The morphologies of the AAO templates and nanobrush are shown in Fig. . Figure , show the typical top view and side view of AAO template, respectively. The straight nanoholes with average diameter around 50 nm demonstrate a symmetrical distance for each other, which is propitious to gain regular nanowire arrays. It can be found in Fig. , that nanowires are filling into the porous of AAO template and parts of nanowires appear after etching AAO template by using NaOH solution. After sputtering Fe70Co30 layer on one side of the nanowire arrays membrane, magnetic nanobrush with AAO template can be obtained nominally as in Fig. . It is worthy to note that nanowire arrays are about 3 μm long with diameter of 50 nm, and the thickness of nanofilm is lower than 60 nm; thus, Co nanowire array is still the main body of nanobrush.
SEM images: a top view, b side view of AAO template, c top view, and d side view of the nanobrush with wire diameter of 50 nm
Figure shows the normalized loops of Co nanowires, nanobrushes with different thickness of Fe70
layer, and Fe70
nanofilm with the applied field perpendicular to the plane of membrane. The coercivity and Sq (Mr
) as a function of the thickness of nanofilm have been described in Fig. . It is found that the coercivity and Sq are the largest for cobalt nanowire arrays, decrease regularly with the increase of thickness of Fe70
layer, and smallest as the thickness reaches 60 nm. Subsequently, the coercivity and Sq of Fe70
nanofilm with the thickness of 60 nm were drawn too, it has lower coercivity and Sq compared with nanobrushes. This indicates that the static magnetic properties of nanobrush can be controlled by changing the thickness of ferromagnetic layer like Fe70
layer. On the other hand, Fig. shows the normalized loops of samples mentioned above under the applied field parallel to the plane of membrane. The coercivity and Sq of nanowire as a function of the thickness of Fe70
layer were drawn in Fig. . It demonstrates that the coercivity of nanobrushes decreases regularly as increasing the thickness of Fe70
layer, which is similar to the results as the applied field perpendicular to the surface of membrane. Furthermore, the Sq increases regularly as the thickness of Fe70
layer increasing. The coercivity and Sq of nanobrush are nearly equal to those of Fe70
film with the thickness of 60 nm. It indicates that the magnetic properties of nanobrush can be seen as the transition from nanowire to nanofilm. As well known, the magnetic moment distribution of ideal magnetic nanowire is almost along the long axis of nanowire for its strong shape anisotropy; thus, its easy magnetization direction will be parallel to the long axis of nanowire and hard magnetization direction perpendicular to its long axis. For nanofilm, its magnetic moments lie in the surface of membrane, which results in an in-plane easy magnetization direction and an out-of-plane hard magnetization direction. To investigate the relationship between magnetic anisotropy and the thickness of Fe70
layer of nanobrush, the effective anisotropy fields of nanobrush were calculated [24
], and listed in Table .
a Normalized hysteresis loops of Co nanowire array, nanobrushes, and nanofilm, b Coercivity and Sq of nanobrush as a function of the thickness of nanofilm with the applied field perpendicular to the surface of membrane
a Normalized hysteresis loops of Co nanowire array, nanobrushes, and nanofilm b Coercivity and Sq of nanobrush as a function of the thickness of nanofilm with the applied field parallel to the surface of membrane
The effective anisotropy field of nanowire, nanofilm, and nanobrush with different thickness of Fe70Co30 layer
Table indicates that nanowire array and nanobrush with 20 nm thickness of Fe70Co30 layer show easy-axis type of anisotropy, and their easy magnetization direction is along the long axis of nanowire, whereas nanobrush with Fe70Co30 layer higher thickness and nanofilm are easy-plane type of anisotropy will be magnetized easily in the film plane. The transition from easy-axis type to easy-plane type can be obtained as the thickness of Fe70Co30 layer increases. We also find that the effective easy-axis anisotropy field decreases from nanowire arrays to Sample A, while the effective easy-plane anisotropy field increases as the thickness of Fe70Co30 layer increases. Therefore, it is an effective method to control the magnetic anisotropy of magnetic nanobrush by adjusting the thickness of magnetic film. In order to prove this result, micromagnetic simulation was applied to study the magnetization reversal processes of magnetic nanobrush and Co nanowire arrays.
Figure shows normalized hysteresis loops of nanobrush and Co nanowire arrays with the applied paralleled to the long axis of wire, and their magnetic moment distributions at the applied field of 2700 Oe were also shown in the Fig. via micromagnetic simulation. The result also indicates that the magnetic property of nanobrush is determined by the competition of magnetic anisotropy between magnetic film and wires. Firstly, the magnetic moment of nanobrush combining film with wires is out-of-plane and does not parallel the long axis of wires. These magnetic moments are the natural nucleation in the magnetization reversal process of nanobrush, which makes the reversal process easier. To make the magnetization reversal processes of magnetic nanobrush clear, we also simulate the hysteresis loop of Co nanowire arrays. It is found that the coercivity and Sq of nanobrush are lower than those of Co nanowire arrays, which agree well with the experimental results. As well known, the magnetic moment of nanofilm lies in the plane for its shape anisotropy, the magnetic moments of nanowire are along the long axis of wire for the same reason. OOMMF simulation result shows that the direction of the magnetic moment in the film relies on the magnetic moment at the end of wire that is close to the film. It will incline to the +Z direction if the magnetic moment of the end part wire is parallel to the +Z direction, while incline to the −Z direction parallel to the −Z direction. Thus, the magnetic moment will be like a consecutive U-shaped semicircle shown in the Fig. for the two wires with anti-parallel magnetic moment direction and film links them, which lead to the interaction of neighbored wires increases. Furthermore, we chose point A and point B corresponding to the magnetic moments of nanowire array and nanobrush at the applied field of −2700 Oe in Fig. . For the nanowire arrays, all the magnetic moments of sixteen nanowires align along the +Z direction at point A, whereas the magnetic moments of eleven nanowire align along the +Z direction and the other five along the −Z direction in nanobrush as shown at point B. The result means that the magnetic moments of five wires reversed in nanobrush and no one reversed in nanowire arrays at the applied field of −2700 Oe. Figure also demonstrates that the adverse fields of nanobrush and nanowire arrays are 1900 Oe and 2700 Oe, respectively. Thus, the magnetic moment of nanowire arrays in nanobrush can be reversed easily compared with the general nanowire arrays. These also agree with the magnetic measurement results that magnetic layer sputtered on the nanowire arrays film will be propitious to the magnetization reversal of nanowire arrays.
Figure 5 Normalized hysteresis loops of FCC Co nanowire arrays and nanobrush with the applied field parallel to the long axis of wire via micromagnetic simulation. Magnetic moments distribute for a nanowire arrays and b nanobrush with the applied field of 2700 (more ...)