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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Simulation of Flying Height and Response Time of Thermal Flying Height Control Sliders With Thermal Insulators Hui Li1 , Hao Zheng2 , Joerg Fritzsche2 , Kensuke Amemiya1 , and Frank E Talke2 Storage Mechanics Laboratory, Hitachi Asia Ltd., 528736 Singapore Center for Magnetic Recording Research, University of California, San Diego, CA 92093-0401 USA Thermal flying height control (TFC) has recently been implemented in magnetic recording disk drives to reduce the flying height at the read/write element of magnetic recording sliders. This paper investigates the flying height and response time of TFC sliders with single and dual TFC heaters and thermal insulators. Simulation results show that the presence of a thermal insulator has little effect on the response time of thermal protrusions. In addition, the effect of the dimensions of a TFC heater on the flying height and thermal response time is important. For a given heater power, the dual TFC heater design with thermal insulators can provide a very flexible control over flying height and response time of TFC sliders. Index Terms—Head/disk interface, thermal flying height control (TFC), thermal insulator.
I. INTRODUCTION HERMAL FLYING HEIGHT CONTROL (TFC) has recently been implemented in magnetic recording disk drives to reduce the flying height at the read/write element [1]–[10]. In a recent investigation, a thermal insulator was proposed to control the temperature distribution inside the slider thereby resulting in a larger flying height reduction than that obtained with a TFC slider without insulators [8]. Moreover, simulation results showed that two separate TFC heaters, each with an individual thermal insulator element, can further reduce the flying height at the read/write element, compared to that of a single TFC heater [9]. In this paper, we investigate the flying height and thermal response time of traditional TFC sliders and TFC sliders with thermal insulators. The response time and flying height reduction of three different TFC slider designs are compared. The effect of the dimensions of the TFC heaters on the flying height and thermal protrusion is studied. The power ratio and location of dual TFC heaters are optimized to improve the performance of the TFC slider with dual heaters.
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II. NUMERICAL MODEL A schematic model of a slider with dual TFC elements is shown in Fig. 1(a). The heater elements with insulators at the top and bottom are depicted in Fig. 1(b). In this study, three types of TFC sliders are investigated. In the first design, a single TFC heater without thermal insulators is positioned near the lower shield of the read/write element. The distance of TFC heater 1 to the air bearing surface (D1) is chosen to be 5.72 m. The second TFC slider has a thermal insulator at the bottom of the structure, i.e., below the heater element. In the third design, two pairs of TFC heaters and insulators are used. TFC heater 1 with an insulator at the bottom of the structure is near the lower shield of the read/write element and TFC heater 2 with an insulator Manuscript received October 11, 2009; revised December 12, 2009; accepted December 25, 2009. Current version published May 19, 2010. Corresponding author: H. Li (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2040025
Fig. 1. (a) Schematic model of TFC slider and (b) TFC heater with thermal insulators.
at the top of the heater element is near the top pole tip. The distance of the heaters to the air bearing surface (i.e., D1 and D2) is chosen to be 1.22 and 9.72 m, respectively. The thickness of the insulators of heater 1 and 2 is denoted as “ ”. The thickness is chosen to be 0.16 m in our model. The parameters of the heater investigated in our study are: (a) heater length; (b) heater width; and (c) heater height, respectively. In is applied for all three the simulation, a constant heat power slider designs. An “in-house-developed” simulation tool is used for all computations. III. SIMULATION RESULTS A. Response Time of Different Types of TFC Sliders The effect of thermal insulator on the thermal response time of different types of TFC sliders was investigated using a transient heat transfer simulation [10]. To allow comparison of the different types of TFC sliders, the same matrix of heat flux coefficients, obtained from a static thermal-structure simulation, was used as boundary condition at each time step. The response time for the three types of TFC slider designs is compared in Fig. 2. We observe that the thermal protrusion of each of the three TFC sliders reaches its steady-state value at approximately the same time, although the peak protrusion differs between the three designs by more than 50%. In particular, the slider with dual TFC heaters shows the largest thermal protrusion. This result is to be expected since thermal insulators can
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LI et al.: SIMULATION OF FLYING HEIGHT AND RESPONSE TIME OF TFC SLIDERS
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Fig. 2. Effect of thermal insulators on response time of thermal protrusion.
concentrate the temperature rise in the region of the read/write element of a TFC slider as shown previously [8], [9]. B. Effects of TFC Heater Dimensions The second slider with one TFC heater and a bottom insulator was chosen to study the effect of heater dimensions on flying height and thermal protrusion response by changing the (a) length and (b) width of TFC heater 1 while keeping its position (D1) and input power unchanged. The length of TFC heater 1 was varied from 2a, 1.5a, a, 0.75a, to 0.5a. The width of the heater was chosen to be 0.5c, 0.67c, c, 1.33c, and 2c, respectively, to keep the mass of TFC heater 1 unchanged. The simulation results in Fig. 3 show the thermal protrusion profile of the slider as a function of heater dimensions, while Fig. 4 shows the variation of flying height near the trailing edge after thermal deformation. We observe that the flying height variation at the read and write element is less than 0.7 and 0.4 nm, respectively, even if the ratio of heater length to heater width is changed by a factor of four keeping the power input constant. This variation is important in the design of low flying TFC sliders such as those in the present work. Therefore, optimization of TFC heater dimensions should be performed to achieve minimum flying height at a given power input to the TFC heater.
Fig. 3. Thermal protrusion profiles as a function of dimension of TFC heater1: (a) along the center line of the slider and (b) along width direction at the edge of upper shield.
C. Effect of Power Ratio and Location of Dual TFC Heaters The third slider design with dual TFC heaters was chosen to study the effects of power ratio and location of the dual heaters on the thermal protrusion and flying height. The power input of heater 1 and heater 2 are denoted as “ ” and “ ”, respectively. Fig. 5 shows the flying height as a function of the power input into the two TFC heaters. In Fig. 5, the total heater power was kept constant for all cases, i.e., the sum of power into heater 1 and heater 2 was fixed. We observe that the flying height at the read element decreases with an increase in heater power 1 while the flying height at the write element increases with an increase in heater power 1. We furthermore observe that the minimum flying height is strongly affected by the division of power into heater 1 and 2, and that a change in heater power to each individual heater allows control of the flying height of the read and
Fig. 4. Flying height profiles along the center line of the slider as a function of dimensions of TFC heater1.
write element, respectively. Thus, a dual TFC heater design provides a flexible control of flying height of both the read and the write element by changing the ratio of power input into the two heaters. This control of flying height appears to be beneficial for future disk drives with active flying height control sliders. Fig. 6 shows the flying height at the read and the write element as a function of the location of the dual TFC heaters. In our simulation, only the distance of the TFC heaters to the air
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Fig. 5. Flying height as a function of power input into dual TFC heaters.
IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Fig. 7. Effect of location of TFC heaters on response time of thermal protrusion.
in the time required for establishing thermal equilibrium. The effect of dimensions of typical TFC heaters on the flying height and response time of a thermal protrusion is important keeping the power input constant. The location of TFC heaters is an important design parameter for the optimization of TFC sliders. Dual TFC heaters and thermal insulators can provide a flexible control of flying height and thermal response time.
REFERENCES Fig. 6. Flying height as a function of location of dual TFC heaters.
bearing surface of the slider (i.e., D1 and D2) was studied. We observe that the flying height of both the read and the write element increases with an increase of the distance of either one of the heaters to the air bearing surface of the slider (i.e., D1 or D2). This result is related to a reduction of the thermal protrusion when the TFC heaters are moved away from the air bearing surface of the slider, i.e., the location of the individual heaters in a dual TFC slider is another important parameter to control the flying height of the read and the write element. Fig. 7 shows the effect of the location of TFC heaters on the time required to establish thermal equilibrium of the thermal protrusion. Four cases are compared and the results show that the thermal protrusion reaches its steady-state value in the 1.22 m and 9.72 m. shortest time in case a) with The time of the thermal protrusion to reach equilibrium decreases if the TFC heaters are moved closer to the air bearing surface of the slider. Therefore, the location of the TFC heaters is an important parameter in controlling the thermal response time of TFC sliders. IV. CONCLUSION TFC sliders with thermal insulators can achieve a larger thermal protrusion than traditional TFC sliders. The incorporation of thermal insulators does not seem to cause an increase
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