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 a width equal to the cross-track distance between outer sides of the longitudinal bias layers. In another embodiment, the PM layer adjoins a backside of the top shield.
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 free layer (FL);
(d) a permanent magnet (PM) layer with a front side that faces a backside of the FL, and separated from the FL by an insulation layer, and wherein the PM layer has a magnetization initialized in a direction that forms an angle from less than +90 degrees to greater than −90 degrees with respect to the first cross-track direction in order to adjust 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 initialized PM layer; and
(e) a top shield that contacts a top surface of the TMR sensor.
2. The magnetic read head structure of claim 1 wherein the TMR sensor is part of a combined read/write head.
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 PM layer is separated from the FL backside by a distance greater than 0 nm and less than 100 nm.
5. The magnetic read head structure of claim 4 wherein the FL backside and a backside of each L biasing layer are a first height (h 1 ) from the ABS.
6. The magnetic read head structure of claim 5 wherein the PM layer has a width w 1 that is bisected by the center plane, and each L biasing layer has an outer side that is ½ w 1 from the center plane.
7. The magnetic read head structure of claim 4 wherein the FL backside is at a first height (h 1 ) from the ABS, and each of the PM layer and L biasing layers has a backside at a second height (h) where h>h 1 .
8. The magnetic read head structure of claim 7 wherein the FL has a width substantially equal to a width of the PM layer.
9. 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.
10. A magnetic recording apparatus, comprising:
(a) the HGA of claim 9 ;
(b) a magnetic recording medium positioned opposite to a slider on which the magnetic 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.