Vortex and magnetoresistance in nanomagnet

The vortex structure in nanomagnetic thin film is studied by means of the relaxation simulation using classical spin dynamics. The fast Fourier transformation (FFT) method is implemented to accelerate the simulation. It is shown that the vortex-like structures can occur of which the spins (a) in the core of the vortex and (b) near the comers of square sample will turn out-of-plane as the thickness goes beyond some appropriate layers. But the reason is different for these two cases: (a) for very thin films the spin's turning-up is dominated by decreasing the exchange energy, but when the thickness of sample increases, the role of decreasing dipolar energy becomes more important; (b) it is always dominated by lowering the dipolar energy. When the spin vortex configuration appears by tuning external magnetic field, the MR(magnetoresistance) may acquire a sharp increase.