Role of crystal-field ground state in the classical spin-liquid behavior of the quasi-one-dimensional spin-chain system Sr3 NiPtO6

The spin-chain compound Sr3NiPtO6 is known to have a nonmagnetic ground state. We investigate the nature of the ground state of Sr3NiPtO6 using the magnetic susceptibility χ(T), heat capacity Cp(T), muon spin relaxation (μSR), and inelastic neutron scattering (INS) measurements. χ(T) and Cp(T) do not exhibit any pronounced anomaly that can be associated with a phase transition to a magnetically ordered state. Our μSR data confirm the absence of long-range magnetic ordering down to 0.04 K. Furthermore, the muon spin relaxation rate increases below 20 K and exhibits temperature-independent behavior at low temperature, very similar to that observed in a quantum spin-liquid system. The INS data show a large excitation near 8 meV, and the analysis of the INS data reveals a singlet crystal-electric-field (CEF) ground state with a first excited CEF doublet state at ΔCEF=7.7 meV. The estimated CEF parameters reveal strong planar anisotropy in the calculated χ(T), consistent with the reported behavior of χ(T) of single-crystal Sr3NiPtO6. We propose that the nonmagnetic singlet ground state and a large ΔCEF (much larger than the exchange interaction Jex) are responsible for the absence of long-range magnetic ordering and can mimic a classical spin-liquid behavior in this quasi-one-dimensional spin-chain system Sr3NiPtO6. The classical spin-liquid ground state observed in Sr3NiPtO6 is due to the single-ion property, which is different from the quantum spin-liquid ground state observed in geometrically frustrated systems, in which two-ion exchanges play an important role.