Abstract The effects of wet-milling mixtures of α-Fe2O3 and cobalt hydroxide Co(OH)2 over a range of Co/Fe ratios for 215 h have been investigated by neutron diffraction and Mössbauer spectroscopy. The starting materials were mixed according to the stoichiometric formula (CoxFe1 –x)3O4 for values of x = 0.037, 0.071, 0.133, 0.234 and 0.380 (i. e., from ≈ Co0.1Fe2.9O4 to the cobalt spinel CoFe2O4). These studies reveal the formation of a nanostructured, mixed Co–Fe spinel phase with non-stoichiometric composition (CoxFe1– x)3 –yO4; the defect spinels have refined values xc = 0.04, 0.08 and 0.14 for the mixtures with the lowest Co content (x = 0.037, 0.071 and 0.133) and defect concentrations in the range y ≈ 0.1 –0.2. Both the spinel phase and un-reacted α-Fe2O3 are found to occur in the neutron diffraction patterns and Mössbauer spectra for the high Co content mixtures x = 0.234 and x = 0.380. Rietveld refinements of the neutron data indicate that the Co atoms predominantly occupy the octahedral B sites with vacancies also found to be located on the octahedral B sites. Analyses of the Mössbauer spectra of the milled samples confirm the existence of vacancy defects in the B sites and reveal that the vacancies cause similar effects to those of the Co ions, leading to a higher average charge state per iron atom.

Abstract The effects of wet-milling mixtures of α-Fe2O3 and cobalt hydroxide Co(OH)2 over a range of Co/Fe ratios for 215 h have been investigated by neutron diffraction and Mössbauer spectroscopy. The starting materials were mixed according to the stoichiometric formula (Co x Fe1 –x )3O4 for values of x = 0.037, 0.071, 0.133, 0.234 and 0.380 (i. e., from ≈ Co0.1Fe2.9O4 to the cobalt spinel CoFe2O4). These studies reveal the formation of a nanostructured, mixed Co–Fe spinel phase with non-stoichiometric composition (Co x Fe1– x )3 –y O4; the defect spinels have refined values x c = 0.04, 0.08 and 0.14 for the mixtures with the lowest Co content (x = 0.037, 0.071 and 0.133) and defect concentrations in the range y ≈ 0.1 –0.2. Both the spinel phase and un-reacted α-Fe2O3 are found to occur in the neutron diffraction patterns and Mössbauer spectra for the high Co content mixtures x = 0.234 and x = 0.380. Rietveld refinements of the neutron data indicate that the Co atoms predominantly occupy the octahedral B sites with vacancies also found to be located on the octahedral B sites. Analyses of the Mössbauer spectra of the milled samples confirm the existence of vacancy defects in the B sites and reveal that the vacancies cause similar effects to those of the Co ions, leading to a higher average charge state per iron atom.

Melt-spun Fe10Cu90 shows a negative magnetoresistance with a nearly linear dependence on external field. The microstructure has Fe-atoms on fee-sites, corresponding to CuFe solid solution, and in bcc alpha-Fe precipitates. Magnetic data indicate the presence of magnetic couplings between magnetic clusters. It is argued that both microstructures, corresponding to the two locations of Fe, yield a mechanism for magnetoresistance. The resistivity in zero field shows a minimum at a temperature above the magnetic freezing or ordering temperature determined as the peak of ac-susceptibility. Glassy dynamics is seen from a creep of the resistivity after cooling below the freezing temperature. A simple micromagnetic model is applied to describe the increase of magnetoresistance with the amount of precipitated alpha-Fe.

The structural and magnetic behaviour of BaFe12O19 subjected to milling in vacuum for 1000 h has been investigated by x-ray powder diffraction and Mössbauer effect spectroscopy techniques. Pronounced structural disorder is obtained along with partial decomposition of BaFe12O19 to α-Fe2O3 and evidence for superparamagnetic relaxation effects due to the fine particles produced on milling. Restoration of the fully crystallised BaFe12O19 structure on annealing at 1000 °C is accompanied by a six fold enhancement in the magnetic coercivity. This behaviour is linked with the fine crystallites.

Analysis of the room temperature and 4.2 K Mossbauer spectra of a quaternary Nd-Co-Si-B phase doped with Fe-57 is presented. A predominant single line is observed at room temperature indicating a high symmetry Co sublattice. This has led to a model for the structure of the phase based on a f.c.c.-like symmetry sublattice. The analysis indicates that the single line is the result of a preferred occupancy of the Fe-57 atoms in high symmetry 2c sites in the structure. The Fe-57 doped compound has a magnetic ordering temperature of T-c approximate to 170 K as determined by a thermal scan of the Mossbauer resonance with a predominant magnetic hyperfine field of (B-hf) approximate to 28 T and electric quadrupole interaction approximate to 0 mm/s at 4.2 K. The site location of the remaining Fe-57 atoms which are identified by a quadrupole doublet (area approximate to 20%, QS approximate to 0.7 mm/s) at room temperature and a corresponding magnetically split subspectrum of (B-hf) approximate to 15 T at 4.2 K is also discussed.

Ferromagnetic Fe86Zr7B6Cu1 alloys have been prepared by both mechanical alloying of elemental powders and rapid quenching. The mechanically alloyed powders are magnetically harder than their rapidly quenched counterparts. The fractions of the main phases-an alpha-Fe type phase, an amorphous phase and a grain boundary region-present in the as prepared and the heat treated materials have been estimated from the spectral components in their room temperature Mossbauer spectra. The magnetic hardening of the mechanically alloyed material is likely to be due to a number of effects including: an inhomogeneous microstructure; distortions in the grain boundary region, and inhomogeneous long-range stress as well as microstrain at the atomic level.

Prolonged milling of SrFe12O19 in air and vacuum results in mixtures of nanostructured iron oxides. The hexagonal ferrite structure is altered to form mixtures of nanoparticles of alpha-Fe2O3 and vacancy defected Fe3-xO4. Mechanical activation in air for 800 h results in the formation of similar to 10 nm alpha-Fe2O3 particles with both similar to 10 nm alpha-Fe2O3 (similar to 56%) and similar to 3 nm Fe3-xO4 particles (similar to 44%) being obtained in the vacuum milled state. The vacancy defected magnetite particles are likely to form as a result of mechanochemical reduction of alpha-Fe2O3 in the oxygen free environment. The range of nanoparticle sizes obtained on milling is reflected by the relaxation effects observed at room temperature and 4.2 K in the Mossbauer spectra of the milled products. A spin glass-like irreversibility in magnetisation is observed below similar to 150 K.

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The novel quaternary rare-earth transition-metal intermetallic compound Nd3Ni29Si4B10 of space group P4/nmm (No. 129) has been synthesised and its structure determined. The crystal structure has been investigated by scanning electron microscopy, X-ray and neutron powder diffraction measurements. Rietveld refinements of the powder X-ray and neutron diffraction patterns reveal a tetragonal structure of lattice parameters a=b=11.2327(7) Å, c=7.8754(3) Å with two formula units of Nd3Ni29Si4B10 per unit cell. The structure contains two rare-earth crystallographic sites of high symmetry, four B sites, seven transition metal sites and one Si site. The Ni atoms at the 2c 4 mm crystallographic site have a quasi-spherical polyhedral coordination.

The magnetic behaviour of a novel quaternary rare-earth compound Nd3Co29Si4B10 has been investigated by AC magnetic susceptibility and Fe-57 Mossbauer effect spectroscopy measurements. The Nd3Co29Si4B10 compound exhibits a magnetic transition around T-c1 similar to 200 K with a second magnetic transition occurring around T-c2 similar to 43 K. Addition of similar to 0.5 wt% Fe-57 is found to decrease the magnetic ordering temperatures to T-c1 similar to 167 K and T-c2 similar to 38 K. Similar to other Co-rich, light rare-earth compounds, Nd3Co29Si4B10 is likely to exhibit ferromagnetic ordering below T-c1. (C) 1998 Elsevier Science B.V. All rights reserved.

High-resolution synchrotron-radiation powder data have been obtained for a new quaternary Nd–Ni–B–Si compound with the nominal composition of NdNi8SiB3. The data were collected on a (573 mm radius) cylindrical cassette diffractometer using imaging plates. The crystal structure of the new compound has been determined and refined by Rietveld analysis. The initial structural model and space group for the refinement is based on insight derived from earlier Mössbauer and TEM analyses. Refinement of the powder data leads to a tetragonal unit-cell with lattice parameters a=11.2260(2) Å and c=7.8731(2) Å in the space group P4/nmm. The refined compound is found to have a composition of NdNi9.7Si1.3B∼4. The atomic coordinates for the Nd, Ni and Si atoms have been determined. Small amounts of the impurity phases, NdNi5 and Nd3Ni13B2, have also been determined and refined in the analysis.

The effects of milling SrFe12O19 in air and vacuum for 800 h have been investigated by X-ray diffraction and Mössbauer effect spectroscopy measurements. Different levels of structural disorder along with partial decomposition of SrFe12O19 to nanocrystalline grains of haematite, α-Fe2O3, and magnetite, Fe3O4, are obtained for the air and vacuum milling. Superparamagnetic relaxation effects due to the fine particles produced on milling are observed for a significant fraction (∼ 10% air milled, ∼ 30% vacuum milled) of the milled samples.

The quaternary compound NdNi8SiB3 has been synthesised by arc melting. The structure and magnetic behavior were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and 57Fe Mössbauer spectroscopy. The XRD study indicates that the dominant phase for the system can be indexed based on a tetragonal structure with the lattice parameters a = 11.137(5) Å, and c = 7.930(5) Å, respectively. Selected area diffraction (SAD) information from TEM analysis further confirms the above proposal. The Mössbauer spectrum indicates that the system exhibits paramagnetism at room temperature, and the 57Fe atoms preferentially occupy the local cubic symmetric positions.

The effects of dry-milling BaFe12O19 in air for periods of 190, 360, 590, 690 and 1000 h have been investigated by X-ray diffraction and Mössbauer effect measurements. The sizes of the BaFe12O19 particles decrease on milling, as expected, although a partial decomposition of BaFe12O19 to α-Fe2O3 is found to take place on extended milling (1000 h). The room temperature Mössbauer spectra are consistent with superparamagnetic relaxation associated with the fine BaFe12O19 and α-Fe2O3 particles. The X-ray diffraction patterns of all the milled samples exhibit features indicative of a disordered structural state, consistent with the nanoscale particles and a nanostructured state.

The effects of wet-milling barium ferrite in the presence of cationic or anionic surface active substances have been investigated by x-ray diffraction and Mossbauer spectroscopy measurements. Evidence for superparamagnetic relaxation of fine particles, both in the surfactant assisted ball-milled samples and in samples dry-milled in air is obtained from the room temperature and 4.2 K Mossbauer spectra. The similarity between samples milled at pH 3 with the cationic or anionic surfactants is due to electrostatic interactions of the surfactants' hydrocarbon groups with the positively charged barium ferrite surface.

Mössbauer spectra of LaCo12B6 doped with57Fe have been obtained at various temperatures between 4.2 and 293 K. The observed splitting of the subspectrum associated with the 18h site belowTC indicates that the Co sublattice of LaCo2B6 remains ordered in the crystallographic basal plane, with no discernible change in magnetic ordering direction belowTC, contrary to the recent suggestion of a change in magnetic states based on NMR data.