The short/long-range order formation in Fe1-x-Co-x (x < 0.3) alloys has been studied by the nuclear magnetic resonance (NMR) technique, as well as ab initio based atomistic simulation. The NMR measurements show up the formation of a certain short-range order (SRO) in dilute limit above the Curie temperature T-C and of the D0(3)-type SRO (with the dominance of 3rd Co-Co nearest neighbors) in the concentration range 0.2 < x <0.3 after quenching and subsequent annealing in the ferromagnetic state. The results of Monte Carlo simulations of binary Fe-Co alloys with ab initio interatomic interactions predict SRO in agreement with the experiment for small concentrations of Co (C-Co < 0.1), while the B2-type ordering is preferable in binary alloy in the ferromagnetic state. We demonstrate that the presence of point defects (vacancies, interstitial) can change essentially the ordering in alloys with the Co content 20-30% and result in the D0(3)-type SRO formation in a reasonable agreement with the experiment. (C) 2018 Elsevier B.V. All rights reserved.

ABSTRACT X-ray diffraction, magnetic measurements, and Mössbauer spectroscopy were employed to comparatively analyse the relation of the basic characteristics of highly anisotropic materials– coercive force Hc and Curie temperature TC to the peculiarities of short-range atomic order that forms upon annealing of disordered samples of equaiatomic FePd alloys produced by different techniques (casting, melt-spinning, severe deformation). It is shown that for all samples, independently of methods of their preparation, the ordered states with the maximal values of coercivity are inhomogeneous in the composition of short-ordered regions, type of atomic ordering, and degree of tetragonality. The Curie temperature depends on the temperature and duration of annealing for ordering similarly to the conventional course of coercivity, which is peculiar to these alloys. The behaviour of these macroscopic characteristics (Hc and TC) in the course of annealing is shown to correlate with changes in the local atomic configurations revealed in the Mössbauer spectroscopy experiments.

AbstractA comparative analysis of the magnetic properties and Mössbauer spectra of two Ce_2Fe_17 samples prepared by different methods and demonstrating different magnetic behavior is carried out. A model for processing the spectra is proposed, which gives a good description of the Mössbauer spectra of samples in different magnetic states by a superposition of eight subspectra. It is shown that in the samples in the ferromagnetic state there are regions with antiferromagnetic ordering at the local level. The relative fractions of iron atoms in these regions differs for different samples and increases when approaching the temperature of the transition from the ferromagnetic to the antiferromagnetic state.

A comparative analysis of the magnetic properties and Mössbauer spectra of two Ce 2 Fe 17 samples prepared by different methods and demonstrating different magnetic behavior is carried out. A model for processing the spectra is proposed, which gives a good description of the Mössbauer spectra of samples in different magnetic states by a superposition of eight subspectra. It is shown that in the samples in the ferromagnetic state there are regions with antiferromagnetic ordering at the local level. The relative fractions of iron atoms in these regions differs for different samples and increases when approaching the temperature of the transition from the ferromagnetic to the antiferromagnetic state.

A comparative Mossbauer study of structural inhomogeneities that arise in the course of ?1 L1(0) phase transformation in nondeformed (as cast and quenched from 950 degrees C and melt-spun from the ingot) and severely deformed samples of equiatomic FePd alloy upon ordering annealing at ?=450 degrees C has been performed. According to the known experimental works, the chosen temperature of annealing is optimal for achieving the highest coercive force H-c in both quenched and deformed samples. It is shown that in the high-coercivity state both quenched and deformed samples FePd possess an inhomogeneous tetragonal structure, which is preserved even after quite a prolonged (40-100h) annealing. All the samples contain, along with the configurations of the nearest neighbourhood that are assigned to the ordered L1(0) phase, significant volume fractions of configurations typical of nonequiatomic compositions. This conclusion is inconsistent with the commonly accepted concept on single-phase L1(0) type alloys with maximal values of H-c. An inference is made that the structural inhomogeneities detected in the samples under study result from the mismatch of the position of the point of congruent ?1 L1(0) transformation (approximate to 58 at.% Pd) in the phase diagram of the FePd system to equiatomic composition.

Mössbauer spectra of two samples of the Ce2Fe17 compound have been analyzed and hyperfine parameters, compared. The samples were subjected to different preparation techniques and display different magnetic properties. It is established that to well fit the Mössbauer spectra of these samples, an appropriate model should be used that takes into account an additional subspectrum differing in hyperfine parameters from the subspectra employed in conventional models. It is shown that in the ferromagnetic state, the samples contain local regions with an antiferromagnetic order, the volume fraction of which, being different for two samples, increases on approaching the temperature of ferro-to-antiferromagnet transition.

The Mössbauer effect is used to study changes in the structure of Fe50Pd50 alloy in the course of annealing for ordering at T = 450°C from different initial states: cast and quenched from 950°C, then subjected to severe plastic deformation by shear under pressure, and that obtained by fast quenching from the melt. Differences in the kinetics of phase transformations are observed depending on the initial state of the material.

The structural and hysteresis properties of ternary Fe(50)Pd(50-x)Nix (x = 4 and 8) alloys have been studied. In order to accelerate the formation of the hard magnetic L1(o) phase in these alloys, they were deformed by rolling with a reduction of 96% and by the method of high-pressure torsion (HPT). The annealing treatment for ordering for up to 100 h was performed at 400-500 degrees C. It is shown that the structure transformation of the initial fcc phase into the ordered tetragonal phase at 400 degrees C proceeds very slowly. After annealing for 100 h at this temperature, the Fe50Pd46Ni4 and Fe50Pd42Ni8 alloys are multiphase and contain residues of the cubic phase (Fm3m), cubic phase ordered by the L1(2) type (Pm3m), and tetragonal phase Ll(o)* appropriately described by the symmetry space group C4/mmm. The isothermal annealing at 400-500 degrees C for up to 100 h does not result in the maximum of He for the as-rolled Fe50Pd46Ni4 and Fe50Pd42Ni8 alloys. Implementation of the annealing routine with a temperature decreasing stepwise from 500 to 400 degrees C allowed us to increase H-c to 851 and 527 Oe for as-rolled samples and to 1192 and 980 Oe for the HPT samples of the Fe50Pd46Ni4 and Fe50Pd42Ni8 alloys, respectively. (C) 2017 Elsevier B.V. All rights reserved.

Fe–Co alloys with an Fe content of 20–95 at % have been investigated using NMR and Mössbauer spectroscopy. A comparison of the hyperfine fields measured at 59Со and 57Fe nuclei on the same samples allowed us to determine that the magnetic moment at an Fe atom in the disordered alloys with Fe concentrations of 20–80 at % varies from 2.8 to 2.4 μB, as well as to ascertain peculiarities of the formation of an ordered state. The data obtained indicate the existence of regions with a short-range order of the D03 type in alloys with an Fe content of 70–80 at %. In alloys with iron concentrations greater that 80 at %, the phase separation of the structure into regions of elemental iron and short-range-ordered Fe89Co11 regions is observed. The obtained results explain the contradictions in the values of the magnetic moment at the Fe atom and the anomalous dependence of the lattice parameter of the alloy with a Со content less than 20 at %, as well as prove the accuracy of the phase diagram containing regions of three bcc phases, rather than one, as is commonly accepted.

Mössbauer spectroscopy study of Ce(Fe1–x Si x )2 compounds with x = 0 and 0.07 was performed at different temperatures. Easy magnetization axis of the CeFe2 ferromagnet at 130 K was shown to be in the {110} plane and to deviate from the [001] axis by ∼10°. Upon cooling, the Ce(Fe0.93Si0.07)2 compound undergoes the ferromagnet–antiferromagnet phase transition in a temperature range of 120–125 K, which is accompanied by the reduction of the lattice symmetry. The Debye temperature of the Ce(Fe0.93Si0.07)2 compound was estimated using temperature dependences of the integral intensity of Mössbauer spectrum; it is T D ≈ 310 K. When analyzing the P(H) hyperfine field distributions P(H) derived from the Mössbauer spectra of Ce(Fe0.93Si0.07)2, it was found that in the cubic structure of this compound in the ferromagnetic state there occur local rhomboherdal distortions typical of the antiferromagnetic state.

Magnetic properties have been measured for the Y1−xTbxMn6Sn6, La1−xSmxMn2Si2, Lu2(Fe1−хMnx)17, and La(Fe0.88SixAl0.12–x)13 systems which show up transitions from antiferromagnetic to ferromagnetic state upon changing concentration of the constituents or application of magnetic field. We determined the concentrations and temperatures of the magnetic phase transitions and plotted magnetic phase diagrams. Near a critical concentration, the AF–F transition can be realized in low magnetic fields, which makes these compounds attractive for magnetothermal applications. Using the data of the magnetization measurement, we determined the isothermal magnetic entropy change in a wide temperature range. All the studied systems have a layered magnetic structure with the positive intralayer exchange interaction and the interlayer exchange integrals of different signs depending on the composition and temperature. For the compounds La(Fe0.88SixAl0.12−x)13 with the cubic crystal structure, the origin of formation of a layered magnetic structure is discussed based on the data of Mössbauer studies which revealed a difference in the local surrounding of resonant atoms in the compounds with different magnetic orders.

Magnetic properties, structure parameters, linear thermal expansion, and Mossbauer effect have been measured on intermetallic compounds La(Fe0.88SixAl0.12 (x))(13) (x = 0.033 and 0.096) with the ground ferromagnetic state. The compound with x = 0.033, on growing temperature, demonstrates a transition to antiferromagnetic state at TF-AF similar to 140 K and then paramagnetic state at T-N = 190 K. The compound with x = 0.096 is a ferromagnet with T-C = 190 K, and the transition to paramagnetic state is of the first order. The ferromagnetic ordering is accompanied by the lattice expansion by 0.5% and 1.2% for compositions with x = 0.033 and 0.096, respectively. It is established, based on the results of fitting of Mossbauer spectra, that the antiferromagnetic state is predominantly featured by the subspectrum with a positive quadrupole shift, whereas in the ferromagnetic state approximately equal contributions with the quadrupole shifts of different signs are observed. (C) 2014 Elsevier B. V. All rights reserved.

Investigation of Ga influence on the structure of Fe-Cr and Fe-Co alloys was performed with the use of Fe-57 Mossbauer spectroscopy and X-ray diffraction methods. In the alloys of the Fe-Cr system, doping with Ga handicaps the decomposition of solid solutions, observed in the binary alloys, and increases its stability. In the alloys with Co, Ga also favors the uniformity of solid solutions. The analysis of Mossbauer experiments gives some grounds to conclude that if, owing to liquation, clusterization, or initial stages of phase separation, there exist regions enriched in iron, some amount of Ga atoms prefer to enter the nearest surroundings of iron atoms, thus forming binary Fe-Ga regions (or phases).

The structure of the Ce(Fe1 − x Si x )2 compounds (with x ≤ 0.075) has been studied and, magnetic susceptibility, heat capacity, and Mössbauer effect have been measured. The compounds with x ≥ 0.05 are antiferromagnetic at low temperatures; as the temperature increases, the compounds become, at first, ferromagnetic and next paramagnetic. The temperatures of magnetic phase transitions have been determined using data on the magnetic susceptibility, and the magnetic phase diagram of the system has been constructed. The heat capacity has been measured and the data were used to calculate the entropy change upon magnetic phase transitions; it is 7.9 and 6.0 J/kg K for CeFe2 and Ce(Fe0.93Si0.07)2, respectively. An analysis of Mössbauer spectra for the alloys in the paramagnetic state allowed us to find that silicon atoms statistically substitute for iron atoms in the crystal lattice intermetallic.

Using Mossbauer spectroscopy based on modern methods of processing spectra, new fine features of the structure of equiatomic alloy FePd, which is formed upon the A1 -> L1(0) phase transformation in the course of severe plastic deformation by torsion and annealing for ordering at the temperature T = 450 degrees C, have been studied. It is shown that the structure of the alloy in the as-deformed state cannot be treated as a homogeneous solid solution; it is more likely to consider it as an alternation of disordered regions that are different in composition and in the sign and degree of tetragonal distortions of the initial fcc lattice. Already at early stages of ordering annealing, there are detected low-symmetry regions, inhomogeneous in composition and in the degree of ordering, with tetragonal distortions of the same sign but essentially different in magnitude. These regions can be identified as the structure constituents of the A6 and L1(0) type with the tetragonal lattice. Such a multi-phase structure is retained even after a long-time (t(ann) = 40 h) annealing. A conclusion is made that the equilibrium tetragonal L1(0) phase is formed as a result of long-term redistribution of Fe atoms between the initial cubic and newly formed tetragonal components. The effect of the fine-structure peculiarities on the coercivity of the FePd alloys is considered. (C) 2013 Elsevier B.V. All rights reserved.

Investigation of Ga influence on the structure of Fe-Cr and Fe-Co alloys was performed with the use of Mossbauer spectroscopy and X-ray diffraction methods. The experimental results are compared with results of first-principles calculations of the mixing and solubility energies for Ga in an Fe-X (X = Co, Cr) alloy both in ferromagnetic and paramagnetic states. It is shown that Ga mainly goes into the solid solutions of the base alloys. In the alloys of the Fe-Cr system, doping with Ga handicaps the decomposition of solid solutions, observed in the binary alloys, and increases its stability. In the alloys with Co, Ga also favors the uniformity of solid solutions. The results of the first-principles calculations testify in favor of a preferable dissolution of Ga in the FeCo regions of a multicomponent structure rather than FeCr regions, both types of regions being in the ferromagnetic state at the temperature of annealing. The analysis of Mossbauer experiments gives some grounds to conclude that if, owing to liquation, clusterization, or initial stages of phase separation, there exist regions enriched in iron, some amount of Ga atoms prefer to enter the nearest surroundings of iron atoms, thus forming binary Fe-Ga regions (or phases). (C) 2014 Elsevier B.V. All rights reserved.