Muon Spin Relaxation and Magnetic Susceptibility Measurements in the Haldane System (Y2−xCax)Ba(Ni1−yMgy)O5

We have performed dc magnetic susceptibility $\left(\chi \right)$ and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements of the Haldane system $\left({\mathrm{Y}}_{2-x}{\mathrm{Ca}}_x\right)\mathrm{Ba}\left({\mathrm{Ni}}_{1-y}{\mathrm{Mg}}_y\right){\mathrm{O}}_5$, in which the chain length is controlled by ${\mathrm{Ca}}^{2+}$ and/or ${\mathrm{Mg}}^{2+}$ doping. In the nominally pure system $(x=y=0\mathrm{})$, we confirmed the absence of static magnetic order down to 100 mK. Similarly, the Mg doped systems ( $y=1.7\mathrm{}$% and 4.1%) stay paramagnetic down to 50 mK. In the Ca doped systems ( $x=9.5\mathrm{}$% and 14.9%), the doping-induced unpaired moments exhibit a spin-glass-like cusp in $\chi$, but $\mu \mathrm{SR}$ has revealed spin fluctuations persisting at 50 mK, suggesting an unconventional ground state.