Shaomin Xiong

Observation of piezoelectricity in free-standing monolayer MoS2

Piezoelectricity allows precise and robust conversion between electricity and mechanical force, and arises from the broken inversion symmetry in the atomic structure. Reducing the dimensionality of bulk materials has been suggested to enhance piezoelectricity. However, when the thickness of a material approaches a single molecular layer, the large surface energy can cause piezoelectric structures to be thermodynamically unstable. Transition-metal dichalcogenides can retain their atomic structures down to the single-layer limit without lattice reconstruction, even under ambient conditions. Recent calculations have predicted the existence of piezoelectricity in these two-dimensional crystals due to their broken inversion symmetry. Here, we report experimental evidence of piezoelectricity in a free-standing single layer of molybdenum disulphide (MoS₂) and a measured piezoelectric coefficient of e₁₁ = 2.9 × 10(-10) C m(-1). The measurement of the intrinsic piezoelectricity in such free-standing crystals is free from substrate effects such as doping and parasitic charges. We observed a finite and zero piezoelectric response in MoS₂ in odd and even number of layers, respectively, in sharp contrast to bulk piezoelectric materials. This oscillation is due to the breaking and recovery of the inversion symmetry of the two-dimensional crystal. Through the angular dependence of electromechanical coupling, we determined the two-dimensional crystal orientation. The piezoelectricity discovered in this single molecular membrane promises new applications in low-power logic switches for computing and ultrasensitive biological sensors scaled down to a single atomic unit cell.

Maskless Plasmonic Lithography at 22 nm Resolution

Optical imaging and photolithography promise broad applications in nano-electronics, metrologies, and single-molecule biology. Light diffraction however sets a fundamental limit on optical resolution, and it poses a critical challenge to the down-scaling of nano-scale manufacturing. Surface plasmons have been used to circumvent the diffraction limit as they have shorter wavelengths. However, this approach has a trade-off between resolution and energy efficiency that arises from the substantial momentum mismatch. Here we report a novel multi-stage scheme that is capable of efficiently compressing the optical energy at deep sub-wavelength scales through the progressive coupling of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). Combining this with airbearing surface technology, we demonstrate a plasmonic lithography with 22 nm half-pitch resolution at scanning speeds up to 10 m/s. This low-cost scheme has the potential of higher throughput than current photolithography, and it opens a new approach towards the next generation semiconductor manufacturing.

A Novel Approach of Carbon Embedding in Magnetic Media for Future Head/Disk Interface

A novel method of carbon embedding (≤1 nm) is used as a surface modification technique to produce overcoat free media surfaces. The filtered cathodic vacuum arc technique at ion energy of 90 eV is used to embed carbon in the top surface of a ~25 nm iron/platinum (FePt) film. Transport of ions in matter (TRIM) simulations and X-ray photoelectron spectroscopy (XPS) are used to study carbon embedding profiles and surface chemical composition. XPS results show that carbon embedding is effective in improving the oxidation resistance of FePt. Conductive atomic force microscopy (CAFM) is done on samples after exposure to a 780 nm IR laser with an effective output power of 40 mW to study the thermal stability. No change in the conductivity is observed in the case of carbon embedded FePt surface. Ball-on-disk tribological tests are conducted at a contact pressure of 0.26 GPa on bare and modified FePt surfaces. It is observed that the coefficient of friction is reduced considerably from a value of approximately 0.8 to ~0.27 after the surface modification.

Investigation of the Local Temperature Increase for Heat Assisted Magnetic Recording (HAMR)

Heat assisted magnetic recording (HAMR) has been proposed for the next generation hard disk drive due to its potentially higher storage areal density. In HAMR systems, a laser is focused onto the disk to heat the magnetic storage layer to the Curie temperature. In this paper, a novel method is proposed to study the temperature increase in a perpendicular magnetic recording media. The disk was prerecorded with a uniform single tone magnetic pattern by flying a magnetic head above it and using a magnetic read/write device. The decay of the magnetic pattern resulting from laser heating under different laser power levels was evaluated quantitatively using a magnetic force microscopy. The temperature increase was estimated by use of an Arrhenius-Neel model of thermally activated magnetization. In addition, a 3-D numerical model was created to obtain the disks thermal response under the laser heating. The thermally erased area was calculated from the temperature distribution based on the numerical results. The calculated erased area was compared with the experimental results from the magnetic images. A good agreement was observed between the experimental and the simulation results. The method should be reliable when used with HAMR media also whenever it becomes available.

A two-stage heating scheme for heat assisted magnetic recording

Heat Assisted Magnetic Recording (HAMR) has been proposed to extend the storage areal density beyond 1 Tb/in.2 for the next generation magnetic storage. A near field transducer (NFT) is widely used in HAMR systems to locally heat the magnetic disk during the writing process. However, much of the laser power is absorbed around the NFT, which causes overheating of the NFT and reduces its reliability. In this work, a two-stage heating scheme is proposed to reduce the thermal load by separating the NFT heating process into two individual heating stages from an optical waveguide and a NFT, respectively. As the first stage, the optical waveguide is placed in front of the NFT and delivers part of laser energy directly onto the disk surface to heat it up to a peak temperature somewhat lower than the Curie temperature of the magnetic material. Then, the NFT works as the second heating stage to heat a smaller area inside the waveguide heated area further to reach the Curie point. The energy applied to the NFT in th...

Experimental Study of Head-Disk Interface in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) has been proposed to achieve magnetic recording density. In HAMR systems, a laser is used to locally heat the magnetic disk to the Curie temperature, which is ~ 400°C-600°C, to assist in the data writing process. The high temperature working condition is a great challenge for the head-disk interface. Lubricant can be depleted by evaporation or decomposition. The protective carbon overcoat can be graphitized and oxidized. The surface quality, such as its roughness, can be changed as well. The near field transducer (NFT) structure is also vulnerable under the large number of heating cycles. All of those effects of the heating will present challenges for the reliability of the HAMR performance. In this paper, an experimental HAMR platform is introduced. The free space laser exposure of the media is calibrated based on the magnetization decay of special data tracks. Lubricant loss is determined from the optical surface analyzer. The lubricant changes as a function of different thermal load cycles are investigated as well. The disk topography is measured by an atomic force microscope after the mobile lubricant is removed. The hard overcoats topography dependence on the laser exposure conditions is also studied. With regard to the media disk, we establish three distinct laser power thresholds for lubricant loss, disk surface degradation, and magnetization decay. As the laser power increases, the lubricant starts to change first before the magnetization starts to decay, indicating that lubricant could be the most vulnerable component to the laser heating in HAMR systems. Then, the disk overcoats surface changes, becoming rougher after the magnetization starts to decay. Furthermore, the lubricant degradation is more serious under cyclic loading conditions. With regard to the transducer, the metal film of the NFT gets removed due to high energy laser ablation, and then the NFT loses its capability to focus the light, as required for HAMR systems.

Nanoscale heat transfer in the head-disk interface for heat assisted magnetic recording

Laser heating has been introduced in heat-assisted magnetic recording in order to reduce the magnetic coercivity and enable data writing. However, the heat flow inside a couple of nanometers head-disk gap is still not well understood. An experimental stage was built for studying heat transfer in the head-disk interface (HDI) and the heat-induced instability of the HDI. A laser heating system is included to produce a heated spot on the disk at the position of the slider. A floating air bearing slider is implemented in the stage for sensing the temperature change of the slider due to the heat transfer from the disk by the use of an embedded contact sensor, and the gap between the two surfaces is controlled by the use of a thermal fly-height control actuator. By using this system, we explore the dependency of the heat transfer on the gap spacing as well as the disk temperature.

Lubricant reflow after laser heating in heat assisted magnetic recording

In heat assisted magnetic recording (HAMR) technology for hard disk drives, the media will be heated to about 500 °C during the writing process in order to reduce its magnetic coercivity and thus allow data writing with the magnetic head transducers. The traditional lubricants such as Z-dol and Z-tetraol may not be able to perform in such harsh heating conditions due to evaporation, decomposition and thermal depletion. However, some of the lubricant depletion can be recovered due to reflow after a period of time, which can help to reduce the chance of head disk interface failure. In this study, experiments of lubricant thermal depletion and reflow were performed using a HAMR test stage for a Z-tetraol type lubricant. Various lubricant depletion profiles were generated using different laser heating conditions. The lubricant reflow process after thermal depletion was monitored by use of an optical surface analyzer. In addition, a continuum based lubrication model was developed to simulate the lubricant refl...

Hard Disk Drive Servo System Based on Field-Programmable Gate Arrays

Servo systems are essential components of current magnetic storage hard disk drives (HDDs). In this paper, we demonstrate a novel servo scheme using a field-programmable gate array (FPGA). Adjacent magnetic tracks with two different frequencies are recorded on the magnetic disk and are used as servo tracks to encode the position information. A discrete Fourier transform is employed to decode the magnetic head position and to obtain the position error signal (PES). The PES is defined in terms of the difference of the magnitudes of the two frequencies. The relationship between the displacement and the PES is obtained by offline calibration. A real-time signal acquisition and processing system using a commercial FPGA, analog-to-digital converter chips, and digital-to-analog converter chips is built to do the high-speed spectrum analysis and generate the PES. The PES is transmitted to a laboratory virtual instrument engineering workbench (LabVIEW) real-time control target to implement the control algorithm. The computed control output from the real-time control target is amplified to drive a piezoelectric stage, which moves the magnetic head along the radial direction to compensate for the position error. A simple repetitive and proportional-integral-derivative control algorithm is implemented for head positioning and to verify this new servo scheme in this paper. With the controller, most of the major disturbances, such as the harmonics of the spindle runout, have been effectively attenuated compared with an open-loop control scheme.

Thermal Response Time of Media in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording product. A laser is introduced to the HAMR system to heat magnetic media to reduce the media coercivity. The thermal response of the media becomes very critical for the success of the magnetic writing process. The study of thermal response time in HAMR relies on the setup configurations, such as laser spot sizes, the way that laser energy is delivered to media and the media structures. In this paper, the thermal response time of HAMR media under three different heating methods is systematically investigated through experiments and numerical analysis. A lumped model is built to simplify the heat conduction problem to understand the difference in thermal responses under various experimental conditions. Dominant layers are identified under those experimental conditions. The transient thermal response is mainly determined by the dominant layers. Engineering the dominant layers helps the most in optimizing the thermal performance of the media. Our study clearly suggests that for HAMR systems, optimizing the thermal properties of the heat sink layer is the key to reducing variations in the transient thermal process resulting from changes in the linear speed.