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

Yinwan Li ANL/ UI Chicago

P. A. MontanoUIChicago/US DOE

J. MitchellANL

B. Barbiellini, P.E. Mijnarends, S. Kaprzyk and A. Bansil

NU Boston

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

Outline

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

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

Bilayer Manganite La 1.2 Sr 1.8 Mn2O7

Metal-ferromagnetic ~ insulator-paramagnetic Tc=129K

La, Sr atoms

MnO octahedra

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

Momentum density of d-orbitals

Magnetic momentum density

MCP along (110)

Temperature dependence

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

This minimum gives the x2-y2 occupation

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.

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.