Individually grown cobalt nanowires as magnetic force microscopy probes

Abstract

AC electric fields were utilized in the growth of individual high-aspect ratio cobalt nanowires from simple salt solutions using the Directed Electrochemical Nanowire Assembly method. Nanowire diameters were tuned from the submicron scale to 40 nm by adjusting the AC voltage frequency and the growth solution concentration. The structural properties of the nanowires, including shape and crystallinity, were identified using electron microscopy. Hysteresis loops obtained along different directions of an individual nanowire using vibrating sample magnetometry showed that the magnetocrystalline anisotropy energy has the same order of magnitude as the shape anisotropy energy. Additionally, the saturation magnetization of an individual cobalt nanowire was estimated to be close to the bulk single crystal value. A small cobalt nanowire segment was grown from a conductive atomic force microscope cantilever tip that was utilized in magnetic force microscopy (MFM) imaging. The fabricated MFM tip provided moderate quality magnetic images of an iron-cobalt thin-film sample.

FIG. 1.

(a) Computer rendered image of the DENA growth setup, showing a pair of electrochemically etched tungsten electrodes immersed in the growth solution. An individual nanowire grows from one electrode to the other with the application of AC potential. The inset shows an optical image of a 400 nm diameter cobalt wire that was extracted from the growth solution and structurally analyzed as shown in Fig. 2. (b) An illustration of the permanent magnet assisted wire extraction method.

FIG. 2.

(a) SEM image of a DENA grown 100 μm long cobalt nanowire transferred onto a holey carbon TEM grid. The scale bar is 20 μm. The tilted and higher magnification SEM image in the inset shows that the wire has a square cross-sectional profile with a width of 400 nm. The scale bar is 500 nm for the inset. (b) High resolution TEM image of the edge of the same cobalt wire shows the highly ordered atomic planes. (c) Simulated electron diffraction pattern over the observed electron diffraction pattern of the crystalline cobalt nanowire viewed along the incident electron beam direction [$1\overline{1}0$]. (d) Real space schematic corresponding to the electron diffraction pattern in panel (c).

FIG. 3.

Cobalt nanowire diameter control from the submicron to nanoscale by varying the AC voltage frequency (red curve, squares) and solution concentration (blue curve, triangles). The insets show the same magnification SEM images of cobalt nanowires grown with one molar (left) and two molar (right) cobalt salt solutions, visually showing the decrease in the wire diameter with the decreasing concentration. Scale bars in the insets represent 1 μm.

FIG. 4.

Room temperature cobalt wire in-plane hysteresis curves with the applied field parallel (0°) and perpendicular (90°) to the wire axis. The inset shows the SEM image of the 1 mm long cobalt wire used in this measurement. The scale bar represents 200 μm.

FIG. 5.

(a) Optical microscopy image of the cobalt wire grown from the tip of the CAFM cantilever on the left (enclosed in the white dashed circle). The tungsten counter electrode is visible on the right side. (b) SEM image of another cobalt wire segment grown from the tip of the platinum coated AFM cantilever. The scale bar represents 1 μm. (c) Magnetic force microscopy (MFM) image of a FePt thin film sample obtained with the manufactured probe in panel (b). The scale bar is 2 μm. (d) SEM elemental analysis mapping shows the composition of the structure in panel (b), where silicon (blue), platinum (yellow), and cobalt (red) were identified, showing that the wire segment is mainly composed of cobalt.