Michael L. Baker, Oliver Waldmann, Stergios Piligkos, Roland Bircher, Olivier Cador, Stefano Carretta, David Collison, Felix Fernandez-Alonso, Eric J. L. McInnes, Hannu Mutka, Andrew Podlesnyak, Floriana Tuna, Stefan Ochsenbein, Roberta Sessoli, Andreas Sieber, Grigore A. Timco, Høgni Weihe, Hans U. Güdel and Richard E. P. Winpenny
Inelastic neutron scattering studies on the odd-membered antiferromagnetic wheel Cr₈Ni
Phys. Rev. B 86, 064405/1-11 (2012)                 

Abstract: [(ⁱC₃H₇)₂NH₂][Cr₈NiF₉(O₂CCMe)₁₈], or Cr₈Ni, is a prominent example of an odd-membered antiferromagnetic “wheel.” A detailed characterization of the magnetic properties of Cr₈Ni has been conducted. Inelastic neutron scattering (INS) is used to investigate the energy and momentum transfer dependence of the low-lying spin excitations, including excited states inaccessible by other experimental techniques. The richness of the INS data, in conjunction with microscopic spin Hamiltonian simulations, enables an accurate characterization of the magnetic properties of Cr₈Ni. Nearest-neighbor exchange constants of J_CrCr = 1.31 meV and J_CrNi = 3.22 meV are determined, and clear evidence of axial single-ion anisotropy is found. The parameters determined by INS are shown to fit magnetic susceptibility. The spectroscopic identification of several successive S=1 excited total spin states and lowest spin band excitations show that the rotational band picture, valid for bipartite AFM wheels, breaks down for this odd-numbered wheel. The exchange constants determined here differ from previous efforts based on bulk measurements, and possible reasons are discussed. The large J_CrNi/J_CrCr ratio in Cr₈Ni puts this wheel into a regime with strong quantum fluctuations in which the ground state can be described with a valence bond solid state picture.