Write current and media heater control for heat or microwave assisted magnetic recording

#The present invention relates to data storage devices, and in particular to data storage devices with rotating disks above which read/write heads flying with a small head-to-disk clearance (“flying height”) that may be controlled to preserve the integrity of a heat assisted magnetic recording (HAMR) process or a microwave assisted magnetic recording process (MAMR).

#Data storage devices employ rotating data storage media such as hard disk drives. In a hard drive, data is written to the disk medium using a write head which generates a high localized magnetic field which aligns magnetic domains within the disk in one of two directions. In some cases, the magnetization direction is up or down relative to the plane of the disk (perpendicular magnetic recording, or PMR). In other cases, the magnetization direction is within the plane of the disk. In all cases, this data may then be read-out with a read head. The write and read heads are typically integrated within a single assembly. To achieve steadily increasing data storage densities (typically measured in bits/inch2), which are now achieving levels near 1012 bits/in2, the sizes of magnetic regions storing individual bits have been reduced to nm levels. Writing to, and reading from, such small regions may include shrinking the sizes of the read and write heads and also having them “flying” closer to the disk surface (since the magnetic forces drop rapidly with increasing distance between the disk and the head). The distance between the head and the disk is called the “flying height” since the head is said to “flying” above the disk on a cushion of compressed air which is entrained by the rapid rotation of the disk and then squeezed between the head (often called a “sled”) and the disk. Very precise control of the flying height is achieved using “thermal flying height control” (TFC) which employs an electrical heater (with mW powers) to heat the pole pieces of the head, resulting in nm-level thermal expansion which pushes the pole pieces slightly closer to the spinning disk surface. In more recent hard drive data storage devices, such as that described in U.S. Pat. No. 7,990,647 B2, issued Aug. 2, 2011, the read/write head may also incorporate a near field light source or a microwave source. In the following description of embodiments, “light source” may be interpreted to also comprise “microwave source”, and the term “Heat Assisted Magnetic Recording” (HAMR) may be interpreted to also comprise “Microwave Assisted Magnetic Recording” (MAMR). One purpose for this light source may be to locally heat the disk medium, thereby momentarily lowering the coercivity and thus reducing the required writing current. This writing process is known as “Heat Assisted Magnetic Recording”, or HAMR. However, near field light sources are typically only 10% efficient in heating the disk medium, with the remainder of the laser power heating the write head. Thus the laser light source represents another potential method for heating the write head in order to increase protrusion and reduce the flying height for improved writing. There are three different sources of heat which may heat the write head, where all of these sources of heat may cause the pole pieces to expand and thus reduce the flying height of the write head above the disk medium: 1) the current flowing through the write head coil, 2) the current flowing through the flying height control heater, and 3) the majority (˜90%) of the power flowing into the near field light source. Each of these three sources of heat to the write head has a different time constant for heating up (when power is applied) and for cooling down (when power is removed).

#A goal of some embodiments is to provide a method for improved control of the head protrusion in a heat assisted magnetic recording or microwave assisted magnetic recording disk drive.

#FIG. 1 is a schematic isometric diagram of a read/write head flying above a moving disk which has written data;

#FIG. 2 is a partial cross-sectional view of a thin film head having resistive heating for thermal flying height control (TFC), and a laser for heat assisted magnetic recording (HAMR), in accordance with an embodiment;

#FIG. 3 is a timing diagram illustrating three sources of heat input to a write head during pre-heating and data writing according to an embodiment;

#Embodiments can provide one or more advantages over previous methods for dynamically controlling the flying height of read/write heads with respect to disk storage media. Not all embodiments may provide all the benefits. The embodiments will be described with respect to these benefits, but these embodiments are not intended to be limiting. Various modifications, alternatives, and equivalents fall within the spirit and scope of the embodiments herein and as defined in the claims.

#FIG. 1 is a schematic isometric diagram 100 of a read/write head 102 flying above a disk 104 moving in a direction 108. Some magnetized bits on the data track (each typically comprising a multiplicity of magnetic domains) are illustrated as clear rectangles 106. The read/write head 102 “flies” above the disk medium 104 typically at a distance of roughly ten nanometers. Aerodynamic effects between the lower surface of the head 102 and the upper surface of the disk 104 aid in maintaining this distance, termed the “flying height”. Magnetic pole pieces in the lower surface of the head 102 detect magnetic transitions in the disk 104 such as bits 106. There are three possible relative motions of the head 102 with respect to the disk 104: 1) rotation of the disk 104 (arrow 108), 2) vertical motions corresponding to changes in the flying height (arrow 112), and 3) side-to-side motions of the head 102 relative to the data track represented by regions 106 (arrow 110). The head flying height affects the detected signal strength: increased flying height decreases the signal, while decreased flying height increases the signal while also increasing the probability of mechanical contact between the head 102 and the disk 104, potentially resulting in physical damage to disk 104 (a “head crash”). Thermal Flying height control (TFC) enables the distance of the write head to be dynamically controlled during disk operation.

#FIG. 2 is a partial cross-sectional view 200 of a thin film head having resistive heating for thermal flying height control (TFC), and a laser for heat assisted magnetic recording (HAMR), in accordance with an embodiment. Some details such as coils, spacers, shielding, and read sensors have been omitted to simplify the figure and emphasize the features of the head with respect to some embodiments. Typical configurations and structures of these omitted details are familiar to those skilled in the art. The TFC resistive heaters have also been omitted in FIG. 2 since these would typically be outside the field of view of FIG. 2. The write head comprises lower return pole layer 202, back gap layers 204, upper return pole layer 206, and upper pole tip layer 208. Lower return pole layer 202 may also have a lower pole tip (not shown) at the air-bearing surface (ABS) 226. An optical waveguide for heat-assisted magnetic recording (HAMR) comprises an optical waveguide core 210, surrounded by cladding layers 212 at top and bottom in FIG. 2. As shown, layers 210 and 212 extend through at least a portion of back-gap layers 204, although other placements of back gap layer 204 are possible. The end portion of optical waveguide at which the laser light is directed to the disk medium surface is enclosed by near field transducer (NFT) 201. Within the volume enclosed by the ABS, back gap layer 204, lower return pole 202, and layers 206, 208, and 212, would be located the following (not shown): coil layers, and insulating and spacer layers, as would be familiar to those skilled in the art. Typically, layers 202, 204, 206, and 208 would be comprised of magnetic alloys or materials, such as ternary alloys of Co, Ni, and Fe, or binary alloys of Co/Fe and Ni/Fe. The optical waveguide 210 would typically be comprised of light transmitting materials such as tantalum pentoxide and/or titanium dioxide. Within some embodiments, the optical waveguide may have other designs such as a planar solid immersion mirror, or a planar solid immersion lens.