Changlin Tang

Structure and magneto-history behavior of DyNi2Mn

The compound DyNi2Mn with a MgCu2-type structure was synthesized, even though the atom ratio of Dy to transition metal was confirmed to be 1:3, by using the energy dispersive spectrum method. The X-ray diffraction pattern (XRD) could be well indexed with a cubic Laves cell with the lattice parameter a = 7.1406(2) Angstrom and space group Fd3m. The refinement result of the XRD pattern indicates that 73.8% of the 8a sites (Dy sites) are occupied by Dy atoms, 21.4% by Mn atoms, and 4.8% are empty. The compound shows ferrimagnetic coupling of the Dy and Mn moments with an ordering temperature T-C of 94 K. The spontaneous magnetization at 5 K is 6.7mu(B)/f.u. A large coercive field of 0.73 T at 5 K is found for the bulk sample, which can be understood in terms of narrow-domain-wall pinning

Synthesis and magnetic properties of compounds

The synthesis and magnetic properties of have been investigated. The formation of is found to depend strongly on annealing temperature. Pure single-phase compound can be synthesized at ambient pressure when . The main phase of the Laves structure can still be observed up to x = 0.8. The study of crystalline parameters, magnetic moments and Mossbauer spectra for single-phase is performed. Curie temperature and local Fe magnetic moment increase with the increasing Pr concentration. Extrapolating the trends the spontaneous magnetization and anisotropy constant of are 4.98 and at 1.5 K, respectively. Magnetostriction increases with increasing x and the largest saturation magnetostriction, 200 ppm, is observed for x = 0.5. With the increase of Pr concentration, lattice parameter and spontaneous magnetization anomalies exist in a specific range 0.2 < x < 0.5 that can be attributed to the change of Ce ion valence toward tetravalent.

Dirac equation description of the electronic states and magnetic properties of a square graphene quantum dot

Electronic eigenstates of a square graphene quantum dot (GQD) terminated by both zigzag and armchair edges are derived in the theoretical framework of the Dirac equation. We find that the Dirac equation can determine the eigenenergy spectrum of a GQD with high accuracy even if its size is reduced to a few nanometers. More importantly, from the Dirac equation description we can readily work out the number and energy gap of the conjugate surface states, which are intimately associated with the magnetic properties of the GQD. By using the Hartree-Fock mean field approach, we study the size dependence of the magnetic ordering formation in this square GQD. We find that there exists a critical size of the width between the two zigzag edges to indicate the onset of the stable magnetic ordering. On the other hand, when such a width increases further, the magnetic ground state energy of a charge neutral GQD tends to a saturated value. These results coincide with the previous results obtained from the first-principles calculation. Then, based on the Dirac equation solution about the surface state, we establish a simple two-state model which can quantitatively explain the size dependence of the magnetic ordering in the square GQD.

Effects of rare-earth substitution in : mixed-valence and magnetic properties

The lattice parameters and magnetic properties of (R = Ho, Er) have been investigated. The deviation of the lattice parameters from Vegards law indicates that the Ce valence in both systems fluctuates with the concentration of the dopant R. This fluctuation is directly relevant to the degree of localization of Ce 4f electrons, and is independent of the atomic size of the dopant element. The observed metamagnetic transition of has been analysed from the description in the band picture. The Ce valence fluctuation from the mixed-valence state to the localized state increases the moments of the Ce and Fe ions, and the Curie temperature. The changes in the mixed-valence state also affect the properties of the Ce-Fe coupling.

Anomalous magnetic properties of cerium ions in the compounds (R = Tb, Dy)

The lattice parameters and spontaneous magnetizations of the pseudobinary compounds (R = Tb, Dy) are reported. The analysis of these results revealed the mutability of the Ce 4f ferromagnetism. The change of the degree of delocalization of the Ce 4f electron is used to analyse the changes of the lattice parameter and the spontaneous magnetic moment. A metamagnetic phase transition can be observed for each of the systems investigated, , when the Ce concentration x is about 0.5. The critical phase field increases with the Ce concentration, and the itinerant-electron model has been applied to analyse the tendency towards change of the critical phase field with x.

Magnetic properties of Tb2Fe15Si2 single crystal

The crystal structure, exchange interaction, magneto-crystalline anisotropy and demagnetization process are analyzed for single crystal Tb2Fe15Si2 Domain wall pinning is demonstrated as silicon is introduced to substitute for Fe in Tb2Fe17 compound. The temperature dependence of H-C are represented quite satisfactorily by a formalism derived by Egami, and the existence of a narrow domain wall-dominated pinning process is derived

MAGNETIC PROPERTIES OF TB2(FE, SI)17 SINGLE CRYSTALS

magnetic properties of single-crystalline Tb2Fe17-xSix (x = 0, 1, 2, 3, and 3.3) have been investigated. The Si substitution constricted the lattices by 1.5% and caused the Th2Ni17 transfer to Th2Zn17. The Curie temperature increased from 413 to 526 K, and the spontaneous magnetic moment decreased from 82.6 to 46.4 emu/g with the increase of Si. The stronger anisotropy and coercivity were generated by Si occupying the Fe sublattices. A domain wall pinning-dominated mechanism was responsible for increasing the coercivity force from 0.01 T (x = 1) to about 0.36 T (x = 3.3) at 1.5 K.