In spin-valve type MR sensor used as read head for high density magnetic recording, the magnetization behaviors of a pinned layer and a free layer should be stably controlled so that sensors have a wide dynamic range and a good linear response. In this point of view, the sensors with a synthetic antiferromagnet layer(Co/Ru/Co) have improved thermal and magnetic stabilities compared to the conventional spin-valve sensors with a single pinned layer. In the present study, MR transfer behaviors of the permanent magnet biased MR heads which have a AF/Co/Ru/Co/Cu/NiFe-type SyAF(synthetic antiferromagnet) spin-valve were studied by micromagnetics modeling. Shielded MR sensors and unshielded MR sensors were simulated in the two different models, i.e. a single domain model(SD model) and a discretized cell model(Non-SD model). Sense current distribution in these SyAF SV layers was calculated by a semiclassical GMR theory proposed by Calmey-Barnas[24], which is more realistic than the conventional fixed-distribution as parallel resistors. Although there have been some changes in the current distribution throughout the multilayers due to the different magnetizations in the freelayer and SyAF during the MR transfer, those seem to have not much effects on the MR response.
For small sized MR heads, various width to height ratios were considered together with strength of permanent magnets which stabilizes the free layer. As the MR sensor width is reduced to 0.12㎛, sensor height less than 0.09㎛ is needed to show good linearity and the Mr·t of permanent magnets smaller than 0.2 memu/㎠ is sufficient for the domain stabilization. It was found that there is a boundary where the free layer behaviors as a nearly single domained particle depending on the strength of permanent magnet, the shield gap and the aspect ratio of MR sensor. When the MR sensor width is 0.12㎛ and the Mr·t of the permanent magnets is 0.384 memu/㎠ , the results of the two models coincide for sensor height less than 0.09㎛. However, when the Mr·t of the permanent magnets decreases to 0.2 memu/㎠ the sensor height also should be reduced to 0.06㎛ or less to show a single domained behavior for the free layer.
