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

Abstract

Because of the potential application of Co(OH)2 in a magnetic cooling system as a result of its superior magnetocaloric effect many people have investigated magnetic properties of Co(OH)2. Unfortunately, most of the works have been carried out on the β-Co(OH)2 phase due to the fact that the α-Co(OH)2 phase is very unstable and continuously transformed into the stable β-Co(OH)2 phase. However, in the present work, using a MoS2 sheet as a two-dimensional template, we have been able to synthesize a stable α-Co(OH)2 phase in addition to a β-Co(OH)2 phase by varying the layer thickness. It is seen that for thinner samples the β phase, while for thicker samples α phase, is grown on the MoS2 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 β 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 α-Co(OH)2 phase. Complete saturation in the hysteresis curve under application of very low field having coercivity of ∼162 Oe at 2 K and 60 Oe at 50 K is obtained. A thin stable α-Co(OH)2 phase grown on MoS2 surface with very soft ferromagnetic ordering will be very useful as the core material in electromagnets.