H.R. Kirchmayr

Magnetic phase transitions and magnetic anisotropy in Nd2Fe14 − xCoxB compounds

Abstract The temperature dependence of the anisotropy field of Nd 2 Fe 14 − x -Co x B has been measured between 77 K and the Curie temperature, using the singular point detection (SPD) technique. The anisotropy field at room temperature is found to decrease with increasing cobalt content. Measurements of the temperature dependence of the initial susceptibility show that at a temperature T 1 , which is below room temperature, the easy magnetization direction changes. The origin of the change is attributed to the neodymium-sublattice anisotropy. For the cobalt-rich samples ( x ⩾ 5) it is found that at temperatures T 2 higher than ambient temperatures a second spin reorientation takes place, which is attributed to the competing effect of the neodymium sublattice anisotropy and the 3d sublattice anisotropy.

Magnetic anisotropy in Pr2(Fe1−xCox)14B compounds

Abstract We have measured the concentration dependence of the Curie temperature, magnetization, anisotropy field and lattice parameters in Pr 2 (Fe 1− x Co x ) 14 B for 0 ⩽ x ⩽ 1. The Curie temperature strongly increases with x . The saturation magnetization at 4.2 K decreases with x after remaining approximately concentration independent up to x = 0.2. The room temperature anisotropy field first decreases with x but after passing through a minimum at about x = 0.7 strongly increases with x . The experimental results are compared with results obtained previously for Nd 2 (Fe 1− x Co x ) 14 B and are discussed in terms of crystal-field effects.

Structure and magnetic properties of R2Fe14B1-xCx compounds (R=Y,Nd)

Abstract Magnetization curves parallel and perpendicular to aligned direction were measured in an extraction magnetometer (magnetic field up to 80 kOe) on R 2 Fe 14 B 0.5 C 0.5 compounds (R = Y,Nd) from 1.5 to 300 K. Modified Sucksmith-Thompson method was used to determine anisotropy constants K 1 and K 2 . It was found that the mean Fe moment, Curie temperature and spin reorientation temperature of the Nd-containing compound were decreased upon substitution of carbon for boron, however, the anisotropy field H a was increased with such substitution. X-ray diffraction analysis showed that values of (1 - α( c / a ) 2 ) of the compound in which boron was partially substituted by carbon was larger than that of the unsubstituted one. It is thought that the crystal field acting on the Fe sublattice is increased with carbon substitution. The decrease of T sr of Nd 2 Fe 14 B 0.5 C 0.5 compared with that of Nd 2 Fe 14 B is probably due to the increase of the Fe sublattice anisotropy. The value of H a determined by the SPD method differs significantly from that derived by 2( K 1 + 2 K 2 )/ M s . It seems that the anisotropy constants K 1 , K 2 determined by the Sucksmith-Thompson method does not completely represent the anisotropy behaviour of this type of compound. The nominal R 2 Fe 14 C samples are a mixture of two phases (R 2 Fe 17 C x + α − Fe) and the Nd 2 Fe 14 B type compound was not found in this case. The fact that R 2 Fe 17 C x has a higher Curie temperature compared to that of R 2 Fe 17 is worth noticing.

Thermodynamics and kinetics of Zr(Fe1−xMnx)2Hx and the storage compound Ti0.8Zr0.2MnCrH3

Abstract Thermodynamic and kinetic data for hydrogen absorption in Zr(Fe1−xMnx)2 and Ti0.8Zr0.2CrMn, which is one of the most promising hydrogen storage materials, are presented. The data were obtained from measurements performed using a high pressure microbalance. The macroscopic rate constants obey an Arrhenius law, and comparison with the hydrogen self-diffusion coefficient of Ti0.8Zr0.2MnCrH3 recently measured by Hempelmann and coworkers suggests that the rate-limiting step in the absorption kinetics is the α-β phase transition rather than the comparatively fast hydrogen diffusion in the bulk.

The effect of substitutions on the hard magnetic properties of Nd-Fe-B based materials

Abstract The effect of different substitutions (Co, Ni, Al, Si, Ga) was investigated by studies of the magnetization process and by measurements of the temperature dependence of the anisotropy field H a and the hysteresis loop (77 K T T c ). Additionally 57 Fe Mossbauer studies were performed. The Co substitution increases the Curie temperature and reduces the anisotropy. The substitution of nonmagnetic elements such as Al, Si or Ga increases I H c via the microstructure as well as the intrinsic properties. Si and Ga substitution increases I H c and also the Curie temperature; this effect is generally attributed to a preferential substitution of the Fe sites. The rise in H A with substitution of Nd by Dy is in agreement with the increase of I H c . In this case a linear relation between I H c ( T ) and H A ( T ) was found and also the magnet parameters are less sensitive to the heat treatment.

Homogeneous precipitation in Co5Sm crystals

Abstract The microstructure of Co 5 Sm crystals shows evidence of continuous precipitation with homogeneous nucleation centres of Co 7 Sm 2 as well as Co 17 Sm 2 . The nucleation and growth mechanism, which is dependent on the lowering rate of the crystals, indicates that Co 5 Sm does not decompose by a classic eutectoidal transformation. Transmission electron microscopy and microprobe analyses show regions with an inhomogeneity of the samarium concentration within and specially near the interphase boundaries of the coherently precipitated Co 7 Sm 2 and Co 17 Sm 2 phases. This exsolution reaction as well as the low free nucleation energy and the relatively high free activation energy below 800 °C for the diffusion process determine the magnetic properties such as the initial susceptibility and the temperature depen dence of the coercive force in Co 5 Sm crystals.

Magnetic anisotropy and magnetic transitions in RFe10Mo2

Compounds of the composition RFe10Mo2 with R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu were studied by means of the ac-initial susceptibility (chi) and the singular point detection (SPD) technique in the temperature range from 4.2 to 300 K. Spin reorientation transitions were detected for the compounds RFe10Mo2 with R = Nd and Dy. For the compounds RFe10Mo2 with R = Y, Ho, Er, Tm and Lu the ac-initial susceptibility data indicate a magnetic transition of unidentified nature. For SmFe10Mo2 a first-order magnetization process (FOMP) is observed below 170 K.

Exchange interaction and magnetic anisotropy in Nd2(Fe13M)B compounds (M = Ga, Si, Al)

Abstract Static and pulse magnetic field measurements were performed to determine the spin reorientation temperature ( T sr ) and the magnetic anisotropy field ( H a ) of Nd 2 (Fe 13 M)B compounds (M = Ga, Si, Al). The changes in T sr in the wake of Fe-substitution is attributed to the changes of the exchange field which could be explained satisfactorily based on the CEF theory by taking account of CEF parameters up to sixth order. Estimation of the value of exchange field (2μ B H ex ) was made by fitting calculated data with experimental ones of T sr on the assumption that CEF parameters remain unchanged with small amount of substitution for Fe in Nd 2 Fe 14 B. The easy c -axis stability energy of Nd ion was calculated based on second-order crystal field model. It is thought that all substitutions for Fe in Nd 2 Fe 14 B lead to lowering of its T sr due to the decrease of exchange field which in turn results in deterioration of intrinsic uniaxial ferromagnetic thermostability of substituted compounds.

Comparison of melt-spun Nd4Fe77B19 with neodymium-rich isotropic permanent magnets

Abstract Initial magnetization and demagnetization data are reported for three types of isotropic Nd2Fe14B containing permanent magnet materials: annealed melt-spun Nd4Fe77B19, melt-spun Nd14Fe81B5, and sintered Nd16Fe78B6. A substantial magnetic interaction of the grains in Nd4Fe77B19 is indicated by the temperature dependence of the coercivity, by the remanence to saturation polarization ratio being 0.75, and by an unusual relation of the remanence after magnetization to remanence after demagnetization.

Electrical resistivity, thermal conductivity and thermopower of nonmagnetic REAL2 compounds☆

Abstract The temperature dependence of the electrical and the thermal resistivity, as well as the thermopower of the three nonmagnetic REAl 2 compounds, are compared (RE  Y, La, Lu). As with other physical properties, LaAl 2 shows a different behaviour in transport phenomena compared with the Lu and Y compounds. From the analysis of ρ ( T ) and λ ( T ) data we show that the anomalous behaviour of LaAl 2 can be understood by assuming that in this case the electron-phonon coupling is about twice as strong. The temperature dependence of the thermopower of LaAl 2 also shows its exceptional position. This fact, however, cannot be simply referred to the enhanced electron-phonon coupling, because the thermopower depends much more on electronic-state properties at the Fermi energy than do the electrical and thermal resistivities.