Effect of azimuth angle and bias field on second harmonic distortion in differentially biased MR heads

• Main Author: Dee, R.H.
• Format: Conference Proceeding
• Language: English
• Subjects: Azimuth; Bismuth; Current measurement; Density measurement; Difference equations; Harmonic distortion; Magnetic flux; Magnetic heads; Magnetoresistance; Saturation magnetization
• DOI: 10.1109/INTMAG.1992.696272

The magnetoresistance effect in a thin permalloy film is a quadratic function of the applied field until the onset of saturation and is often written as R = Ro + dR cos 2 A where A is the angle the internal field B 1 makes with the sense current direction due to an externally applied field. Ro the resistance at zero applied field and dR is the magnetoresistance. As saturation approaches, the gradient of the MR characteristic begins to reduce a t which point a quasi-linear region exists for small inputs. This point also is the point of maximum gradient and is often called the optimum bias point as it delivers the maximum output per unit input current. A DC bias field. Bb, is normally applied to operate close to this region. In the purely quadratic region of the characteristic the use of opposing bias fields and differential sensing from two indentical (half) elements eliminates the second harmonic terms by perfect cancellation. The equations governing the read response for each half element below the onset of saturation derived from equation 1, V1 & V2 are:- V(1,2) = I Ro 2 (dR Ro) Bt 2 (dBy z ± 2Bb dBy) dBy is the input field, Bb the bias field applied in the + or - direction and B1 the internal field. As can be seen for differential sensing the dBy 2 term would disappear if they were equal. However, this cancellation is only "perfect" under the following conditions (a) each half element carries the same bias current, (b) each half element is magnetically identical or (c) the input flux to each half element is the same. Condition (c) is not met when there is some azimuth angle between the MR element and the recorded data on a tape or disk. For even a small azimuth angle the value of dB, for each half would differ as the sinusoidal wavefront would arrive at the element at slightly different times. The effect of azimuth angle.