Antiferromagnetic molecular nanomagnets with odd-numbered coupled spins

¹ Département de Physique, Université de Montréal - Montréal, Québec H3C 3J7, Canada
² Perimeter Institute for Theoretical Physics - 31 Caroline St. N., Waterloo, Ontario N2L 2Y5, Canada
³ Institute for Quantum Computing, University of Waterloo - Waterloo, Ontario N2L 3G1, Canada
⁴ Department of Physics and Astronomy, University of Waterloo - Waterloo, Ontario N2L 3G1, Canada

Received: 25 March 2015
Accepted: 11 May 2015

Abstract

In recent years, studies on cyclic molecular nanomagnets have captivated the attention of researchers. These magnets are finite in size and contain very large spins. They are interesting because they possess macroscopic quantum tunneling of Néel vectors. For antiferromagnetic molecular nanomagnets with finite number of even-numbered coupled spins, tunneling involves two classical localized Néel ground states separated by a magnetic energy barrier. The question is: can such phenomena be observed in nanomagnets with odd number of magnetic ions? The answer is not directly obvious because cyclic chains with odd-numbered coupled spins are frustrated as one cannot obtain a perfect Néel order. These frustrated spins can indeed be observed experimentally, so they are of interest. In this letter, we theoretically investigate macroscopic quantum tunneling in this odd spin system with arbitrary spins s, in the presence of a magnetic field applied along the plane of the magnet. In contrast to systems with an even-numbered coupled spins, the ground state of the cyclic odd-spin system contains a topological soliton due to spin frustration. Thus, the classical ground state is 2N-fold degenerate as the soliton can be placed anywhere along the ring with total . Small quantum fluctuations delocalize the soliton with a formation of an energy band. We obtain this energy band using degenerate perturbation theory at order 2s. We show that the soliton ground state is chiral for half-odd integer spins and non-chiral for integer spins. From the structure of the energy band we infer that as the value of the spin increases the inelastic polarized neutron-scattering intensity may increase or decrease depending on the strengths of the parameters of the Hamiltonian.