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A systematic study of the magnetic and structural properties dependence on the particle size was realized. For this, commercial NdFeB powder was separated into five different mean particle sizes using sieves. Besides, from the original powder, eleven samples were also produced by mechanical milling assisted by surfactant, using various milling times. A total of sixteen samples were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and Mossbauer spectrometry (MS). The particle sizes of the samples vary from the micrometer to the nanometer scale. The crystallite size decreases with decreasing particle size. XRD result indicates that the Nd2Fe14B phase is found in all the samples, and the presence of this phase is also corroborated by MS using six sextets for fitting their spectra, with an additional singlet corresponding to the Nd1.1Fe4B4 phase. The mean hyperfine magnetic field increases with increasing particle size because the magnetic dipolar interaction between the magnetic moment of the particles increases with particle size. From the VSM measurements the magnetic energy density (BH)max values were calculated for different particle sizes, and their maximum value of 34.45 MGOe is obtained for the sample with the particle size of 60 μm.

This study reports the stabilization of the RFe12-type based compounds where part of R and Fe are substituted with Zr and Co and Si, respectively, in order to examine whether these rare-earth-lean materials are suitable for applications as permanent magnets. Structural and magnetic characterization of the family of alloys with the general formula Nd0.4Zr0.6Fe10–xCoxSi2 (x = 0–3) and their melt-spun ribbons were carried out using X-ray diffraction and Mossbauer spectroscopy. The ThMn12-type structure is obtained for all samples as the majority phase with a minority α-Fe(CoSi) phase (less than 5 wt%) as it was estimated by XRD for x = 1 and 2. The Curie temperature increases linearly with Co substitution from 561 K for x = 0 to 712 K for x = 3. The saturation magnetization decreases slightly from 130.5 (x = 1) to 129.1 A·m2/kg (x = 3), while the anisotropy field is following the same trend.

In this work, we examined the effects of the of Nd by Ce substitution and of the nitrogenation in the structural and hyperfine magnetic properties of (Nd1-xCex)1.1Fe10CoTi with x = 0, 0.5 and 1 systems. The alloys were prepared, with high pure elements, by arc-melting and the pieces were then homogenized at 1100 °C for 48 h. The nitrogenation was carried out at 450 °C for 1 h with a pressure of 0.1 MPa. The X-ray diffraction analysis allowed to identify the ThMn12-type structure mainly with average weigh fraction of 68%. The obtained tetragonal ThMn12 structure type presented an increased lattice parameter after nitrogenation for all alloys. The rhombohedral structure Th2Zn17-type is also present in the alloys and their lattice parameters do not show important changes after the nitrogenation. The Mossbauer spectra were fitted with seven sextets and a small quadrupolar component, the sextets were associated to the Fe sites in the ThMn12-type and Th2Zn17-type phases. The mean field, 〈Bhf〉, improve for all alloys after nitrogenation process. The Nd1.1Fe10CoTi (x = 0) and (Nd0.5Ce0.5)1.1Fe10CoTi (x = 0.5) alloys that present weigh fractions of 75 and 73%, respectively, with the ThMn12−type tetragonal structure present a 〈Bhf〉 of 30.9 and 31.0 T, respectively, after nitrogenation. Then, they can be possible candidates for rare earth-lean permanent magnets.

In this work, we study the possibility of obtaining the pure Fe3Sn phase by means of two solid-state reactions (2-SSR) method in which two thermal treatments are made. Samples of Fe3Sn1-xInx were prepared with x = 0.00, 0.05, 0.10, 0.15 and 0.20 by means of two solid state reactions. These samples were studied by X-ray diffraction (DRX) and Mossbauer spectrometry (MS). Rietveld analysis showed that the DRX patterns of the samples were refined with the hexagonal crystalline phases: Fe3Sn, Fe2Sn, FeSn2, FeSn, In0.2Sn0.8 and the cubic phase Fe1.84Sn0.16. With this structural analysis, it was shown that the Fe3Sn phase appears in the Fe3Sn1-xInx sample with x = 0.00 in an approximately 90% wt. In addition, it was found that the weight percentage (wt%) of the Fe3Sn phase present in each of the 5 samples decreases as the doping or insertion of In increases in the Fe3Sn1-xInx samples, while the weight percentage of the In0.2Sn0.8 phase increases as indium doping increases. The other crystalline phases present in the samples do not change monotonously or follow some pattern. The Mossbauer spectra for the samples were adjusted with sextets associated with the ferromagnetic phases such as: Fe3Sn, Fe3Sn2, FeSn and Fe1.84Sn0.16. The Mossbauer adjustments showed that the percentage of spectral area of the Fe3Sn magnetic phase decreases as indium doping increases and the percentage of spectral area of the Fe3Sn2, FeSn and Fe1.84Sn0.16 phases increase.

(Zn0.9Fe0.1)O powders were produced with different ball to powder weight ratios (BPR) by ball milling of ZnO and Fe elemental powders. The BPR used were 15:1, 20:1, 25:1, and 30:1. The prepared samples were studied by x-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Mossbauer spectroscopy (MS). The XRD patterns showed a zinc oxide (ZnO) single phase with wurtzite structure, except for the 15:1 ratio, which exhibits an additionally bcc structure associated to α-Fe. The mean crystallite sizes were found to be around 100 A. The Mossbauer spectra were fitted using two doublets, associated to the incorporation of Fe ions into the ZnO lattice, which correspond to the Fe3+ and Fe2+ oxidation states except for the 15:1 ratio, for which an additional sextet was necessary to obtain a good fit, this sextet was associated to segregated Fe into the sample. The increase of the BPR allowed that all Fe penetrated into the ZnO matrix. The magnetization measurement using VSM reveals that the system exhibits ferromagnetic behavior at room temperature. For sample with BPR 15:1, the ferromagnetism is due to the segregated Fe, and for higher BPRs, from 20:1 up to 30:1, we propose that their ferromagnetism is due to the percolation of polarons. These polarons appear due to the oxygen vacancies (detected by XRD) which are occupied by electrons, which appear in the charge balance. Exchange interaction between the electron spins located in the vacancies or holes and those of the magnetic impurities (Fe atoms) leads to the formation of bound magnetic polarons, which percolate, producing in this way magnetic behavior.

Polycrystalline τ-MnAlC thin films were produced by thermal evaporation technique in one-step deposition and using as target materials a homogeneous τ-MnAlC(100%) and a heterogeneous e-MnAlC(73.9%)/τ-MnAlC(22.0%)/Al6Mn(4.1%) alloy, both keeping the same atomic percent for each constituent element. X-ray diffraction patterns evidence all films present a high concentration of the desired τ-phase. A microscopy analysis shows the thermal evaporation induces an island-type deposition of nanocrystalline grains with an average size of 9 nm. A hysteresis loop shows the highest concentration τ-MnAlC film exhibits a superparamagnetic-like behavior, an extremely important nanomagnetic state for the development of actuators and magnetic field sensor.

Fe0.70Al0.30 alloy is a bcc and ferromagnetic phase, being the Al atoms magnetic dilutor. In this work, we study the effect of the Nb on the structural and hyperfine behavior of the Fe0.70Al0.30 alloy when atoms of Nb substitute atoms of Fe or Al. The nanostructured system of (Fe0.70Al0.30)1-xNbx (x = 0, 0.05, 0.10, 0.20, at. %) was obtained by alloying Fe, Al and Nb powders in a planetary ball mill during 12 h, 24 h and 36 h, and a ball mass to powder mass relation of 10:1. The magnetic and hyperfine properties of the samples were studied by X-ray diffraction (XRD) and Mössbauer Spectrometry (MS) at room temperature, respectively. The X-ray diffraction patterns for x=0 showed the bcc-α FeAl structure and its lattice parameter is approximately constant with milling times (∼ 2.91 Å). For x=0.05, 0.10 and 0.20 the patterns showed the coexistence of the α-FeAl, Nb(Fe,Al)2 structural phases with an amorphous component. The Mössbauer spectra of x=0 samples were fitted using hyperfine magnetic field distributions (HMFDs), and the obtained mean hyperfine fields (MHF) were 23.4, 24.2, and 24.3 T for 12, 24, and 36 h of milling time, respectively, which correspond to the α-FeAl structure. The spectra of the samples with x=0.05 and 0.10 were fitted using a model with two components, the first one is a HMFD attributed to the bcc-FeAlNb structure and the second with two doublets attributed to the Nb(Fe,Al)2 structure. When atomic percentage of Nb increases up to 20 at. % the ferromagnetic behavior is diluted due to substitution of Fe-atoms by Nb and Al atoms in the bcc-FeAlNb structure. The magnetic behavior becomes paramagnetic at x=0.20, the spectra were fitted with three doublets, one of them related with bcc-FeAlNb structure and the others to the Nb(Fe,Al)2 structural phase. The alloying of Nb to the Fe0.70Al0.30 system destroyed the magnetism due the substitution of Fe by Nb atoms and generates an amorphization into the system.

The structural and magnetic properties of Fe-doped ZnO are reported in this study, as obtained by mechanical alloying from elemental powders of ZnO and Fe. The properties of Zn0.90Fe0.10O samples alloying while varying the milling time (6, 12, 24 and 36 h) are also reported. The Rietveld refinement of X-ray Diffraction (XRD) patterns revealed that the system presents two structures: the wurtzite structure of ZnO and the bcc structure of α-Fe. The Mossbauer spectra show that the samples present three components: a ferromagnetic component, associated with the Fe phase and two paramagnetic components, associated with the Fe atoms, which penetrate inside the ZnO matrix behaving as Fe3+ and Fe2+. The milling time contributes to an increase in the paramagnetic sites, and a solubility limit of the Fe atoms in the ZnO lattice was detected. The VSM measurements at room temperature detected ferromagnetic behavior with a saturation magnetization of 11 emu/g and a coercive field of 330 Oe for the sample alloyed over 24 h. A similar behavior was shown by the other samples.

A study was made of the structural and magnetic properties of the Fe2Nb alloy using X-ray diffraction, Mossbauer spectrometry at different temperatures, and ac susceptibility techniques. The alloy was obtained after 30 hours of milling in a high energy planetary mill and subsequently sintered at 700 K for 72 hours. The alloy is paramagnetic and presents the bcc-Fe and hcp-Fe2Nb structural phases, with a mean crystallite size of 75 and 91 nm, respectively. From the magnetic measurements, it was found that the Fe2Nb hexagonal structure exhibits: reentrant spin-glass behavior with a freezing temperature of 67 K, ferromagnetic and paramagnetic phases which coexist between 67 and 265 K, and paramagnetic behavior above 265 K.

In this paper we carried out the structural and magnetic study of the Fe2Nb stoichiometric alloy by means of x-rays diffraction and Mspectrometry. The samples were obtained by two different techniques, arc- melting and mechanical alloying, and then they were thermally treated. The structural properties of the alloy depend on the method of preparation: by the melting method it was possible to obtain the hcp-Fe2Nb ordered structure; while by mechanical alloying besides of the hcp structure also were formed the fcc-NbO and bcc- FeNb structures. After heat treatment of the melted alloy, this was submitted to mechanical alloying during 16 and 32 hours. Mechanical alloying allows the formation of bcc-FeNb grains which behave as superparamagnetic due to their small size and they are surrounded by hcp grains. Before heat treatment of the alloy obtained by mechanical alloying it was also found the existence of superparamagnetic bcc grains and the heat treatment increases the size of these bcc grains and the superparamagnetic-ferromagnetic transition is induced.

Zn100−x OFe x samples were obtained by mechanical alloying (MA) with different milling times. The purpose of this work was to analyze the variation of hyperfine parameters of the system with milling time. X-ray patterns show that pure ZnO under mechanical milling is not contaminated; additionally the system obtained by mechanical alloying has a wurtzite structure similar to pure ZnO. Mossbauer spectra show that all samples present two phases: paramagnetic and ferromagnetic. The milling time contributes to increase the paramagnetic phase; however, the Mossbauer spectra taken for samples milled during 36 and 99 hours indicates that there is a solubility limit for Fe atoms in ZnO lattice due that the spectra area remains nearly constant after 36 h. Mossbauer spectrometry shows that the Fe atoms which penetrate inside the ZnO matrix behave like Fe3+ and Fe2+.

Samples of nominal composition Fe50Co50 were produced by mechanical alloying byusing a planetary ball mill and different milling times. The samples were studied via X-raydiffraction, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy to characterize thephase distribution resulting from the milling process. The Mössbauer data indicated that Co startsdiffusing into Fe after 8 h of milling. Between t = 8h and t = 24 h the sample has a heterogeneouscomposition, presenting a bimodal hyperfine field distribution with maxima centred at 34.3 T and35.8 - 36.4 T, compatible with the presence of different Fe environments (richer in Co and richerin Fe, respectively). After 48 h of milling, the sample presents a more homogeneous compositionshowing an almost symmetric hyperfine magnetic field distribution centred at H=34.9 T,indicating that a disordered equiatomic FeCo solid solution has already been formed. The X-rayphotoelectron spectroscopy data indicate that the native oxide layer formed on the freshly milledsamples contains Co2+, Fe2+ and Fe3+ oxides. After complete removal of this native oxide layer byAr ion bombardment, X-ray photoelectron spectroscopy analysis yields the composition of thenominal equiatomic Fe50Co50 alloy.

A study was made on the alloy Fe65Si35 using x-ray diffraction and Mossbauer spectrometry. The alloy was obtained by mechanical alloying in a high energy planetary mill, with milling times of 15, 30, 50, 75 and 100 h. The results show that in the alloys two structural phases are present, a Fe-Si BCC disordered phase and ferromagnetic, and a Fe-Si SC phase, whose nature is paramagnetic and which decreases with milling time. In the temporal evolution of the milling two stages are differentiated: one between 15 and 75 h of milling, in which silicon atoms diffuse into the bcc matrix of iron and its effect is to reduce the hyperfine magnetic field; the other, after 75 h of milling, where the alloy is consolidated, the effect of the milling is only to increase the disorder of the system, increasing the magnetic order.

Fe 2 Nb samples were obtained by arc melting, ordered by heat treatment, and finally disordered by means of mechanical alloying with milling times of 0.5, 1, 2 and 4 h. The ordered alloy shows only the hcp-Fe 2 Nb structure which is paramagnetic at room temperature, and at low temperatures exhibits spin glass and antiferromagnetic behaviours. Mechanical alloying destroys the ordered structure and permits the formation of bcc iron-rich grains. Atomic disorder maintains the antiferromagnetic behaviour of the alloy, but increases its freezing temperature. The Neel temperature of the hcp grains is increased by the apparition of bcc grains.

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 Mossbauer 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 Mossbauer 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.

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 Mossbauer 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 Mossbauer spectral area of this phase increases when the milling time increases and it was associated to an FCC phase.

Nanostructured powders of melted and 48 h-mechanically alloyed (MA) samples of the Fe50Mn10Al40 system were studied by XRD, SEM and Mossbauer spectroscopy (MS), and their properties were compared. The samples present BCC structure with similar mean lattice parameter (near 2.92 A) and grain size (around 22 nm). The MA sample presents additionally the α-Fe phase. Mossbauer spectra of the samples present a hyperfine field distribution (HFD), a broad paramagnetic (P) site and a sextet showing that the BCC ternary phase is disordered with Fe sites in environments rich in Fe (HFD) and rich in Al and Mn (P), respectively. The mean hyperfine magnetic field vs. temperature curve of melted sample presents two kinks, one at 28 K and other at 210 K and a Curie temperature at 340 K. A similar curve is observed for the milled sample but the kinks occur at 65 and 265K. Mossbauer spectra at different temperatures with and without applied field permit to conclude that the low temperature anomaly corresponds to the freezing temperature of a re-entrant spin-glass phase (RSG) and that of the second one corresponds to a blocking temperature of a superparamagnetic (SP) phase. These phases are possible in the samples due to their disordered character induced by the preparation conditions and the competitive interactions of the Fe and Mn atoms. The enhancement of the magnetic behaviour of the MA sample is due its larger disorder induced by the preparation method that can also explain the increase of the RSG and SP transition temperatures.

In this work we report the magnetic and structural properties obtained by Mossbauer spectrometry, Vibrating Sample Magnetometry and X-ray diffraction, of powder of Nd2Fe14B prepared by mechanical alloying. The mixtures were ball milled during 48 hours and submitted to heat treatments between 500 and 900 °C under an argon atmosphere. Structural parameters were derived from high statistics X-ray patterns. The Mossbauer spectra registered at 300 K for samples without heat treatment were fitted by means of a sextet and a hyperfine field distribution, associated to a residual pure iron phase (α-Fe) and a disordered iron-based phase, respectively. From the spectra at 300 K the formation of the Fe3C phase is observed for samples heat treated at 900 °C. A quenching above 900 °C accelerates the formation of the Fe3C phase. The hysteresis loops allow to conclude that these samples behave as soft ferromagnets. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Monte Carlo simulations on a diluted Ising Hamiltonian were used to obtain the susceptibility of virtual binary samples (conformed by ferromagnetic atoms and non-magnetic atoms) on a bcc lattice. Samples size of L = 10 were constructed with different configuration order using random mixtures. The susceptibility curves illustrate that the critical temperature decreases when the disorder in the samples increase. From fittings of the exchange interaction versus the concentration of non-magnetic atoms it was possible to reasonably describe the magnetic phase diagram of the Fe 1 − q Al q alloys.

Melted alloys of the Fe x Mn 0.65− x Al 0.35 disordered system, 0.25⩽ x ⩽0.65, were experimentally studied by Mossbauer spectrometry, vibrating sample magnetometry and AC magnetic susceptibility. All the alloys exhibit the BCC structure with a nearly constant lattice parameter (2.92 A). Mossbauer studies at room temperature (RT) show that in the 0.25 ⩽ x ⩽0.45 range the alloys are paramagnetic (P) while in the 0.50⩽ x ⩽0.65 range, they are ferromagnetic. At 77 K, Mossbauer studies show that the alloy with x = 0.25 presents weak magnetic character that is consistent with an antiferromagnetic (AF) behavior due to the high Mn content, while those with 0.30⩽ x ⩽0.40 are paramagnetic, and those in the 0.45⩽ x ⩽0.65 range are ferromagnetic (F) with a mean field increasing with the Fe content. Hysteresis cycles at RT prove the paramagnetic character of the alloys between x = 0.25 and 0.40 and the ferromagnetic character for x ⩾ 0.45 . Complementary measurements using AC magnetic susceptibility permit a magnetic phase diagram to be proposed, with the P phase for high temperature and all the compositions, the AF phase for low Fe content and at low temperature, the F phase for high Fe content above RT and the spin glass phase for all the compositions and at temperatures lower than 46 K. In addition, the mean field renormalization group (MFRG) method, applied to a random competitive and site dilute Ising model with nearest-neighbor, gives rise to magnetic phase diagram, which fairly agrees with previous experimental one.

In this work the magnetic and structural properties are investigated by Mossbauer spectrometry, Vibrating Sample Magnetometry and X-ray diffraction of Nd2(Fe100−x Nb x )14B powdered alloys with x=0, 2 and 4 prepared by arc melting. The Mossbauer spectra of the samples were fitted with several contributions from: Nd2Fe14B, α-Fe and a paramagnetic phase associated with Nd1.1Fe4B4 for x=O and additionally from NbFeB and Nd2Fe17 for x=2 and x=4. The relative fractions of α-Fe and Nd2Fe14B are smaller for x=4 than for x=0, indicating that the amount of these two phases is reduced with increasing Nb content, while the relative fraction of Nd2Fe17 increases. The α-Fe grain size slightly decreases while that ofthe Nd2Fel4B phase is increasing, when the Nb content increases. The hysteresis loops indicate that these samples behave as hard ferromagnets, with a coercive field which decreases when the Nb content increases, but with rather low remanent magnetization.

Fe60Mn10Al20Nb10, (Fe60Mn10Al30)95Nb5 and (Fe60Mn10Al30)90Nb10 ball milled powdered alloys were investigated using X-ray diffraction, Mossbauer spectrometry, thermomagnetic (TGM) and magnetization measurements. We studied the influence of Nb content and of different milling times on the structural and magnetic properties. Two main features can be concluded: (1) the FeAIMn induces a BCC phase whatever the Nb content is, and (2) as both increasing Nb content and milling time give rise to an highly disordered state in conjunction with a decrease of the ferromagnetic behavior.

In this work we report the magnetic and structural properties obtained by Mossbauer spectrometry, Vibrating Sample Magnetometer and X-ray diffraction of milled powders with initial composition Nd2(Fe100−x Nbx)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 Mossbauer 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.

Results are reported about the phase distribution and magnetic properties of high-energy ball milled samples prepared from pure Fe and B powders and having nominal equiatomic composition. After milling the precursor powders for times from 40 to 270 h, the milling product consists of a majority amorphous phase and of milling-time-dependent small percentages of α-Fe, Fe 2 B and FeB. The coercivities measured in the as-milled samples were of the order of thousands of A/m and decreased to tens of A/m after a short time; low temperature treatments decreased the coercivity. We propose that this softening process is linked to a combination of stress relaxation and of enhancement of the exchange coupling between the minority crystalline phases and amorphous matrix, this last fact leading to the elimination of hindrances to the domain wall motion.

Fe60Mn10Al30−x Nb x alloys (2

The magnetic properties of the FexMn0.600 −xAl0.400(0.200 ≤ x ≤ 0.600) disordered alloys were investigated by means of Mossbauer spectrometry, ac magnetic susceptibility, and ac calorimetry measurements. Our results are summarized in a magnetic phase diagram, which includes an antiferromagnetic phase for x ≤ 0.300 at T ≤ 225 K, a ferromagnetic one for x ≥ 0.425 at T ≤ 340 K, and a paramagnetic phase for high temperatures and all the compositions. This phase diagram also shows for the first time both a re-entrant spin-glass–ferromagnetic transition line for x ≥ 0.425 and T ≤ 60 K and a re-entrant spin-glass–antiferromagnetic one for x ≤ 0.300 and T ≤ 40 K. The occurrence of superparamagnetic-like blocked events (for 0.300 ≤ x ≤ 0.600 and at T ≈ 200 K) is associated with the presence of Fe-rich magnetic clusters which can originate from the disordered atomic character of the alloys.

Fe82Nb6B12 alloy was synthesized from the elemental powders by a planetary high-energy ball mill, at room temperature, using different milling times and different surfactants. The products were measured by X-ray diffraction, magnetic thermogravimetry, vibrating sample magnetometer, and Mossbauer spectroscopy. Amorphization was observed from the X-ray diffraction on a sample ball milled for about 220 h using 595 rpm and cyclohexane as surfactant. Nevertheless, using the same conditions without surfactant, only the bcc Fe was found. The results are compared with data for rapidly quenched materials.

A simple site-diluted Ising model is proposed to study the magnetic properties of ${\mathrm{Fe}}_{p}{\mathrm{Al}}_{q}$ alloys (with $p+q=1)$ in the structural disordered phase. It is assumed that Al atoms, which are nonmagnetic, can induce an extra ferromagnetic interaction between second-neighbor Fe atoms. It is further assumed that this second-neighbor interaction, as well as the nearest-neighbor one, decreases as q increases. The critical properties are obtained by the variational approach based on Bogoliubov inequality for the free energy in the pair approximation. Quite good fittings to the experimental results of the ordering temperature are obtained as a function of q. A negative value of the extra exchange interaction for some range of the Al concentration is obtained. It is argued that this negative exchange can drive a spin-glass-like phase in these compounds, a fact that should be sought experimentally.

We report on the magnetic properties of Fex Mn0.3 Al0.7-x (0.275 ≤ x ≤ 0.525) disordered alloys, which were investigated by means of Mossbauer spectroscopy, ac magnetic susceptibility and magnetization measurements. Our results are summarized in a magnetic phase diagram which includes both a re-entrant spin glass-ferromagnetic transition line for x > 0.425 and a re-entrant spin glass-antiferromagnetic transition line for x < 0.375.

Fe70Al30)100� xSix (x ¼ 0, 5, 10 and 20) alloys were obtained by mechanical milling for durations of 12, 24 and 36 h. They were studied by X-ray diffraction (XRD), Mossbauer spectrometry and magnetic measurements at room temperature. The XRD experiments show that all the samples have the Fe-BCC phase and for x ¼ 20 an additional Fe0.34Si0.66 phase appears. The obtained Mossbauer spectra show a decreasing broad magnetic component and increasing broad paramagnetic component when the Si concentration increases. The spectra were fitted by using a hyperfine field distribution and a quadrupolar splitting distribution, respectively. The broad spectra show the disordered character of the samples. When the milling time increases the mean HF increases. These results suggest that Si acts as magnetic dilutor and that the milling time increases the magnetism. All the samples present magnetic hysteresis cycles in which the coercive field decreases with the Si increase, improving in this way the semi-soft character of the alloy. r 2006 Elsevier B.V. All rights reserved.

The magnetic properties of the FexMn0.7−xAl0.30 (0.40⩽x⩽0.58) alloy series have been investigated by means of 57Fe Mossbauer spectroscopy and ac magnetic susceptibility measurements. From our data, we propose a magnetic phase diagram for the system which is adequately reproduced by a diluted and random-bond Ising model which was analyzed on the basis of the mean-field renormalization-group method.

By Mossbauer spectroscopy the magnetic phase diagram of the FexMn0.7-xAl0.3 spin glass alloy has been obtained. X-ray diffraction (XRD) has shown that all the alloys are in the BCC phase. The broad spectral lines observed in the Mossbauer spectra are indicative of the disordered character of these alloys. Depending on the composition and the temperature the alloys behave as paramagnetic (P), ferromagnetic (F) and reentrant spin glass (RSG). For the alloys rich in iron the only detected magnetic transition is from F to P. For medium iron content two transitions were observed, namely from RSG to F and from F to P. The RSG phase is obtained as a consequence of the disordered character of the alloys and the competitive exchanges due to iron and manganese atoms.

Fe2Nb alloy samples obtained by arc-melting and mechanical alloying followed by heat treatment (700—925 K, 3—7 d) are characterized by XRD and Moessbauer spectroscopy.

Powders of melted disordered Fe 50Mn 10Al 40 alloy were separated at different mean particle sizes as well as magnetically and structurally characterized. All the samples are BCC and show the same nanostructure. Particles larger than 250 οm showed a lamellar shape compared to smaller particles, which exhibited a more regular form. All the samples are ferromagnetic at room temperature and showed reentrant spin-glass (RSG) and superparamagnetic (SP)-like behaviors between 30 and 60 K and 265 and > 280 K, respectively, as a function of frequency and particle size. The freezing temperature increases with increasing particle size while the blocking one decreases with particle size. The origin of these magnetic phenomena relies in the internal disordered character of samples and the competitive interaction of Fe and Mn atoms. The increase of their critical freezing temperature with increasing mean particle size is due to the increase of the magnetic dipolar interaction between the magnetic moment of each particle with the field produced by the other magnetic moments of their surrounding particles. © 2012 Elsevier B.V.

Melted Fe 50Mn 10Al 40 alloy powder with particle size less than 40 μm was characterized at room temperature by XRD, SEM and XPS; and at low temperatures by Mössbauer spectrometry, ac susceptibility, and magnetization analysis. The results show that the sample is BCC ferromagnetic but with a big contribution of paramagnetic sites, and presents super-paramagnetic and re-entrant spin-glass phases with critical temperatures of 265 and 35 K, respectively. The presence of the different phases detected is due to the disordered character of the sample and the competitive magnetic interactions. The obtained values of the saturation magnetization and the coercive field as a function of temperature present a behavior which indicates a ferromagnetic phase. However, the behavior of the FC curve and that of the coercive field as a function of temperature suggest that the dipolar magnetic interaction between particles contributes to the internal magnetic field in the same way as was reported for nanoparticulate powders. © 2012 Elsevier B.V. All rights reserved.

A study by Mossbauer spectroscopy and magnetization of the Fe 0.45 Mn 0.25 Al 0.30 disordered alloy is reported. The curves of magnetization versus temperature for zero-filled cooling (ZFC) and field cooling (FC) are different above 60 K. Also, this alloy showed a remanent magnetization and a temporal decrease when the field turns off. This spin-glass behavior was confirmed as a re-entrant spin-glass in the ferromagnetic phase by Mossbauer spectroscopy.

The thermodynamic properties of a diluted and random-bond Ising model are calculated using the variational method based on the Bogolyubov's inequality for the ferro- and antiferromagnetic regions. It is assumed that the system is composed of clusters of one and two spins. Previously reported results for Ising models pure and diluted by site and by bond are recovered with this model. Es werden die thermodynamischen Eigenschaften fur ferro- und antiferromagnetische Bereiche eines bindungs-verdunnten und statistisch verteilten Ising-Modells mit Hilfe eines Variationverfahrens berechnet, das auf der Bogoljubovschen Ungleichung beruht. Dabei wird angenommen, das das System aus Clustern von ein und zwei Spins aufgebaut ist. Es wird gezeigt, das das Modell gut mit Ergebnissen anderer Autoren ubereinstimmt.

In this work we present a theoretical study of the magnetic behavior of disordered Fe–Al alloys on the basis of a simple-diluted quantum Heisenberg spin model with the assumption that the exchange interaction J depends on the Al concentration q. We calculated the critical temperature and exponents through the mean field renormalization group method. An acceptable fit to the experimental phase diagram for Al concentration in the range 0.30⩽q⩽0.45 is obtained.

A quenched disordered Ising model with a distribution function that includes dilution and random bonds is studied through the mean-field renormalization-group approach. Random bonds are considered through a ferromagnetic exchange interaction between nearest-neighbor spins (NN) and an antiferromagnetic exchange interaction between next-nearest-neighbor spins (NNN). The phase diagrams are obtained and the results are applied to Fe-Al disordered alloys.

A random-bond Blume-Capel model is proposed in order to study the ternary FeNiMn and FeAlMn alloys. The extended mean-field renormalization group approach and a variational procedure based on Bogoliubov inequality for the free energy are employed in the pair approximation. The phase diagrams are obtained for a symmetric distribution as well as for asymmetric distributions compatible to these alloys. The obtained phase diagram for the FeNiMn system is in a much better accord with the experimental data than the previous traditional mean-field renormalization group applied to the Ising model, mainly concerning the spin-glass and antiferromagnetic phases. Better results are also achieved for the order parameter of the FeAlMn alloys.

We report on the magnetic properties of Fe1�xAlx alloys 0.2x 0.4 produced by mechanical alloying by milling pure element powders for t = 12, 24, and 36 h. The alloys present a bcc lattice with compositional disorder and are ferromagnetic at room temperature, independently of the milling time. The lattice parameter of the x = 0.2 sample presents a small decrease with t, whereas those of the x = 0.3 and 0.4 samples remain constant independently of the milling time. The magnetic properties of the alloys with x = 0.2 and 0.3 do not show important variations with t, while those of x = 0.4 are strongly dependent on the milling time. For this latter alloy it was found that: i despite being the most diluted of the series, it presents a well developed ferromagnetic order at room temperature as the Mossbauer and hysteretic data have shown; ii the temperature dependence of the ac susceptibility and the Mossbauer spectra recorded at different temperatures evidence the occurrence of reentrant spin-glass and superparamagnetic phenomena. The enhancement of the ferromagnetic behavior and the presence of reentrant spin-glass freezing temperature and of a superparamagnetic blocking process are interpreted in terms of a simple localized model based on the disorder present in that alloy and on the occurrence of competitive interactions, namely, the ferromagnetic nearest-neighbor Fe-Fe interactions and the antiferromagnetic near-nearest-neighbor Fe-Fe ones. Taken together, these results evidence that the stabilization of the magnetic order takes place in the x = 0.4 sample exclusively through the induction of compositional disorder and without any contribution from the lattice expansion.

In this work, we report an ac susceptibility study of Fe0.5Mn0.2Al0.3, Fe0.5Mn0.1Al0.4, and Fe0.225Mn0.375Al0.40 disordered alloys. All the samples exhibit only one well defined peak near 58, 18, and 31 K, respectively. When the frequency of the ac field increases, the measured intensities of the peaks decrease and the maxima shift to higher temperatures, and when the dc field increases the intensity of the peaks decreases. This dependence of the position and intensity of the peaks on the field and frequency is evidence of the spin glass character of the transitions in these alloys. Mossbauer spectra at different temperatures were performed for the sample with 30 at. % Al and the plot of the adjusted mean hyperfine field versus temperature shows a kink at 53 K which is further evidence that this sample shows spin glass behavior.

The Bogoliubov inequality approach in the pair approximation and Monte Carlo simulations are used in a site-disordered Ising model in order to obtain the critical temperature of the Fe1�qAlq system. A similar study is also made for a classical Heisenberg model using Monte Carlo simulations. The results are compared with experimental data and the best fit shows a nonlinear relationship between the effective exchange parameter JFeFe and Al concentration q. The results obtained for the critical temperature by Monte Carlo simulations on both models Ising and Heisenberg, despite being better than those from the pair approximation, still give the same general overview of the experimental phase diagram.