Tham khảo tài liệu 'neutron scattering from magnetic materials part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 358 T. Chatterji interactions at higher temperature. One must emphasize that the high temperature diffuse scattering involves the bare J from the spin Hamiltonian whereas the value obtained by fitting the low temperature spin wave dispersion depends upon the accuracy in 1 S to which the spectrum was calculated. The anisotropy in the exchange interactions Ja and Jc which can be attributed to the orbital overlaps is expected to reduce at higher temperature. One can understand qualitatively the reduction of the both intra- and inter-layer exchange couplings as well as of their ratio at higher temperature on the basis of following arguments. The exchange coupling in the ferromagnetic state scales with the kinetic energy of the electrons in the double-exchange model. This result could be extended to the paramagnetic state as well provided that the short-range ferromagnetic correlations still exist while the long-range order is suppressed by the thermal fluctuations. The kinetic energy of the electrons is expected to be reduced from zero temperature value due to the scattering from the disordered spins as well as due to the polaronic effects which become more important at high temperatures when the number of phonons increases. So the exchange couplings are expected to be reduced at high temperatures as seen in experiment. Let us discuss next the effect of the temperature on the ratio between the in-plane and inter-plane coupling. Of the two eg orbitals on the Mn sites dx2 _y2 and d3z2 _r2 only the latter is responsible for the inter-layer within the bilayer exchange while the former one is localized in the plane. The bilayer structure favors the occupation of the dx2_y2 orbitals that has the higher in-plane transfer amplitude and hence removing the orbital degeneracy leads to the anisotropy in orbital occupation. Actually this anisotropy has been recently detected experimentally in the bilayer manganites by the magnetic Compton-profile measurements 61 . One could expect
