The effect of stress induced anistotropy on magnetoresistive heads with soft adjacent layer

Abstract

The modification of film anisotropy (Hk) after the establishment of an air bearing surface (ABS) due to the interaction of anisotropic stress and film magnetostriction is well known.' A resultant increase in Hk has been found to degrade the sensitivity of an unshielded magnetoresistive (MR) sensor. Similar results have been reported for barber pole biased MR heads.2 For M R sensors biased by a soft adjacent layer (SAL) the effect of the stress induced Hk takes on additional ci~mplexities. The overall device transfer function can be profoundly affected due to the interplay between the two films, resulting in severe nonlinear distortions. We have studied unshielded SAL biased M R sensors with the MR film thickness at 300 A and the magnetostriction coefficients (A) near zero and at -3~10-6. After ABS fabrication the sensor geometry is 3.5 pm in height and 120 pm in length. With near-zero 1 for both films the transfer function assumes the normal quadratic shape (Fig.1). With the negative MR L the transfer function develop5 a kink near the operating point with the sensitivity reduced abruptly beyond the kink (Fig.2). In a recording application this will result in severely distorted pulse\ of the negative polarity. T o understand the phenomenon we have constructed an uniform rotation model with elliptical approximation for the demagnetizing field. The model describes very well the device behavior for near-zero MR I (Fig.3). For the negathc MR 1 we have assumed a stress induced Hk of 10 Oe in the MR film along the stripe (Fig.4), which corresponds to a compressive stress of 0.9~109 dyne/cm2. The modrling results reveal that the kink in the transfer function develops when the SAL comes out of saturation (Figs). The operation of the device thus can be divided into two regions. In one region the SAL stay saturated and only the MR layer responds to the external field, resulting in the expected quadratic response. In the other region thr SAL is no longer saturated, and both layers respond to the field, resulting in an nbrupt change in senritivity. The two regions for SAL biased MR heads always exist as evidrnced in even the de\.ice with near-zero MR 1. The transition between the two regions is a result of energy balance between the MR layer and the SAL. The transition can be affected by many parameters, and rhould be an important consideration in SAL biased MR heads designs. Field (20 Oe/div) Fig I Transfer function of zero 1 *---_ -8W 0 tW Field (Oe) Fig 3 Modeling of the transfer functlon with zero MR 1 Field (20 Oejdiv) Fig.2 Transfer function of nagalive MR 1 I o / i Field (Oe) Fig 4 Modeling of the transfer function with negative MR 1 ' : / I i ' I -7 0 0 0 IW Field (@I Fig 5 Rotation of the MR and SAL magnetmtion.