J. Wecker

High coercivity in Sm2Fe17Nx magnets

Using mechanical alloying and a subsequent two‐step heat treatment we produced magnetically isotropic microcrystalline Sm2Fe17Nx samples with room‐temperature coercivities up to 24 kA/cm (30 kOe). The remanence and the energy product are equivalent to similarly prepared Nd‐Fe‐B samples, but the properties at elevated temperatures are superior because of the high Curie temperature of 470 °C and the large anisotropy field of 14 T at room temperature. From differential scanning calorimetry it is concluded that the 2:17 nitride is metastable. It decomposes into Sm nitride and α‐Fe above 600 °C.

Formation and properties of NdFeB prepared by mechanical alloying and solid‐state reaction

Magnetically isotropic Nd15Fe77B8 powder is prepared by mechanical alloying and a solid‐state reaction from the elemental powders. Due to the extremely short reaction time (for example, 10 min at 700 °C) the magnetically isotropic particles have a very fine microstructure comparable to rapidly quenched samples, show a pinning‐type behavior and possess excellent magnetic values such as IHC up to 13 kOe and BHmax up to 12.8 MGOe. Details on the development of the microstructure during milling, the reaction kinetics, and the magnetic properties are reported.

Structural and hard magnetic properties of rapidly solidified Sm–Fe–N

Sm–Fe alloys have been produced by rapid quenching and the resulting phases have been investigated in the as‐quenched state and after nitriding. Besides the well‐known equilibrium phases of the binary Sm–Fe system (Fe, Sm2Fe17, SmFe3, SmFe2 and Sm), a hexagonal TbCu7‐type phase shows up in melt spun ribbons (a=4.88 A, c=4.23 A). Its stoichiometry is about Sm1Fe9 and it is formed only at wheel velocities above 15 m/s. The Curie temperature and the saturation polarization of this new phase is 210 °C and 1.25 T, respectively. At higher Sm concentrations or lower quenching rates the structure changes to the Th2Zn17‐type. The Th2Zn17‐type ribbons are magnetically soft whereas the TbCu7‐type samples show moderate coercivities of up to 1.7 kA/cm. Nitrogenation leads to an expansion of the lattice and to an overall improvement of the hard magnetic properties for both phases. Their Curie temperatures are increased to 470 °C and the saturation polarizations are raised to 1.40 and 1.51 T for the TbCu7‐ and the Th2Zn...

Giant magnetoresistance in melt spun Cu‐Co alloys

Cu1−xCox alloys with x=0.1 and x=0.2 have been prepared by conventional melt spinning. The rapid solidification process results in an extended solubility of Co in Cu although some Co precipitates already during quenching. In the as‐quenched ribbons, the magnetoresistance (MR) is only of the order of 1.5%. It increases dramatically with the controlled nucleation and growth of Co precipitates from the supersaturated Cu matrix. The highest MR of 11% at 300 K occurs for Cu90Co10 after an aging at about 440 °C when the Co clusters are superparamagnetic. Saturation is possible only after a higher annealing or at lower measuring temperatures. For the optimally annealed samples the MR increases to 36% at 30 K.

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.

Permanent magnets by mechanical alloying (invited)

Mechanical alloying is applied to prepare Nd‐Fe‐B, Sm‐Fe‐TM type (TM: V, Ti, Zr), and interstitial nitride and carbide permanent magnets. Starting from elemental powders, the hard magnetic phases are formed by milling in a planetary ball mill and a following solid‐state reaction at relatively low temperatures. For Nd‐Fe‐B, the magnetically isotropic particles are microcrystalline, show a high coercivity (up to 16 kA/cm for ternary alloys and above for Dy‐substituted samples), and can be either used for making bonded magnets or compacted to dense isotropic magnets by hot uniaxial pressing. Magnetically anisotropic samples with a remanence up to 1.31 T and an energy product up to 326 kJ/m3 are formed by die upsetting. The mechanical alloying process has also been applied to prepare magnetic material of three new Sm‐Fe‐TM phases: Sm‐Fe‐V with the ThMn12 structure, Sm‐Fe‐Zr with the PuNi3 structure, and Sm‐Fe‐Ti with the A2 structure. They all show high or ultrahigh coercivities (up to 51.6 kA/cm for Sm‐Fe‐Ti...

GMR angle detector with an artificial antiferromagnetic subsystem (AAF)

Abstract A magnetoresistive GMR sensor scheme for detection of angular positions is demonstrated and analyzed in which the hard magnetic layers are replaced by artificial antiferromagnetic subsystems (AAFs). These consist of ferromagnetic layers (Co) that are antiferromagnetically coupled via interlayers (Cu or Ru). The rigidity of this AAF is improved by an order of magnitude as compared to the individual Co layers. Operational field windows for 360° angle detectors of 15–20 kA/m have been realized. The maximal sensor signal Δϱ/ϱ is 6%. The temperature operation range extends up to 150°C. The angle resolution is about 1°.

Intrinsic giant magnetoresistance of mixed valence La‐A‐Mn oxide (A=Ca,Sr,Ba) (invited)

A large intrinsic magnetoresistance has been found near the ferromagnetic transition of metallic manganese oxides with perovskite‐type crystal structure. The magnetic and transport properties were measured on bulk and thin‐film La1−xAxMnO3+δ with A=Ca,Sr,Ba. Assuming the double‐exchange model proposed by Zener [Phys. Rev. 81, 440 (1951); 82, 403 (1951)], the strong dependence of the transport properties on the magnetic field and also on the chemical composition is attributed to the mixed Mn3+/Mn4+ valence.

Orientation relationships of epitaxial oxide buffer layers on silicon (100) for high‐temperature superconducting YBa2Cu3O7−x films

The preparation parameters of epitaxially grown buffer layers on silicon (100) wafers were investigated. We found that an in situ removal of the native amorphous SiO2 layer from the Si surface is possible, avoiding the etching of the wafer prior to the deposition. YSZ and Y2O3 were chosen as buffer layers for subsequent YBa2Cu3O7−x thin‐film deposition. The orientation of the thin films during the deposition process was analyzed by RHEED. Different orientations on the substrates are obtained depending on the evaporation parameters. TEM studies of the interfaces, x‐ray diffraction analysis, and measurements of the superconducting properties were made after the deposition of the films.

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.