Remanence of Ni nanowire arrays: Influence of size and labyrinth magnetic structure

The influence of the macroscopic size of the Ni nanowire array system on their remanence state has been investigated. A simple magnetic phenomenological model has been developed to obtain the remanence as a function of the magnetostatic interactions in the array. We observe that, due to the long range of the dipolar interactions between the wires, the size of the sample strongly influence the remanence of the array. On the other hand, the magnetic state of nanowires has been studied by variable field magnetic force microscopy for different remanent states. The distribution of nanowires with the magnetization in up or down directions and the subsequent remanent magnetization has been deduced from the magnetic images. The existence of two short-range magnetic orderings with similar energies can explain the typical labyrinth pattern observed in magnetic force microscopy images of the nanowire arrays.

Figure 1

(a) Hysteresis loops as measured by SQUID with the external field applied parallel to the wire direction. The gray dots correspond to the magnetic moment deduced from the MFM images (see Sec. 4C). (b) Hysteresis loops performed by numerical simulations considering different sizes of the sample.

Figure 2

Magnetostatic interaction per wire in the array (solid line) and remanent magnetization (dotted line) as a function of the size of the sample. Black dots correspond to the remanent magnetization obtained from hysteresis loops simulated in Fig. 1b.

Figure 3

MFM images of the (a) initial state and after applying fields of (b) $90\mathrm{Oe}$, (c) $190\mathrm{Oe}$, (d) $290\mathrm{Oe}$, (e) $400\mathrm{Oe}$, and (f) $500\mathrm{Oe}$ parallel to the tip field.

Figure 4

Magnetic configuration of a typical hexagonal cell with (a) minimum energy and (b) first excited state. Black (white) dots represent a wire with its magnetization pointing up (down). The energy difference between both configurations is 10%. (c) Simulated patterned domain structure at remanence state. Image size: $15\times 15\mu {\mathrm{m}}^2$.