Xue‐Fu Zhong

CALCULATION OF SPIN-REORIENTATION TEMPERATURE IN ND2FE14B

The spin-reorientation phenomenon in Nd2Fe14B has been investigated using an angular dependent free energy approach. A magnetic Hamiltonian which includes the crystal electric field term and the exchange term has been established using realistic band structure results. The temperature dependence of the molecular field is accounted for by introducing the Brillouin function and the magnetic Hamiltonian is diagonalized within the ground state multiplet of the Nd ion. The eigenstates are then used to form the partition function for the free energy. At each temperature, the direction of the molecular field is obtained by searching for the minimum in the angular parameter space of the free energy. Our calculations show that for Nd2Fe14B, the net magnetic anisotropy direction is canted away from the c axis at a temperature close to the experimentally reported spin-reorientation temperature of 150 K. The temperature dependence of the magnetic structure is found to be very sensitive to the size of the second order crystal field parameter B20.

Theoretical analysis of the spin‐density distributions in Y2Fe17N3 and Y2Fe17C3

The electronic structures and spin‐density distributions in Y2Fe17N3 and Y2Fe17C3 are calculated using the self‐consistent spin‐polarized orthogonalized‐linear‐combination‐of‐atomic‐orbitals method. The N or C atoms are assumed to occupy the (9e) sites in the rhombohedral structure. The Fe (18f) and N or C at the (9e) site form covalent bonds which result in a very nonspherically symmetric spin‐density distribution. The calculation shows a reduced moment for the Fe (18f) sites due to doping. However, the moments at other sites are increased due to lattice expansion. There are some differences in the spin‐density distribution between N and C at the (9e) site. By comparing with the results of calculation on the pure Y2Fe17 at different volumes, changes in moment enhancement and density of states at the Fermi level due to lattice expansion alone and that due to the chemical effect of introducing N or C are separated.

Electronic structure of Nd2Fe17N

The electronic structures of ternary compound Nd2Fe17N with N atoms on 9e, 3b, and 18g sites are calculated and compared. The local moments on different Fe sites are in good agreement with experiments. The mechanism of increasing Curie temperature by N doping is checked by additional calculations with lattice expansion. The results show that the change in interatomic interaction is more important than the lattice expansion effect.The electronic structure of Nd/sub 2/Fe/sub 14/B has been calculated using the first-principles orthogonalized linear combination of atomic orbitals method. The f electrons in Nd are included in the calculation but are treated as localized states in the determination of the Fermi energy. Results are presented for the total density of states (DOS), orbital-decomposed and spin-decomposed partial DOS. The calculated spin-magnetic moments on each of the six Fe sites are in good agreement with the values deduced from neutron scattering experiment. The charge density map and the spin density map on the basal plane of the tetragonal cell show the evidence for covalent bonding between Fe and B atoms and reveal the distortion from the spherically symmetric distribution around the atomic sites.

Calculation of local orbital moments of conduction electrons in Nd2Fe14B

Based on the spin‐polarized band structure of Nd2Fe14B using the OLCAO method and the inclusion of spin‐orbit interaction in the Hamiltonian, the contribution to the local moments have been calculated from the orbital angular momenta of the conduction electrons. A site decomposition of partial density of states provides all the essential information about the local orbital moments on each site. The calculation involves diagonalization of complex band matrix equations for all wave vectors in several stars of k in the Brillouin zone. The calculated orbital moments on different Fe sites are in general agreement with other calculations and are much smaller than the spin magnetic moments. The calculation presented here also reveals that for the crystallographically equivalent sites, the local orbital moments may not be the same depending on the local environment of the site.

First‐principles calculation of orbital moment distribution in amorphous Fe

A model calculation on a large periodic unit cell of amorphous Fe containing 200 atoms is carried out to determine the distribution of its local orbital moments. A spin‐polarized orthogonalized linear combination of atomic orbitals method including spin‐orbit coupling is used for the calculation of the electronic structure. It is shown that spin‐orbit coupling brings about a small change in the density of states for amorphous Fe. The average orbital moment in amorphous Fe is found to be 0.01 μB, which is much smaller than the value of 0.09 μB for crystalline fcc Fe. The distribution of orbital moments over various sites is rather broad, indicating a strong quenching effect as a result of variation in the local structure in the amorphous case. It is speculated that such ground‐state properties are favorable to the formation of the spin glass phase.

On the characteristics of magnetic order of Nd2Fe14B‐type compounds

Based on the angle‐dependent free‐energy formalism, a general analysis of the characteristics of magnetic order in Nd2Fe14B‐type compounds is given. The focus is on the symmetry in magnetic order and its relation to the crystal structure. The spin‐reorientation and the first‐order magnetization process are discussed from the viewpoint of phase transition with the emphasis on the change in the symmetry of magnetic order.

Calculation of crystal field parameters in Nd2Fe14B using realistic energy‐band results

There have been several attempts to estimate the crystal field parameters of the rare‐earth‐iron‐boron permanent magnets because of its pivotal role in understanding the energy anisotropy, spin reorientation, etc. Usually, a simple point charge model (PCM) was employed which depends on some arbitrary parameters such as the values of ionic charge and screening factors. The crystal field parameters obtained from the PCM generally have correct signs but are too large in magnitude. We report a more accurate preliminary calculation of the crystal field parameters in Nd2Fe14B based on the results of realistic energy‐band calculation. Three levels of calculations are carried out: (1) The PCM calculation which essentially reproduces the published values of other workers; (2) use of effective charges on each ion from the band calculation instead of fully ionic values, which reduces the magnitude of parameters but have signs of some parameters reversed; (3) use of charge distribution calculated from the Bloch wave ...