Influence of Nd Substitution on the Phase Constitution in (Zr,Ce)Fe10Si2 Alloys with the ThMn12 Structure.

Iron-based compounds with a ThMn12-type structure have the potential to bridge the gap between ferrites and high performance Nd2Fe14B magnets. From the point of view of possible applications, the main advantage is their composition, with about 10 wt.% less rare earth elements in comparison with the 2:14:1 phase. On the other hand, the main issue delaying the development of Fe-rich alloys with a ThMn12-type structure is their structural stability. Therefore, various synthesis methods and stabilizing elements have been proposed to stabilize the structure. In this work, the influence of increasing Nd substitution on the phase constitution of Zr0.4-xNdxCe0.6Fe10Si2 (0 ≤ x ≤ 0.3) alloys was analyzed. X-ray diffraction and 57Fe Mössbauer spectrometry were used as the main methods to derive the stability range and destabilization routes of the 1:12 structure. For the arc-melted samples, an increase in the lattice parameters of the ThMn12-type structure was observed with the simultaneous growth of bcc-(Fe,Si) content with increasing Nd substitution. After isothermal annealing, the ThMn12-type structure (and the coexisting bcc-(Fe,Si)) were stable over the whole composition range. While the formation of a 1:12 phase was totally suppressed in the as-cast state for x = 0.3, further heat treatment resulted in the growth of about 45% of the ThMn12-type phase. The results confirmed that the stability range of ThMn12-type structure in the Nd-containing alloys was well improved by other substitutions and the heat treatment, which in turn, is also needed to homogenize the ThMn12-type phase. After further characterization of the magnetic properties and optimization of microstructure, such hard/soft magnetic composites can show their potential by exploiting the exchange spring mechanism.

Site Occupancy Determination in Th2Zn17- and TbCu7-types Sm2Fe17-xCox Compounds using Synchrotron Resonant Diffraction.

Sm2Fe17 compounds are high-performance permanent magnets. Cobalt substitution allows us to further improve their magnetic properties. Depending on the thermal treatment, cobalt-substituted compounds can be synthesized either in the TbCu7 (disordered) or in the Th2Zn17 (ordered) structure type. Rietveld refinement of the number of transition metal dumbbells replacing rare-earth atoms from synchrotron powder diffraction data shows that the TbCu7 disordered structure has the same composition as the ordered one (a transition metal-to-rare earth ratio of 8.5). Then, cobalt site occupancies have been determined in both structures using synchrotron resonant (anomalous) diffraction. Cobalt is found to be absent from the dumbbell sites. The diffraction results are confirmed by Mössbauer spectroscopy.

Structure and Magnetic Properties of Intermetallic Rare-Earth-Transition-Metal Compounds: A Review.

This review discusses the properties of candidate compounds for semi-hard and hard magnetic applications. Their general formula is R1-sT5+2s with R = rare earth, T = transition metal and 0≤s≤0.5 and among them, the focus will be on the ThMn12- and Th2Zn17-type structures. Not only will the influence of the structure on the magnetic properties be shown, but also the influence of various R and T elements on the intrinsic magnetic properties will be discussed (R = Y, Pr, Nd, Sm, Gd, … and T = Fe, Co, Si, Al, Ga, Mo, Zr, Cr, Ti, V, …). The influence of the microstructure on the extrinsic magnetic properties of these R-T based intermetallic nanomaterials, prepared by high energy ball milling followed by short annealing, will be also be shown. In addition, the electronic structure studied by DFT will be presented and compared to the results of experimental magnetic measurements as well as the hyperfine parameter determined by Mössbauer spectrometry.

Magnetic and spectroscopic properties of Ni-Zn-Al ferrite spinel: from the nanoscale to microscale.

This article presents the annealing effect on the structural, elastic, thermodynamic, optical, magnetic, and electric properties of Ni0.6Zn0.4Fe1.5Al0.5O4 (NZFAO) nanoparticles (NPs). The samples were successfully synthesized by the sol-gel method followed by annealing of the as-synthesized at 600, 800, 900, 1050, and 1200 °C. This approach yielded the formation of a highly crystalline structure with crystallite size ranging from 17 nm to 40 nm. X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques, as well as energy disperse spectroscopy (EDS), Fourier transform infrared (FTIR) and Raman spectroscopy, were used in order to determine the structural and morphological properties of the prepared samples. Rietveld XRD refinement reveals that Ni-Zn-Al ferrite nanoparticles crystallize in inverse cubic (Fd3̄m) spinel structure. Using FTIR spectra, the elastic and thermodynamic properties were estimated. It was observed that the particle size had a pronounced effect on elastic and thermodynamic properties. Magnetic measurements were performed up to 700 K. The prepared ferrite samples present the highest Curie temperature, which decreases with increasing particle size and which is consistent with finite-size scaling. The thickness of the surface shell of about 1 nm was estimated from size-dependent magnetization measurements using the core-shell model. Besides, spin resonance, magnetostriction, temperature coefficient of resistance (TCR), and electrical resistivity properties have been scientifically studied and appear to be different according to their size. The optical properties of synthesized NZFAO nanoparticles were investigated, and the differences caused by the particle sizes are discussed on the basis of the phonon confinement effect. This effect was also inspected by the Raman analysis. Tuning of the physical properties suggests that the Ni-Zn-Al ferrite samples may be promising for multifunctional diverse applications.

Differently shaped nanocrystalline (Fe, Y)3O4 and its adsorption efficiency toward inorganic arsenic species.

Herein we report effects of partial substitution of Fe3+ by Y3+ in magnetite (Fe3O4) on morphology and inorganic arsenic species adsorption efficiency of the Fe3-x Y x O4 nanoparticles formed. The series of Fe3-x Y x O4 (x = 0.00, 0.042 and 0.084, labeled as Y00, Y05 and Y10, respectively) was synthesized using co-precipitation followed by microwave-hydrothermal treatment (MW) at 200 °C. With increase of yttrium content (x value), both the morphological inhomogeneity of the samples and the fraction of spinel nanorods as compared to spinel pseudospherical particles increased. By both transmission electron microscopy and x-ray powder diffraction analyses, it was determined that the direction of growth of the spinel nanorods is along the [110] crystallographic direction. The Fe3-x Y x O4 affinities of adsorption toward the inorganic arsenic species, As(III) (arsenite, AsO3 3-) and As(V) (arsenate, AsO4 3-), were investigated. Increased Y3+ content related to changes in sample morphology was followed by a decrease of As(III) removal efficiency and vice versa for As(V). The increase in Y3+ content, in addition to increasing the adsorption capacity for As(V), significantly expanded the optimum pH range for the maximum removal and decreased the contact time for necessary 50% removal (t 1/2) of As(V) (Y00: pH 2-3, t 1/2 = 3.12 min; Y05: pH 2-6, t 1/2 = 2.12 min and Y10: pH 2-10, t 1/2 = 1.12 min). The results point to incorporation of Y3+ in the crystal lattice of magnetite, inducing nanorod spinel structure formation with significant changes in sorption properties important for the removal of inorganic arsenic from waters.

Electronic structure and magnetic properties of Pr-Co intermetallics: ab initio FP-LAPW calculations and correlation with experiments.

First-principle calculations combining density functional theory and the full-potential linearized augmented plane wave (FP-LAPW) method are performed to investigate the electronic and magnetic structure of Pr2Co7 in its two polymorphic forms, (2:7 H) and (2:7 R), for the first time. This type of calculation was also performed for PrCo5 and PrCo2 intermetallics. We have computed the valence density of states separately for spin-up and spin-down states in order to investigate the electronic band structure. This is governed by the strong contribution of the partial DOS of 3d-Co bands compared to the partial DOS of the 4f-Pr bands. Such a high ferromagnetic state is discussed in terms of the strong spin polarization observed in the total DOS. The magnetic moments carried by the Co and Pr atoms located in several sites for all compounds are computed. These results mainly indicate that cobalt atoms make a dominant contribution to the magnetic moments. The notable difference in the atomic moments of Pr and Co atoms between different structural slabs is explained in terms of the magnetic characteristics of the PrCo2 and PrCo5 compounds and the local chemical environments of the Pr and Co atoms in different structural slabs of Pr2Co7. From spin-polarized calculations we have simulated the 3d and 4f band population to estimate the local magnetic moments. These results are in accordance with the magnetic moments calculated using the FP-LAPW method. In addition, the exchange interactions J ij are calculated and used as input for M(T) simulations. Involving the data obtained from the electronic structure calculations, the appropriate Padé Table is applied to simulate the magnetization M(T) and to estimate the mean-field Curie temperature. We report a fairly good agreement between the ab initio calculation of magnetization and Curie temperature with the experimental data.

Study of the magnetic and electronic properties of nanocrystalline PrCo3 by neutron powder diffraction and density functional theory.

Nanocrystalline PrCo(3) powder has been synthesized by high-energy milling and was subsequently annealed from 873 to 1273 K for 30 min to optimize the extrinsic properties. The structure and magnetic properties of the nanocrystalline PrCo(3) have been investigated by means of x-ray and neutron diffraction as well as magnetization measurements. All compounds crystallize in the same PuNi(3) type structure, with grain sizes between 28 and 47 nm. As the annealing temperature increases, a maximum coercive field of 12 kOe at 300 K (55 kOe at 10 K) was obtained by annealing at 1023 K for a grain size of 35 nm. The refinement of the neutron powder diffraction patterns (NPD) of PrCo(3) from 1.8 to 300 K shows an expansion of the parameter a and a contraction of the parameter c, leading to a decrease of the ratio c/a. The evolution of the Co and Pr magnetic sublattices measured by NPD indicates that this compound is a highly anisotropic uniaxial ferromagnet with the easy magnetization axis parallel to c(-->). This experimental study has been completed by a theoretical investigation of the electronic structure of the PrCo(x) (x = 2, 3 and 5) compounds. Band structure calculations with collinear spin polarization were performed by using the local approximation of the density functional theory scheme implemented in the projector-augmented wave method. The electronic structure of PrCo(3) compound in both directions of spin shows that the majority of occupied states are dominated by the 3d states of Co, with a strong electronic charge transfer from Pr to Co. The PrCo(3) electronic structure can be explained by a superimposition of those of PrCo(2) and PrCo(5), as expected from its crystal structure. The magnetic anisotropy has been confirmed for PrCo(3), as a non-collinear spin calculation with the polarization along the c axis is shown to be more stable than with the polarization in the (a(-->),b(-->)) plane.

An integrated study of thermal treatment effects on the microstructure and magnetic properties of Zn-ferrite nanoparticles.

The evolution of the magnetic state, crystal structure and microstructure parameters of nanocrystalline zinc-ferrite, tuned by thermal annealing of ∼4 nm nanoparticles, was systematically studied by complementary characterization methods. Structural analysis of neutron and synchrotron x-ray radiation data revealed a mixed cation distribution in the nanoparticle samples, with the degree of inversion systematically decreasing from 0.25 in an as-prepared nanocrystalline sample to a non-inverted spinel structure with a normal cation distribution in the bulk counterpart. The results of DC magnetization and Mössbauer spectroscopy experiments indicated a superparamagnetic relaxation in ∼4 nm nanoparticles, albeit with different freezing temperatures T(f) of 27.5 K and 46 K, respectively. The quadrupole splitting parameter decreases with the annealing temperature due to cation redistribution between the tetrahedral and octahedral sites of the spinel structure and the associated defects. DC magnetization measurements indicated the existence of significant interparticle interactions among nanoparticles ('superspins'). Additional confirmation for the presence of interparticle interactions was found from the fit of the T(f)(H) dependence to the AT line, from which a value of the anisotropy constant of K(eff) = 5.6 × 10(5) erg cm(-3) was deduced. Further evidence for strong interparticle interactions was found from AC susceptibility measurements, where the frequency dependence of the freezing temperature T(f)(f) was satisfactory described by both Vogel-Fulcher and dynamic scaling theory, both applicable for interacting systems. The parameters obtained from these fits suggest collective freezing of magnetic moments at T(f).