The Importance of Measuring Magnetostriction
Why is the measurement of magnetostriction so critical to the performance of thin film heads?
In thin film head structures, the magnetic thin films perform a relaying function between magnetic signals from the recorded track and electrical signals to the read Electronics. The critical performance parameter in this function is the magnetic permeability of the film material along the magnetic path followed by the flux coming from the track.
At today's high data rates, it is the high frequency magnetic permeability of the films that is critical.
In order to achieve the highest possible magnetic permeability along the path, head designers have learned to determine the orientation of the domain walls in the films. Somewhat paradoxically , the highest permeability is obtained when the domain walls are essentially perpendicular to the path of the varying flux. This is because magnetization rotation within the domains is is less costly (in terms of energy expenditure) than domain wall motion, which can be seriously impeded by film inhomogeneities and other microscopic metallurgical defects.
In conjunction with the shape effects, the preferred direction of domain walls is determined by the magnetic anisotropy of the film material. The anisotropy control is a complex combination of process parameters during film deposition in wafer production. Later down the line the wafers will be diced and the surfaces that are critical from an aerodynamic point of view will be polished. Both of these processes disturb the ceramic material, which will inevitably change the stress level between the ceramic substrate and the magnetic films. (most of the stress change is due to the cutting, forcing the ceramic to relieve some of the deposition stress).
The magnetostriction of the film material will change the anisotropy of the magnetic layer. Since the degree of stress relief is for the greatest part unpredictable, the best way to maintain the correct anisotropy is to keep the magnetostriction close to zero, in any case down to 10E-7 or less.
Another reason to keep magnetostriction low is to minimize the parasitic signals caused by ultrasonic disturbances at the head/media interface, which the magnetostriction would transduce into magnetic signals, which the head will further transduce into electrical noise, thus decreasing the safety margins of the bit detectors, and increasing the read error rate. Since all these head performance parameters are critical to the acceptance of the component, magnetostriction has a strong impact on wafer yield if it is allowed to get out of control.
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