Magnetic Properties of Layered Ni/Cu Nanowires

The magnetic properties of layered nanowires (NWs) composed of alternating nickel and copper layers were studied. In such structures, magnetic properties are governed by several factors, such as the aspect ratio of alternating layers, the dipole interaction between neighboring layers within a single NW, and the interaction of neighboring NWs. NW arrays were formed by matrix synthesis. Nickel layers had a fixed thickness of 400 nm, and the thickness of copper layers varied from 25 to 300 nm. The magnetic characteristics of such NWs were studied in two states: in a matrix (integral magnetic characteristics determined by vibrating sample magnetometry) and for individual NWs (local magnetization visualized by magnetic force microscopy (MFM)). For NWs in a matrix, the hysteresis loops measured for two magnetic field directions became identical when the thickness of a Cu layer increased to 300 nm due to the weakening of dipole interaction between Ni layers inside NWs and the growing role of dipole interaction between neighboring NWs. In this case, the residual magnetization grew after a field parallel to the matrix plane was applied. The samples with a Cu layer thickness of 300 nm were studied by MFM. It was step-by-step demonstrated how the application of an external magnetic field led to magnetization reversal. Magnetization reversal in a pair of NWs was revealed to occur in two stages like in a two-phase system with two characteristic fields: H_c1 = 40–50 Oe for the formation of a pair with the opposite magnetization direction and H_c2 =160 Oe for complete magnetization reversal. The latter value was close to the coercive force for an array of NWs in a matrix.