Search

We have designed a readback-signal generator to provide noise-corrupted signals to a read channel simulator. It is implemented in a Xilinx Virtex-E field-programmable gate array (FPGA) device. The generator simulates in hardware the noise processes and distortions observed in hard drives. It uses embedded nonuniform random number generators to simulate the random characteristics of various disturbances in the read/write process. The signal generator can simulate readback pulses, intersymbol interference, transition noise, electronics noise, head and media nonlinearity, intertrack interference, and write timing error according to the characteristics specified by the user. A sample implementation operates at a 70-MHz clock speed. The design can easily be scaled for different error rates. The generator can be reconfigured in real time to give the user flexibility and increase the capacity of the FPGA device. The readback-signal generator can be integrated into an FPGA read channel simulator or serve as a test bench for data-recovery circuits. Index Terms--Field programmable gate array (FPGA), Gaussian pseudorandom number generator, magnetic recording, read channel, read channel modeling, reconfigurable computing, transition noise.

Soft sendust keeper layer has been sputtered on top of longitudinal thin film media which has an [M.sub.r]t value of 0.49 memu/[cm.sup.2]. Measurements of time-decay in both keepered and non-keepered longitudinal thin film media have been performed on a spin-stand tester. Experimental results showed that time-decay is larger in tested media which has lower [M.sub.r]t values compared to those that have a larger [M.sub.r]t of 1.26 memu/[cm.sup.2]. A 50[Angstrom] thick sendust keepered media has about half the decay rate compared to its non-keepered counterpart ([M.sub.r]t = 0.49 memu/[cm.sup.2]) at a recording density of 6000 fr/mm. Also, it was found that time-decay in 100[Angstrom] and 150[Angstrom] thick keepered media is less than that in a 50[Angstrom] keeper media. Index Terms - keepered media, superparamagnetism, thermal decay, time-decay

The time decay of remanent magnetization, remanent coercivity, and readback signal of ultra-thin longitudinal Co[Cr.sub.12][Ta.sub.2]/Cr thin film media have been studied experimentally with a VSM and a recording spin-stand tester using a MR read head. The time decay rate of the remanent magnetization or magnetic viscosity S increases significantly with decreasing magnetic layer thickness, especially for a [M.sub.r]t of less than 0.5 memu/[cm.sup.2] and it is found to be strongly related to the strength of the magnetic interactions between grains. For such thin media (CoCrTa=6 nm) with a [M.sub.r]t] = 0.36 memu/[cm.sup.2], the readback signal amplitude decreases at a high decay rate of about 5% per decade of time (sec) which is found to be essentially independent of the linear recording density from 2000 to 6000 fc/mm. To obtain Co[Cr.sub.12][Ta.sub.2]/Cr media with a time decay rate of less than 10 % in 5 years, the value of the ratio of the anisotropy energy barrier to the thermal energy (Kv/kT) should be greater than 80 which corresponds to a CoCrTa thickness of greater than about 10 nm.

Sendust and CoZrNb were sputtered on longitudinal Co[Cr.sub.12][Ta.sub.2] thin film media as alternative keeper materials to NiFe. Experiments were performed to study the dependence of recording performance on the thickness of the keeper and Cr break layer, head bias current, and recording density. It was found that a thinner (50 [Angstrom]) CoZrNb keepered media can achieve the same amplitude improvement as obtained by a 150 [Angstrom] thick sendust keeper. Spin-stand time decay measurements show that Sendust keepered media has a significantly lower decay rate (1.1% per decade) than its unkeepered counterpart (2.8% per decade). A magnetic force microscope (MFM) was used to observe the flux from transitions under a 50 [Angstrom] and 250 [Angstrom] keeper layers when a external magnetic field of 60 Oe is applied.

As areal density in hard drives increases, the bit aspect ratio (BAR) becomes an increasingly critical design issue. In this paper, we use the detection signal-to-noise ratio (SNR) as a system-level indicator to establish the relationship between the optimal BAR and all major recording system parameters that relate to the head/medium combination and servo as well as the read channel. We address practical and fundamental issues including thermal stability, medium and additive system noise, and equalization and track misregistration. The results of our analysis generally agree with the recent consensus to move toward lower BARs but also caution that the optimal BAR is a strong function of the servo capability. The results of our BAR analysis also indicate that at high areal densities, the ratio of PW50 to the bit cell is considerably smaller than found in today's drives. Index Terms--Bit aspect ratio (BAR), detection signal-to-noise ratio (SNR), intertrack interference, minimum mean square error (MMSE) equalizer.