Extracting d-orbital occupancy from magnetic Compton scattering in bilayer manganites

We consider the shape of the magnetic Compton profile (MCP), J_mag(p_z), in La_1.2Sr_1.8Mn₂O₇ for momentum transfer p_z along the [110] direction and the associated reciprocal form factor B(r) defined by taking the one-dimensional Fourier transform of J_mag(p_z). B(r) is shown to contain a prominent dip at r≈1 Å, where the minimum value B_min of B(r) can be related to the occupancies of the e_g orbitals of dx²−y² and d3z²−r² symmetry in the system. We illustrate our procedure in detail by analyzing the measured MCP at 5 K and the MCP computed within the framework of the local spin density approximation (LSDA) and comment on the differences between the measured and computed e_g occupancies as a reflection of the limitations of the LSDA in treating electron correlation effects.     

Momentum and spin density in magnetite

Measurements are reported of the angular correlation of annihilation radiation from polarized positrons in single crystals of Fe₃O₄. No change has been found in the angular correlations on passing through the metal-nonmetal phase transition. The twophoton momentum distribution, obtained by deconvolution of a set of angular correlations, displays a slight anisotropy. The spin density consists of a negative peak at zero momentum surrounded by a shell of positive density between 2 and 5 mrad. The observed spin density is probably a mixture of contributions from the moment on the ferrous ions and from the spin transfer onto the anions owing to covalency effects.     

Inelastic magnetic X-ray scattering from highly correlated electron systems: La1.2Sr1.8Mn2O7, La0.7Sr0.3MnO3 and Fe3O4

Magnetic Compton profiles have been measured for the colossal magnetoresistance manganites La_1.2Sr_1.8Mn₂O₇ and La_0.7Sr_0.3MnO₃, and for magnetite Fe₃O₄, along various crystallographic directions, over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations for the double layer manganite, La_1.2Sr_1.8Mn₂O₇, and by using a simple model involving atomic d-orbitals and free electrons for the other two compounds. For all three materials a preference for the occupation of e_g orbitals is found, particularly, for orbitals of dx²−y² symmetry. An itinerant electron contribution is adduced at all temperatures in magnetite; such a contribution also appears in La_1.2Sr_1.8Mn₂O₇, but it is present only at low temperatures in La_0.7Sr_0.3MnO₃.     

Temperature-dependent orbital degree of freedom of a bilayer manganite by magnetic compton scattering

We have measured temperature-dependent magnetic Compton profiles (MCPs) from a single crystal of La1.2Sr1.8Mn2O7. The MCPs, which involved the scattering of circularly polarized x rays, are in general related to the momentum density of all the unpaired spins in the system. Nevertheless, we show that when the x-ray scattering vector lies along the [110] direction, the number of magnetic electrons of a specific symmetry, i.e., d electrons of x(2)-y(2) symmetry, yield a distinct signature in the MCP, allowing us to monitor substantial changes in the occupancy of the dx(2)(-y(2)) states over the investigated temperature range of 5-200 K. This study indicates that magnetic Compton scattering can provide a powerful window on the properties of specific magnetic electrons in complex materials.     

Approximate validity of the Lock-Crisp-West theorem in positron annihilation

The validity of the Lock-Crisp-West (LCW) theorem is investigated theoretically. Substantial deviations are found in paramagnetic nickel, used as a model system, when a realistic positron wave function is used. Hybridization effects can play a major role. Use of the theorem in data analysis will often profit from theoretical support.