The effect of write current on thermal flying height control sliders with dual heater/insulator elements
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- Zheng, H., Li, H., Amemiya, K. et al. Microsyst Technol (2011) 17: 959. doi:10.1007/s00542-010-1216-4
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The effect of write induced pole tip protrusion on the magnetic spacing of the head/disk interface has to be taken into consideration as flying heights approach the spacing regime of a few nano-meters. Thermal flying height control (TFC) sliders are presently in common use in hard disk drives to control the flying height at the read/write element during drive operations. In this paper the flying characteristics of TFC sliders with dual heater/insulator elements are investigated. Simulation results are shown for situations where the write current is “on” and where the write current is “off”. The effect of design parameters of two heater/insulator elements is studied to optimize the performance of TFC slider.
In order to achieve areal densities beyond 1 Tb/in.2, a stable head/disk interface at an ultra-low magnetic spacing is required for magnetic hard disk drives. Pole tip protrusion caused by the write current may lead to flying instability of the slider and to head/disk contact (Tian et al. 1997; Pust et al. 2002; Imamura et al. 2002; Xu et al. 2009; Song et al. 2008; Li et al. 2005).
Thermal flying height control (TFC) sliders are presently in common use in hard disk drives to improve the flying characteristics of the read/write element. Numerical simulations have shown that TFC sliders with a single heater element embedded near the read/write element exhibit reduced flying heights at the reader and writer compared to sliders without any heater elements. Furthermore, simulation results indicate that a TFC slider with two heater/insulator elements achieves a larger flying height reduction at the read/write element than single TFC heater designs, keeping the power input the same for the two types of TFC sliders. In addition, the thermal actuation efficiency (ratio of flying height reduction to the maximum thermal protrusion) of the read/write element is found to be improved by the implementation of two heaters (Kurita et al. 2005, 2006; Suk et al. 2005; Juang and Bogy 2007; Li et al. 2009, 2010; Zheng et al. 2009).
In this paper, the flying characteristics of a typical TFC slider with dual heater/insulator elements are investigated numerically and compared with the flying characteristics of a slider without thermal flying height control. The flying height of the two types of sliders is evaluated for situations where the write current is “on” and where the write current is “off”. The design parameters for the dual heater/insulator elements, such as the location and the heat input of the two heaters, are optimized.
2 Numerical model
An iterative solution is required to obtain the reduction in flying height caused by the thermal protrusion (Kurita et al. 2005). The solution proceeds as follows. First, the air bearing pressure and the flying height of the slider are obtained by solving the equations of motion of the slider and the Reynolds equation simultaneously. Then, the heat transfer coefficient between the slider and the air bearing is determined (Juang and Bogy 2007; Li et al. 2009, 2010). The results serve as boundary condition at the ABS for the thermal analysis. The thermal deformation causes a protrusion of the read/write element, which, in turn, alters the geometry of the ABS. A new air bearing calculation is performed to obtain the updated pressure distribution and flying height. Iteration continues until convergence is reached, i.e., until the change in flying height at the read/write element between succeeding iterations is ≤0.1 nm.
3 Simulation result
3.1 The effect of write current
We observe from Fig. 2 that a small thermal protrusion is present if the write current is turned “on” without the heaters. A much larger thermal protrusion is obtained if the heaters are “on” without the writer being operating. The thermal protrusion is the largest among the three cases when both the write element and the heaters are turned “on”. However, this result (“Writer & DH ON”) does not agree with the mathematical sum of the thermal protrusion profiles of case (a) and (b), which is indicated as “Writer + DH”. This is due to the nonlinearity of the air bearing. The maximum difference between the two profiles is 0.5 nm.
Figure 3 shows that a small decrease in flying height occurs if the write element is turned “on”. A much larger decrease is observed if the dual heater elements are “on”. The largest decrease in flying height is observed for the case that both the heaters and the write element are turned “on”. This case leads to an additional decrease in flying height of 0.7 and 1.3 nm at the read and write elements, respectively, compared to the case that only the heaters were turned on. However, the thermal actuation efficiency at the reader reduces from 61%, when both the heaters and the write element are “on”, to 56%, when only the two heaters are “on” and the writer is “off”. It can also be seen from Fig. 3 that the flying height prediction for the case that both the writer and the heaters are “on” (“Writer & DH ON”) does not equal the flying height calculated from the ABS with the thermal protrusion profile as the case “Writer + DH”. The difference at the write and read elements is 12% (0.4 nm) and 8% (0.2 nm), respectively.
3.2 The effect of distance between TFC heater 1 and the ABS
3.3 The effect of distance between TFC heater 2 and the ABS
3.4 The effect of the power ratio of dual TFC heaters
A TFC slider with two separate TFC heaters and two individual insulator elements was investigated for the two cases that the write current was either “on” or “off”. It was found that the write current has a smaller effect on the flying height change at the reader than at the writer. In order to reduce the flying height and improve the thermal actuation efficiency of the read/write element, both heater elements of a dual TFC slider must be positioned in close proximity to the ABS. For a given position of heater 1 and heater 2, the power ratio of the heaters must be carefully selected to achieve low flying height, high thermal efficiency, and reduced dependence of the write current on spacing changes at the read and write elements.
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