Heat assisted media recording apparatus with compensating heater

An apparatus includes an optical pathway configured to deliver energy to heat a magnetic recording medium via a slider body. The optical pathway generates heat in the slider body when delivering energy to the magnetic recording medium. The apparatus includes a compensating heater with a thermal characteristic that matches a thermal characteristic of the optical pathway. The compensating heater is activated at least part of the time when the optical pathway is not delivering the energy to the magnetic recording medium.

SUMMARY

Examples described herein are directed to a heat-assisted media recording device. In one embodiment, an apparatus includes an optical pathway configured to deliver energy to heat a magnetic recording medium via a slider body. The optical pathway generates heat in the slider body when delivering energy to the magnetic recording medium. The apparatus includes a compensating heater with a thermal characteristic that matches a thermal characteristic of the optical pathway. The compensating heater is activated at least part of the time when the optical pathway is not delivering the energy to the magnetic recording medium.

DETAILED DESCRIPTION

This disclosure describes use of compensating heaters in heat-assisted magnetic recording (HAMR) devices. In HAMR devices, also sometimes referred to as thermal-assisted magnetic recording (TAMR) devices or energy assisted magnetic recording (EAMR), a magnetic recording medium (e.g., hard drive disk) is able to overcome superparamagnetic effects that limit the areal data density of typical magnetic media. In a HAMR recording device, information bits are recorded on a storage layer at elevated temperatures. The heated area in the storage layer determines the data bit dimension, and linear recording density is determined by the magnetic transitions between the data bits.

In order to achieve desired data density, a HAMR recording head (e.g., slider) includes optical components that direct light from a laser to the recording media. The HAMR media hotspot may need to be smaller than a half-wavelength of light available from current sources (e.g., laser diodes). Due to what is known as the diffraction limit, optical components cannot focus the light at this scale. One way to achieve tiny confined hot spots is to use an optical near-field transducer (NFT), such as a plasmonic optical antenna. The NFT is designed to support local surface-plasmon at a designed light wavelength. At resonance, high electric field surrounds the NFT due to the collective oscillation of electrons in the metal. Part of the field will tunnel into a storage medium and get absorbed, raising the temperature of the medium locally for recording. During recording, a write element (e.g., write pole) applies a magnetic field to the heated portion of the medium. The heat lowers the magnetic coercivity of the media, allowing the applied field to change the magnetic orientation of heated portion. The magnetic orientation of the heated portion determines whether a one or a zero is recorded. By varying the magnetic field applied to the magnetic recording medium while it is moving, data is encoded onto the medium.

A HAMR drive uses a laser to heat the media to aid in the recording process. Due to inefficiencies of the optical transmission path, the laser also heats the head/slider. To illustrate possible optical transmission paths,FIGS. 1 and 2show perspective views of HAMR configurations according to example embodiments. InFIG. 1A, slider100is a laser-in-slider (LIS) configuration. In this configuration, slider100includes body101having an edge-emitting laser diode102integrated into a trailing edge surface104of the slider body101. In this example, the laser diode102is disposed within a cavity formed in the trailing edge surface104. The laser diode102is proximate to a HAMR read/write element106, which has one edge on an air bearing surface108of the slider100. The air bearing surface108faces and is held proximate to a moving media surface (not shown) during device operation.

While here the read/write element106is shown as a single unit, this type of device may have a physically and electrically separate read element (e.g., magnetoresistive stack) and write element (e.g., a write coil and pole) that are located in the same general region of the slider100. The separate read and write portion of the read/write element106may be separately controlled (e.g., having different signal lines, different head-to-media spacing control elements, etc.) although may share some common elements (e.g., common signal return path). It will be understood that the concepts described herein described relative to the read/write element106may be applicable to individual read or write portions thereof, and may be also applicable where multiple ones of the read write portions are used, e.g., two or more read elements, two or more write elements, etc.

The laser diode102provides electromagnetic energy to heat the media surface at a point near to the read/write element106. Optical pathway components, such as a waveguide110, are formed integrally within the slider device100to deliver light from the laser102to the media. In particular, a local waveguide and NFT112may be located proximate the read/write element106to provide local heating of the media during write operations. These components106,110,112may also experience significant heating from the laser102due to coupling and transmission inefficiencies.

InFIG. 1B, a laser-on-slider (LOS) configuration120is illustrated. This example includes a laser diode122that is mounted on a top surface of a slider body121The laser122is coupled to an optical pathway of the slider body121that includes, among other things, a straight waveguide124. In this configuration, the laser122may also be edge-emitting, such that the light is emitted from the laser122in a direction normal to the trailing edge surface104. In order to direct the light towards the air bearing surface108, the laser122(or other component) may include optical pathway elements such as a mirror (not shown) that redirects the light emitted from the laser122towards the air bearing surface108. In other configurations, an edge-emitting, top-mounted laser may be oriented so that the light emitted directly downwards toward the air-bearing surface108. This may involve placing the laser on a submount (not shown) on the top of the slider body121, the submount orienting the laser output in the desired direction.

While other components shown inFIG. 1B, such as the NFT112and read/write element106are referenced using the same numbers asFIG. 1A, the physical configuration of these and other components may differ in the different slider arrangements, e.g., due to the differences in optical coupling pathways, materials, laser power, etc. However, similar to the configuration shown inFIG. 1A, components106,112,124shown inFIG. 1Bmay also experience heating from the laser102due to coupling and transmission inefficiencies.

While not illustrated inFIGS. 1A and 1B, other slider configurations may utilize different types of lasers, such as vertical cavity emitting lasers. Other slider configurations may use free space light delivery, wherein light is coupled to the slider from an external laser. A free-space light delivery configuration may not experience direct heating from laser itself, but the optical pathway components that propagate the light may experience similar heating. The embodiments described below may be applicable to any of these energy delivery configurations.

The optical pathway heating in these examples can be localized at the NFT, the light delivery optics and/or at the laser itself. Light absorbed in these components is converted to heat, which is conducted to the surrounding materials. This heating causes thermal expansion, which can lead to head-media spacing (HMS) changes. For example, heat-induced expansion can change the shape of the slider and changing the air bearing characteristics, which can cause the writer to fly closer to the disk. An example of this is shown inFIG. 2, which illustrates a side view of a slider202according to an example embodiment.

The slider202is coupled to an arm204by way of a suspension206(e.g., gimbal) that allows some relative motion between the slider202and arm204. The slider202includes read/write elements208(e.g., transducers) at a trailing edge that are held proximate to a surface210of a magnetic recording medium, e.g., disk211. When the slider202is located over surface210of disk211, a flying height212is maintained between the slider202and the surface210by a downward force of arm204. This downward force is counterbalanced by an air cushion that exists between the surface210and an air bearing surface203of the slider202when the disk211is rotating.

It is desirable to maintain a predetermined slider flying height212over a range of disk rotational speeds during both reading and writing operations to ensure consistent performance. A region214is a “close point” of the slider202, which is generally understood to be the closest point of contact between the slider202and the magnetic recording medium211, and generally defines the HMS213. As described above, heating from HAMR optical components can affect the HMS213. This is shown inFIG. 2by dotted line that represents a change in geometry of the region214. In this example, the geometry change may be induced, in whole or in part, by an increase or decrease in temperature of the region214due to different thermal expansion properties of the respective materials surrounding the region.

In various embodiments described below, the slider202may include one or more heaters216that are designed to compensate for HAMR heating effects. The heater216may be positioned close to a heat-generating component, e.g., a top mounted laser219as shown here. A controller218can be coupled to the heater216to control when the heater216is switched on, and optionally to control an amount of power applied to the heater216.

The controller218includes a write control module220that controls various aspects of the device during write operations. For a HAMR device, writing involves activating the laser219while writing to the media, which is indicated by way of laser control module222. The laser control module222includes circuitry that switches the laser219on and off, e.g., in response to a command from write control module220. A compensating heater control224switches heater216on and off inversely to the laser219to minimalize thermal changes within the slider202when the laser219is switched on and off.

The slider202may also include other heaters (not shown) that actively control HMS213during device operation, as indicated by HMS control module226. The other heaters may be associated with one or both of the read/write elements208. The HMS control module actively adjusts HMS213during respective read and write operations. The activities of the HMS control module226may be coordinated with the compensating heater control module224. For example, a magnitude of signals sent from the compensating heater control224may be modified so as to complement (or at least not interfere with) HMS heating operations.

In reference now toFIG. 3A, a graph shows an example of how the recording process may change as a result of HAMR heating. The graph inFIG. 3Ashows the bit error rate (BER) for 1011 consecutive sectors. For the first 50 sectors the BER improves until it reaches the steady state value of −2.75. This is due to the thermal effects of the laser. During the first 50 sectors, the spacing between the NFT and the media is being reduced because the head is running hotter. As the record head comes into thermal equilibrium, the head flies closer to the media and the performance improves.

InFIG. 3B, a graph shows an in-drive HMS measurement of the clearance between the media and the head with an accompanying graph of normalized protrusion and laser power. It takes nearly 1000 μs for this system to reach steady state. This represents an issue that may need to be addressed in the drive, because 1000 μs may represent hundreds of sectors that may need to be compensated for. In addition, the compensation method must also account for situations where smaller sector bursts are written. For example, the thermal expansion must compensate for situations where 100 sectors are written, followed by 50 sectors of not writing and then another 200 sectors being written. In this example, during the 50 sectors of not writing, the head is cooling but has not reached room temperature before the 200 sectors start up again.

In order to compensate for these heating effects, a special heater may be used to mimic the heating from the laser. The heating effect from the laser may be due to a number of components of the optical pathway (e.g., waveguides, NFT) acting in combination, or one of the components may dominate. If the heating is found to come from more than one source, multiple heaters (e.g., different locations, time constants, thermal power, etc.) may be used if one heater cannot be designed to compensate all the effects. The heater or heaters could have one or more of the same thermal characteristics as heat generated by the optical pathway, or at least a portion of the heat or heat profile that is being compensated for. The thermal characteristics may include, but are not limited to, an amount of thermal energy or power, thermal time constant, location, thermal transfer paths to sinks or sources (conductive, convective and/or radiative), and shape of a heat profile of at least part of the heated optical pathway.

As described herein, a HAMR recording device may define a write/recording mode as a signal that is activated when writing occurs and/or is expected to occur. The laser and write pole may be deactivated in write mode, such as when passing over servo gates or sectors that are not to be written. In such a case, a compensating heater can be activated to simulate the effects of the laser heating when the laser is off, thereby maintaining thermal equilibrium. This is shown inFIG. 4A, which illustrates a signal timing diagram according to an example embodiment.

Looking at the diagram inFIG. 4Afrom left to right, at some time401defined by the drive firmware the drive goes into write mode, which is indicated by signal404. In this mode the drive is not writing data but it is preparing to write, e.g., waiting for the affected sectors to be under the write element. The heater control (signal408) engages and the protrusion begins as is shown by trace412in the figure. Once the protrusion has reached equilibrium, the write gate is enabled (e.g., as indicated by trace406at time403) and the laser is turned on (e.g., as indicated by trace410at time403).

As seen by comparing traces408and410, whenever the laser is on during recording mode, the heater control is off. While there may configurations where both the laser and heater are contemporaneously active during the recording mode (e.g., where the compensating heater also performs active HMS control), in this example the laser and heater are not contemporaneously active during the recording mode. Since the heating/cooling from the laser is exactly or approximately compensated by the cooling/heating of the heater, the protrusion412maintains equilibrium (and thereby stabilizes head-media spacing) after time403. Also shown in the figure is the servo gate (trace407). When a servo operation is being performed the laser is off and the heater is on. When the drive leaves write mode and goes back into read mode or standby mode, the heater is turned off

In the diagram ofFIG. 4A, it may be assumed that the thermal characteristics of the laser and compensating heater are reasonably well-matched, so that the heater control signal408can be an inverse of the laser current signal any time after write is enabled, e.g. while write mode signal404is asserted high. If one of the thermal characteristics of the laser and compensating heater are not matched, then the control signals can be adjusted to maintain a more steady thermal equilibrium. An example of this is shown in the timing diagram ofFIG. 4B.

InFIG. 4B, trace420represents a laser current signal similar to signal410inFIG. 4A. In this case, although the heater may be matched to, e.g., an output power, shape, etc., of the laser, it may be the case that the heater has a different time constant, such as where the laser diode is slower to come up to operating temperature than it takes for the compensating heater to cool from operating temperature. As such, if the heater current were disengaged at the same time the laser current was engaged, there might be a detectable dip in temperature, which would cause a momentary change in protrusion. In this example, two heater traces422,424show how the control signals can prevent this type of protrusion change by overlapping heater activation with laser activation.

As indicated by, e.g., period between times426and428, the compensating heater is activated during a period when the laser on and delivering energy to the magnetic recording medium via the optical pathway. In trace422, the heater is on at full power between times426and428. In trace424, the heater is on at less than full power between times426and428. A similar overlap may occur (instead of or in combination with the previously describe overlap) after the laser is deactivated, e.g., just before time430. For example, if the laser was faster to cool down that the laser was to heat up, then the heater could be engaged (either at partial or full power) before time430.

In other examples, there may be a “gap” period where neither the laser or compensating heater are activated. This is indicated by trace432. At time434, when the laser current has not yet been activated, the heater current has been deactivated. This may be a zero-power-level deactivation as shown or stepped/intermediate power level similar to the transitions shown in track424. This type of transition may be used, e.g., where the heater is slow to cool from operating temperature and the laser comes up to operating temperature quickly. Similar adjustments may be made when the laser is turned off, e.g., time430, such that there is a delay after time430before the heater is turned back on.

Although the example compensation techniques shown inFIG. 4Bare described as being used where there is a mismatch between thermal time constants, the technique may be applied to correct for a number of mismatched thermal characteristics. For example, even if the time constants of the heater and laser are well-matched, other thermal factors (e.g., shape of heater or optical path, separation distance, different intervening thermal path materials) may case protrusion to fluctuate when transitioning between the heater being activated and the laser being deactivated, and vice versa. The overlap and/or gap techniques shown inFIG. 4Bcan be used singly or in combination to compensate for any mismatch in thermal characteristics between the laser, optical pathway, and/or heater. These overlaps and/or gaps may be applied in any combination during one or both of laser activation and deactivation.

While an optical path may have a number of thermal hotspots, a heater may be chosen to compensate for a dominant source of heat. If the dominate source of heating is from the laser, a resistive element can be placed either under the laser for the LIS case or on the laser carrier or submount for the LOS case. This is shown inFIGS. 5A and 5B. InFIG. 5A, a view from the trailing edge of slider502shows a resistive heater504located within cavity506where the laser is mounted (see, e.g., laser-in-slider configuration inFIG. 1A). InFIG. 5B, a resistive heater514is located proximate laser512on carrier510(see, e.g., laser-on-slider configuration inFIG. 1A). The heater514may also be placed under the laser512. It will be appreciated that other types of heaters may be utilized besides a resistive heater, such as an inductive heater, optical heater, etc. The illustrated heaters504,514are coupled to respective controller circuits508,518as known in the art.

If the dominate source of heating is from losses in the light path, a heater can be placed under the light path as is shown inFIG. 6A. A heater602is placed on or near a waveguide604and/or other optical component (e.g., mirror, collimator, coupler, decoupler, mode shifter, etc.) that generates significant heat. A heater can also be placed above the light path or in close proximity to it. In some instances, the heater can even be part of the light path. For example, as shown inFIG. 6B, a solid-immersion mirror (SIM) design includes an integrated heater according to an example embodiment.

InFIG. 6B, a SIM610includes two sidewalls612,614formed of a reflective material (e.g., Au, Ag). The sidewalls612,614have a shape (e.g., parabolic) that focuses light615onto a near-field transducer616, which is located at a focus of the SIM610and proximate a media-facing surface620(e.g., air bearing surface). The areas within (and outside) the sidewalls612,614may be filled with a dielectric material through which the light615can propagate. In this example, regions622,624near the sidewalls612are filled with a resistive heater material, and conductive leads626,628are electrically coupled to the regions622,624. The leads626,628can terminate outside a slider that houses the SIM610and NFT616. From there, the leads626,628can be coupled to a controller that activates the heating regions622,624. In another arrangement, the sidewalls612,614themselves may be coupled to the leads626,628, in which case the side walls can have the dual purpose of being a reflector and a heater, either alone or in combination with other regions622,624.

If the dominate source of heating is the NFT, a heater similar to what is used for the writer and reader heaters can be used. Reader and writer heaters may be provided for active control of the HMS, e.g., a feedback loop include an HMS sensor and HMS adjusting heater. In some instances it may be possible to design the reader or writer heater to have the same thermal profile and time constant as NFT heating. In such a case, the reader or writer heater can perform two functions—controlling the HMS and compensating for protrusion from laser heating during write mode. In this case a multilevel heater controller may be utilized, because a different heater current may be required when the laser is on and off.

InFIG. 7, a flowchart illustrates a method according to an example embodiment. The method involves delivering704energy via an optical pathway of a slider body to heat a magnetic recording medium (e.g., HAMR media). The optical pathway generates heat in the slider body when delivering energy to the magnetic recording medium. A compensating heater with a thermal characteristic that matches a thermal characteristic of the optical pathway is activated704when the optical pathway is not delivering the energy to the magnetic recording medium.

InFIG. 8, a flowchart illustrates another method according to an example embodiment. As shown here, this method may be entered at regular intervals via block800, and exit immediately at block804if it is determined at block802that the apparatus is not in a recording mode. It will be understood that this type of operation is for purposes of illustration, and similar result may be obtain by performing the illustrated operations in response to events, e.g., in response to signals as shown inFIGS. 4Aand/or4B.

While in the recording mode, block806determines whether a region of a magnetic recording medium that is to be written is currently under a recording element (e.g., record head). If so, energy is delivered808to the recording medium via an optical pathway, and data is encoded810to the magnetic recording medium via the record element. If it is determined at block806that a region of a magnetic recording medium that is to be written is not currently under a recording element, a compensating heater is activated812. The compensating heater has one or more thermal characteristics that match characteristics of heat generated by the optical pathway. The thermal characteristics may include, but are not limited to, an amount of thermal energy, thermal time constant, location, and shape of a heat profile of at least part of the heated optical pathway.