Antivortex state in crosslike nanomagnets

We report on results of computer micromodelling of antivortex states in asymmetrical crosslike ferromagnetic nanoparticles and their practical realization. The arrays of cobalt crosses with $1\mu \text{m}$ branches, 100 nm widths of the branches and 40 nm thicknesses, were fabricated using e-beam lithography and ion etching. Each branch of the cross was tapered at one end and bulbous at the other. The stable formation of antivortex magnetic states in these nanostructures during magnetization reversal was demonstrated experimentally using magnetic force microscopy.

Figure 1

The distributions of magnetization for the magnetic states with winding number equal to one. (a) is a vortex state $\nu =1,{\phi }_0=+\pi /2$, (b) is a hedgehoglike state $\nu =1,{\phi }_0=0$, and (c) is an antivortex state $\nu =-1,{\phi }_0=0$.

Figure 2

The model distributions of magnetization in crosslike elements. (a) is for quasivortex state, (b) is for quasiuniform state, and (c) is for antivortex state.

Figure 3

The formation of an antivortex state during the remagnetization of an asymmetrical cross in an external magnetic field. (a) is the initial distribution of magnetization, (b) is the nucleation of vortices in the reversed external magnetic field, (c) is the nucleation of an antivortex core and formation of additional vortices, and (d) is the final antivortex state. The lateral size of the crosses $a$ was about $1\mu \text{m}$; the width of the segments $b$ was 100 nm and size of the bulb was 150 nm.

Figure 4

(Color online) (a) is SEM image of an array of symmetric crosslike cobalt nanostructures. The scale bar is $1\mu \text{m}$. The normalized experimental (b) and model (c) MFM contrast distributions corresponding to the A type of quasiuniform state [Fig. 2b] of magnetization (arbitrary units).

Figure 5

(Color online) (a) is the B-type quasiuniform state with three incoming and one outgoing magnetization vectors (simulation). The normalized experimental (b) and model (c) MFM contrast distribution corresponding to the B type of quasiuniform state of magnetization (arbitrary units).

Figure 6

The model MFM contrast distribution corresponding to the antivortex state [Fig. 2c] of magnetization (arbitrary units).

Figure 7

(Color online) The normalized experimental (a) and model (b) MFM contrast distributions corresponding to the quasivortex distribution [Fig. 2a] of magnetization (arbitrary units).

Figure 8

SEM image of asymmetric Co nanocrosses. The white scale bar is $1\mu \text{m}$.

Figure 9

(Color online) Transformation of magnetic states in asymmetric Co crosses from quasiuniform states to antivortex states in an external magnetic field. (a) is MFM image of the initial quasiuniform state. (b) is MFM image of the same array of crosses in a weak (250 Oe) reversed magnetic field. The cross with antivortex state is indicated by a white circle. (c) is MFM image of the final antivortex states. The directions of the applied magnetic fields are indicated by white arrows. The frame size is $6\times 12\mu {\text{m}}^2$.