Temperature-dependent orbital degree of freedom of abilayer 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
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
Ionization
Chamber
Slits
Silicon
Monochromator
50keV, 1=100keV,
74keV, 125keV
3-element Ge
Solid 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
BESSRC
Elliptical Multipole
Wiggler
Measurements are carried out
by flipping the photon polarization
Synchrotron
Radiation
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 couplingbetween Mn3+ and Mn4+ gives chargedelocalization
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 MCPOverlap integral along (110)
This minimum gives the x2-y2 occupation
Important result
The gain of d(z^2) results in an expansion ofthe apical distance Mn-O (observed in PRB55, 63 (1997)) and, below Tc, it correlateswith FM order.
Summary
•We have used a large magnetic field of 7 T whichensures a ferromagnetic homogeneous phase forall studied temperatures.
•The choice of the [110] MCP direction makes ouroccupancy analysis particularly robust because ofsymmetry constraints.
•Changes in occupancy give dramatic structuralresponse to the onset of ferromagnetism.