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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

Yinwan Li ANL/ UI Chicago P. A. Montano UIChicago/US DOE J. Mitchell ANL

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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. - PowerPoint PPT Presentation

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Page 1: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

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

Yinwan Li ANL/ UI Chicago P. A. Montano

UIChicago/US DOEJ. Mitchell

ANLB. Barbiellini, P.E. Mijnarends, S. Kaprzyk

and A. Bansil NU Boston

*Project supported by the U. S. Department of Energy

Page 2: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

Outline

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

Page 3: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

Experimental set-up

B

Superconductor

Magnet

IonizationChamber

Slits

SiliconMonochromato

r50keV, 1=

100keV,74keV, 125keV

3-element GeSolid State Detector

Res.~0.4 a.u. at 100keV

Pc=0.64 at Kx=1, Ky=14.4

11º

B=0~7 Tesla,T=4.2K~300K

BESSRCElliptical Multipole

Wiggler

Measurements are carried out by flipping the photon polarization

SynchrotronRadiation

Page 4: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

Bilayer Manganite La 1.2 Sr 1.8 Mn2O7

Metal-ferromagnetic ~ insulator-paramagnetic Tc=129K

La, Sr atoms

MnO octahedra

Page 5: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

Ferromagnetic phase 3d orbitals in a perovskite environnement

ferromagnetic double exchange coupling between Mn3+ and Mn4+ gives charge delocalization

t2g

eg

Control: Temperature, Magnetic field H. At H=7 T we have a homogeneous ferromagnetic phase.

t2g

eg

Page 6: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

Momentum density of d-orbitals

Page 7: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

Magnetic momentum density

Page 8: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

MCP along (110)

Page 9: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

Temperature dependence

Page 10: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

B(r):Fourier transform of MCP Overlap integral along (110)

This minimum gives the x2-y2 occupation

Page 11: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

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.

Page 12: Yinwan Li  ANL/ UI Chicago  P. A. Montano UIChicago/US DOE J. Mitchell ANL

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.