Diameter-dependent coercivity of cobalt nanowires

We show that the coercivity of electrochemically grown cobalt nanowires (NWs) within the pores of a polycarbonate membrane can be changed to a large extent by tuning their diameters. The face centered cubic crystalline structure of the NWs having diameter in the range of 10 to 200 nm could be retained. Smaller diameter wires (below 30 nm) are found to be single crystalline and oriented in the [110] growth direction, but for higher diameter wires the crystallite size became very small. Magnetization measurements with an applied field parallel to the axis of the NWs show that the nature of the M–H loop changes from square to linear as the diameter of the NWs increases. The coercivity was found to be 1700 Oe and 480 Oe at 5 K (1000 Oe and 250 Oe at 300 K) for 10 nm and 100 nm wires, respectively. The observed changes in the nature of the M–H loop and in coercivity could be explained following the Stoner–Wohlfarth model and using the fact that the domain size reduces as the diameter of the wires increases.