Patent Application
20150199987
1. Field of the Invention
Embodiments described herein generally relate to methods for controlling flying height of a head.
2. Description of the Related Art
Hard disk drives (HDD) include read and write transducers that reside within a slider, which flies over a recording media/disk. Increasing demand in data density requires that the read and write transducers fly closer to the media. As flying heights diminish, it becomes more relevant to accurately control the head-disk distance (i.e., the distance between the read-write heads and the disk). Accordingly, the fly-height between the slider and disk is increasingly important as storage densities also increase.
Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording media to reduce the coercivity of the media so that an applied magnetic writing field can more easily direct the magnetization of the media during the temporary magnetic softening of the media caused by the heat source. This technique is broadly referred to as “thermally assisted (magnetic) recording” (TAR or TAMR), “energy assisted magnetic recording” (EAMR), or HAMR which are used interchangeably herein. A tightly confined, high power laser light spot is used to heat a portion of the recording media to substantially reduce the coercivity of the heated portion. In one approach, a beam of light is condensed to a small optical spot on the storage medium using a near field transducer (NFT) to heat a portion of the medium and reduce the magnetic coercivity of the heated portion. Then, the heated portion is subjected to a magnetic field that sets the direction of magnetization of the heated portion. In this manner, the coercivity of the media at ambient temperature can be much higher than the coercivity during recording, thereby enabling stability of the recorded bits at much higher storage densities and with much smaller bit cells.
The NFT is designed to reach local surface-plasmon resonance at a designed light wavelength. The surface plasmon is excited in a small conducting antenna that is incorporated within the read/write head structure. At resonance, a high electric field surrounding the NFT appears, due to the collective oscillation of electrons in the metal. A portion of the field will tunnel into a storage medium and get absorbed, raising the temperature of the medium locally for recording.
As the plasmon antenna heats up due to the absorption of optical energy from the laser, it very quickly (approximately 50 μs to 100 μs) creates a transient protrusion which protrudes from the surface of the read/write head and approaches the medium surface. In principle, the thermal response of the thermal flying height control (TFC) element can compensate for the transient protrusion by slightly lifting the head away from the disk surface to increase fly height. However, the time constant for transient protrusion is less by a factor between 10 and 50 than the time constant for TFC response so the TFC element cannot adequately compensate for the antenna protrusion. This large difference in response times leads to a transient protrusion at the beginning of a write process which can lead to head/disk interference.
Thus, there is a need for better control of flying height in magnetic recording devices.
Embodiments described herein generally relate to controlling the flying height of a head during operation. In one embodiment, a method for controlling flying height can include positioning a head at a flying height which is a first distance from a magnetic disk, the head comprising a near field transducer; applying a voltage to the head or the magnetic disk, the voltage changing the flying height between the head and the magnetic disk from the first distance to a second distance; delivering a radiation to the magnetic disk through the near field transducer, the radiation heating the near field transducer and changing the flying height from the second distance to a third distance; and altering the voltage to the head to change the flying height to the second distance.
In another embodiment, a method for controlling flying height can include delivering an electrical bias to a head, the head comprising a first surface facing a magnetic disk and a near field transducer positioned at the first surface at a first distance from the magnetic disk; delivering radiation to the magnetic disk through the near-field transducer, the near field transducer changing shape in response to the radiation, wherein the near field transducer is positioned at a second distance from the magnetic disk; and reducing the electrical bias to the head to move the near field transducer to a third distance from the magnetic disk.
In another embodiment, a system for controlling flying height can include at least one processing unit. The processing unit is adapted to perform steps including positioning a head at a flying height which is a first distance from a magnetic disk, the head comprising a near field transducer, applying a voltage to the head, the voltage changing the flying height between the head and the magnetic disk from the first distance to a second distance, delivering a radiation to the magnetic disk through the near field transducer, the radiation heating the near field transducer and changing the flying height from the second distance to a third distance and altering the voltage to the head to change the flying height to the second distance.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).
Embodiments described herein generally relate to controlling the flying height of a head during a writing operation. More specifically, embodiments relate to controlling the flying height of a HAMR head with relation to transient changes in the air bearing surface. The NFT in HAMR recording has a transient protrusion time constant much faster than the flying height accommodation provided by many TFC elements. The speed of the NFT transient protrusion inhibits adequate compensation at write start. By controlling the flying height of the slider either fully or partially through electrostatic effects, the NFT transient protrusion can be compensated for in a near instantaneous fashion. The methods described herein adjust the flight height of NFT element using an electrostatic control mechanism generating a physical force through electrostatic induction. In this way, the speed of actuation is only limited by the air bearing P2 frequency. Therefore, NFT time constants of the order of 50-100 us can be compensated. Embodiments disclosed herein are more clearly described with reference to the figures below.
The HDD 100 according to the present embodiment supplies a driving current to the VCM 110, thereby rotating the HSA 112 using the bearing 108 as a rotation center. A rotation angle of the HSA 112 is limited to a given range. An adherent substance might be adhered to a part of the slider 104. The HDD 100 supplies a driving current to the VCM 110 and thus rotates the HSA 112, thereby removing the adherent substance from the slider 104. In many cases, the adherent substance is lubricating oil or the like applied onto the magnetic disk.
The magnetic disk 102 is fixed to the SPM 114, and is rotated by driving the SPM 114. At least one surface of the magnetic disk 102 serves as a recording surface on which information is magnetically recorded.
The slider 104 is provided at one end of the arm 106 so as to be associated with the recording surface of the magnetic disk 102. The read head on the slider 104 reads a signal magnetically recorded on the recording surface of the magnetic disk 102, and outputs the read signal to the head IC 118. Furthermore, in response to a write signal (write current) fed from the head IC 118, the write head on the slider 104 magnetically records information on the recording surface of the magnetic disk 102. The slider 104 slides over the recording surface of the magnetic disk 102.
The arm 106 is provided at its one end with the slider 104. In response to the supply of a driving current to the VCM 110, the arm 106 rotates using the bearing 108 as a rotation center, and moves the slider 104 radially over the recording surface of the magnetic disk 102.
The bearing 108 serves as the rotation center of the HSA 112 by inserting a shaft (not shown) to be fixed to an enclosure of the HDD 100. The VCM 110 is driven in response to a driving signal (current) supplied from the motor driver 116, thereby rotating the arm 106 on the shaft. The HSA 112 is the structure integrally constituted by the slider 104, the arm 106, the bearing 108 and the VCM 110. In response to the supply of a driving current to the VCM 110, the HSA 112 moves the slider 104, provided at one end of the arm 106, using the bearing 108 as the rotation center. The rotation angle of the HSA 112 is limited to a given range.
The SPM 114 is driven in response to a driving signal (current) supplied from the motor driver 116, thereby rotating the magnetic disk 102. Based on control carried out by the CPU 122, the motor driver 116 supplies, to the VCM 110 and the SPM 114, the driving signals for driving the VCM 110 and the SPM 114, respectively.
The head IC 118 amplifies a signal fed from a read head (not shown) provided at the slider 104, and outputs, as read information, the amplified signal to the RDC 120. Further, the head IC 118 outputs, to a write head (shown in
The RDC 120 performs a given process on the read information, fed from the head IC 118, to decode the read information, and outputs, as transfer information, the decoded information to the HDC 128. Furthermore, the RDC 120 performs a given process on information, which has been fed from the HDC 128 and should be recorded, to encode the information, and outputs, as recording information, the encoded information to the head IC 118. The RDC 120 utilizes the RAM 124 as a work memory in performing the given processes for encoding and decoding. The RAM 124 is a work memory for the RDC 120, the CPU 122 and the HDC 128. In one embodiment, the RAM 124 is a DRAM serving as a volatile memory.
The NVRAM 126 is a nonvolatile memory for storing a program executed by the CPU 122. The program stored in the NVRAM 126 is updatable. In accordance with a program stored in the NVRAM 126, the CPU 122 controls each block included in the HDD 100. The CPU 122 is a processor for controlling rotational operations of the VCM 110 and the SPM 114. The CPU 122 utilizes the RAM 124 as a work memory in executing the program. In the present embodiment, with the aim of removing an adherent substance adhered to the slider 104, the CPU 122 performs control so as to rotate the VCM 110 to a position at which the adherent substance does not interfere with the recording surface of the magnetic disk 102. This control is carried out using given timing as a trigger.
The HDC 128 carries out a communication process for transmitting and receiving information to and from the host system 150. The HDC 128 performs a given process on the transfer information, fed from the RDC 120, to encode the transfer information, and transmits, as transmission information, the encoded information to the host system 150. Moreover, the HDC 128 performs a given process on reception information, received from the host system 150, to decode the reception information, and outputs, as information that should be recorded, the decoded information to the RDC 120. For example, the HDC 128 carries out the communication process with the host system 150 in accordance with a SATA (Serial Advanced Technology Attachment) standard.
The above description of a typical magnetic disk storage system and the accompanying illustration are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders.
The waveguide 210 is a channel that transmits the radiation through the height of the head 200 to the near-field transducer 212—e.g., a plasmonic device—which is located at or near the air-bearing surface (ABS). The near-field transducer 212 further focuses the beamspot to avoid heating neighboring tracks of data on a magnetic disk 250—i.e., creates a beamspot much smaller than the diffraction limit. As shown by arrows 214, this optical energy emits from the near-field transducer 212 to the surface of the magnetic disk 250 below the ABS of the head 200. The embodiments herein are not limited to any particular type of near-field transducer and may operate with, for example, either a c-aperture, e-antenna plasmonic near-field source, or any other shaped transducer.
The magnetic disk 250 is positioned adjacent to or under the head 200. The magnetic disk 250 includes a substrate 252, which may be made of any suitable material, such as ceramic glass or amorphous glass. A soft magnetic underlayer 254 is deposited on the substrate 252. The soft magnetic underlayer 254 may be made of any suitable material such as, for example, alloys or multilayers having Co, Fe, Ni, Pd, Pt or Ru. A hard magnetic recording layer 256 is deposited on the soft underlayer 254, with the perpendicular oriented magnetic domains contained in the hard layer 256. Suitable hard magnetic materials for the hard magnetic recording layer 256 can include at least one material having a relatively high anisotropy at ambient temperature, such as FePt or CoCrPt alloys.
A power source 206 is shown here as connected to the head 200. The power source 206 can provide a bias to the head 200, which creates a Coulomb force between the head 200 and the magnetic disk 250 (which is referred to hereafter as interface voltage control). Though shown here as biasing the head 200 to create the interface voltage control, the bias may also be applied to the magnetic disk 250. The Coulomb force between two or more charged bodies is the force between them due to Coulomb's law. If the charged bodies are both positively or negatively charged, the force is repulsive. If the charged bodies are of opposite charge, the force is attractive. The Coulomb force created between the head 200 and the magnetic disk 250 counterbalances the opposing force created by the air under the air bearing surface during the rotation of the disk, the force applied by the arm 106, the forces above as augmented by a TFC element or other lifting forces on the head 200.
The head 200 directs the radiation source 202 to heating the magnetic disk 250 proximate to where the write pole applies the magnetic write field to the magnetic disk 250. The transmitted radiant energy, generally designated by arrows 214, is delivered through the near-field transducer 212 to the surface of the magnetic disk 250 for heating a localized area of the magnetic disk 250, and particularly for heating a localized area of the hard magnetic layer 256.
During operation of a HAMR/TAR enabled head 200, the rotation of the magnetic disk 250 generates an air cushion between the ABS of the slider 104 and the surface of the magnetic disk 250 which exerts an upward force or lift on the head 200. The air flow thus counter-balances the slight spring force of the arm 106, shown with reference to
In
The interface voltage control through the power source 206 acts in conjunction with the actuator arm, the TFC and other flying height control elements to maintain a constant height. To prevent the expansion of the NFT 212 and other components from reducing the overall flying height of the head 200, the power source 206 reduces the charge delivered to the head 200. This reduced charge increases the flying height of the head 200, as the interface voltage control at the head 200 from the power source 206 creates an attractive force (e.g. Coulomb force) which counterbalances against other forces involved with the flying height of the head 200.
The Coulomb force from the interface voltage control is time responsive on the order of μs, allowing changes in flying height over a time period ranging from 2 μs to 5 μs. The speed of actuation is only limited by the air bearing surface P2 (Pitch 2) frequency. The ABS surface is typically designed with two pitch nodal lines, designated P1 (Pitch 1) and P2 (Pitch 2), at which the ABS rotates in a pitch direction relative to the air flow. Further, based on the dynamics of rigid body motion, each pitch nodal line corresponds to a particular frequency of vibration of the slider. This frequency limits the speed of interface voltage control response. In one embodiment, the P2 frequency is above 200 kHz, leading to an interface voltage control response time of 5 μs or less. In another embodiment, the P2 frequency is less than 400 kHz, leading to an interface voltage control response time of 2 μs or greater. Since, the NFT time constants for expansion are between 50 μs and 100 μs, the interface voltage control can be adjust in real time to respond to changes in temperature in the near-field transducer 212.
The interface voltage control voltage can be between −2V and 2V. In one embodiment, the interface voltage control voltage applied to the head 200 is about −0.5V. The voltage applied from the power source 206 can either be a direct current (DC) or an alternating current (AC). When using AC, the AC frequency should be much higher than the P2 frequency, such as a 10 fold increase or greater over the P2 frequency. To determine the flying height of the head 200, the head 200 further includes an ECS (not shown). To control the head/disk clearance, a relationship between the signal from the ECS and the head/disk clearance is calculated.
The change in flying height can be determined by subtracting the flying height 218a from the flying height 218b. The head 200 is then repositioned such that the transient extension from the near-field transducer 212 is at the flying height 218a. The temperature of the NFT 212 is expected to equilibrate with nearby components over longer period of time, thus leading to expansion of the nearby components. During shorter operations, the NFT 212 is expected to cool quickly after the radiation source 202 is turned off and based on air flow at the air bearing surface. In either case, the extension of the near-field transducer is expected to be transient. The interface voltage control may be used to maintain the flying height 218a during the entire operation of the magnetic disk 250 and head 200, such as during any operations requiring a constant flying height. The interface voltage control may be used to maintain the flying height 218a only during a portion of the operation of the magnetic disk 250 and head 200, such as only during write operations or only during the initial radiation delivery of a HAMR/TAR write operation. Further, the interface voltage control can act in conjunction with or in the absence of the TFC.
The head and the slider are positioned at the flying height which is a first distance from a magnetic disk, at element 302. The first distance is a flying height which is created by a combination of the air flowing under the air bearing surface, the TFC, the actuator arm and other factors which act in conjunction to create the initial height. The first distance may be a distance from the magnetic disk which is slightly higher than an appropriate flying height for a head, such as between 1 nm and 10 nm. The head includes one or more components, such as the NFT, depicted with reference to
At element 304, a plurality of interface voltages are applied to the head or the magnetic disk to bias either the head, the magnetic disk or combinations thereof. The interface voltages applied cause a coulomb attraction and affect the spacing between the head and the magnetic disk. The plurality of interface voltages may cause a plurality of changes in the head/disk clearance (e.g., the first distance). The interface voltages may be applied to the magnetic disk or to the head, and may have a range between −2 V to 2 V. In further embodiments, the flying height can be modified by an interface voltage control from the first distance to the second distance or the interface voltage control can supplant one or more of the above factors in modifying the first distance to the second distance.
Next, the change in head/disk clearance for each interface voltage applied to the disk is calculated. The calculation may be based on any suitable technique, such as techniques based on Wallace Spacing Loss relationship, where the change in amplitude of the measured read-back signal harmonics directly relate to the head/disk clearance change. If a TD is performed, the actual head/disk clearance may be obtained.
Then, a radiation is delivered through the NFT to the magnetic disk, at element 306. As described above, the radiation delivered through the NFT heats the NFT simultaneously. Thus, there is a transient extension of the NFT from the air bearing surface, which reduces the second distance to a third distance. The transient extension can be less than 2 nm, such as less than 1 nm. The third distance is the second distance as changed by the transient extension of the NFT. The third distance is directly related to the temperature of the NFT and the material composition of the NFT. Thus, the amount of expansion, and thereby the third distance, can be determined based on empirical data and temperature measurement.
At element 308, the interface voltages delivered to the head and/or the magnetic disk are altered to reposition the head including the transient extension. Using the correlation between voltage and flying height, the voltage of either the head or the magnetic disk can be adjusted to reposition the head including the transient extension at the second distance. The adjustment of the voltage may be continuous to maintain the transient extension at the second distance during expected changes in the head, such as based on heating of components of the head or slider which might affect flying height or based on changes in the transient extension itself.
As the transient extension is expected to occur within 50-100 μs of receiving the radiation from the radiation source, it is believed that the accommodation can occur at a variety of time points around or in that time frame. In one embodiment, the interface voltages are modulated at a time point prior to the transient extension or simultaneous with the transient extension. Further, one or more of the interface voltages may be changed, either up or down, such that the overall flying height change results in the transient extension being at the second distance from the magnetic disk. The interface voltages may be used transiently, such as the interface voltages accommodating for the flying height changes until the thermal height control can accommodate during longer write cycles.
In another embodiment, a method for controlling flying height can include delivering an electrical bias to a head, delivering radiation to the magnetic disk through the near-field transducer, the NFT changing shape in response to the radiation, wherein the NFT is positioned at a second distance from the magnetic disk, and reducing the electrical bias to the head to move the NFT to a third distance from the magnetic disk. The head can include a first surface facing a magnetic disk and a NFT positioned at the first surface at a first distance from the magnetic disk.
In a further embodiment, a system is adapted to control the flying height of the slider using the method 300 described above. The system can include at least one processing unit. The processing unit can be adapted to perform steps including positioning a head at a flying height which is a first distance from a magnetic disk, the head comprising a near field transducer; applying a voltage to the head, the voltage changing the flying height between the head and the magnetic disk from the first distance to a second distance; delivering a radiation to the magnetic disk through the near field transducer, the radiation heating the near field transducer and changing the flying height from the second distance to a third distance; and altering the voltage to the head to change the flying height to the second distance.
Embodiments described herein generally relate to use of interface voltage control to control for short transient changes in flying height in a head. When radiation is delivered thought the NFT, the NFT heats up. This heating can lead to a transient extension from the air bearing surface toward the magnetic disk. Interface voltage control can be used to compensate the potential difference between head and disk. The voltage can be supplied to the entire slider body or to the magnetic head and therefore will automatically produce an attractive force. This attractive force is used to control the flying height, i.e., the distance between the head and disk. By controlling the interface voltage between the head and the magnetic disk in response to changes in the NFT, the distance between the head and the disk can be tightly and responsively controlled during write operations.
In order to achieve a fast retract when writing starts (and the NFT starts protruding), the head should be biased first with a proper interface voltage. While applying this bias voltage, the TFC power needs to be reduced accordingly to maintain same clearance. The TFC actuation may occur before, during or after the electrical actuation using the interface voltage control. In one embodiment, the TFC power could retract first and then the interface voltage can then be applied. When the NFT starts protruding, the voltage is reduced in a controlled fashion to keep the actual HMS constant. As soon as the transient extension has diminished, the TFC can overtake the compensation, allowing the interface voltage to return to a nominal value or a starting value.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
