A new magnetic phase diagram for the quasi-one-dimensional (1D) spin chain compound Ca$_{3}$Co$_{2}$O$_{6}$*

The spin chain cobaltite system Ca$_{3}$Co$_{2}$O$_{6}$ combines geometric frustration with intrinsic low-dimensionality, giving rise to complex physical phenomena that continue to attract a great deal of interest. A long-wavelength spin-density wave (SDW) has recently been observed in Ca$_{3}$Co$_{2}$O$_{6}$ at zero field, stabilized by a helical exchange pathway among neighboring chains. We establish a new and more comprehensive phase diagram for this exotic system through the evolution of the magnetic entropy change $\Delta $S$_{\mathrm{M}}$ (T,H) associated with the magnetocaloric effect. $\Delta $S$_{\mathrm{M}}$ measurements in a single crystal of Ca$_{3}$Co$_{2}$O$_{6}$ prepared by the flux method demonstrate the suppression of the SDW modulation by small applied magnetic fields (\textless 1T). Metamagnetic transitions to a ferrimagnetic up-up-down spin chain arrangement and full ferromagnetic alignment are observed below 25 K. Short-range ordered (SRO) correlations with an antiferromagnetic character grow in volume as the temperature is lowered below 15 K, resulting in a crossover from $\Delta $S$_{\mathrm{M}}$ (H) \textless 0 to $\Delta $S$_{\mathrm{M}}$ (H) \textgreater 0 at 12 K. Our entropy--based analysis reflects current understanding of the role of SDW and SRO phases in Ca$_{3}$Co$_{2}$O$_{6}$, resolves new sub-features of the ferrimagnetic phase, and extends previous results below the onset of slow dynamics ($\sim$ 10 K).