Berlin 2008 – scientific programme

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MA: Fachverband Magnetismus

MA 27: Bio- and Molecular Magnetism

MA 27.2: Talk

Thursday, February 28, 2008, 14:45–15:00, EB 202

Inelastic neutron scattering on an antiferromagnetic tetrahedral Ni₄ molecule — •Joscha Nehrkorn¹, Oliver Waldmann¹, and Hannu Mutka² — ¹Physikalisches Institut, Universität Freiburg, D-79104 Freiburg, Germany — ²Institut Laue Langevin, BP 156-38042, Grenoble Cedex 9, France

We study the magnetic excitations in a tetrahedral Ni₄ molecule by inelastic neutron scattering (INS). The Ni₄ cluster consists of four antiferromagnetically coupled spin-1 Ni²⁺ ions in an almost perfect tetrahedral arrangement. The antiferromagnetic Heisenberg coupling should lead to a S = 0 ground state with higher-lying S = 1 and S = 2 levels. In our INS measurements we see peaks at transition energies in accord with the Heisenberg spectrum, but which violate the INS selection rule Δ S = 0, ± 1. That is, we observe not only the peaks which would correspond to the S = 0 → S = 1 and S = 1 → S = 2 transitions, but also a peak which would have to be assigned as a S = 0 → S = 2 transition. However, Ni²⁺ ions are known to exhibit single-ion anisotropies as large as several 10 K, which hence, apparently, should not be neglected. Therefore, the effects of a single-ion anisotropy in a tetrahedral symmetry on the INS spectrum is explored by exact numerical calculations.