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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.

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.

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.

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.

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.

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.

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.

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.

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.