1. Temperature-dependent orbital degree of freedom of a bilayer manganite by magnetic Compton scattering Yinwan Li ANL/ UI Chicago P. A. Montano UIChicago/US DOE J. Mitchell ANL B. Barbiellini, P.E. Mijnarends, S. Kaprzyk and A. Bansil NU Boston *Project supported by the U. S. Department of Energy

2. Outline Bi-layer manganite Ferromagnetic phase Magnetic Momentum density Magnetic Compton Profile (MCP) along (1 1 0) B(r) overlap integral Main results Summary

3. Experimental set-up B Superconductor Magnet Ionization Chamber Slits Silicon Monochromator 50keV,  1 = 100keV, 74keV, 125keV 3-element Ge Solid State Detector Res.~0.4 a.u. at 100keV P c =0.64 at K x =1, K y =14.4 11º B=0~7 Tesla, T=4.2K~300K BESSRC Elliptical Multipole Wiggler Measurements are carried out by flipping the photon polarization Synchrotron Radiation

4. Bilayer Manganite La 1.2 Sr 1.8 Mn 2 O 7 Metal-ferromagnetic ~ insulator-paramagnetic Tc=129K La, Sr atoms MnO octahedra

5. Ferromagnetic phase 3d orbitals in a perovskite environnement ferromagnetic double exchange coupling between Mn 3+ and Mn 4+ gives charge delocalization t 2g egeg Control: Temperature, Magnetic field H. At H=7 T we have a homogeneous ferromagnetic phase. t 2g egeg

6. Momentum density of d-orbitals

7. Magnetic momentum density

8. MCP along (110)

9. Temperature dependence

10. B(r):Fourier transform of MCP Overlap integral along (110) This minimum gives the x2-y2 occupation

11. Important result The gain of d(z^2) results in an expansion of the apical distance Mn-O (observed in PRB 55, 63 (1997)) and, below Tc, it correlates with FM order.

12. Summary We have used a large magnetic field of 7 T which ensures a ferromagnetic homogeneous phase for all studied temperatures. The choice of the [110] MCP direction makes our occupancy analysis particularly robust because of symmetry constraints. Changes in occupancy give dramatic structural response to the onset of ferromagnetism.