Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

Effect of Ni on the lattice parameter and the magnetic hyperfine field in (Fe70Al30)100 − x Ni x alloys

Samples of the (Fe₇₀Al₃₀)_100???x Ni _x system, with x?=?5, 10, 15, and 20, were prepared by mechanical alloying using milling times of 12, 24, 36, 48, and 72 h and characterized by X-ray diffraction (XRD) and Mössbauer spectrometry (MS). XRD of all the samples allowed us to identify the BCC structure as the main component. A decreasing lattice parameter, as the milling time and Ni content increase, was obtained. The MS experiments were carried out at room temperature. The spectra were fitted with a hyperfine magnetic field distribution (HMFD). The Mean Hyperfine Fields (MHF) are ranged between 27 and 29 T with a small dependence with the milling time and was not strongly influenced by Ni content. An additional paramagnetic single line in the spectra of samples with x?=?20 and that of sample with x?=?15 and milled during 72 h must be included. The Mössbauer spectral area of this phase increases when the milling time increases and it was associated to an FCC phase.     

Influence of silicon and atomic order on the magnetic properties of (Fe80Al20)100– x Si x nanostructured system

Mechanically alloyed (Fe₈₀Al₂₀)_100???x Si _x alloys (with x?=?0, 10, 15 and 20) were prepared by using a high energy planetary ball mill, with milling times of 12, 24 and 36 h. The structural and magnetic study was conducted by X-rays diffraction and Mössbauer spectrometry. The system is nanostructured and presents only the BCC disordered phase, whose lattice parameter remains constant with milling time, and decreases when the Si content increases. We found that lattice contraction is influenced 39% by the iron substitution and 61% by the aluminum substitution, by silicon atoms. The Mössbauer spectra and their respective hyperfine magnetic field distributions show that for every milling time used here, the ferromagnetism decreases when x increases. For samples with x?≥?15 a paramagnetic component appears. From the shape of the magnetic field distributions we stated that the larger ferromagnetic phase observed in the samples alloyed during 24 and 36 h is a consequence of the structural disorder induced by mechanical alloying.     

Magnetic and structural properties of mechanically alloyed Tb0.257−x Nd x Fe0.743 alloys, with x = 0 and 0.257

The alloys between a transition metal and a rare earth present magnetic and magneto optical properties of exceptional interest for the production of magnetic devices for information storage. In this work we report the magnetic and structural properties, obtained by Mössbauer spectrometry (MS) and X-ray diffraction (XRD), of Tb_0.257?x Nd _x Fe_0.743 alloys with x?=?0 and 0.257 prepared by mechanical alloying during 12, 24 and 48 h, to study the influence of the milling time in their magnetic and structural properties. The X-rays results show for all the samples that the α-Fe and an amorphous phase are always present. The first decreases and the second increases with the increase of the milling time. Mössbauer results show that the amorphous phase in samples with Nd is ferromagnetic and appears as a hyperfine field distribution and a broad doublet, and that as the milling time increases the paramagnetic contribution increases. For samples with Tb the amorphous phase is paramagnetic and appears as a broad doublet which increases with the milling time and for 48 h milling it appears an additional broad singlet.     

X-ray diffraction,microstructure, Mössbauer and magnetization studies of nanostructured Fe50Ni50 alloy prepared by mechanical alloying

Nanocrystalline Fe₅₀Ni₅₀ alloy samples were prepared by the mechanical alloying process using planetary high-energy ball mill. The alloy formation and different physical properties were investigated as a function of milling time, t, (in the 0–50 h range) by means of the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), energy dispersive X-ray (EDAX), Mössbauer spectroscopy and the vibrating sample magnetometer (VSM). The complete formation of γ-FeNi is observed after 24 h milling. When milling time increases from 0 to 50 h, the lattice parameter increases towards the Fe₅₀Ni₅₀ bulk value, the grain size decreases from 67 to 13 nm, while the strain increases from 0.09% to 0.41%. Grain morphologies at different formation stages were observed by SEM. Saturation magnetization and coercive fields derived from the hysteresis curves are discussed as a function of milling time.     

Giant magnetic impedance of film nanostructures adapted for biodetection

A comparative study of the magnetic properties and giant magnetic impedance of Fe₁₉Ni₈₁/Su/Fe₁₉Ni₈₁ film structures fabricated by the method of ion-plasma evaporation is performed versus the geometry of an impedance element. The geometrical parameters of the Fe₁₉Ni₈₁/Сu/Fe₁₉Ni₈₁ structures are estimated from the viewpoint of obtaining strong magnetic impedance effect and possible application of these structures for detection of bioelements with magnetic markers. Widening of the structures necessary for increasing their active surface area weakens the magnetic impedance effect, and the length increase is limited by the optimal working biodetector frequencies and electromagnetic wave propagation processes.     

Mössbauer and X-ray study of the Fe 65 Ni 35 invar alloy obtained by mechanical alloying

Fe₆₅Ni₃₅ samples were prepared by mechanical alloying (MA) with milling times of 5, 6, 7, 10 and 11 h, using a ball mass to powder mass ratio of 20:1 and at 280 rpm. The samples were characterized by X-ray diffraction (XRD) and transmission ⁵⁷Fe Mössbauer spectrometry. The X-ray diffraction pattern showed the coexistence of one body centered cubic (BCC) and two face centered cubic (FCC1 and FCC2) structural phases. The lattice parameters of these phases did not change significantly with the milling time (2.866 Å, 3.597 Å and 3.538 Å, respectively). After 10 h of milling, the X-ray diffraction pattern showed clearly the coexistence of these three phases. Hence, Mössbauer spectrometry measurements at low temperatures from 20 to 300 K of this sample were also carried out. The Mössbauer spectra were fitted using a model with three components: the first one is a hyperfine magnetic field distributions at high fields, related to the BCC phase; the second one is a hyperfine magnetic field distribution involving low hyperfine fields related to a FCC phase rich in Ni, and the third one is a singlet related to a FCC phase rich in Fe, with paramagnetic behavior. As proposed by some authors, the last phase is related with the antitaenite phase.     

Magnetic and Structural Properties of the Mechanically Alloyed Nd2(Fe100 − x Nb x )14B System

In this work we report the magnetic and structural properties obtained by Mössbauer spectrometry, Vibrating Sample Magnetometer and X-ray diffraction of milled powders with initial composition Nd₂(Fe_{100 ? x} Nb _x )₁₄B with x = 0 and x = 4. The mixtures were ball milled for different times up to 240 h. Structural and microstructural parameters were derived from high statistics X-ray patterns and discussed as a function of milling time. The Mössbauer spectra of the samples were fitted by means of a sextet and an hyperfine field distribution, associated to a pure iron phase (α-Fe) and a disordered iron-based phase, respectively. The α-Fe grain size decreases from 50 nm for 6 h up to 5 nm for 240 h milling time. The Vibrating Sample Magnetometer results allow to conclude that these samples behave as soft ferromagnets.     

Structure,magnetic properties and magnetostriction of Fe81Ga19 thin films

The structure, magnetic properties and magnetostriction of Fe₈₁Ga₁₉ thin films have been investigated by using X-ray diffraction analysis, scanning electron microscope (SEM), vibrating sample magnetometer and capacitive cantilever method. It was found that the grain size of as-deposited Fe₈₁Ga₁₉ thin films is 50–60 nm and the grain size increases with increase in the annealing temperature. The remanence ratio (M_r/M_s) of the thin films slowly decreases with increase in the annealing temperature. However, the coercivity of the thin films goes the opposite way with increase in the annealing temperature. A preferential orientation of the Fe₈₁Ga₁₉ thin film fabricated under an applied magnetic field exists along 〈1 0 0〉 direction due to the function of magnetic field during sputtering. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-plane-induced anisotropy results in 90° rotations of the magnetic domains during magnetization and in the increase of magnetostriction for the thin film.     

Formation of nanostructured ω-Al 7 Cu 2 Fe crystalline phase by the ball milling technique

Crystalline ω-Al₇Cu₂Fe bulk samples were prepared by arc furnace and then by means of milling, the average grain size of these samples is reduced to the nanometer scale. The structural and magnetic properties of the nanostructured ω-Al₇Cu₂Fe phase have been studied by X-ray diffraction employing Rietveld method, Mössbauer spectroscopy and vibrating sample magnetometry. The results indicate that the average grain size of the synthesized sample (ω-phase) rapidly decreases from 79 to 12 nm after 5 h of milling. Furthermore, the hyperfine parameters of the nanostructured samples are higher than the values for the bulk ω-phase. Magnetic measurements show a weak ferromagnetic behavior with M _s ?=?0.46 emu.g^???1 for the bulk ω-phase. After the milling process this value increases to M _s ?=?1.50 emu.g^???1 due to the formation and growth of a magnetic interstitial region after reducing the average grain size of the sample.     

Magnetic and structural studies of mechanically alloyed nanostructured Fe49Co49V2 powders

Nanostructured Fe₄₉Co₄₉V₂ powders were produced by high energy milling at different milling times and then examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The saturation magnetization and coercivity of samples were measured at room temperature by a vibration sample magnetometer (VSM). Structural studies show that as the milling time increases from 0 to 125 h, the average grain size reduces from 130 to about 8-10 nm, while the microstrain increases up to 1.7%. The lattice parameter decreases from 0 to 36 h and then increases up to 125 h. According to the XRD patterns, the formation of intermetallic compound of (Fe, Co)V after about 16 h affects the magnetic properties. The coercivity totally increases up to 61 Oe due to the introduction of microstrain during the milling process. Magnetic measurements reveal that the saturation magnetization has some fluctuations during the milling treatment and finally at 125 h reaches about 180 emu/g     

Magnetic exchange bias enhancement through seed layer variation in FeMn/NiFe layered structures

The exchange bias and crystalline texture of the multilayer structure (Ta/Al/seed/Fe₅₀Mn₅₀/Ni₈₁Fe₁₉/Al₂O₃/Ni₈₁Fe₁₉/Al/Ta with seed=Ni₈₁Fe₁₉ or Ni₈₁Fe₁₉/Cu) has been characterized. Measurements indicate an abrupt decrease in exchange bias of the Ni₈₁Fe₁₉ pinned layer for samples with very thin seed layers, and exchange bias as high as 325 Oe for thicker seed layers. Fluctuation of exchange bias with thickness was greatly reduced for the Ni₈₁Fe₁₉/Cu seed configuration. X-ray diffraction measurements demonstrate a correlation between exchange bias and strong (1 1 1) texture of FeMn. The results suggest a high sensitivity of Ni₈₁Fe₁₉ roughness and texture on deposition conditions, and corroborate previous observations of roughness in ultrathin NiFe films.     

Multilevel interaction between layers in layered film structures

An analysis was carried out of the mechanism underlying magnetic interlayer interaction in film structures. The investigation was based on the assumption that interlayer bonding affects film hysteresis. This was based on experimental data on the coercive force, the domain structure parameters, and the microstructure of Fe₁₉Ni₈₁/Cr/Fe₁₉Ni₈₁ and Fe₁₅Co₂₀Ni₆₅/Cr/Fe₁₅Co₂₀Ni₆₅ films. Theoretical estimates show that, as the thickness of the Cr interlayer increases, the exchange interaction between the ferromagnetic layers can be replaced by the magnetostatic interaction whose effectiveness is determined by surface irregularities and layer ‘magnetization ripples’. Fiz. Tverd. Tela (St. Petersburg) 39, 2191–2194 (December 1997)     

Effect of boron in Fe 70 Al 30 nanostructured alloys produced by mechanical alloying

The substitution of aluminum by boron in the Fe₇₀Al₃₀ system prepared by high energy ball milling is studied when the B content ranged from 0 up to 20 at. %, and the milling times were 24, 48 and 72 h. X-ray diffraction (XRD) patterns of Fe₇₀Al₃₀ showed a predominant bcc structural phase with a lattice parameter larger than that of α-Fe. A second (tetragonal) phase arose with the addition of boron. It is associated to the existence of (Fe, Al)₂B, although the values of the lattice parameters are slightly different from those found in the literature. This phase shows high stability; its lattice parameters and the Mössbauer parameters do not show notable variations, either with milling time or composition. It was also evidenced that an increase of boron content and of milling time produced a decrease of the lattice parameter of the Fe-Al bcc structure. This is in agreement with the small atomic radius of boron in comparison with that of aluminum. This also allows boron to occupy interstitial sites in the lattice, increasing the grain size and giving rise to the ductile character of the alloy. On the other hand, 300 K transmission Mössbauer spectra (TMS) were fitted, for low boron concentrations (<8 at.%), with a hyperfine field distribution (HFD) associated with the bcc phase. For high boron content (≥8 at.%), a magnetic component related to the tetragonal phase is added and its broadened lines are attributed to the disordered character of Fe₂B, probably induced by the milling process.     

The evolution of electronic configuration and magnetic characterization of Fe<Subscript>9</Subscript>Ni<Subscript>1</Subscript>, Fe<Subscript>8</Subscript>Ni<Subscript>2</Subscript> alloy in theoretical calculation

To analyze the origin of the magnetic enhancement of Fe-Ni alloy, the electronicconfigurations and magnetic properties were investigated using density functional theorybased on the first-principle. The supercell (5 × 1 × 1) of Fe,Fe₉Ni₁ and Fe₈Ni₂ were constructed. Thedefect formation energy, band structure, density of states and electron density differencewere calculated. The results showed that Ni doping changed the electronic configuration ofFe atoms, resulting in the enhancement of spin polarization of Fe and the larger Bohrmagnetic moment in Fe-Ni alloys (Fe₉Ni₁). The results showed thatthe charge transfer and the atomic spacing between Fe atoms and the dopant Ni atoms playedan important role in determination of magnetic moment. The value of Fe supercell(5 × 1 × 1), Fe₉Ni₁ and Fe₈Ni₂ were 23.14,23.34 and 22.61μ _B, respectively.     

Thickness-dependent magnetic properties of Ni81Fe19, Co90Fe10 and Ni65Fe15Co20 thin films

We present results of magnetization and magnetic anisotropy measurements in thin magnetic films of the alloys Ni₈₁Fe₁₉, Co₉₀Fe₁₀ and Ni₆₅Fe₁₅Co₂₀ that are commonly used in magnetoelectronic devices. The films were sandwiched between layers of Ta. At room temperature the critical thickness for all the films to become ferromagnetic is in the range 11–13 Å. In Co₉₀Fe₁₀ the coercivity and the anisotropy field both depend strongly on layer thickness.     

g-Factors of magnetic–rotational states in 85Zr

The properties of the double iron and tungsten carbide prepared by mechanical alloying technique (MA) from elemental powders are reported. The samples were milled for 1, 3, 5, 10, 15, 20, 25 and 30 h. The alloy progress for each milling time was evaluated by X-ray diffraction (XRD) and ⁵⁷Fe Mössbauer spectrometry. Once the alloy was consolidated two sorts of paramagnetic sites and a magnetic distribution were detected according to the Mössbauer fitting. The majority doublet could correspond to Fe₆W₆C ternary carbide as X-ray diffraction suggests, and the other could be Fe₃W₃C. The hyper fine parameters are reported. Vickers microhardness measurements of 30 h milled sample was conducted at room temperature with a load of 0.245 N for 20 s.     

Hyperfine interactions, structure and magnetic properties of nanocrystalline Co–Fe–Ni alloys prepared by mechanical alloying

Mechanical alloying method was used to prepare nanocrystalline Co₅₀Fe₄₀Ni₁₀, Co₅₂Fe₂₆Ni₂₂ and Co₆₅Fe₂₃Ni₁₂ alloys. X-ray diffraction proved that during milling Co–Fe-based solid solution with b.c.c. lattice was formed in the case of Co₅₀Fe₄₀Ni₁₀, while for Co₅₂Fe₂₆Ni₂₂ and Co₆₅Fe₂₃Ni₁₂ compositions Co–Ni-based solid solutions with f.c.c. lattice were obtained. Mössbauer spectroscopy revealed similar values of the average hyperfine magnetic fields for all alloys, e.g. 32.17, 32.24 and 31.21 T for Co₅₀Fe₄₀Ni₁₀, Co₅₂Fe₂₆Ni₂₂ and Co₆₅Fe₂₃Ni₁₂ alloys, respectively. Magnetization measurements allowed to determine the effective magnetic moment, Curie temperature, saturation magnetization and coercive field for the obtained alloys.     

Magnetic, structural and micro-structural properties of mechanically alloyed nano-structured Fe48Co48V4 powder containing inter-metallic Co3V

The Fe₄₈Co₄₈V₄ alloy was synthesized in a planetary high-energy ball-mill under an argon atmosphere. The structure, microstructure and magnetic properties of the mechanically alloyed powders were investigated by X-ray diffraction, Scanning Electron Microscopy and a Vibration Sample Magnetometer, respectively. During the mechanical alloying of Fe₄₈Co₄₈V₄, inter-metallic Co₃V appears. The lattice parameter decreases up to 55 h of milling time with an oscillation and then increases from 55 to 125 h of milling time. The coercivity increases during the milling treatment from 49 to 58 Oe. The saturation magnetization has some fluctuations during the milling treatment and finally reaches ∼190 emu/g at 125 h.     

Structural and magnetic properties of Fe–Ni mecanosynthesized alloys

Fe_100???x Ni _x samples with x?=?22.5, 30.0 and 40.0 at.% Ni were prepared by mechanical alloying (MA) with milling times of 10, 24, 48 and 72 h, a ball mass to powder mass (BM/PM) ratio of 20:1 and rotation velocity of 280 rev/min. Then the samples were sintered at 1,000°C and characterized by X-ray diffraction (XRD) and transmission Mössbauer spectrometry (TMS). From the refinement of the X ray patterns we found in this composition range two crystalline phases, one body centered cubic (BCC), one face centered cubic (FCC) and some samples show FeO and Fe₃O₄ phases. The obtained grain size of the samples shows their nanostructured character. Mössbauer spectra were fitted using a model with two hyperfine magnetic field distributions (HMFDs), and a narrow singlet. One hyperfine field distribution corresponds to the ferromagnetic BCC grains, the other to the ferromagnetic FCC grains (Taenite), and the narrow singlet to the paramagnetic FCC grains (antitaenite). Some samples shows a paramagnetic doublet which corresponds to FeO and two sextets corresponding to the ferrimagnetic Fe₃O₄ phase. In this fit model we used a texture correction in order to take into account the interaction between the particles with flake shape and the Mössbauer $\upgamma$ -rays.