Field Angle and Thickness Dependence of Coercivity in Electrodeposited CoNi–Cu Multilayer Nanowires

Variation in the coercivity of CoNi-Cu multilayer nanowires grown by electrodeposition using a hole pattern of alumina templates has been systematically studied as a function of the CoNi layer thickness and the angle between an applied magnetic field and the nanowire axis. We have found that the magnetization reversal for the nanowires with thin disk-shaped CoNi layers is in the coherent rotation mode, while that for the nanowires with long rod-shaped CoNi layers can be interpreted by a combination of the coherent rotation and curling modes. With decreasing CoNi layer thickness t(CoNi) from 6.8plusmn0.8 to 1.7plusmn0.2 nm, the coercivity rapidly decreases from 305 to 10 Oe, indicating that for t(CoNi)<1.7plusmn0.2 nm, the CoNi layers of the nanowires become superparamagnetic at room temperature. The room temperature superparamagnetism is further confirmed by magnetization measurements at low temperatures using a superconducting quantum interference device. The estimated blocking temperatures for t(CoNi)=1.7plusmn0.2 and 1.0plusmn0.1 nm are 181 and 136 K, respectively