Papers by Author: Guang Duo Lu

Abstract: The microwave magnetic dynamic properties of the [NiFe(40nm)/IrMn(15nm)]₆/[NiFe(30nm)/IrMn(15nm)]7/[NiFe(20nm)/IrMn(15nm)]₁₀ multi-stacks structure films, prepared by high vacuum DC magnetron sputtering deposition, have been investigated at the frequency range from 10 MHz to 6 GHz. By changing the thickness of the ferromagnetic (FM) NiFe layer in bottom [NiFe(t nm)/IrMn(15nm)]₆ stack part, a tunable lower frequency range used magnetic multilayer can be realized. The influence of the NiFe layer thickness in bottom stack to static and dynamic magnetic properties were obtained and analyzed. Combining with the top [NiFe(30nm)/IrMn(15nm)]₇/[NiFe(20nm)/IrMn(15nm)]₁₀ two stacks used in the higher frequency, a kind of multi-stacks structure magnetic thin films were fabricated for the wideband microwave noise filter.

Abstract: The local structure and the spin Hamiltonian parameters g factors gi (i =x, y and z) and the hyperfine structure constants Ai for Co2+ in orthorhombic CaTiO3 are theoretically investigated from diagonalization of 6 × 6 energy matrix within the 4T1 ground state for a 3d7 ion in orthorhombic symmetry. In the calculations, the contributions from the admixtures of various J states (J=1/2, 3/2, 5/2), the ligand orbitals and spin-orbit coupling and the fourth-order orthorhombic field parameter, which were usually neglected in the previous works, are taken into account. The crystal-field parameters are determined from the superposition model in consideration of the suitable lattice distortion due to the charge and size mismatching substitution of Ti4+ by Co2+. Based on the studies, the bond lengths R1 and R2 in the xy plane are estimated to suffer the relative alternation R ≈ 5.4%, yielding more significant orthorhombic distortion in the impurity center as compared with that for the host Ti4+ site in pure crystal. The calculation results based on the above local lattice distortion show reasonable agreement with the observed values. The various contributions to the spin Hamiltonian parameters are discussed. Present studies may theoretically verify that the impurity Co2+ occupies the 6-fold coordinated Ti4+ site rather than the 12-fold coordinated Ca2+ site, associated with the enhanced orthorhombic distortion due to the mismatching substitution.

Abstract: The axial displacements for Co2+ and Ni3+ in Al2O3 are theoretically investigated starting from the perturbation formulas of the EPR parameters for a 3d7 ion with high spin (S=3/2) and low spin (S=1/2) in trigonal symmetry, respectively. Based upon these studies, the Co2+ is found to shift towards the center of the oxygen octahedron by an amount
ZCo (≈ 0.03 Å) along the C3 axis, while the Ni3+ may suffer another axial displacement
ZNi (≈ −0.38 Å) away from the center of the octahedron. The calculated EPR parameters based upon the above axial displacements show good agreement with the observed values. The differences in the EPR parameters and the axial displacements for the two 3d7 ions are discussed.