Magnetism of spin-chain systems and the role of spin-orbital coupling

Low-dimensional and geometrically frustrated spin systems exhibit some of the most interesting physical phenomena seen in condensed matter physics. Due to the low site connectivity and competing interactions, classical order is often suppressed by quantum and thermal fluctuations, giving rise to novel ground states and quasiparticle excitations. Among the low-dimensional systems, the spin-chain systems with general formula A3MM’O6 (A = alkaline-earth metal, M/M’ = transition metals) have attracted much attention in recent years, due to their reduced dimensionality in the presence of geometrical frustration [1-4]. The crystal structure consists of one-dimensional (1D) chains that are oriented along the c-axis and arranged in a triangular lattice in the ab plane. Beside the strongly one-dimensional crystal structure these compounds show strong spin anisotropy and possibility to investigate the role of spin-orbital coupling on the magnetism. We have been investigating the magnetism of Sr3NiIrO6, Sr3ZnIrO6 and Sr3NiPtO6 using neutron scattering, both elastic and inelastic, as well as muon spin rotation/relaxation (µSR) measurements. Despite the presence of two magnetic phase transitions (TN1 = 75 K and TN2 = 20 K) in the magnetization of Sr3NiIrO6, neutron diffraction study reveals only one long range magnetic ordering below 70 K with antiferromagnetic coupling between Ni2+ and Ir4+ moment along the c-axis. On the other hand the non-collinear magnetic structure of Sr3ZnIrO6 reveals that the moments of Ir4+ are tilted away from the c-axis, indicating the role of Dzyaloshinskii-Moriya (DM) interaction. Our inelastic neutron scattering study reveals a quasi-1D magnon excitations with a giant spin gap of 30 meV in Sr3NiIrO6. The analysis of the spin wave spectrum reveals strongly coupled Ising-like chains (Jz >> Jxy) along the c-axis that are weakly coupled into a frustrated triangular lattice in the ab-plane. The magnetic excitations survive up to 200 K well above the magnetic ordering temperature of TN ∼ 75 K, also indicating a quasi-1D nature of the magnetic interactions in Sr3NiIrO6. Our microscopic model is in agreement with ab initio electronic structure calculations [5] and explains the giant spin-flip field observed in bulk magnetization measurements. We will compare the magnetic excitations observed in Sr3NiIrO6 with that of Sr3ZnIrO6 and Sr3NiPtO6 and discuss the nature of spin-liquid ground state in Sr3NiPtO6 [6].