Muon Spin Relaxation and Magnetic Susceptibility Measurements in the Haldane System (Y 2-xCa x)Ba(Ni 1-yMg y)O5

We have performed dc magnetic susceptibility (χ) and muon spin relaxation (μSR) measurements of the Haldane system (Y2-xCax)Ba(Ni1-yMgy)O5, in which the chain length is controlled by Ca2+ and/or Mg2+ doping. In the nominally pure system (x=y=0), we confirmed the absence of static magnetic order down to 100 mK. Similarly, the Mg doped systems ( y=1.7% and 4.1%) stay paramagnetic down to 50 mK. In the Ca doped systems ( x=9.5% and 14.9%), the doping-induced unpaired moments exhibit a spin-glass-like cusp in χ, but μSR has revealed spin fluctuations persisting at 50 mK, suggesting an unconventional ground state.