One of the oldest unresolved problems in physics is the mechanism of charge exchange between contacting surfaces when at least one of them is insulating. We describe a new technique, using force microscopy, for studying this problem with greater lateral resolution than has been previously possible. The force microscope is shown to have 0.2-jwm lateral resolution and the sensitivity to detect 3 electronic charges. In contact-charging experiments between the microscope tip and polymethyl methacrylate, the charged region was much larger than the expected contact area and bipolar charge exchange was observed.PACS numbers: 73.40. Bf, 61.16.Di, 73.25.+i, 73.40.Ns When two surfaces are brought into contact, they will generally exchange charge and when subsequently separated will be oppositely charged. This type of charge exchange between objects is observed in such diverse areas as ice particles in clouds, shoes on carpets, and toner particles in electrophotographic copiers and printers, and has been implicated in grain elevator explosions. ^ Despite its everyday occurrence, the physical explanation of this basic phenomenon of contact electrification, or triboelectrification, has eluded researchers since the times of the ancient Greeks. ^ Traditionally, triboelectrification has been classified into three categories: metal-metal, metal-insulator, and insulator-insulator contact. In the first case, when two metals are contacted. Harper^ has demonstrated that electrons flow until the two Fermi levels equilibrate. The process in the later two cases, however, is not well understood and there are contradictory data in the literature. Lowell, Rose-Innes, and El-Kazzaz,"^ for example, have claimed that on contacting a polymer with a series of metals the net insulator charge depends only on the last metal contacted. In similar experiments, Fabish and Duke^ found that the charge on the polymer accumulates with each contacting metal and that each metal acts independently. Based on their observations they proposed a theory of contact-charge spectroscopy wherein each metal accesses different electronic states in the polymer.At the center of this dispute is the nature of the charge sites. If it were possible to identify such sites with near atomic resolution, then a deeper understanding of the triboelectrification process might result. Most current experiments, however, measure the charge exchanged averaged over millimeter-sized areas. In this Letter we describe a novel approach to studying this problem, where, by using a force microscope, the spatial distribution of charge from a single contact can be imaged with higher resolution than has been possible with other techniques. With further refinement, the technique holds promise for being able to image single charges and to locate the charge sites.

This paper discusses the principles of magnetic force microscopy (MFM) and its application to magnetic recording studies. We use the ac detection method which senses the force gradient acting on a small magnetic tip due to fields emanating from the domain structure in the sample. Tip fabrication procedures are described for two types of magnetic tips: etched tungsten wires with a sputter-deposited magnetic coating and etched nickel wires. The etched nickel wires are shown to have an apex radius on the order of 30 nm and a taper half-angle of approximately 3°. Lorentz-mode transmission electron microscopy of the nickel tips reveals that the final 20 μm is essentially single domain with magnetization approximately parallel with the tip axis. Images of written bit transitions are presented for several types of magnetic media, including CoPtCr, CoSm, and CoCr thin films, as well as γ-Fe2O3 particulate media. In general, the written magnetization patterns are seen with high contrast and with resolution better than 100 nm. A number of magnetic recording applications are discussed, including the investigation of overwrite behavior and the writing characteristics in CoSm media at high data density. Computer calculations were performed to simulate the MFM response to written magnetic transitions. By including the extended geometry of the tip, the nonparallel orientation of the cantilever, and the finite width of the magnetic transitions, good agreement with experiment was obtained. The model calculations correctly predict the experimentally observed change in image contrast that occurs as a function of tip orientation. Computer calculations showing the dependence of resolution on tip geometry are also presented.

A force microscope has been used in a new application to deposit and image localized surface charge on insulators. The lateral resolution for imaging surpasses that of currently available techniques. By applying voltage pulses to an etched nickel microscope tip, micron-sized regions of approximately 2×10−16 C were created on polymethylmethacrylate and single-crystal sapphire surfaces. After depositing the charge, high-contrast images of the charged region were obtained as contours of constant force gradient. The contrast was observed to decay over approximately 1 h, providing evidence for surface charge mobility. The minimum detectable surface charge was estimated to be on the order of 100 electrons.

A force microscope is described which uses a fiber-optic interferometer as the cantilever displacement sensor. Low thermal drift and reduced susceptibility to laser frequency variation are achieved due to the small (several micrometer) size of the interferometer cavity. A sensitivity of 1.7×10−4 Å/(Hz)1/2 is observed for frequencies above 2 kHz. The drift rate of the sensor is on the order of 3 Å/min. As an initial demonstration, laser-written magnetic domains in a thin film sample of TbFeCo were imaged.

We have imaged naturally occurring domains in soft magnetic films using the force microscope. Classic closure structures were seen in both sputtered and plated Permalloy films. These structures were compared to optical Kerr micrographs of the same films, with generally good correspondence. The force microscope had sufficient sensitivity and resolution to observe magnetic details such as ripple structure and a Bloch line. Clear evidence of tip-induced wall motion was seen when the tip-to-sample separation was less than about 100 nm.