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)

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

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.