Domain Wall Tip for Manipulation of Magnetic Particles

We demonstrate a method for manipulation of single magnetic microparticles based on a domain wall tip displaced in a controlled manner. By applying an external magnetic field, the tip can either drag or push magnetic particles. This kind of tweezers has potential applications in probing and manipulating colloidal systems.

Figure 1

Two different ways to obtain a domain wall tip. In (a) roughnesses at the edge generate the characteristic domain. We found that it is most efficient to simply make a small scratch (typically 0.1 mm or less) at the edge. In (b) a small inclusion in the film generates typical Néel spikes, which result in four characteristic domains.

Figure 2

Two of the possible modes of operation are shown in (a) and (b). Here the tip can be used to drag or push colloidal particles in the presence of a strong magnetic field. The black scale bar is $30\mu \mathrm{m}$. In (c) two pictures are superimposed on each other to show how the tip can be used to drag the particles. The black arrows indicate the particle displacement. (d) shows another experiment with another domain wall tip. Here we drag a single particle along the domain wall tip without moving the surrounding particles. To show that only one particle has been dragged, we have made the second layer more transparent so that the particle appears weaker after being dragged. Note that only one particle appears weaker (i.e., has been moved). The black scale bar in (d) is $20\mu \mathrm{m}$.

Figure 3

The normalized $x$ component of the force [Eq. (2)] as a function of distance from the tip under the assumption that $z=a$. Here the solid line corresponds to $H_{\mathrm{e}\mathrm{x}}=500\mathrm{A}/\mathrm{m}$, the dashed line to $H_{\mathrm{e}\mathrm{x}}=250\mathrm{A}/\mathrm{m}$, and the dash-dotted line to $H_{\mathrm{e}\mathrm{x}}=25\mathrm{A}/\mathrm{m}$.