R. Grössinger

MAGNETOCRYSTALLINE ANISOTROPY OF SM2FE17N2

Abstract The magnetocrystalline anisotropy of Sm2Fe17N2 was investigated from room temperature up to the Curie temperature, Tc. By introducing interstitial N in Sm2Fe17, which has the Th2Zn17 crystal structure, the easy magnetization direction (EMD) changes from the basal plane to the c-axis. The anisotropy field, μ0HA, at temperature is found to be 14 T. The temperature dependence of the anisotropy field was measured up to 700 K by the singular point detection (SPD) method and by magnetization measurements on aligned powder samples. The anisotropy field, HA, decreases monotonically with increasing temperature up to Tc and thus the EMD lies always parallel to the c-axis. The high anisotropy field in combination with the high saturation magnetization and Curie temperature of this material is very promising with respect to its application as a new permanent magnet.

Metal-semiconductor transition in NiFe2O4 nanoparticles due to reverse cationic distribution by impedance spectroscopy

We have investigated the magnetic and electrical response of the sol-gel synthesized NiFe2O4 nanoparticles. Changes in the impedance plane plots with temperature have been discussed and correlated to the microstructure of the material. Thermally activated hopping carriers between Fe3+-Fe2+ and Ni2+-Ni3+ ions have been determined for a decrease in the resistance of the sample and a change in the conduction mechanism around 318 K. The mixed spinel structure and broken exchange bonds due to small size effects are due to the canted spin structure at the surface of the nanoparticles. The magnetization is found to be influenced by the surface spin canting and anisotropy. We have established the semiconducting to metallic transition (SMT) temperature to be around 358 K in terms of localized and delocalized eg electrons along with a transition from less conductive [Fe3+–O2−–Fe3+] and [Ni2+–O2−–Ni2+] linkage to more conductive [Fe3+–Fe2+] and [Ni2+–Ni3+] linkage at the octahedral B site. A decrease in the dielectr...

Temperature dependence of anisotropy fields and initial susceptibilities in R2Fe14B compounds

Abstract The temperature dependence of the anisotropy field was determined in various R 2 Fe 14 B compounds (R = Y, La, Ce, Pr, Nd, Gd, Ho, Lu and Th). The magneto-crystalline anisotropy in materials where the R component has an orbital moment consists of two different contributions that have an opposite temperature dependence. At temperatures well below the Curie temperature the 4f sublattice anisotropy decreases with temperature while the 3d lattice anisotropy increases with temperature. We measured the temperature dependence of the initial susceptibility of all R 2 Fe 14 B compounds mentioned, including the compounds with R = Sm, Dy, Er and Tm in the range from 4.2 to 300 K. Indications for the presence of a spin reorientation were found in Pr 2 Fe 14 B, Nd 2 Fe 14 B and Ho 2 Fe 14 B.

Magnetic properties and thermal stability of Sm2Fe17Nx with intermediate nitrogen concentrations

Abstract The existence of interstitial Sm2Fe17Nx with intermediate nitrogen contents (0 ≤ x ≤ 2.94) is revealed by a continuous increase of the unit cell volume, the Curie temperature, the saturation polarization and the anisotropy field with increasing N concentration. The thermal stability of Sm2Fe17Nx increases also with increasing x. The anisotropy constants K1 and K2 at room temperature were deduced by fitting magnetization curves of oriented powders. The easy magnetization direction changes from the basal plane (x = 0) via an easy cone (δ = 24 ° forx = 0.4) to the c-axis (x ≥ 0.55). The intrinsic magnetic properties of Sm2Fe17N2.94 were found to be Tc = 473 ° C, Js = 1.51 T and HA = 21.0 T where K1 = 8.4 and K2 = 2.1 MJ/m3.

Large enhancement of magnetostriction due to compaction hydrostatic pressure and magnetic annealing in CoFe2O4

Polycrystalline CoFe2O4 was produced by a ceramic method. The heat-treated powder was pressed, varying the hydrostatic pressure between 87 and 278 MPa, and was heat-treated again at 1350 °C for 24 h. All magnetic parameters showed a clear dependence on this hydrostatic pressure. The saturation magnetization showed a minimum, and the coercivity, the anisotropy, and the magnetostriction showed a maximum at compaction pressures around 150 MPa. This pressure dependence of the magnetic parameters can be explained by a cation redistribution due to the hydrostatic pressure and heat treatment. Additionally, all samples were field-annealed in an external field of 10 T (at 300 °C for 3 h). The field-annealing process causes an induced uniaxial anisotropy, which results in a reduction of the coercivity (in the easy axis) as well as a dramatic increase in the magnitude of the magnetostriction along the hard axis. Maximum magnetostriction value of -400×10-6 was obtained. Additionally, dλ/dH is increased within a facto...

Structural and intrinsic magnetic properties of Sm2(Fe1−xCox)17Ny

Formation and magnetic properties of the interstitial Th2Zn17-type nitrides were investigated over the whole concentration range for Sm2(Fe1−xCox)17. The decomposition temperature, Td, and the nitrogenation kinetics decrease with increasing Co content x. The substitution of Co for Fe, for x < 0.2, leads to an overall improvement of the intrinsic magnetic properties of Sm2Fe17Ny. The Curie temperature, the room temperature saturation polarization and the anisotropy field show a maximum in dependence on composition for x = 0.5 (TC = 901 K), x = 0.15 (Js = 1.55 T) and x = 0.3 (HA = 25.0 T), respectively. For Sm2Co17, nitrogenation considerably decreases the Curie temperature and the saturation polarization whereas the anisotropy field is almost doubled. For x = 0.2 the compound has excellent intrinsic magnetic properties for hard magnetic applications: TC = 842 K, Js = 1.55 T and HA = 23.7 T at room temperature.

Pulsed field magnetometry

The merits and limitations of a pulsed field system for the characterization of permanent magnets are discussed. The most significant limitation is the effect of eddy currents which circulate in electrically conductive magnets. To minimize the errors due to eddy current flow, a correction strategy is presented and supported by simulations. >

Colossal resistivity with diminished tangent loss in Zn?Ni ferrite nanoparticles

We have investigated the electrical and magnetic response of the sol‐gel synthesized ZnxNi1−xFe2O4 (x = 0.0, 0.5 and 1) nanoparticles. The ratio of A-site sextet intensity to that of B-site sextet is featured in terms of divergence in coordination of Fe 3+ ions from four-fold (A-site) to six-fold (B-site). Canted spin structure and weakening of Fe 3+ (A)‐Fe 3+ (B) interactions at the surface of the nanoparticles assign the reduced value of room temperature magnetization in these nanoparticles. Shift of the blocking temperature with Zn content is ascribed to the change in the magnetic anisotropy. Colossal resistivity and reduced dielectric constant are discussed on the basis of dangling bond, superparamagnetic character, canted spin structure and polarizability of the cations. Diminished tangent loss is stipulated in terms of decrease in magnetocrystalline anisotropy and collapse of long-range magnetic order. We report colossal resistivity (i.e. 3.15 × 10 9 � cm), reduced dielectric constant (3.97) and diminished tangent loss (0.07) for Ni0.5Zn0.5Fe2O4 nanoparticles. (Some figures in this article are in colour only in the electronic version)

Differential T-cell responses and allergen uptake after exposure of dendritic cells to the birch pollen allergens Bet v 1.0101, Bet v 1.0401 and Bet v 1.1001

The major birch pollen allergen Bet v 1 is present in pollen as a mixture of at least 14 isoforms that share high sequence and structural identities. These isoforms possess either a high or a low IgE-binding capacity which defines them as allergenic or hypoallergenic. Recently, we could demonstrate that only the allergenic isoform Bet v 1.0101 was able to induce an IgE response in birch pollen allergic individuals. The hypoallergenic isoforms Bet v 1.0401 and Bet v 1.1001 were unable to induce IgE synthesis. T-helper cell responses against allergens are characterised by increased levels of Th2 cytokines. Therefore, we examined extent and polarisation of the Th cell response and the kinetics of the allergen uptake after exposure of dendritic cells (DCs) to these isoforms. Monocyte-derived DCs (MDDCs) from birch pollen allergic and non-atopic individuals stimulated with Bet v 1.0101, Bet v 1.0401 or Bet v 1.1001 in combination with the maturation factors TNF-α and IL-1β resulted in a mature DC phenotype as measured by costimulatory molecule up-regulation. Only Bet v 1.0101-stimulated MDDCs from allergic subjects enhanced proliferation of autologous Th cells and the expression of the Th2 cytokines IL-5 and IL-13. Immature MDDCs of allergic individuals internalised equivalent amounts of the allergenic Bet v 1.0101 and the hypoallergenic Bet v 1.0401. In contrast, the uptake of the hypoallergenic Bet v 1.0401 by immature MDDCs of non-atopic individuals was significantly higher. These results provide evidence that DCs discriminate between allergens and highly related hypoallergens. This process may have an impact on the early phase of sensitisation.

Optical and neutron domain structure studies of amorphous ribbons

Abstract The domain configuration of amorphous ribbons with positive, vanishing and negative magnetostriction constant λ s has been investigated by means of the longitudinal magneto-optical Kerr-effect and a three-dimensional neutron depolarization analysis. The influence of an external tensile stress on the domain arrangement depends on the sign and the magnitude of the magnetostriction constant and could be measured quantitatively in the depolarization experiment. With the aid of the optical observations domain models have been established; these models were tested and completed by means of an analytic calculation of the corresponding depolarization matrix and a comparison with experimental data. Domain reorientation processes within the ribbons under an imposed tensile stress were found to agree with theoretical predictions. The nearly nonmagnetostrictive alloys shows neither stripe domains nor simple wide domains but a more complicated structure with several layers of domains.