In hard disk technology, read/write heads are used, which are referred to in the following text as sliders, which are in the form of flying bodies and float on a thin air cushion above the storage disks of the hard disk. The air cushion is in this case produced by rapid rotation of the hard disk. The slider is attached to the end of a spring arm and is moved by it to the respective write/read positions for the hard disk. Efforts are being made to use sliders such as these for optical and/or magneto-optical recording and replay systems, as well.
A slider-based recording system allows to achieve high storage densities by moving either a focusing lens or a magnet coil very close to the surface of a material which is suitable for data storage. This allows the use of optics with a high numerical aperture (NA). Furthermore, in the case of magneto-optical recording, methods for magnet superresolution (MSR, e.g. MAMMOS (magnetic amplifying magneto-optical system), DWDD (domain wall displacement detection)) may be combined with the high NA. In a recording system with a rotating storage medium, a slider is raised by means of an air cushion between the surface of the storage medium and the slider base surface. There is a force equilibrium between an externally applied holding-down force and the air cushion, with a correspondingly associated flying height of the slider. The lift force on the slider produced by the air cushion is dependent on the linear velocity at which the storage medium moves under the slider. In order to keep the flying height of the slider constant from the smallest to the largest radius over the entire area of the storage medium, it would be possible to use a constant linear velocity by controlling the rotation frequency of the drive motor for the storage medium as a function of the radius (CLV mode, constant linear velocity). This solution has the disadvantage, however, of reduced access speeds in comparison to the use of a constant angular velocity of the storage medium (CAV mode), since the storage medium must be braked or accelerated appropriately when large sudden radial changes occur. However, the linear velocity of the storage medium relative to the slider varies in the CAV mode, so that the lift force on the slider also varies as a function of the respective radius on the storage medium. This is illustrated in FIG. 1, which shows a measurement of the flying height of the slider as a function of the rotation speed. In the CAV mode, this is equivalent to the relationship between the flying height and the radius since, in this case, the speed of flight rises linearly as the radius increases. However, for reading and/or writing, the flying height of the slider must be kept virtually constant over the entire area of the storage medium.
In one known solution which is used as standard for hard disks, the tracking is carried out by means of a pivoting arm with pivot suspension. This means that there is only one radius on the storage medium at which the slider center point is tangential with respect to the track. The angular position of the slider with respect to the data track varies as a result of pivoting to other radii, so that the incident flow direction resulting from the air flow produced by the rotating storage medium also varies. The air cushion between the slider and the storage medium is thus formed differently, which is used to vary and to compensate for the flying height. However, this method works only in conjunction with a pivot spring arm suspension, and not with linear tracking mechanisms.
US 2001/0033546 A1 discloses a further method for controlling the flying height. The aim of the slider method is to move a lens or coil element to a constant working distance from the surface of the storage medium, corresponding to near-field or far-field recording. If the flying height of the slider varies as a result of a change in the linear velocity, the change in the flying height is compensated for in this solution by moving the write/read element on the slider in the opposite direction to this, in the direction of the optical axis. The distance between the write/read unit and the surface is thus kept constant by means of a thermal expansion element or piezo-actuator. One disadvantage of this solution is the costly and complicated manufacture as a result of assembly, contact making and driving an additional microactuator on the slider.
In U.S. Pat. No. 6,178,157 B1, the contact pressure force of the slider, which determines the flying height when there is a force equilibrium with the lift force from the air cushion, is controlled by an actuator. In systems with focusing optics, the focus error signal is used as the controlled variable. This solution requires a costly and complicated actuator mechanism and control.
U.S. Pat. No. 5,986,850 and U.S. Pat. No. 6,317,294 B1 disclose solutions in which the lower face of the slider is designed such that overpressure zones, which result in the flying height of the slider increasing, and underpressure zones, which result in the flying height of the slider decreasing, are formed. When the linear velocity of the storage medium changes, the resultant lift force produced by these two components remains constant, so that the flying height of the slider also remains constant. This solution involves complicated manufacture of the structures on the lower face of the slider.
A further solution is known from WO 03/034416 A2. During operation, the slider has a pitch angle in the direction of flight. As the linear velocity rises, that is to say as the flying height rises, the pitch angle also increases. In this case, a slider type was chosen whose trailing edge falls and whose leading edge rises. This results in the slider having an imaginary rotation axis about which it is rotated. A lens was placed accurately on the slider such that its focal point lies on this imaginary rotation axis. When the linear velocity changes, the slider now rotates about this axis, on which the focal point is located. The distance between the focal point and the surface of the storage medium thus remains constant. This solution has the disadvantage that a specific slider design must be used with a rotation axis which is governed by the change in the pitch angle, taking account of the geometry and weight of the lens that is used. Furthermore, the method works only when spherical lenses are used on the slider.