Coherent Excitations at Ferromagnetic Gd(0001) and Tb(0001) Surfaces

Abstract

It is shown that ferromagnetic lanthanide surfaces like Gd(0001) and Tb(0001) present good model systems to investigate excitation, interaction, and relaxation of electronic, phononic, and magnetic subsystems in the time domain. In particular, analysis of an optically excited coherent phonon (CP)-magnon mode by pump-probe experiments gives detailed insight into the intriguing interaction between lattice and magnetization dynamics on femtosecond timescales.The lanthanide series of elements (Ce-Lu) is well known for its consecutive filling of the 4f shell, which is localized at the ion core. Due to the poorly screened 4f orbitals, an effective increase of nuclear charge occurs with increasing electron count, which leads to the reduction in the ionic radius with increasing electron count. The orbital and the spin magnetic moments per ion are also related to the number of the 4f electrons by Hunds rules. Accordingly, the spin magnetic moment is largest for the half-filled configuration 4f 7 of gadolinium (Gd), S ¼ 7/2. The orbital magnetic moment vanishes (L ¼ 0) for this configuration. In terbium (Tb) with 4f 8 , the spin and orbital magnetic moments are given by S ¼ 3 and L ¼ 3, respectively. 1) Both elements crystallize in the hexagonal close-packed structure with the lattice constant a Gd ¼ 3:63 A and a Tb ¼ 3:60 A . Their structural and vibrational properties are accordingly similar [1]. The lanthanide elements exhibit rich and interesting magnetic ordering phenomena [2], which are closely related to the indirect (or RKKY) exchange interaction. This type of exchange couples the magnetic moments that are localized at the ion cores and, hence, have no direct wave function overlap. This indirect exchange acts through polarization of the delocalized conduction band electrons. As depicted in Figure 12.1b, for the simplified situation of a spin impurity embedded in free electron gas, the spin polarization (SP) surrounding the localized spin exhibits a damped oscillation as a function of distance r from the ion core. Since 1) The ionic configuration in Tb is actually 4f 9 6s 2 while it is 4f 7 5d 1 6s 2 for Gd. In a solid, which is of interest here, the number of 4f electrons increases as an integer count along the lanthanide series [1].

Ultrafast magnetization dynamics of Gd(0001): Bulk versus surface

“…The signal starts to recover subsequently and returns to its value before optical excitation after several 100 ps. The oscillatory part has been explained before as a coupled phonon–magnon mode localized to the surface 15, 16. It is therefore expected, that it is not found in the bulk signal.…”

“…19 ). The oscillatory part has been explained before as a coupled phonon-magnon mode localized to the surface 15,16 . It is therefore expected, that it is not found in the bulk signal.…”

“…For details see Refs. 15–17. Gd was evaporated from an electron beam heated crucible under UHV conditions onto a W(110) substrate at 300 K. Annealing to 700 K subsequent to the deposition results in smooth epitaxial films 18.…”