neutron scattering from magnetic single crystals i. …center for materials for information...
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Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Neutron Scattering from Magnetic Single Crystals
I. Zoto, F.D. Mackey, V.V. Krishnamurthy, J.L. Robertson, N. Cavadini and
Gary J. MankeyMINT Center and Physics Department
The University of AlabamaThis project was funded by grant DE-FG02-02ER45966 and share equipment from NSF-DMR-0213985.
MINT Fall Review 2002.
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Why Neutrons?• The de Broglie wavelength of thermal neutrons is
comparable to atomic spacing.• Neutrons and phonons have energies of the same
order of magnitude.• Neutrons have magnetic moment → we can study
magnetic structures and dynamics.• They interact strongly with nuclei.• Neutrons are highly penetrating.
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Triple Axis Spectrometer (TAS)
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HB1
HFIR Center for Neutron Scattering
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
BAMA – Bragg Angle Multicrystal Analyzer• Analyzer consists of a
number of equidistant graphite monochromator crystals which can be rotated separately to form different modes.
• The neutrons Bragg diffract from each crystal to select the wavelength by the Bragg angle formula:
λθ nd =sin2
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
BAMA MODES
• These are the 3 different focusing modes.
• A is the conventional TAS mode with a flat analyser and straight collimators.
• B is the monochromatic point-to-point focusing mode.
• C is the constant-q⊥ point-to-point focusing mode.
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Neutron Scattering Experiment at SINQ
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Structural, Magnetic and Transport Properties of Pr0.5Sr0.5MnO3
(H. Kawano et al., PRL 78, 4253, 1997) • Curie Temperature TC ~ 265 K • Ferromagnetism and metallic behavior above 140 K due to double exchange interaction • Néel Temperature TN ~ 140 K ( A-type Antiferromagnetic structure) • Space group: P21/n. Lattice constants; a= 5.360 Å, b= 7.813 Å , c= 5.377 Å (T=110 K) • Antiferromagnetic state is semiconducting
A-type Antiferromagnet
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Elastic Diffraction From AF Peak
• Py0.5Sr0.5MnO3 crystal was grown by John Mitchell of Argonne National Lab.
• Magnetic diffraction reveals three components, a twin + a off-stoichiometrycomponents.
• Elastic diffraction probes AF order, (only direct technique available).
• Inelastic scattering probes dynamics.
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Phase Transition
• .
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Inelastic Neutron Diffraction• Measures spin wave dispersion
relations, Eg, Dsw.• Eg is energy gap →information
about the strength of the coupling (exchange constant) J (E=J*S1*S2)
• Dsw is “spin wave stiffness”. E=Eg+Dsw*q2 tells about dynamics.
• Anisotropic spin waves are expected.
Center for Materials for Information Technologyan NSF Materials Science and Engineering Center
Conclusions
• Elastic neutron scattering probes antiferromagnetic order in crystals.
• Inelastic neutron scattering is a powerful method to probe spin-wave excitations in magnetic single crystals.
• We will build the BAMA device for use with the TAS spectrometers at ORNL.
• BAMA will enable the measurements of spin dynamics in magnetic thin films.