Magnetic momentum density, Fermi surface and directional magnetic Compton profiles in LaSr$_{2}$Mn$_{2}$O$_{7}$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

摘要

We have carried out first principles, all-electron computations of themagnetic momentum density $\rho_{mag}({\mathbf p})$ and magnetic Comptonprofiles (MCPs) for momentum transfer along the [100], [001], and [110]directions in LaSr$_2$Mn$_2$O$_7$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ withinthe local spin density approximation (LSDA) based band theory framework.Parallel measurements of these three MCPs from a single crystal ofLa$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ at 5 K in a magnetic field of 7 T are alsoreported. $\rho_{mag}({\mathbf p})$ is shown to contain distinct peaks arisingfrom the occupied majority-spin $t_{2g}$ electrons and to display images of theFermi surface (FS) in the first and higher Brillouin zones (BZs). The overallshape of the MCPs, $J_{mag}(p_z)$, obtained by integrating $\rho_{mag}({\mathbfp})$ over $p_x$ and $p_y$, is found to be dominated by the majority-spin$t_{2g}$ states. The FS-related fine structure in the MCPs is howeversubstantial only in the [100] MCP, which contains features arising from thelarge majority-spin hole sheets. The overall shapes and widths of theexperimental MCPs along all three directions investigated are in reasonablygood accord with theoretical predictions, although some discrepanciesindicating inadequacy of the LSDA in treating the magnetic states can beidentified. We discuss details of the FS-related signatures in the first andhigher BZs in the [100] MCP and show that high resolution magnetic Comptonscattering experiments with a momentum resolution of 0.1 a.u. FWHM(full-width-at-half-maximum) or better will be necessary to observe this finestructure. We comment also on the feasibility of using positron annihilationspectroscopy in this connection.