Patent Publication Number: US-9431043-B2

Title: Integrated compound DBR laser for HAMR applications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/307,451, filed Jun. 17, 2014, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments disclosed herein generally relate to data storage systems, and more particularly, to heat-assisted magnetic recording (HAMR) system. 
     2. Description of the Related Art 
     Higher storage bit densities in magnetic media used in disk drives have reduced the size (volume) of magnetic bits to the point where the magnetic bit dimensions are limited by the grain size of the magnetic material. Although grain size can be reduced further, the data stored within the magnetic bits may not be thermally stable. That is, random thermal fluctuations at ambient temperatures may be sufficient to erase data. This state is described as the superparamagnetic limit, which determines the maximum theoretical storage density for a given magnetic media. This limit may be raised by increasing the coercivity of the magnetic media or by lowering the temperature. Lowering the temperature may not always be practical when designing hard disk drives for commercial and consumer use. Raising the coercivity, on the other hand, requires write heads that incorporate higher magnetic moment materials, or techniques such as perpendicular recording (or both). 
     One additional solution has been proposed, which uses heat to lower the effective coercivity of a localized region on the magnetic media surface and writes data within this heated region. The data state becomes “fixed” once the media cools to ambient temperatures. This technique is broadly referred to as “thermally assisted (magnetic) recording” (TAR or TAMR), “energy assisted magnetic recording” (EAMR), or “heat-assisted magnetic recording” (HAMR) which are used interchangeably herein. It can be applied to longitudinal and perpendicular recording systems as well as “bit patterned media”. Heating of the media surface has been accomplished by a number of techniques such as focused laser beams or near-field optical sources. 
     Typically, the laser beam used to heat the media surface is generated from Fabry Perot (FP) laser diodes, but the FP laser diodes suffer from mode hopping which leads to significant power fluctuations, such as power fluctuations greater than two percent. High frequency pulsing of the laser diode can reduce the impact of mode hopping by forcing the device to operate multimode. However, the quality of the magnetic recording is degraded by using the high frequency pulsing of the laser diode. 
     Therefore, an improved HAMR system is needed. 
     SUMMARY 
     Embodiments disclosed herein generally relate to a magnetic write head including a media facing surface and a surface opposite the media facing surface. The magnetic write head further includes a reflector extending from the surface opposite the media facing surface toward the media facing surface. A semiconductor laser diode gain region protrudes out of the surface opposite the media facing surface, and the reflector optimizes the optical energy generated in the semiconductor laser diode gain region to be a single lasing optical mode over a large current and temperature range. 
     In one embodiment, a magnetic write head is disclosed. The magnetic write head includes a media facing surface, a surface opposite the media facing surface, a semiconductor laser diode gain region protruding out of the surface opposite the media facing surface, and a reflector extending from the surface opposite the media facing surface toward the media facing surface. The semiconductor laser diode gain region is aligned with the reflector. 
     In another embodiment, a magnetic write head is disclosed. The magnetic write head includes a media facing surface, a surface opposite the media facing surface, a near field transducer disposed proximate the media facing surface, a semiconductor laser diode gain region disposed between the near field transducer and the surface opposite the media facing surface, and a reflector disposed between the near field transducer and the semiconductor laser diode gain region. The semiconductor laser diode gain region is aligned with the reflector. 
     In another embodiment, a magnetic write head is disclosed. The magnetic write head includes a media facing surface, a surface opposite the media facing surface, an optical light generating device disposed over the surface, a spot size converter extending from the structure to the media facing surface, and a guided mode resonance structure coupled to the spot size converter. The guide mode resonance structure has a single mode planar waveguide that is oriented in a direction that is substantially parallel to the media facing surface and the direction is a cross-track direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 in any field involving magnetic sensors. 
         FIG. 1  illustrates a disk drive system, according to embodiments of the described herein. 
         FIG. 2  illustrates a HAMR system according to one embodiment described herein. 
         FIG. 3A  illustrates a gain region according to one embodiment described herein. 
         FIG. 3B  illustrates a semiconductor laser diode having an antireflective coating according to one embodiment described herein. 
         FIG. 4  illustrates a magnetic write head according to one embodiment described herein. 
         FIG. 5  illustrates a magnetic write head according to one embodiment described herein. 
         FIGS. 6A-6B  illustrate magnetic write heads according to embodiments described herein. 
     
    
    
     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. 
     DETAILED DESCRIPTION 
     In the following, reference is made to embodiments. 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 disclosed herein generally relate to a magnetic write head including a media facing surface and a surface opposite the media facing surface. The magnetic write head further includes a reflector extending from the surface opposite the media facing surface toward the media facing surface. A semiconductor laser diode gain region protrudes out of the surface opposite the media facing surface, and the reflector helps optimizing the optical energy generated in the semiconductor laser diode gain region to be a single mode over a large current and temperature range. 
       FIG. 1  illustrates a disk drive  100  according to one embodiment disclosed herein. As shown, at least one rotatable magnetic media  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording on each disk is in the form of annular patterns of concentric data tracks (not shown) on the magnetic media  112 . 
     At least one slider  113  is positioned near the magnetic media  112 , each slider  113  supporting one or more magnetic head assemblies  121  that may include a radiation source (e.g., a laser and/or electrically resistive heater) for heating the disk surface  122 . As the magnetic disk rotates, the slider  113  moves radially in and out over the disk surface  122  so that the magnetic head assembly  121  may access different tracks of the magnetic media  112  where desired data are written. Each slider  113  is attached to an actuator arm  119  by way of a suspension  115 . The suspension  115  provides a slight spring force which biases the slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 . The actuator means  127  as shown in  FIG. 1  may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by control unit  129 . 
     During operation of a TAR or HAMR enabled disk drive  100 , the rotation of the magnetic media  112  generates an air bearing between the slider  113  and the disk surface  122  which exerts an upward force or lift on the slider  113 . The air bearing thus counter-balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the media  112  surface by a small, substantially constant spacing during normal operation. The radiation source heats up the high-coercivity media so that the write elements of the magnetic head assemblies  121  may correctly magnetize the data bits in the media. 
     The various components of the disk drive  100  are controlled in operation by control signals generated by control unit  129 , such as access control signals and internal clock signals. Typically, the control unit  129  comprises logic control circuits, storage means and a microprocessor. The control unit  129  generates control signals to control various system operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide the desired current profiles to optimally move and position slider  113  to the desired data track on media  112 . Write and read signals are communicated to and from write and read heads on the assembly  121  by way of recording channel  125 . 
     The above description of a typical magnetic disk storage system and the accompanying illustration of  FIG. 1  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. 
       FIG. 2  is a cross sectional schematic of a HAMR system  200 , according to one embodiment of the invention. The HAMR system  200  includes a magnetic write head  202 , a magnetic read head  204  and a magnetic media  206 , such as a disk. The magnetic media  206  includes a substrate and a perpendicular magnetic recording layer (RL)  246 . In one embodiment, the magnetic media  206  may include an optional “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). However, the SUL is not required for a HAMR system  200 . 
     The RL  246  may be any media with perpendicular magnetic anisotropy, such as a cobalt-chromium (CoCr) alloy granular layer grown on a special growth-enhancing sublayer, or a multilayer of alternating films of Co with films of platinum (Pt) or palladium (Pd). The RL  246  may also be an L 1   0  ordered alloy such as FePt or FeNiPt. The disk may also include a protective overcoat (not shown) over the RL  246 . 
     The HAMR system  200  has a substrate trailing surface  211  and a media facing surface (MFS)  208  oriented generally perpendicular to trailing surface  211 . The HAMR system  200  also includes a surface  210  opposite the MFS  208 . The trailing surface  211  is typically formed of a composite material, such as a composite of alumina/titanium-carbide (Al 2 O 3 /TiC), and supports the read head  204  and the write head  202 , which are typically formed as a series of thin films and structures on the trailing surface  211 . The magnetic media  206  may spin in a direction  223  away from the trailing surface  211  and towards the other layers of the HAMR system  200 . The MFS  208  is the recording-layer-facing surface of the slider that faces the media  206 . Note that  FIG. 2  is not drawn to scale because of the difficulty in showing the very small features and, for the sake of clarity, omits structures from the head such as spacing and insulating layers. 
     The magnetic read head  204  includes a magnetoresistive read pole  215  located between shields S 1  and S 2 . The magnetic write head  202  includes a magnetic yoke  220  with a write pole  220   a , a return pole  220   b , and an electrically conductive coil  225 . The write pole  220   a  is formed of a high-moment material, such as a NiFe or FeCoNi alloy. The write coil  225  is wrapped around the yoke  220  with the electrical current directions being shown as into the paper by the coil cross-sections marked with an “X” and out of the paper by the coil cross-sections marked with a solid circle. When write-current pulses are directed through the coil  225 , the write pole  220   a  directs magnetic flux, represented by arrow  230 , to the RL  246 . Further, the magnetic flux  230  continues through the substrate or a SUL layer before arriving at the return pole  220   b . However, the invention is not limited to the structure and material discussed above. For example, the coil  225  may be a helical coil or the write pole  220   a  may include a wrap-around shield. Further, embodiments may operate with any recording head that can perform the functions discussed herein. 
     The magnetic head  202  may also include a non-magnetic material  251  between the return pole  220   b  and the write pole  220   a . The non-magnetic material  251  may include SiO 2  and Al 2 O 3 . A reflector  212  is embedded in the non-magnetic material  251 . The reflector  212  has a first end  214  extending to the surface  210  and a second end  216  proximate a spot size converter (SSC)  218 . The reflector  212  may be a spatial periodic structure that includes a plurality of alternating layers  213 ,  215 , and the alternating layers  213 ,  215  have different indices of refraction. One of the two layers  213 ,  215  may be made of III-V semiconductor materials, such as AlGaAs or GaAs, and the other layer of the two layers  213 ,  215  may be made of a material having a lower refractive index than III-V semiconductor materials. Examples of the material may include tantalum oxide, tantalum nitride, silicon nitride, silicon dioxide, and the like. In one embodiment, the reflector  212  is a distributed Bragg reflector (DBR). The reflector  212  may extend through the yoke  220  and may be located between the write pole  220   a  and the return pole  220   b . As noted by the ghosted lines, the yoke  220  may continuously connect the write pole  220   a  to the return pole  220   b.    
     A semiconductor laser diode gain region  217  may be protruded out of the surface  210 . The semiconductor laser diode gain region  217  may be optically aligned with the reflector  212 . A cross sectional view of the semiconductor laser diode gain region  217  is illustrated in  FIG. 3A . As shown in  FIG. 3A , the semiconductor laser diode gain region  217  includes a mirror  302 , a p-doped layer  304 , an n-doped layer  306  and a p-n junction  308 . When an electrical current is applied to the semiconductor laser diode gain region  217 , an optical light  310 , such as a laser beam, is generated. The optical light  310  generated by the semiconductor laser diode gain region  217  may switch wavelengths, also known as mode hopping, which leads to significant power fluctuations. Referring back to  FIG. 2 , the reflector  212  embedded in the non-magnetic material  251  functions as a wavelength selective element that helps reducing the number of modes available for lasing. 
     Alternatively, the semiconductor laser diode gain region  217  may be replaced with a semiconductor laser diode having an antireflective (AR) coating. As shown in  FIG. 3B , a semiconductor laser diode  320  includes a first mirror  322 , a p-doped layer  324 , an n-type layer  326 , a p-n junction  328 , a second mirror  330  and an AR coating  332 . The semiconductor laser diode  320  may be disposed over the surface  210 , forming a gap between the semiconductor laser diode  320  and the surface  210 . The AR coating  332  may be facing the surface  210 . The AR coating  332  may be made of SiO X N. The AR coating  332  prevents the semiconductor laser diode  320  from generating a laser beam having a single mode. However, the reflector  212  as shown in  FIG. 2  helps reducing the number of modes available for lasing. 
     As shown in  FIG. 2 , a near field transducer (NFT)  219 —e.g., a plasmonic device or an optical transducer—is located at or proximate the MFS  208 . The single mode optical light  310  coming out of the reflector  212  is focused on the NFT  219  by the SSC  218 . The NFT  219  further focuses the optical light  310  to avoid heating neighboring tracks of data on the media  206 —i.e., creates a beamspot much smaller than the diffraction limit. As shown by arrows  221 , this optical energy emits from the NFT  219  to the surface of the media  206  below the MFS  208 . The embodiments herein are not limited to any particular type of NFT and may operate with, for example, either a c-aperature, e-antenna, nanobeak, lollypop, split ring resonator, near-field source, or any other shaped transducer known in the art. The reflector  212 , the SSC  218  and the NFT  219  may be fabricated at any location such that the NFT  219  passes over a portion of the spinning magnetic media  206  prior to that portion passing below the write pole  220   a . Specifically, the reflector  212  may be located between shield S 2  and return pole  220   b , or between the write pole  220   a  and the outer face  231  of the HAMR system  200  (if the media  206  rotates opposite of the direction  223  shown). 
     While writing to the media  206 , the RL  246  moves relative to the read/write heads  204 ,  202  in the direction shown by arrow  223 . In HAMR, the optical energy  221  emitted from the NFT  219  temporarily lowers the coercivity (H c ) of the RL  246  so that the magnetic recording regions  227 ,  228 ,  229  may be oriented by the write field from write pole  220   a . The magnetic recording regions  227 ,  228 ,  229  become oriented by the write field if the write field (H w ) is greater than H c . After a region of the RL  246  in the data track has been exposed to H w  from the write pole  220   a  and the resulting heat from the optical energy  221  from the NFT  219 , the region&#39;s temperature falls below the Curie temperature and the data associated with the magnetic orientations is recorded. Specifically, the transitions between recorded regions (such as previously recorded regions  227 ,  228 , and  229 ) represent written data “bits” that can be read by the read pole  215 . In this manner, the NFT  219  uses the optical energy  221  to heat the RL layer  246  and lower its magnetic coercivity. 
       FIG. 4  illustrates a magnetic write head  400  according to one embodiment described herein. The magnetic write head  400  may be the magnetic write head  202  described in  FIG. 2 . For better clarity, components such as the write pole, the return pole and NFT and the magnetic yoke are omitted in  FIG. 4 . In order to further improve wavelength selectivity, a heating element  402  may be disposed proximate the reflector  212 . The heating element  402  will raise the temperature of the wavelength selecting reflector  212 , which changes the index of refraction of the various materials within the reflector  212 . The resonant wavelength of the laser beam is highly dependent on the indices of refraction of the materials within the reflector  212 . Therefore, the spectrum for the reflected light can be accurately controlled if the temperature of the reflector  212  is controlled. The heating element  402  may be a resistive heating element made of resistive metal alloy. The heating element  402  may have a serpentine shape, as shown in  FIG. 4 . The reflector  212  and the heating element  402  may be aligned in the down track direction (along the y-axis), or aligned in the cross track direction (along the x-axis), as shown in  FIG. 4 . 
       FIG. 5  illustrates a magnetic write head  500  according to one embodiment described herein. The magnetic write head  500  may be the magnetic write head  202  described in  FIG. 2 . For better clarity, components such as the write pole, the return pole and NFT and the magnetic yoke are omitted in  FIG. 5 . The magnetic write head  500  may include the MFS  208  and the surface  210  opposite the MFS  208 . A semiconductor laser diode gain region  502  is disposed between the surface  210  and the MFS  208 . The semiconductor laser diode gain region  502  can be transferred to the AlTiC substrate using epitaxial layer transfer, wafer bonding, or flip chip bonding. The semiconductor laser diode gain region  502  may be the same as the semiconductor laser diode gain region  217  shown in  FIG. 3 . The semiconductor laser diode gain region  502  has a first end  501  extending to the surface  210  and a second end  503  recessed from the surface  210  and towards the MFS  208 . A reflector  504  is disposed proximate the second end  503  of the semiconductor laser diode gain region  502  and is aligned with the semiconductor laser diode gain region  502 . The reflector  504  may be the same as the reflector  212  described in  FIG. 2 , except the reflector  504  does not extend to the surface  210 . The reflector  504  may include alternating layers  506 ,  508 , and the alternating layers  506 ,  508  may have different indices of refraction. The layers  506 ,  508  may be made of III-V semiconductor materials. The SSC  218  is disposed between the reflector  504  and the MFS  208 . 
       FIGS. 2-5  illustrate a HAMR system having a reflector embedded in the magnetic write head. The reflector is oriented parallel to the laser beam and suppresses the number of modes of the laser beam. Alternatively, a guided mode resonance structure constructed from grating coupler and a single mode waveguide having an optical axis parallel to the MFS (perpendicular to the laser beam) and is embedded in the magnetic write head may be utilized to suppress the number of modes of the laser beam.  FIGS. 6A-6B  illustrate such design. As shown in  FIG. 6A , a magnetic write head  600  includes an optical light generating device  602  disposed over the surface  210 . The optical light generating device  602  may be the semiconductor laser diode gain region  217  shown in  FIG. 2 , where the semiconductor laser diode gain region  217  may be in contact with the surface  210 . Alternatively, the optical light generating device  602  may be the semiconductor laser diode  320  shown in  FIG. 3B , where a gap may be formed between the semiconductor laser diode  320  and the surface  210 . The optical light generating device  602  is optically aligned with a SSC  604 , which also functioning as a waveguide for directing the optical light, such as a laser beam, to the NFT  219 . The SSC  604  may be made of TaO and may be surrounded by a cladding material  603 , such as SiO2 or Al 2 O 3 . 
     To generate a guided mode resonance structure  601 , a grating coupler  610  may be used to couple the SSC  604  to a single mode planar waveguide  606 . The single mode planar waveguide  606  may be made of TaO and the cladding  607  may be made of SiO2 or Al 2 O 3 . The single mode planar waveguide  606  may be disposed on both sides of the SSC  604  in a cross-track direction (X-direction). The grating coupler  610  may include a first plurality of protrusions  609  facing the surface  210  and a second plurality of protrusions  611  facing the MFS  208 . Each protrusion  609  is aligned with a corresponding protrusion  611 . The size of the grating coupler  610  should closely match the extent of the diverging laser beam to maximize the coupling efficiency. A reflector  612 , such as a DBR, may be disposed at each end of the single mode planar waveguide  606 . Each reflector  612  may include a comb structure  614  and cladding material  616 . In one embodiment, the comb structure  614  is made of TaO and the cladding material  616  is made of SiO2 or Al 2 O 3 . The single mode planar waveguide  606  and the reflectors  612  may be extending in the cross-track direction and substantially parallel to the MFS  208 , as shown in  FIG. 6A . 
     During the guided mode resonance structure  601  operation, a laser beam is generated by the optical light generating device  602  and is coupled to the SSC  604 . The laser beam, which may suffer from mode hopping, is coupled to the single mode planar waveguide  606  via the grating coupler  610  and travels in a direction that is substantially parallel to the MFS  208 . The reflectors  612  reflect the laser beam backward towards the SSC  604 , creating a resonant cavity supporting light oscillating in the direction transverse to the incident beam. Since the grating coupler and the single mode waveguide are highly wavelength selective, only a single well defined mode is supported by the guided mode resonance structure  601 . The laser beam returned to the optical light generating device  602  from the guided mode resonance structure  601  has a very narrow linewidth on the order of  10   s  of picometers. The effective laser cavity is thus formed by the laser diode cavity and the guided mode resonance structure. Such configuration results in a narrow bandwidth reflection filter for a broad spectral ranges. The bandwidth is determined by the quality factor of the resonator, i.e., the dimensions of the core region of the single mode planar waveguide  606 . The reflectors  612  can be made significantly longer than 200 microns and can be as long as half of the slider spacing on the fabricated wafer. Also, the reflectance of the reflectors  612  can be enhanced to make highly reflective mirrors, creating a compact, high-Q cavity. As the laser beam is reflected back to the SSC  604  by the reflectors  612 , the resulting laser beam traveling towards the NFT  219  via the SSC  604  will have a single mode. 
     In one embodiment, instead of using the reflectors  612  to reflect the laser beam back towards the SSC  604 , the single mode planar waveguide  606  is extended to metal plates  622 , as shown in  FIG. 6B . The metal plate  622  may be made of silver. The length “L 1 ” of a portion of the single mode planar waveguide  606  (the distance between the metal plate  622  and the grating coupler  610 ) may be between about 200 microns and about 400 microns. The laser beam is coupled to the single mode planar waveguide  606  via the grating coupler  610  and is reflected back to the SSC  604  by the metal plate  622 . Again the resulting laser beam traveling towards the NFT  219  via the SSC  604  also will have a single mode. 
     In summary, a HAMR system is disclosed. The HAMR system includes a magnetic write head having a MFS and a surface opposite the MFS. A reflector is disposed between the MFS and the surface opposite the MFS and a semiconductor laser diode gain region is protruding out of the surface opposite the MFS. The reflector suppresses the number of modes of optical light generated in the semiconductor laser diode gain region and functions as a wavelength selective element. Having a single mode optical light helps reducing power fluctuation of the power used to generate the optical light. 
     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.