Small field excitation of flux closure state
Fig. 1: Time resolved domain images of a 50 nm thin, 40 µm x 40 µm wide Permalloy square.
The small-field excitation of a flux closure domain structure is shown in Fig. 1. The first row displays the actual domain state at the indicated time. The change of magnetization with respect to the initial state is shown in the next two lines and a qualitative sketch of the magnetization pattern is given in the last row.
The first reaction of the magnetization to the pulsed magnetic field is a rotation of magnetization in the upper and lower domain. In these domains the torque acting onto the magnetization is larger than in the other two domains, where the magnetization is parallel or antiparallel to the pulsed field. As the amplitude of the exciting field is rather small (900 A/m) and the duration of the field is less than 1 ns, the domain state of the square is only slightly excited and stays in general in the flux closure state. Due to the rotation of magnetization the two domain walls on the left side are not a sharp line anymore but a rather broad region where the magnetization rotates by 180° between upper and lower edge of the element. The magnetization in the right domain is oriented antiparallel to the pulsed magnetic field and should not react to it. But due to small deviations from a perfectly homogeneous magnetization direction a torque is acting on this magnetization which results in a clock- and counter clockwise rotation of magnetization. At t=0.64 ns two spike domains develop at the upper and lower left corner. These spike domains develop by a clock and counterclockwise rotation of magnetization at the bottom and top of the element, respectively. The formation of these spike domains reduces the magnetostatic energy that increases due to the rotation of magnetization in the center of the element. Due to these spike domains the relaxation process back into the initial state is rather slow compared with the creation of these domains (15 ns vs. less than 1 ns).
A. Neudert et al. Phys. Rev. B 71, 134405 (2005)