G. Wiesinger

Sublattice occupation in Sr1−xLaxFe12−xCoxO19 hexagonal ferrite analyzed by Mössbauer spectrometry and Raman spectroscopy

Abstract The cationic distribution of Co 2+ in Sr 1− x La x Fe 12− x Co x O 19 hexagonal ferrite is investigated by Mossbauer spectrometry and Raman spectroscopy. The two complementary techniques lead to the same conclusion: the majority of the Co 2+ is substituted for Fe 3+ in the octahedral 4f 2 and 2a sites, in agreement with the magnetization measurements. A valence change of some Fe 3+ to Fe 2+ in the 2a site is also evidenced.

Volume magnetostriction and structure of copper mold-cast polycrystalline Fe-Ga alloys

The effect of the volume expansion on the total linear magnetostriction of Fe85Ga15, Fe80Ga20, and Fe71Ga29 mold-cast bulk alloys was investigated by measuring the change in length with applied field longitudinal and perpendicular to the temperature gradient during solidification. In the magnetically saturated state, due to the contribution of the volume expansion, the saturation magnetostriction for all three alloys was about 66 % of the total linear magnetostriction. The magnetostriction is strongly dependent on the direction of the temperature gradient, being larger in this direction. The substitution of Fe by Ga atoms increases the lattice constant and causes a change to the A2 crystal structure, which was confirmed by Mossbauer spectroscopy. The thermal-expansion coefficient increases with Ga content at temperatures between 4.2 and 150K.

Mossbauer investigation of Sr-Fe-O hexaferrites with La-Co addition

M-type Sr/sub 1-x/La/sub x/Fe/sub 12-x/Co/sub x/O/sub 19/ ferrite powders with x=0 to 0.4 were investigated using Mossbauer spectrometry in the 77 K-550 K temperature range. The Mossbauer spectra were fitted by assuming that Co/sup 2+/ is present in both 4f/sub 2/ and 2a octahedral sites, and that some Fe/sup 2+/ is present in the 2a site. On increasing x, the main changes concern the Mossbauer contributions of the 12k, 4f/sub 2/ and 2b sites. These changes are interpreted in relation with the Co/sup 2+/-Fe/sup 3+/ substitution at the 4f/sub 2/ site, the Fe/sup 3+/-Fe/sup 2+/ valence change at the 2a site and the La/sup 3+/-Sr/sup 2+/ substitution. The temperature dependence of the hyperfine field at the Fe sites of the Sr/sub 0.6/La/sub 0.4/Fe/sub 11.6/Co/sub 0.4/O/sub 19/ compound is similar to that of the unsubstituted compound. However, the curvature of the temperature dependence of the hyperfine field at the 12k site slightly increases with x. This reflects the increase of the curvature of the temperature dependence of the magnetization of the substituted compounds.

Substituted Ferrites Studied by Nuclear Methods

We report on recent investigations on substituted hexaferrites of the type SrM, Sr 1-x La x Fe 12-x Co x O 19 (0 ≤ x ≤ 0.4). It was already proved that the simultaneous substitution of Sr and Fe by La and Co, respectively, significantly improves the permanent magnetic properties of the material. In order to get a deeper insight into the magetic order from an atomistic point of view, 57 Fe Mossbauer spectroscopy and NMR ( 57 Fe, 59 Co) studies were carried out. It is demonstrated that by applying local probe hyperfine techniques the knowledge about this complex material can be considerably improved. The main results concern the preferred Co 2+ occupation on the 4f 2 and the 2a sites and the occurrence of Fe 2+ ions in the substituted material.

Magnetic and anisotropy studies of Nd-Fe-B based permanent magnets

Abstract Magnetocrystalline anisotropy and coercivity of sintered and rapidly solidified Nd-Fe-B magnets are compared. The coercivity for melt-spun ribbons with optimum cooling rate appears to be dominated by domain wall pinning, while for sintered Nd-Fe-B the coercivity is mainly controlled by nucleation of reverse domains.

Magnetic properties of a new family of rare-earth substituted ferrites

The magnetic properties of substituted hard-magnetic M-type ferrites are surveyed. A combined La–Co substitution leads to magnets of compositions Ba1−xLaxFe12−xCoxO19 and Sr1−xLaxFe12−xCoxO19 (x= 0, 0.1, 0.2, 0.3, 0.4). Due to this substitution, the anisotropy as well as the coercivity increases. The effect of the ion size was studied on the system (Sr1−yCay)1−xLaxFe12−x Co0.75xFe2+0.25xO19 (x= 0.2, 0.4; y= 0, 0.33, 0.66, 1). Additionally, Sm substitution which leads to a strong increase of the coercivity was studied, but the effect on the anisotropy is small (1%).

Structural and magnetic properties of RFe2Dx deuterides (R = Zr, Y and x ≥ 3.5) studied by means of neutron diffraction and 57Fe Mössbauer spectroscopy

The structure of Y Fe2D3.5 at 290 K has been refined in a monoclinic superstructure using synchrotron radiation and neutron diffraction experiments. The evolution of the 57Fe Mossbauer spectra versus temperature for ZrFe2Dx and Y Fe2Dx deuterides with large deuterium content has been studied. For x = 3.5 both ZrFe2Dx and Y Fe2Dx show an increase of the Fe magnetization compared to the parent intermetallic compound, related to the increase of the cell volume. For Y Fe2Dx the Fe moment decreases to complete disappearance as x increases from 3.5 to 5 due to the stronger influence of the Fe–D bonding compared to the volume increase. The evolution of the isomer shift can be attributed to an almost pure volumetric effect in the case of the Y Fe2 deuterides, whereas for ZrFe2D3.5 additional D–Fe charge transfer has to be taken into account.

Complete spin crossover in tris(pyridylbenzimidazole) iron(II)

Abstract The magnetic properties of a hexacoordinated iron(II) complex with the bidentate ligand pyridylbenzimidazole (pybzim), [Fe(pybzim)3](ClO4)2 · H2O, (1), have been investigated by AC susceptometry, 57Fe-Mossbauer, ESR and NMR spectroscopy. The temperature dependence of the AC magnetic susceptibility for a powder sample showed essentially diamagnetic behaviour below 85 K and paramagnetism (μeff/μB = 4.5−5.3, S = 2) above 160 K, obeying the Curie-Weiss law. Spin crossover behaviour can be found within the temperature range 90–160 K; it is centered at 141 K. The Mossbauer spectra confirm the susceptibility measurements: the area fraction of the high-spin quadrupole doublet plotted against temperature shows a curve similar to that deduced from susceptibility measurements. The complex is ESR silent in the X-band mode: neither the powder sample (77 K and 300 K) nor a methanolic solution exhibit an ESR signal. NMR measurements in methanol, acetone, nitromethane, acetonitrile and dimethylsulfoxide show an increasing magnetic moment μeff/μB from 1.6 (220 K, acetone) to 5.2 (325 K, DMSO), respectively. The spin-crossover transition of a freshly prepared powder sample is complete. However, time evolution causes (within months) an increasing admixture of iron(III), which was proved by magnetic susceptibility measurement, 57Fe-Mossbauer, ESR and electronic spectra of the complex.

Rare earth substitutions in M-type ferrites

In this paper, the effects of a partial substitution of Sr by either Sm or Nd but also by La in SrFe/sub 12/O/sub 19/ on the fundamental magnetic properties is investigated. Hysteresis loop measurements were performed using a pulsed field magnetometer. The anisotropy fields of the samples was measured using the SPD method.

Temperature and concentration dependence of magnetization, magnetocrystalline anisotropy and hyperfine parameters in Zr(Fe1−xAlx)2

Abstract Hyperfine interactions and bulk magnetic properties have been investigated in the system Zr(Fe1−xAlx)2 as a function of concentration, temperature and stoichiometry. The similarity of both the concentration and the temperature dependence of the magnetization is pointed out. The influence of clusters, short range order and magnetocrystalline anisotropy on the magnetization process will be discussed. From the temperature dependence of the hyperfine field and the magnetization spin wave contributions are calculated. The applicability of the model of itinerant electron magnetism will be compared with the formation of local moments.