Specific Features of Synthesis, Structure, Magnetometry, and NMR Spectroscopy of Different-Type Nanowires

Abstract

Nanowires of different types (homogeneous (made of iron) and heterogeneous (layered)), obtained by the template synthesis method, have been investigated. A technique for preparation of layered nanowire arrays with alternating thin layers of magnetic and nonmagnetic metals (Co/Cu and Ni/Cu) has been developed and described. Microscopy methods (SEM and TEM with elemental analysis) are used to study topography of the obtained structures, nanowire diameters, individual-layer thicknesses, and specific features of layer interfaces. Several ways of synthesis of nanowires with thin layers and sharp interfaces are proposed: dilution of electrolyte, use of a reference electrode, and monitoring of the charge passage. Layered nanowires are investigated by magnetometry; it is shown that the magnetic properties of a layered nanowire array (in particular, direction of the easy magnetization axis in the Co/Cu nanowire array) depend not only on the aspect ratio of the magnetic layer but also on the thickness ratio between the magnetic-metal layer and the nonmagnetic spacer (copper interlayer). Nanowires of the two types are studied using the nuclear magnetic resonance (NMR) method. Layered Co/Cu structures are analyzed by ⁵⁹Co NMR: it is shown that nanowires with smaller-thickness layers (and, accordingly, with a larger contribution from the interfaces) are characterized by a larger fraction of Co atoms coordinated by Cu atoms. The large fraction of the Cu-coordinated atoms suggests that copper impurity enters cobalt layers. Homogeneous nanowires made of iron are compared with bulk iron samples (⁵⁷Fe NMR). The line is found to be shifted to higher frequencies (by 0.3 MHz), which is indicative of an increase in the field by about 0.2 T. A significant line broadening and a decrease in the spin–lattice relaxation time may indicate that there is a large dispersion between the local magnetic field values.