Dresden 2009 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 29: Correlated Electrons: Low-dimensional Systems - Materials 2

TT 29.4: Vortrag

Mittwoch, 25. März 2009, 14:45–15:00, HSZ 301

Quantized Spin Waves in a Mesoscopic Antiferromagnetic Molecular Ring — •J. Dreiser¹, G. Carver², C. Dobe², H. U. Güdel², A. L. Barra³, J. Taylor⁴, and O. Waldmann¹ — ¹Physikalisches Institut, Universität Freiburg, D-79104 Freiburg, Germany — ²Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland — ³Grenoble High Magnetic Field Laboratory, CNRS, 38042 Grenoble Cedex 9, France — ⁴ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom

We present inelastic neutron scattering (INS) data as well as numerical simulations on the antiferromagnetic (AF) CsFe₈ molecule, in which eight spin-5/2 Fe(III) ions are arranged in a ring with nearest-neighbor AF Heisenberg interaction. In such even-numbered rings the lowest-lying excitations are formed by rotational modes of the Néel vector (L Band) while spin waves (E Band) occur at higher energies. This is in contrast to bulk antiferromagnets where spin waves are the lowest observed excitations. Further, spin waves become quantized, i.e., occur at discrete energies due to the mesoscopic size of the ring.

We recorded INS data up to high energies at the time-of-flight spectrometer MARI at ISIS, which clearly allowed us to observe the excitation of discrete spin waves. The data is modeled using a Heisenberg-exchange Hamiltonian together with a single-ion anisotropy term. Due to the molecules’ symmetry, only two parameters J, D are needed and exact numerical diagonalization yields excellent agreement with the data.