Since its introduction in 1956, hard disk drives have become one of the dominant products in the industry of data storage. The capacity of the hard disk drives must keep evolving to store the exploding data generated in the era of big data. This demand pushes the development of technologies including heat assisted magnetic recording (HAMR), microwave assisted magnetic recording (MAMR) and bit-patterned media (BPM) to increase the areal density beyond 1Tb/in2. In the development of these technologies, it is essential to have a clear understanding of the dynamics and nanoscale heat transfer behavior across the head-disk interface.

In this dissertation, dynamics and nano-scale heat transfer in the head disk interface are discussed. Experimental study of nano-scale heat transfer is conducted with the specifically designed static touchdown experiment. Simulation strategy that incorporates the wave-based phonon conduction theory was also developed.

In the flying condition, correlation between the temperature and head disk spacing was found at both passive flying stage and modulation stage. When the flying height increases due to either disk surface microwaviness or contact induced modulation, head temperature will increase, with a slight time delay, indicating the existence of a cooling effect as the head approaches the disk.

The static touchdown experiment, which decouples the complicated air bearing from the nano-scale interface was further designed and performed. The heat transfer behavior across a closing nano-scale gap between head and disk was observed and measured. Experimental and simulation results showed general agreement with the theoretical predictions of the wave based theory for radiation and phonon conduction. The effect of different factors including humidity, air pressure, lubricant layer and disk substrate in the static touchdown experiment were also studied separately.

Furthermore, the dynamics of HAMR condition was studied with waveguide heads. The laser induced protrusion was found to be around 1~2 nm in height.

The findings of this dissertation could be applied to future HAMR head/media design, and the static touchdown experiment could be potentially improved to be a new approach to measure material conduction coefficient and emissivity with high special resolution.

As data generated worldwide are growing explosively, it is crucial to increase the areal density of traditional storage devices to satisfy the requirements. Conventional hard disk drive (HDD) technology, perpendicular magnetic recording (PMR), has reached the superparamagnetic limit of ~ 1 Tb/in2. To realize the areal density over 1 Tb/in2, the size of the media bits must be further decreased to tens of nanometers, which requires high coercivity magnetic media. The high coercivity can avoid superparamagnetism and thus store data safely at the small bit size under room temperature, but it makes data writing challenging. To assist the writing process, energy is input to the media to lower its coercivity temporarily. Current technologies such as heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) utilize two different methods to lower the coercivity. HAMR integrates a laser to locally heat the media to its Curie temperature (400–500 °C), while MAMR uses a spin torque oscillator to induce ferromagnetic resonance in the media grains.

In the HAMR head-disk interface (HDI), a recording head flies over a rotating disk with a relative velocity of 5–40 m/s and an initial spacing of 10–15 nm controlled by an air bearing. Then, the spacing is reduced by energizing a joule heater inside the head. The heater generates a protrusion on the head surface to lower the initial spacing to 1–2 nm so that data reading/writing can be performed using the read/write transducers in the head. The head is also integrated with a laser diode, a waveguide (WG) and a near-field transducer (NFT) for laser delivery. The laser beam is launched from the recording head and is focused on the recording disk to locally heat the disk (400–500 °C), which is even hotter than the head temperature (150–250 °C). Therefore, the head-disk interface of HAMR is a system that combines nanoscale spacing (< 15 nm), high temperatures (head ∼ 150–250 °C, disk ∼ 400–500 °C), steep thermal gradient (∼ 10 K/nm), and a high-speed sliding condition (5–40 m/s). Furthermore, the introduction of the laser affects thermal transport and thermal protrusion, and causes thermally-induced material transfer in the interface, which needs to be investigated both for fundamental understanding and for practical applications such as HAMR and other microelectronics devices.

To study the thermal transport across a closing gap between the head and the disk, we conducted static touchdown experiments using a custom-made setup where the disk is not rotating to exclude the air cooling effect. The head temperature rise was measured as a function of the heater power under various conditions such as different substrate materials, relative humidity and laser on/off. An enhanced thermal transport due to phonon heat conduction is observed for the gap < ∼ 2 nm. The thermal transport across the gap becomes stronger when a better thermal conductor is used as the substrate or when the humidity is higher than 75%. With the presence of the laser, the head undergoes a joule heat dissipation inside the head and a back-heating from the hot spot on the substrate.

In the HAMR operations, the laser delivery involves energy loss, which leads to a localized angstrom-level laser-induced protrusion (LIP) and a fly height change (FHC). They need to be considered and compensated in the spacing control. Flying touchdown experiments were performed to evaluate their overall effect on the spacing change, then they were separated using their different time constants in microseconds and milliseconds. In addition, HAMR operations may utilize two heaters in the head. It is demonstrated that the head protrusion shape can be modulated by use of the dual heaters, and that the touchdown area can be controlled precisely.

During the laser exposure under HAMR operations, material transfer also happens due to the high level of thermal transport. The temperature of the hot spot on the disk is much higher than the lubricant evaporation temperature, so the lubricant is evaporated from the disk and then condenses on the head surface. The material accumulation on the head surface, also known as smear, is a challenging reliability issue for HAMR. We experimentally investigate the smear formation mechanism and propose two smear mitigation strategies. The results show that the smear forms when the lubricant evaporation occurs for a certain time, and that the smear can be mitigated by a mechanical burnishing approach or a thermal approach.

Next, we report a thermal mapping technique using a phase change material Ge2Sb2Te5. Ge2Sb2Te5 undergoes a crystalline transition at 149 °C with changes in its density and optical reflectivity. By use of these changes, we can map surface temperatures from nanoscale to microscale with minimal calibration, which is demonstrated using a recording head.

Finally, we propose a near-field thermal transport based scheme for lubricant thickness measurement. The thermal effect of the lubricant is investigated when the head approaches the disk in the flying touchdown experiments, which is then used to determine the lubricant thickness. Most previous lubricant measurements require an ex-situ tool such as optical surface analyzer (OSA), but the proposed scheme is an in-situ method with a sub-angstrom resolution and a faster response time. Using the scheme, we performed in-situ measurements of the lubricant depletion and reflow dynamics under HAMR operations.

This dissertation investigates the effects of shock and the disk surface pattern on the head disk interface (HDI) response in hard disk drives (HDDs). A new local adaptive mesh method is proposed at the end to improve the accuracy and efficiency of the algorithm to simulate the sliders' steady flying attitudes.

Over the past decades, there has been an increase in the demand of HDDs used in portable devices. In such applications, the work performance of a HDD mainly depends on its ability to withstand external disturbances. Studies of the HDD's responses and failures during external shocks can be very beneficial for improving the HDD's design.

A multi-body operational shock (op-shock) model is developed for this purpose in this thesis. The Guyan reduction method is used to model all the components considered in the op-shock model (a disk, a spindle motor, a base plate, a pivot and a head actuator assembly (HAA)). A fluid dynamic bearing (FDB), between the rotating and stationary units in the spindle motor, is simplified as a spring-dashpot system to save computation efforts. The same simplification is applied to a ball bearing (BB) system between the rotating and stationary units in the actuator pivot. Then the reduced models for all the components are assembled to obtain a complete multi-body op-shock model. Four models which include different components are introduced in this thesis to investigate various components' effects on the HDD's operating performance. The HDDs' failure mechanisms are also studied. It is found that different components influence the HDI responses in different ways.

The ramp load/unload (LUL) technology has been proved to be a better alternative to the contact start-stop (CSS) approach due to the advantages of increasing areal density and greater durability. However, the application of the LUL ramps in the HDDs increases the possibility of collisions between the disk and the ramps since the ramps sit closely to the disk's outer radius. Therefore, it is important to study the ramp effects on the HDD's response during a shock. A reduced model of a deformable ramp is developed and implemented to the multi-body op-shock model. Numerical analyses using three ramp models (no-ramp model, rigid ramp model and deformable ramp model) are carried out to study the HDD's failure dependence on different ramp models.

Bit patterned media (BPM) recording is one of the promising techniques for future disk drives in order to increase the areal density above 4 Tbit/in2. In patterned media, an individual recorded bit is stored in a distinct magnetic island. Thus, the BPM can change the topography of the disk surface and has an effect on the flying characteristics of the air bearing sliders. Proper designs of sliders and disks in the HDDs are required in order to achieve a stable work performance. So a simulator to model a slider's flying condition over a BPM disk is particularly important. Three methods (the averaging method, the Homogenization method and the Taylor expansion Homogenization methods) are implemented to simulate a slider's flying attitude, and finally an economical accurate method is chosen (the Taylor expansion Homogenization method) to investigate the slider's dynamics on partially planarized patterned media.

In modern HDDs, the requirement of small and steady head disk spacing leads to more complicated air bearing surface designs. Thus it is challenging for an air bearing simulator to accurately capture the pressure under a slider's surface. A new local adaptive grid-generating algorithm is developed and is used to simulate the sliders' steady flying attitude. Local finer meshes (mesh's dimension decreases to half) are created on the nodes of the current grids, which have pressure gradients or geometry gradients larger than a pre-defined tolerance. Two sliders are used to demonstrate the applicability of this method. It is found that this new local adaptive grid-generating method improves the stability and efficiency of the simulation scheme.

To compete with solid state drives (SSDs), hard disk drives (HDDs) must improve their performance in capacity, speed and reliability, which requires the spacing between the magnetic disk, used to store information, and the magnetic transducer, used to read information from and write information onto the disk, to decrease. This distance is now approaching 5nm, and, accordingly, the distance between a slider, embedding the transducer, and the disk ranges from several nanometers to several micrometers, which makes the gas flowing between the slider and the disk rarefied. This dissertation applies rarefied gas dynamics to investigate several issues related to HDDs' performance.

Particle contamination on the slider may scratch the disk and induce loss of data. An improved model is proposed to numerically study particle contamination on a thermal flying-height control (TFC) slider, which adjusts the transducer-disk spacing by use of a small heater embedded in the slider near the transducer. It is found that the currently used model is sufficiently accurate despite its simple form.

The temperature increase inside HDDs during operation may affect their reliability. This dissertation derives an analytical formula for the gas-flow induced shear force in the head-disk interface (HDI) and uses it to investigate how the raised temperature affects the slider's flying attitude and the shear forces on the slider and the disk.

Numerical prediction of a TFC slider's flying performance lays the foundation for commercial designs of TFC sliders. An improved model is proposed to calculate the heat flux on the TFC slider and it is found that the currently used model is accurate enough for this purpose. Finally, a general approach is proposed to numerically investigate a TFC slider flying in gas mixtures.

As a recent development to further reduce the flying height of a magnetic head in hard disk drives (HDDs) to nanometers, thermal flying-height (TFC) control technology is now widely applied in the HDD industry because it enables consistent read/write spacing, increased storage density and improved HDD reliability. The fast development of TFC technology presents new challenges to head designers because of the complicated structure of a TFC head, the thermo-mechanical-coupling effects and tribology issues arising at nanometer read/write spacing.

A steady-state TFC solver dedicated to obtaining the steady-state flying attitude of a TFC slider is developed in this thesis. This solver uses a finite volume based solver (CML static solver) to solve the generalized Reynolds equation and obtain the pressure and spacing fields in the air bearing and a commercial coupled-field solver (ANSYS) to obtain the stress and strain fields due to internal heating. An iterative procedure is adopted to consider the cooling effect of the air bearing on the heater-induced protrusion. Accuracy of the solver is verified by drive-level magnetic tests on several combinations of air bearing and heater designs.

TFC sliders' performances under different ambient conditions are investigated based on the TFC solver. It is found that the thermal actuation efficiency of a TFC slider increases with altitude because of the weakened cooling and reduced air bearing stiffness at the transducer area at a higher altitude. In addition, a TFC slider maintains a more consistent read/write spacing at different humidity levels, compared with a non-TFC slider, because the thermal actuation is able to compensate part of the pressure loss caused by water condensation. A TFC slider's flying height in air-helium mixtures is shown to be a highly nonlinear function of the fraction of helium in the gas mixture due to the combined effects of the gas mean free path, viscosity and heat conductivity. These results provide general guidelines for heater and ABS designers to reduce a TFC slider's sensitivity to ambient conditions and improve HDD reliability.

A touchdown numerical model for predicting TFC sliders' dynamics at touchdown and over-pushed conditions is developed and implemented based on the CML dynamic simulator. It extends the solution of the time-varying generalized Reynolds equation to near-contact and contact conditions using a statistical multi-asperity approach. Various interfacial forces are considered by use and further development of a sub-boundary lubrication model to capture important tribological effects occurring at touchdown. This model is able to predict a TFC slider's unstable dynamics at the beginning of touchdown, which has been discovered in many related experimental studies. The effects of different head-disk interface factors are investigated using this numerical model. It is found that the suspension is actively involved in the TFC slider's bouncing vibrations and has a significant influence on the excited second air bearing pitch mode. It is also shown that adhesion force serves as an essential factor in exciting the second air bearing mode whereas other interfacial forces only affect details of the slider's bouncing behaviors. By changing the interfacial properties, namely, the interface roughness and lubricant thickness, the variation of interfacial forces with spacing reduction differs, which leads to very different touchdown patterns. With a rougher interface profile the slider smoothly transfers from a flying stage to a sliding stage. With a smoother interface profile the slider experiences a flying-bouncing-sliding transition. With the smoothest interface the slider goes through a flying-bouncing-surfing-sliding transition.

The touchdown behaviors predicted by the numerical simulator are correlated with experiments conducted on industry-provided head parts with the same ABS and suspension design. Similar touchdown stages and excited modes are also discovered in the experiments. Though experiments showed a slider spectrum with richer frequency components, the modes missed from the numerical simulations are recovered by conducting a harmonic analysis on a full HGA model with air bearing included.

The different touchdown dynamic patterns predicted here result in significant differences in the successful touchdown detection, which is very important for realizing reliable read/write operations, and therefore this work provides guidelines for head disk interface (HDI) optimization. The general approach proposed here is also applicable to studies on the effects of other important HDI factors, such as air bearing geometric features, heater-induced protrusion profiles, and suspension design parameters, and on the slider's touchdown dynamics behaviors, which will assist in obtaining solutions to performance and reliability issues in current hard disk drives.