P. Wolfers

Neutron diffraction studies of some hexagonal ferrites: BaFe12O19, BaMg2W and BaCo2W

Abstract Neutron diffraction studies on powder samples have been carried out on the BaFe 12 O 19 (M-type), BaMg 2 Fe 16 O 27 and BaCo 2 Fe 16 O 27 (W-type) room-temperature ferrimagnetic hexagonal ferrites. From high temperature data, above the Curie temperature ( ≈ 450–500° C), we first determine the Mg 2+ and Co 2+ location among the seven sublattices of the W-type structure; these M 2+ cations lie essentially in the octahedral and tetrahedral sites of the spinel S-blocks. From low temperature (4.2 K) data, we confirm, for the three compounds, the collinear Gorter model; the spin configuration is along the hexagonal c axis in the BaFe 12 O 19 and BaMg 2 W compound whereas it is a quasi-planar one in the BaCo 2 W compound, the angle of the magnetic moments with the c axis being 68.5°. The magnetic moments on each site of the M- and W-type structures are given and we compare the spontaneous magnetizations with the saturation ones.

Mossbauer spectroscopy of R2Fe14B

The authors present results obtained by 57Fe Mossbauer spectroscopy on R2Fe14B, where R represents any of the rare earth elements except Pm, Eu and Yb. Additional measurements have been performed on oriented powder for R=Y, Pr and Ho. For the case of Y the measurements at 4.2 K were performed in applied magnetic fields up to 4 T. From these measurements it is deduced that the only compounds with an easy c axis at room temperature which show a spin reorientation of the iron sublattice at a lower temperature are the Nd and Ho alloys. Iron hyperfine fields are anisotropic and the anisotropy has different signs on different sites. An analysis of the non-4f contribution to the electric field gradient at the 161Dy or 166Er nuclei in Dy2Fe14B and Er2Fe14B is consistent with published results on 155Gd in Gd2Fe14B. In order to explain that a spin rotation is found only for R=Nd, Ho the authors invoke the relative small quadrupole moment of the 4f shell for these two ions and crystal field terms of higher than second order.

Structural and magnetic properties of RE2Fe14BH(D)x; REY, Ce, Er☆

Abstract The structural and magnetic characteristics of RE 2 Fe 14 BH x hydrides with RE ≡ Y, Ce, Er have been studied as a function of x . Both Curie temperature and magnetization are increased on hydriding the ternary bondes. Neutron diffraction experiments performed on deuterated samples have permitted the location of the host sites and the measurement of the magnetic moments of iron and RE. Different anomalies reported in the structural and magnetic behaviour of Ce 2 Fe 14 BH x are related to the change of electronic state on cerium.

The magnetic anisotropy change of BaFe12−2xIrxCoxO19: a single-crystal neutron diffraction study of the accompanying atomic and magnetic structures

Abstract In this paper, we present the atomic and magnetic structures accompanying the change, from axial to planar, of the magnetic anisotropy in BaFe 12−2 x Ir x Co x O 19 using magnetic measurements and single-crystal neutron diffraction. The magnetic measurements allowed us to determine the characteristic anisotropy field H θ and anisotropy constants k i for the different stages. Our structure refinements show that the change of magnetic anisotropy in BaFe 12−2 x Ir x Co x O 19 leads, on a microscopic level, to a collective canting of the moments from 0 to 90° through ferrimagnetic structures, which is in contrast to what has been observed up to now for other substituted single-crystal hexaferrites. The peculiar change of the magnetic anisotropy in BaFe 12−2 x Ir x Co x O 19 is correlated to the substitution of Fe 3+ by Ir 4+ on the 4e bipyramidal site.

Magnetization, 57Fe and 161Dy Mössbauer study of Dy2Fe14BHx with 0 ⩽ x ⩽ 4.7

Abstract From the hydrogen dependence of Mossbauer hyperfine parameters in Dy2Fe14BHx and neutron diffraction results on Y2Fe14BD3.5 we infer the hydrogen localization as a function of hydrogen concentration in Dy2Fe14BHx. Our 161Dy Mossbauer spectroscopy data recorded at 4.2 K put constraints on the magnetic properties of the heavy rare-earth (RE) ions in RE2Fe14BHx. A combination of results from 57Fe Mossbauer spectroscopy and bulk magnetization measurements allows us to determine the magnetic structure of the compounds.

Crystal structure and magnesium location in the W-type hexagonal Ferrite: [Ba]Mg2-W

Abstract The crystal structure of BaMg 2 Fe 16 O 27 W-type hexagonal ferrite has been refined from X-ray single crystal data. This compound is hexagonal: a = 5.9060(7) A and c = 32.915(7) A, space group P6 3 /mmc, with two formula units per cell. The structure can be described by the sequence ⋯ R S SR∗ S ∗S∗ ⋯, where R and S are the structural blocks already met in the hexaferrite M-structure. Moreover, the location of the Mg 2+ cation among the seven sublattices is given: this cation lies essentially in the octahedral and tetrahedral sites of the spinel S-blocks.

A new series of materials for permanent magnets: The W ferrites BaZn2(1−x)(LiFe)xFe16O27

Abstract The possibility of improving the W-type hexagonal ferrite BaZn 2 -W in order to use it as a hard material for permanent magnets will be demonstrated through the partial substitution of Zn 2+ cations according to the scheme 2 Zn 2+ ↔ Li + + Fe 3+ Such a partial substitution leads to a new series of compounds with W-type structure whose magnetic specifications are just as good as those of the previously known Zn 2 -W or Fe 2 -W ferrites. Furthermore, synthesis conditions are less stringent to atmosphere or calcining times. The new phases obtained by a combination of monovalent, bivalent and trivalent cations lead to products suitable to be used as permanent magnet materials (80 uem/g at room temperature with H a = 12 kOe and T C =430°C or 75 uem/g at RT with H a =11.5 kOe and T C =518°C). We propose a model for the distribution of the lithium through the different sublattices which leads to a good agreement between the observed magnetization at 4.2 K and the value we calculated using collinear Gorters model.

Electronic, magnetic structures and neutron diffraction in B1 and B3 phases of MnS: a density functional approach

Abstract The linear combination of atomic orbitals (LCAO) method implemented in the C rystal program for the study of periodic systems has been used to obtain wave functions, charge and spin densities of the most stable antiferromagnetic orderings of the MnS B 1 and B 3 polymorphs. The calculations have been led to the density functional (DF) level of theory by solving the Kohn–Sham equations self-consistently. One local and one gradient exchange-correlation (XC) potentials have been studied for comparison with the Hartree–Fock (HF) results. Electron charge and spin densities have been calculated and the neutron diffraction of the two polymorphic forms of MnS deduced. Comparison of our results with the experimental data is discussed.

Magnetism of Fe2P investigated by neutron experiments and band structure calculations

Abstract Neutron diffraction and magnetisation experiments have been performed on powdered Fe 2 P. The electronic band structure of Fe 2 P and the previously measured FeMnP 1- x As x has been computed. The fully self-consistent KKR-CPA method was used in all calculations. The magnetic structure of Fe 2 P was refined with the new neutron data and ab initio calculated magnetic form factors.

Crystallographic and magnetic properties of fe2p

Summary The crystallographic and magnetic properties of Fe 2 P have been studied by neutron diffraction experiments, magnetization measurements and KKR method for band structure calculations. The measured atomic magnetic moments of tetrahedral Fe(3f) and pyramidal Fe(3g) sites are respectively (0.59±0.02)μ B and (2.22±0.01)μ B , resulting in a net moment of (2.82±0.03)μ B . This value is very close to that obtained from saturation magnetization measurements (H=150kOe, T=4K) which yield 2.87μ B per formula unit. The results obtained from the KKR method are 0.80, 2.33, −0.03(1), −0.02(8)μ B for Fe and P sites respectively. The values are comparable with the measured atomic magnetic moments. Thermal evolution of magnetic moments, magnetization at saturation and cell parameters presents a sharp drop at the Curie temperature (217±2)K indicating a first order ferro - paramagnetic transition.