constant behaved differently from that of the magnetostriction. Moreover, the anisotropy did not disappear by subsequent annealing at high temperatures where the internal stress relief and the crystallization were completed.     

Structure and superconductivity in FexCu1−xBa2YCu2O7+y superconductors synthesized by high pressure

Comprehensive studies of X-ray diffraction, oxygen content, superconductivity and Mössbauer effect have been made on Fe_xCu_1−xBa₂YCu₂O_7+y superconductors (0.00≤x≤0.70) synthesized by ambient (AM) and high pressure (HP). Results indicate that all the HP-samples have tetragonal structure, smaller lattice parameter c and unit-cell volume than the AM-samples. The studies of oxygen content, and Mössbauer spectroscopy indicate that the HP-samples have higher oxygen content, carrier concentration and average valence of Fe than the AM-samples. Moreover, for the HP-samples more Fe atoms located in CuO_x chains have fivefold-oxygen coordination. These are important reasons for the enhancement of T_c in the HP-samples.