ABSTRACT
Magnetic needles can be broadly defined as any needle-shaped structures that produce magnetic fields. Generally, the dimensions of a magnetic needle tip would not exceed a few millimeters in size. Their most basic functionality is to apply a magnetic force on magnetic entities. In some instances, this force might have the intent of capturing magnetic nanoparticles. In other cases, the force could be used to manipulate the position of a magnetic structure. Depending on the targeted application, the design of the needle is critical. Choosing the appropriate material and type for the magnetic needle (electromagnet or permanent magnet) can have a large impact in terms of performance and ease of manufacturing. This chapter begins by describing the three basic types of magnetic needles along with comparative strengths and weaknesses. Finite-element simulations of each type of needle are also provided to visualize the magnetic field patterns produced in each case. Next, some of the common manufacturing techniques used to create small-scale magnetic structures are introduced. Both bottom-up and top-down approaches for fabrication are described. Then, engineering considerations for developing magnetic needles are discussed. These include the optimum geometry of the needle and the material used for manufacturing. Additional simulations of different geometries are included for visualization. Finally, a first-order analytical model of magnetic force is built based on the needle magnetic properties. These calculations are motivated by two of the common applications of magnetic needles, which are applying a magnetic force and capturing magnetic particles.
