Return Field Induced Partial Erasure in Trailing Edge Shielded Perpendicular Writers

Abstract

Trailing edge shielded pole head is the primary writer design for perpendicular recording (PMR) with soft under layer (SUL), due to its higher field gradient compared to a single pole head. [1] However, since the perpendicular field component underneath the trailing shield (TS) is opposite to that from the main pole (MP), the bits will experience this reverse field right after being written by the MP. In this paper, we will demonstrate that, both by modeling and by experiments, with certain combinations of head, media, SUL, and write current, the return field induced partial erasure (RFPE) will occur, causing recording performance degradation. Therefore, head, media and system optimizations are necessary in order to avoid this issue. Fig. 1 shows the finite element modeling results of the on-track Stoner-Wohlfarth field for three write currents. It can be seen that both the recording field from the MP and the return field increase with increasing write current. Spin stand tests have been done to demonstrate RFPE. Fig. 2 shows the media noise power on a DC-erased media as a function of the write current and the current overshoot. The DC noise reaches a minimum in the medium current range and increases at both low and high currents. At low current side, it is due to insufficient write field. For high current, however, it is due to the RFPE, even though the MP is able to switch all the media grains. With high current overshoot, the actual write current becomes much higher than the nominal current Iw, therefore, RFPE occurs at a much lower nominal Iw, as shown in Fig. 2. Fig. 3 shows the actual readback waveforms from a low density bit pattern using different write currents. The media background is AC demagnetized. Similar to what was observed in DC erase, poor writing occurs at low current, and RFPE occurs at high current, with clear amplitude degradation in the center of the bit. For medium write current, clean waveform was obtained, indicating that, in such regime, the MP field is sufficiently high and the TS field is sufficiently low not to cause RFPE. It has been experimentally observed that both reverse overwrite and spectral SNR reach a maximum at some optimal write current in the medium range.[2] This can be explained by the observations above. Notice that the SNR degradation at high write current in this case is not due to poor transitions from the field gradient drop, as usually for a ring head in longitudinal recording, but due to the amplitude drop and noise increase resulting from RFPE. In the presence of demagnetizing field, RFPE could happen at lower return fields. Fig. 4 shows the readback waveform of low density patterns written on a DC-erased media background. The DC magnetization direction is corresponding to the positive polarity in Fig. 4. Because the demagnetizing field from the DC-erased media is in the negative direction, it will strengthen the return field exerted on positive bits, causing RFPE. However, it cancels the return field that tries to partially erase the negative bits, thus leadi...

Influence of Negative Field on Media Noise in Combinations of Capped Medium and Shielded Pole Heads

“…Consequently, we took into account that the phenomenon came from the negative field around the edge of the trailing shield [6], [7]. Fig.…”