Study of magnetic properties of magnetic force microscopy probes using micronscale current rings

Metal rings with inner diameters of 1 and 5 μm, fabricated using electron-beam lithography, were used to calibrate magnetic force microscopy (MFM). A MFM tip's effective magnetic charge, q, and effective magnetic moment along the tip long axis, m_z, can be determined by the current flowing in the ring. The magnetic moments in the directions transverse to the tip's long axis were estimated by a straight current wire. It was found that for a silicon tip coated with 65 nm thick cobalt on the side, q = 2.8 × 10^-6 emu/cm, m_z = 3.8 × 10^-9 emu, and m_x = m_y = 10^-13 emu, which are negligible compared with m_z. Furthermore, the tip's sensitivity to the second derivative of the magnetic field was found to be about 0.1 Oe/nm².