Observation of ferromagnetic ordering in a stable α−Co(OH)2 phase grown on a MoS2 surface

Because of the potential application of $\mathrm{Co}{(\mathrm{OH})}₂$ in a magnetic cooling system as a result of its superior magnetocaloric effect many people have investigated magnetic properties of $\mathrm{Co}{(\mathrm{OH})}₂$. Unfortunately, most of the works have been carried out on the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Co}{(\mathrm{OH})}₂$ phase due to the fact that the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Co}{(\mathrm{OH})}₂$ phase is very unstable and continuously transformed into the stable $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Co}{(\mathrm{OH})}₂$ phase. However, in the present work, using a $\mathrm{Mo}{\mathrm{S}}₂$ sheet as a two-dimensional template, we have been able to synthesize a stable $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Co}{(\mathrm{OH})}₂$ phase in addition to a $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Co}{(\mathrm{OH})}₂$ phase by varying the layer thickness. It is seen that for thinner samples the $\ensuremath{\beta}$ phase, while for thicker samples $\ensuremath{\alpha}$ phase, is grown on the $\mathrm{Mo}{\mathrm{S}}₂$ surface. Magnetic measurements are carried out for the samples over the temperature range from 2 to 300 K and it is seen that for the $\ensuremath{\beta}$ phase, ferromagnetic ordering with fairly large coercivity (1271 Oe) at 2 K is obtained instead of the usual antiferromagnetism. The most interesting result is the observation of ferromagnetic ordering with a transition temperature (Curie temperature) more than 100 K in the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Co}{(\mathrm{OH})}₂$ phase. Complete saturation in the hysteresis curve under application of very low field having coercivity of $\ensuremath{\sim}162\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}$ at 2 K and 60 Oe at 50 K is obtained. A thin stable $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Co}{(\mathrm{OH})}₂$ phase grown on $\mathrm{Mo}{\mathrm{S}}₂$ surface with very soft ferromagnetic ordering will be very useful as the core material in electromagnets.