Kang Kee Ng

Experimental Study of Lubricant Depletion in Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording and further increase the areal recoding density of a hard disk drive. However, HAMR brings about serious lubricant depletion problems on the disk surface due to the high temperature in the heat assisted writing process. Experimental studies of the lubricant depletion under HAMR conditions are still very limited so far. It is essential to do experimental studies under real HAMR conditions or under equivalent conditions if a stand-alone laser is used to emulate the HAMR system. In this work, a self-developed HAMR tester is introduced. A method to control the repeatability of laser heating temperature is explained. Lubricant depletion, accumulation, loss, the percentage of accumulation to depletion, and the depth of depletion are determined quantitatively. The effects of laser power, total laser-on-time and laser-off-time in one laser heating and cooling cycle on lubricant depletion are studied experimentally with the HAMR tester. From the experimental studies, it is found that lubricant accumulation at the edge of the lubricant depletion track takes a considerable percentage of the lubricant depletion and the lower the laser heating temperature, the higher the percentage is. Furthermore, media cooling time plays a significant role in lubricant depletion for the media without a heat sink layer and on glass substrate.

Nonlinear dynamic effects of thermal flying-height control sliders at touch-down

The touch-down test is widely used to calibrate the flying-height (FH) of sliders by gently contacting the disk with the thermal protrusion of thermal FH control (TFC) sliders. However, the dynamic instability of TFC slider during the touch-down may significantly affect the contact detection and FH calibration. This paper highlights the nonlinear dynamics perspectives on the instability of TFC sliders during the touch-down. The mechanism of dynamic instability of TFC sliders is explored by experiments, nonlinear system models and simulations. The impact factors and air bearing surface (ABS) design considerations for minimizing the critical flying-height are also discussed.

Investigations of Light Contact and Lube-Surfing State With Electrical Current

It was reported that a thermal flying height control (TFC) slider often exhibits a strong bouncing vibration at the touchdown power (TDP), which is suddenly suppressed when the TFC power is increased beyond the TDP. This phenomenon is inferred to be the lube-surfing state during which the sliders dynamics are damped out by the lubricant, and its attitude adjusted so as to achieve a second stable flying state with a tiny bulge surfing in the lubricant. In this paper, the measured current with respect to a full view of the second stable flying state during touchdown, lube-surfing, and takeoff processes of the specific TFC slider will be characterized for in-depth understanding of light lubricant contact.

Frequency Analyses of Air Bearing Slider in Near Contact and Contact States

Determining the air bearing frequencies is important and essential to understand the complex flying performances of slider. However, in typical flying height test, it is usually difficult to distinguish the air bearing frequencies in the experimental data, especially when the slider is in full flying state. In such a case, it is optional to employ simulation to help determine the air bearing frequencies and their harmonic components. This paper performs both time and frequency domain simulations to analyze air bearing frequencies of a pemto slider and compare the results with experimental data obtained using laser Doppler vibrometer. It is found that the simulation results are well correlated with the experimental results. Time domain simulation provides not only the air bearing frequencies, but also the harmonic frequencies. Frequency domain simulation, on the other hand, provides clear identification of the three dominant air bearing frequencies, with additional information such as air bearing stiffness and damping obtainable. It is suggested that simulations in both time and frequency domains should be conducted to assist in determining the air bearing frequencies and their harmonic components.

Effect of Interfacial Roughness on Slider-Disk Interactions at Near-Contact Regime

One of the major efforts in reducing fly height (FH) in order to achieve higher recording areal density for hard disk drive, is to reduce slider-disk interfacial roughness. However, as roughness improves to the atomically smooth level, various interaction forces such as van der Waals force, electrostatic force, and intermittent contact force affect slider fly stability. In this paper, an electrostatic force manipulation methodology was employed to investigate roughness effect on slider-disk interactions at near-contact regime. In such methodology, an AC excitation with pre-determined frequency was applied across the slider-disk interface. First harmonic vibration response of the excitation was monitored. Such vibration response is sensitive to various interaction forces, as FH is reduced to near-contact regime. The phenomena observed were verified with static and dynamic simulations, using self-developed ABsolution simulation software. It was found that disk with higher roughness has larger adhesive force gradient and hence larger first harmonic vibration at near-contact. First harmonic vibration dip was observed near touchdown point. Simulation result suggests that the phenomenon is due to time-averaging effect of slider intermittently “snap” to the disk surface. It was found that magnitude of the vibration dip is surface roughness dependent. At contact regime, nonlinear contact stiffness-based contact detection is highly sensitive for smooth surface. Abrupt transition of low to high vibration state of smooth surface during contact is attributed to larger adhesion hysteresis effect.

Heater AC Voltage Induced Flying Height Modulations

For a thermal flying-height control (TFC) slider, its heater is usually provided with DC voltage. However, recently, both DC and AC voltages may be supplied to the heater. Unlike supplying AC voltage to the slider and disk in the past, the AC voltage to the heater will not only produce a thermal protrusion on the slider, but also leaves a part of the AC voltage on the slider/disk interface. The voltage acts as the electrostatic force and can be used for further control of the slider, even in the drive level. Simulations show that the flying height modulation is highly related to the AC frequency. By sweeping the AC frequencies while monitoring the flying height and pitch angle modulations, the first and second pitch modes of air bearing frequencies can be experimentally obtained without slider/disk contact. The roll mode frequency is also obtainable when the skew angle is not zero. The simulation results agree well with the experimental results obtained by a laser Doppler vibrometer (LDV). Therefore, the sweeping AC frequency method provides a practical scheme to obtain the air bearing frequencies without any slider/disk contact, even in the drive level.

Thermoreflection measurement of magnetic thin films

With recent developments in heat assisted magnetic recording (HAMR), there is growing intense interest in understanding the thermal-optical properties of the HAMR media. We study the phase change of the optical beam as it reflects from a FePt thin film under strong laser irradiation by carrying out close-aperture reflection Z-scan experiments. It is found that the far field spatial distribution of the reflected beam is sensitively dependent on the thermal field distribution in the media. We also carry out a temperature dependent pump-probe experiment to study the variation of the differential thermoreflection signal with different media temperature. It is found that the differential signal is sensitively dependent on the media temperature. The present studies show potential to enable noncontact temperature measurement of HAMR media and detect the spatial distribution of the thermal field in the HAMR media.

Preparation and Characterization of Dedicated Servo Media

The dedicated servo media (DSM) is the conventional perpendicular magnetic recording (PMR) media with an inserted servo recording layer between the data recording layer and the soft underlayer. By using the conventional PMR media as the reference, the design consideration on the layer structure, the preparation, the magnetic properties, the microstructure, including the c-axis orientation and the growth of the layers, the surface roughness, and the recording performance for the DSM are reported.

Electrostatic Force Manipulation Methodology: Principles, Mechanisms, and Setup for Head–Disk Interactions Monitoring

To achieve higher hard disk drive areal density, the head-disk clearance had been reduced to <;2 nm recently. Head-disk interaction monitoring with high sensitivity and resolution is desired to monitor the intense interactions at near-contact and to prevent head-disk contact. In this paper, head-disk interaction monitoring based on the electrostatic force manipulation (EFM) principle is discussed. The mechanisms of the two setups to apply alternating electrostatic force (Fes) across the head-disk interface were investigated. It was shown that the distribution of the applied Fes was different for the two setups. This led to further studies of the effects of the Fes distribution on interactions monitoring. The results showed that when an alternating Fes was distributed at the pole region, the magnitude of the electrodynamically excited first-harmonic vibration increased significantly as the gap fly height was reduced. It was very sensitive to spacing change and useful for the head-disk interactions monitoring at the near-contact regime. The second harmonic was found sensitive to head-disk contact. When the alternating Fes was distributed at the slider body, the magnitudes of the first and second harmonics were larger due to larger effective surface area. However, the fly height of the slider body was mainly affected by the protrusion push-up effect, and hence the first and second harmonics showed a reverse trend as compared with the case where Fes was distributed at the pole region. In addition, slider pitch motion was also significantly affected. The effective EFM setup was constructed to demonstrate its effectiveness in head-disk interactions monitoring and contact detection.