PHYSICAL REVIEW B 96, 214433 (2017) Observation of ferromagnetic ordering in a stable α-Co(OH) 2 phase grown on a MoS 2 surface Anup Debnath, Shatabda Bhattacharya, and Shyamal K. Saha * Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India (Received 1 September 2017; revised manuscript received 30 November 2017; published 26 December 2017) 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 MoS 2 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 MoS 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 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 MoS 2 surface with very soft ferromagnetic ordering will be very useful as the core material in electromagnets. DOI: 10.1103/PhysRevB.96.214433 I. INTRODUCTION It is well known that transition-metal hydroxides exhibit interesting and unusual magnetic behavior [1–5]. In particular, Co(OH) 2 is formed to be very useful in magnetic cooling systems because of its superior magnetocaloric effect (MCE) [6,7]. In general, Co(OH) 2 is crystallized into a hexagonal layered type structure with two polymorphs α-Co(OH) 2 and β -Co(OH) 2 , in which β -Co(OH) 2 possesses a brucitelike structure with a = 3.1 ˚ A, c = 4.6 ˚ A, and the octahedral with divalent Co contains sixfold coordinated by hydroxyl ions share edges to produce two-dimensional (2D) charge neutral layers stacked one over the other along the c axis. However, α-Co(OH) 2 consists of slightly positively charged layers with intercalated charge balancing anions (CH 3 COO − , Cl − , CO 3 2− , and NO − 3 , etc.), [8–10] to restore the charge neutrality. As a result, in the case of α-Co(OH) 2 , interlayer separation increases substantially and can have a value 7–27 ˚ A according to the size of charge balancing anions. Therefore, with this marked change in lattice spacing of the c axis and critical interface chemistry, the magnetic property changes drastically in the two structures of α and β phases. Out of two phases, β -Co(OH) 2 is stable, however, the α-Co(OH) 2 phase is metastable and after formation, the α phase is transformed continuously to a β phase [8,9]. As far as magnetism is concerned, the β phase is known to be an antiferromagnetic [6,7,11–15] in nature but the interlayer interaction diminishes with an increase in interlayer separation as in the case of the α phase [1,15]. So far many reports on magnetism in Co(OH) 2 are available in the literature but most of them are on the β phase where its magnetic and magnetocaloric effects are concerned. They investigated the phase transition in β -Co(OH) 2 from the antiferromagnetic to weak ferromagnetic state with application of external magnetic field at low temperature [6,7,13,14]. Also the magnetocaloric * cnssks@iacs.res.in effect associated with magnetic phase transition has been reported [6,7]. In this case we are able to grow the ultrathin β -Co(OH) 2 phase on the MoS 2 surface, which shows complete ferromagnetic behavior with high coercivity. By varying the thickness of the β -Co(OH) 2 phase on the MoS 2 surface, we are able to tailor its magnetic saturation at low concentration. Till now all the α-Co(OH) 2 phase synthesized are poorly crystalline in nature and have a turbostratical disorder along the c axis where the layers are randomly oriented [8–11]. This prevents creation of a stable α-Co(OH) 2 phase. Because of an unstable α phase, magnetic results in α-Co(OH) 2 have not yet been reported. Therefore, to synthesize stable α-Co(OH) 2 on the MoS 2 surface to investigate its magnetic properties is a real challenge. In the previous works [16–18] we have reported many interesting magnetic results on transition metals and their hydroxides grown on graphene and MoS 2 surfaces. Exploiting the interface interaction, in the present work, we have been able to synthesize a stable α-Co(OH) 2 phase considering MoS 2 as a two-dimensional template. The thickness has been controlled by changing the concentration of the Co precursor. We have prepared four samples with different concentrations of Co precursor keeping the MoS 2 concentration constant. It is seen that for lower concentrations the β -Co(OH) 2 phase is grown while for higher concentrations the α-Co(OH) 2 phase is grown. The most interesting result is the synthesis of stable α-Co(OH) 2 layers using the MoS 2 sheet as a 2D template. The magnetic measurements are carried out on all the samples over the temperature from 2 to 300 K. Because of charge transfer [16] from S to Co the usual antiferromagnetic nature has not been obtained in the case of a thin layered β -Co(OH) 2 phase grown on the MoS 2 surface; rather perfect ferromagnetic ordering with fairly large coercivity of 1271 Oe is observed in this case. As the α-Co(OH) 2 phase grown on the MoS 2 sheet is very stable for a higher concentration of Co precursor, we have been able to investigate the detailed magnetic properties in this thin layered α-Co(OH) 2 sample. Perfect ferromagnetic saturation under very low magnetic field (<1000 Oe) with coercivity of 2469-9950/2017/96(21)/214433(10) 214433-1 ©2017 American Physical Society