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September 25, 2002 Probing Local Magnetic Order in Transition Metal OxidesQ. Qian1, T.A. Tyson1, A. Yu. Ignatov1, C.-C. Kao2, and M. Croft2,3 In the transition metal oxide La1-xCaxMnO3 (with 0<x<1), electrical conductivity is strongly influenced by external magnetic fields, making this material an exceptionally good candidate for a new generation of magnetic sensors that may significantly improve magnetic data storage. To probe the local spin arrangement in transition metal oxides, we have developed a method to directly assess the local magnetic ordering about specific metal atoms. This method promises to lead to a better understanding of magnetism in magnetic oxides and to assist in improving theoretical models of correlated electron systems.
High resolution XAS work has provided evidence of intriguing temperature dependencies of the manganese K XAS pre-edges of La1-xCaxMnO3 materials which accompany changes upon crossing phase boundaries involving combinations of insulating, paramagnetic, antiferromagnetic, ferromagnetic, and charge-orbital ordered phases. But these interphase changes are not well understood. Consequently, we used a powerful combination of XAS and XES, called spin-polarized x-ray absorption near edge spectroscopy (SPXANES), to extract spin-polarized x-ray absorption spectra and integrate the results into a model which is sensitive to local magnetic order.
Manganese SPXANES measurements were performed at beamline X21A of
the National Synchrotron Light Source at Brookhaven National
Laboratory. SPXANES spectra were collected by monitoring the Kβ
fluorescence yield at two energies, specific to spin up and down
final states, while the incident energy across the near-edge region
All the K-edge SPXANES spectra of La1-xCaxMnO3 are shown in figure 1. They reveal a marked splitting between the spin up and spin down channels of both the pre-edges (energy E between 6.535 and 6.55 kilo-electronvolts (keV)) and the main edges (energy E between 6.55 and 6.58 keV). In figure 2, we expand the pre-edge region of the SPXANES
spectra. Three peaks (labeled a1, a2, and a3) in In figure 3, we display our transition model, which shows that the changes in the pre-edge region with temperature can be directly linked to changes in magnetic ordering of the manganese ions around absorbing manganese sites. The model also shows a transition from a random local magnetic arrangement to an ordered arrangement.
Our model could be used to investigate a great range of perovskite transition metal materials, and, more generally, octahedrally-coordinated transition metal materials. Our model and SPXANES measurements could also be used to predict magnetic ordering changes in materials such as thin films, single crystal or powders. FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
Intriguing phenomena such as high-temperature superconductivity
(electrical conductivity without resistance) and colossal
magnetoresistance (change in electrical resistance due to the
presence of a magnetic field) have stimulated renewed interest in
the physics of transition metal oxides based on perovskite (CaTiO3),
by using techniques such as x-ray absorption spectroscopy (XAS) and
x-ray emission spectroscopy (XES). 
SPXANES is based on energy resolving the 3p to 1s transition (Kβ
emission) and measuring the emission from the main or satellite
lines of the spectra as a function of the incident x-ray energy.
Previous studies focused on the nature of the splitting between spin
up and spin down channels in the main line, but no emphasis was made
on the pre-edge region and on the temperature-dependent changes in
SPXANES. 
was scanned. Measurements were performed for x = 0, 0.3, 0.5, and 1
(corresponding to LaMnO3, La0.7Ca0.3MnO3, La0.5Ca0.5MnO3 and CaMnO3,
respectively) at two temperatures: 15 and 300 Kelvin (K). 