Spin Singlet Ground State Evidence in Frustrated Antiferromagnetic Ring Cr8Ni
Spin frustration is one of the most exciting frontiers of contemporary research in condensed matter physics, found among iron moments in the new superconducting FeAs materials. Materials, such as magnetic molecules with a small number of spins, offer an opportunity to experimentally measure the effects of spin frustration, and their properties can be accurately determined theoretically, too. The odd number antiferromagnetic spin ring cluster, Cr8Ni, is a newly synthesized spin frustration system with a nontrivial ground state. There are antiferromagnetic nearest neighbor interactions between the spins (s=1 on Ni2+ and s=3/2 on Cr3+). The ground state is non trivial because of the odd number of spin species, where all nearest neighbors cannot be antiferromagnetically aligned. High field magnetization, specific heat, and proton NMR measurements at very low temperatures to 100mK were used to elucidate the magnetic ground state of the spin frustrated Cr8Ni cluster. Absence of low temperature magnetic broadening of the proton NMR spectrum and almost zero magnetization up to ~1 Tesla directly revealed a spin singlet, non-magnetic ground state for the cluster. Level crossings with higher magnetic fields, specific heat measurements, and theoretical analysis revealed the influence of sizeable Dzyaloshinski-Moriya (DM) interactions.