Adaptive bias control for magnetic recording head
A read head includes a permanent magnet (PM) layer formed up to 100 nm behind a free layer where PM layer magnetization may be initialized in a direction that adjusts free layer (FL) bias point, and shifts sensor asymmetry (Asym) closer to 0% for individual heads at slider or Head Gimbal Assembly level to provide a significant improvement in device yield. Asym is adjusted using different initialization schemes and initialization directions. With individual heads, initialization direction is selected based on a prior measurement of asymmetry. The PM layer is CoPt or CoCrPt and has coercivity from 500 Oersted to 1000 Oersted. The PM layer may have a width equal to the FL, or in another embodiment, the PM layer adjoins a backside of the top shield and has a width equal to or greater than that of the FL.
1. A magnetic read head structure, comprising:
(a) a tunneling magnetoresistive (TMR) sensor formed on a bottom shield and with a front side at an air bearing surface (ABS), and wherein the TMR sensor comprises a free layer (FL) with a first magnetization substantially in a first cross-track direction, and has two sidewalls formed equidistant from a center plane that bisects the TMR sensor;
(b) the bottom shield;
(c) a longitudinal (L) biasing layer that is adjacent to each of the two FL sidewalls, and each L biasing layer has a front side at the ABS, a second magnetization in the first cross-track direction, and provides a bias field to stabilize the first magnetization in the FL;
(d) a top shield that contacts a top surface of the TMR sensor; and
(e) a permanent magnet (PM) layer with a front side that adjoins a backside of the top shield, and wherein the PM layer has a magnetization direction that adjusts a FL bias point so that the absolute value of TMR sensor asymmetry is reduced and is closer to 0% than in the absence of the PM layer.
2. The magnetic read head structure of claim 1 wherein the PM layer has a thickness greater than a thickness of the top shield.
3. The magnetic read head structure of claim 1 wherein the PM layer is comprised of CoPt, CoCrPt, or FePt and has a coercivity from about 500 Oersted (Oe) to 1000 Oe.
4. The magnetic read head structure of claim 1 wherein the FL has a backside at a first height (h1), and the PM layer front side is a distance that is greater than h1 but less than a sum (h1+100 nm) from the ABS.
5. The magnetic read head structure of claim 1 wherein the PM layer magnetization direction forms an angle that is from +90 degrees to −90 degrees with respect to the first cross-track direction.
6. The magnetic read head structure of claim 1 wherein the PM layer has a cross-track width equal to or greater than that of the FL.
7. A head gimbal assembly (HGA), comprising:
(a) the magnetic read head structure of claim 1 ; and
(b) a suspension that elastically supports the magnetic read head structure, wherein the suspension has a flexure to which the magnetic read head structure is joined, a load beam with one end connected to the flexure, and a base plate connected to the other end of the load beam.
8. A magnetic recording apparatus, comprising:
(a) the HGA of claim 7 ;
(b) a magnetic recording medium positioned opposite to a slider on which the read head structure is formed;
(c) a spindle motor that rotates and drives the magnetic recording medium; and
(d) a device that supports the slider, and that positions the slider relative to the magnetic recording medium.