D. Rugar

Frequency modulation detection using high‐Q cantilevers for enhanced force microscope sensitivity

A new frequency modulation (FM) technique has been demonstrated which enhances the sensitivity of attractive mode force microscopy by an order of magnitude or more. Increased sensitivity is made possible by operating in a moderate vacuum (<10−3 Torr), which increases the Q of the vibrating cantilever. In the FM technique, the cantilever serves as the frequency determining element of an oscillator. Force gradients acting on the cantilever cause instantaneous frequency modulation of the oscillator output, which is demodulated with a FM detector. Unlike conventional ‘‘slope detection,’’ the FM technique offers increased sensitivity through increased Q without restricting system bandwidth. Experimental comparisons of FM detection in vacuum (Q∼50 000) versus slope detection in air (Q∼100) demonstrated an improvement of more than 10 times in sensitivity for a fixed bandwidth. This improvement is evident in images of magnetic transitions on a thin‐film CoPtCr magnetic disk. In the future, the increased sensitivi...

Magnetic force microscopy : general principles and application to longitudinal recording media

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 ...

Improved fiber-optic interferometer for atomic force microscopy

A high‐sensitivity fiber‐optic displacement sensor for atomic force microscopy is described. The sensor is based on the optical interference occurring in the micron‐sized cavity formed between the cleaved end of a single‐mode optical fiber and the microscope cantilever. As a result of using a diode laser light source and all‐fiber construction, the sensor is compact, mechanically robust, and exhibits good low‐frequency noise behavior. Peak‐to‐peak noise in a dc to 1 kHz bandwidth is less than 0.1 A. Images are presented demonstrating atomic resolution of graphite and magnetic force imaging of bits written on a magnetic disk.

Deposition and imaging of localized charge on insulator surfaces using a force microscope

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.

Thermomechanical writing with an atomic force microscope tip

We have developed a new technique to perform fast, reliable nanoindentation of polymer surfaces for possible applications to high density data storage. In this technique, an infrared laser is focused on an atomic force microscope (AFM) tip, which is in contact with a transparent polymethyl methacrylate (PMMA) substrate. The heat from the tip softens the PMMA in the contact region, at which point the local tip pressure creates a pit. The pits range in size from several hundred angstroms to 1 μm, depending on the size of the laser pulse and the loading force on the tip. Pits have been made with pulses as short as 0.3 μs at loads of 10−7 N. By operating the AFM on a rotating disk, we have shown that the features can be written and read at frequencies up to at least 100 kHz.

Sub-attonewton force detection at millikelvin temperatures

A 290-nm-thick single-crystal silicon cantilever has been cooled in vacuum to a temperature of 110 mK in order reduce its thermal motion and thereby improve the achievable force resolution. Since the thermal conductivity of the silicon cantilever is extremely low at millikelvin temperatures, an improved optical fiber interferometer was developed to measure the subangstrom thermal motion with optical powers as low as 2 nW. At the lowest temperature, the cantilever exhibited a quality factor of 150 000 and achieved a noise temperature of 220 mK, with a corresponding force noise of 820 zN in a 1 Hz bandwidth.

Force microscope using a fiber‐optic displacement sensor

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 A/(Hz)1/2 is observed for frequencies above 2 kHz. The drift rate of the sensor is on the order of 3 A/min. As an initial demonstration, laser‐written magnetic domains in a thin film sample of TbFeCo were imaged.

Near‐field optical data storage

A recently developed near‐field optical technique, the solid immersion lens (SIL), is utilized in a realistic demonstration of near‐field optical data storage. Using 830 nm light, a 360 nm optical spot size is obtained at the exit surface of the SIL and is transferred across a small air gap to the surface of a spinning magneto‐optical disk. Reading and writing of data are achieved at a density of 3.8× 108 bits/cm2 with a data rate of 3.3×106 bits/s. The subwavelength gap between the lens and the disk is maintained at a media velocity of 1.25 m/s by incorporating the lens into an air‐bearing slider.

High‐resolution magnetic imaging of domains in TbFe by force microscopy

High‐resolution images of domains written in a magnetic thin film have been obtained for the first time using force microscopy. The sample consisted of 500‐A‐thick Tb19Fe81 with magnetization of 109 emu/cm3. Micron‐sized magnetic domains were thermomagnetically written in the sample using a focused laser beam. Domain images were obtained by observing the magnetic interaction of the sample with a small vibrating magnetized iron tip. Typical observed force gradients were in the range 0.8×10−4–6×10−4 N/m and the forces were in the range 10−12–10−11 N. The spatial resolution of the technique was on the order of 1000 A. This was sufficient resolution to see irregularities in those laser written marks which were recorded using low bias field.

Force microscopy of magnetization patterns in longitudinal recording media

A force microscope with a magnetic tip has been used to examine magnetization patterns in a thin‐film cobalt‐alloy sample similar to that used in magnetic disk recording. Longitudinal magnetic bits were written on discrete tracks with a recording head flown over the surface of the sample. After minimal sample preparation, images were obtained showing strong magnetic contrast. Model calculations for the expected image contrast were found to be in excellent qualitative agreement with experimental results. By using a constant height imaging mode, enhanced contrast for fine detail was obtained.