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
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.