# Patent

## Title
Multilayer plasmon generator

## Abstract

A plasmon generator has a front end face, a first metal layer, a second meta-field light based on a surface plasmon. The intermediate layer is interposed between the first metal layer and the second metal layer. Each of the first metal layer, the second metal layer and the intermediate layer has an end located in the front end face. Each of the first and second metal layers is formed of a metal material. The intermediate layer is formed of a material higher in Vickers hardness than the metal material used to form the first metal layer and the metal material used to form the second metal layer.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plasmon generator for use in thermally-assisted magnetic recording where a recording medium is irradiated with near-field light to lower the coercivity of the recording medium for data writing, and to a thermally-assisted magnetic recording head including the plasmon generator.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head section including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head section including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, a thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near a medium facing surface of the slider.
The plasmon generator has a front end face located in the medium facing surface. The front end face generates near-field light. Surface plasmons are excited on the plasmon generator and propagate along the surface of the plasmon generator to reach the front end face. As a result, the surface plasmons concentrate at the front end face, and near-field light is generated from the front end face based on the surface plasmons.
U.S. Patent Application Publication No. 2011/0170381 A1 discloses a technology in which the surface of a waveguide and the surface of a metallic structure (plasmon generator) are arranged to face each other with a gap therebetween, and evanescent light that occurs at the surface of the waveguide based on the light propagating through the waveguide is used to excite surface plasmons on the metallic structure, so that near-field light is generated based on the excited surface plasmons. Further, U.S. Patent Application Publication No. 2011/0170381 A1 discloses forming a part of the metallic structure from a material different from that of other parts of the metallic structure.
Materials that are typically employed for plasmon generators are metals having high electrical conductivities, such as Au and Ag. However, Au and Ag are relatively soft and have relatively high thermal expansion coefficients. Thus, if a plasmon generator is formed entirely of Au or Ag, there are problems as described below.
In the process of manufacturing a thermally-assisted magnetic recording head, the medium facing surface is formed by polishing. During polishing, polishing residues of metal materials may grow to cause smears. To remove the smears, the polished surface is slightly etched by, for example, ion beam etching in some cases. If the plasmon generator is formed entirely of Au or Ag, which are relatively soft, the polishing and etching mentioned above may cause the front end face of the plasmon generator to be significantly recessed relative to the other parts of the medium facing surface. In such a case, the front end face of the plasmon generator becomes distant from the recording medium, and the heating performance of the plasmon generator is thus degraded.
Part of the energy of light guided to the plasmon generator through the waveguide is transformed into heat in the plasmon generator. Part of the energy of near-field light generated by the plasmon generator is also transformed into heat in the plasmon generator. The plasmon generator thus increases in temperature during the operation of the thermally-assisted magnetic recording head. A plasmon generator that is formed entirely of Au or Ag will expand and significantly protrude toward the recording medium when the temperature of the plasmon generator increases. As a result, a protective film covering the medium facing surface may come into contact with the recording medium. This may cause damage to the recording medium or cause the protective film to be broken. When the protective film is broken, the plasmon generator may be damaged by contact with the recording medium or may be corroded by contact with high temperature air.
Further, a plasmon generator that is formed entirely of Au or Ag may be deformed due to aggregation when its temperature increases. In addition, such a plasmon generator expands when its temperature increases and then contracts when its temperature decreases. When the plasmon generator undergoes such a process, the front end face of the plasmon generator may be significantly recessed relative to the other parts of the medium facing surface. In such a case, the heating performance of the plasmon generator is degraded as mentioned above.
For the various reasons described above, the plasmon generator formed entirely of Au or Ag has the drawback of being low in reliability.
U.S. Patent Application Publication No. 2011/0170381 A1 discloses a metallic structure composed of a main body and a layer having a greater hardness than the main body (this layer will hereinafter be referred to as the hard layer). In this metallic structure, the main body is not exposed in the medium facing surface, but the hard layer is exposed in the medium facing surface. In this metallic structure, surface plasmons are generated in the main body. The generated surface plasmons propagate to the hard layer, and near-field light is generated from the vertex of the hard layer. This metallic structure has the drawback that there is a great loss of the surface plasmons as they propagate from the main body to the hard layer, and it is thus difficult to allow the surface plasmons to efficiently propagate to the vertex.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a plasmon generator of high reliability that allows surface plasmons to propagate to the front end face efficiently, and to provide a thermally-assisted magnetic recording head having such a plasmon generator.
A plasmon generator of the present invention is configured to excite a surface plasmon, and has a front end face, a first metal layer, a second metal layer, and an intermediate layer. The front end face generates near-field light based on the surface plasmon. The intermediate layer is interposed between the first metal layer and the second metal layer. Each of the first metal layer, the second metal layer and the intermediate layer has an end located in the front end face. Each of the first and second metal layers is formed of a metal material. The intermediate layer is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer and the metal material used to form the second metal layer.
A thermally-assisted magnetic recording head of the present invention includes: a medium facing surface facing a recording medium; a main pole that produces a write magnetic field for writing data on the recording medium; a waveguide; and a plasmon generator. The waveguide includes a core through which light propagates, and a cladding provided around the core. The plasmon generator is configured to excite a surface plasmon based on the light propagating through the core.
In the thermally-assisted magnetic recording head of the present invention, the plasmon generator has a front end face located in the medium facing surface, a first metal layer, a second metal layer, and an intermediate layer. The front end face generates near-field light based on the surface plasmon. The intermediate layer is interposed between the first metal layer and the second metal layer. Each of the first metal layer, the second metal layer and the intermediate layer has an end located in the front end face. Each of the first and second metal layers is formed of a metal material. The intermediate layer is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer and the metal material used to form the second metal layer.
In the plasmon generator and the thermally-assisted magnetic recording head of the present invention, the intermediate layer may be smaller in thickness than the first and second metal layers. The material used to form the intermediate layer may be a metal material different from the metal material used to form the first metal layer and the metal material used to form the second metal layer, or may be a dielectric material.
The plasmon generator of the present invention may further have a core facing surface that faces the surface of the core through which light propagates. In this case, the first metal layer may be located closer to the core facing surface than is the second metal layer. Further, the first metal layer may be greater in thickness than the second metal layer.
In the thermally-assisted magnetic recording head of the present invention, the core may have an evanescent light generating surface that generates evanescent light based on the light propagating through the core. In this case, the plasmon generator may further have a core facing surface that faces the evanescent light generating surface. The cladding may include an interposition part interposed between the evanescent light generating surface and the core facing surface. The first metal layer may be located closer to the core facing surface than is the second metal layer. Further, the first metal layer may be greater in thickness than the second metal layer.
The plasmon generator of the present invention and the plasmon generator in the thermally-assisted magnetic recording head of the present invention may further include a metal portion formed of a metal material and in contact with the first metal layer. No part of the metal portion constitutes part of the front end face.
According to the present invention, it is possible to provide a plasmon generator of high reliability that allows surface plasmons to propagate to the front end face efficiently, and to provide a thermally-assisted magnetic recording head having such a plasmon generator.
Other and further objects, features and advantages of the present invention will appear more fully from the following description.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing the main part of a thermally-assisted magnetic recording head according to a first embodiment of the invention.
FIG. 2 is a cross-sectional view showing the main part of the thermally-assisted magnetic recording head according to the first embodiment of the invention.
FIG. 3 is a front view showing part of the medium facing surface of the thermally-assisted magnetic recording head according to the first embodiment of the invention.
FIG. 4 is a plan view showing the positional relationship between the plasmon generator and the core of the waveguide shown in FIG. 1.
FIG. 5 is a cross-sectional view showing the configuration of the thermally-assisted magnetic recording head according to the first embodiment of the invention.
FIG. 6 is a front view showing the medium facing surface of the thermally-assisted magnetic recording head according to the first embodiment of the invention.
FIG. 7A and FIG. 7B are cross-sectional views showing a step of a method of manufacturing the thermally-assisted magnetic recording head according to the first embodiment of the invention.
FIG. 8A and FIG. 8B are cross-sectional views showing a step that follows the step shown in FIG. 7A and FIG. 7B.
FIG. 9A and FIG. 9B are cross-sectional views showing a step that follows the step shown in FIG. 8A and FIG. 8B.
FIG. 10A and FIG. 10B are cross-sectional views showing a step that follows the step shown in FIG. 9A and FIG. 9B.
FIG. 11A and FIG. 11B are cross-sectional views showing a step that follows the step shown in FIG. 10A and FIG. 10B.
FIG. 12A and FIG. 12B are cross-sectional views showing a step that follows the step shown in FIG. 11A and FIG. 11B.
FIG. 13A and FIG. 13B are cross-sectional views showing a step that follows the step shown in FIG. 12A and FIG. 12B.
FIG. 14 is a perspective view showing a first modification example of the plasmon generator according to the first embodiment of the invention.
FIG. 15 is a perspective view showing a second modification example of the plasmon generator according to the first embodiment of the invention.
FIG. 16 is a perspective view showing the main part of a thermally-assisted magnetic recording head according to a second embodiment of the invention.
FIG. 17 is a cross-sectional view showing the main part of the thermally-assisted magnetic recording head according to the second embodiment of the invention.
FIG. 18 is a front view showing part of the medium facing surface of the thermally-assisted magnetic recording head according to the second embodiment of the invention.
FIG. 19 is a perspective view showing the main part of a thermally-assisted magnetic recording head according to a third embodiment of the invention.
FIG. 20 is a cross-sectional view showing the main part of the thermally-assisted magnetic recording head according to the third embodiment of the invention.
FIG. 21 is a cross-sectional view showing the main part of a thermally-assisted magnetic recording head according to a fourth embodiment of the invention.
FIG. 22 is a front view showing part of the medium facing surface of the thermally-assisted magnetic recording head according to the fourth embodiment of the invention.
FIG. 23 is a cross-sectional view showing the main part of a modification example of the thermally-assisted magnetic recording head according to the fourth embodiment of the invention.
FIG. 24 is a front view showing part of the medium facing surface of the modification example of the thermally-assisted magnetic recording head according to the fourth embodiment of the invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[First Embodiment]
Preferred embodiments of the present invention will now be described in detail with reference to the drawings. First, reference is made to FIG. 5 and FIG. 6 to describe the configuration of a thermally-assisted magnetic recording head according to a first embodiment of the invention. The thermally-assisted magnetic recording head according to the present embodiment includes a plasmon generator according to the present embodiment. FIG. 5 is a cross-sectional view showing the configuration of the thermally-assisted magnetic recording head. FIG. 6 is a front view showing the medium facing surface of the thermally-assisted magnetic recording head.
The thermally-assisted magnetic recording head according to the present embodiment is for use in perpendicular magnetic recording, and is in the form of a slider to fly over the surface of a rotating recording medium. When the recording medium rotates, an airflow passing between the recording medium and the slider causes a lift to be exerted on the slider. The slider is configured to fly over the surface of the recording medium by means of the lift.
As shown in FIG. 5, the thermally-assisted magnetic recording head has a medium facing surface 60 facing a recording medium 80. Here, X direction, Y direction, and Z direction will be defined as follows. The X direction is the direction across the tracks of the recording medium 80, i.e., the track width direction. The Y direction is a direction perpendicular to the medium facing surface 60. The Z direction is the direction of travel of the recording medium 80 as viewed from the slider. The X, Y, and Z directions are orthogonal to one another.
As shown in FIG. 5 and FIG. 6, the thermally-assisted magnetic recording head includes: a substrate 1 formed of a ceramic material such as aluminum oxide-titanium carbide (Al2O3—TiC) and having a top surface 1a; an insulating layer 2 formed of an insulating material such as alumina (Al2O3) and disposed on the top surface 1a of the substrate 1; a bottom shield layer 3 formed of a magnetic material and disposed on the insulating layer 2; a bottom shield gap film 4 which is an insulating film disposed to cover the bottom shield layer 3; a magnetoresistive (MR) element 5 serving as a read element disposed on the bottom shield gap film 4; two leads (not illustrated) connected to the MR element 5; a top shield gap film 6 which is an insulating film disposed on the MR element 5; and a top shield layer 7 formed of a magnetic material and disposed on the top shield gap film 6. The Z direction is also a direction perpendicular to the top surface 1a of the substrate 1.
An end of the MR element 5 is located in the medium facing surface 60. The MR element 5 may be an element formed of a magneto-sensitive film that exhibits a magnetoresistive effect, such as an anisotropic magnetoresistive (AMR) element, a giant magnetoresistive (GMR) element, or a tunneling magnetoresistive (TMR) element. The GMR element may be of either the current-in-plane (CIP) type in which a current used for detecting magnetic signals is fed in a direction generally parallel to the plane of layers constituting the GMR element or the current-perpendicular-to-plane (CPP) type in which the current used for detecting magnetic signals is fed in a direction generally perpendicular to the plane of layers constituting the GMR element.
The parts from the bottom shield layer 3 to the top shield layer 7 constitute a read head section. The thermally-assisted magnetic recording head further includes an insulating layer 8 disposed on the top shield layer 7, a middle shield layer 9 formed of a magnetic material and disposed on the insulating layer 8, and a nonmagnetic layer 10 formed of a nonmagnetic material and disposed on the middle shield layer 9. The insulating layer 8 and the nonmagnetic layer 10 are formed of alumina, for example.
The thermally-assisted magnetic recording head further includes a return pole layer 11 formed of a magnetic material and disposed on the nonmagnetic layer 10, and an insulating layer (not illustrated) disposed on the nonmagnetic layer 10 and surrounding the return pole layer 11. The return pole layer 11 has an end face located in the medium facing surface 60. The non-illustrated insulating layer is formed of alumina, for example.
The thermally-assisted magnetic recording head further includes a shield layer 12 located close to the medium facing surface 60 and lying on a part of the return pole layer 11, a coupling layer 13 located away from the medium facing surface 60 and lying on another part of the return pole layer 11, an insulating layer 14 lying on the remaining part of the return pole layer 11 and on the non-illustrated insulating layer, and a coil 15 lying on the insulating layer 14. The shield layer 12 and the coupling layer 13 are each formed of a magnetic material. The shield layer 12 has an end face located in the medium facing surface 60. The coil 15 is planar spiral-shaped and wound around the coupling layer 13. The coil 15 is formed of a conductive material such as copper. The insulating layer 14 is formed of alumina, for example.
The thermally-assisted magnetic recording head further includes an insulating layer 16 disposed around the shield layer 12, the coupling layer 13 and the coil 15 and in the space between every adjacent turns of the coil 15, and two coupling portions 17A and 17B disposed on the coupling layer 13. The coupling portions 17A and 17B are each formed of a magnetic material. Each of the coupling portions 17A and 17B has a first layer located on the coupling layer 13, and a second and a third layer stacked in this order on the first layer. The first layer of the coupling portion 17A and the first layer of the coupling portion 17B are disposed to be aligned in the track width direction (the X direction). The insulating layer 16 is formed of alumina, for example.
The thermally-assisted magnetic recording head further includes a waveguide. The waveguide includes a core 20 through which light propagates, and a cladding provided around the core 20. The core 20 will be described in detail later.
The cladding includes cladding layers 18, 19, and 21. The cladding layer 18 is disposed over the shield layer 12, the coupling layer 13, the coil 15, and the insulating layer 16. The core 20 is disposed on the cladding layer 18. The cladding layer 19 is disposed on the cladding layer 18 and surrounds the core 20. The cladding layer 21 is disposed over the core 20 and the cladding layer 19.
The core 20 is formed of a dielectric material that transmits laser light to be used for generating near-field light. The laser light emitted from a laser diode (not illustrated) enters the core 20 and propagates through the core 20. The cladding layers 18, 19, and 21 are each formed of a dielectric material that has a refractive index lower than that of the core 20. For example, the core 20 may be formed of tantalum oxide such as Ta2O5 or silicon oxynitride (SiON), while the cladding layers 18, 19, and 21 may be formed of silicon dioxide (SiO2) or alumina.
The first layers of the coupling portions 17A and 17B are embedded in the cladding layer 18. The second layers of the coupling portions 17A and 17B are embedded in the cladding layer 19. The second layer of the coupling portion 17A and the second layer of the coupling portion 17B are located on opposite sides of the core 20 in the track width direction (the X direction), each being at a distance from the core 20.
The thermally-assisted magnetic recording head further includes a main pole 26 disposed above the core 20 in the vicinity of the medium facing surface 60, and a plasmon generator 40 interposed between the core 20 and the main pole 26. The plasmon generator 40 is configured to excite surface plasmons on the principle to be described later. The plasmon generator 40 will be described in detail later.
The main pole 26 has an end face 26a located in the medium facing surface 60. The main pole 26 may include a narrow portion having the end face 26a and an end portion opposite to the end face 26a, and a wide portion connected to the end portion of the narrow portion. The wide portion is greater than the narrow portion in width in the track width direction (the X direction).
The thermally-assisted magnetic recording head further includes a dielectric layer 22 lying on the cladding layer 21 and surrounding the plasmon generator 40, a heat sink 23 lying astride part of the plasmon generator 40 and part of the dielectric layer 22, a dielectric layer 24 disposed to cover the heat sink 23, and a dielectric layer 25 disposed to cover the plasmon generator 40 and the dielectric layer 24. The maximum thickness of the heat sink 23 is in the range of 200 to 500 nm, for example. The dielectric layer 24 has a top surface, and an end face closest to the medium facing surface 60. The distance from the medium facing surface 60 to an arbitrary point on the aforementioned end face of the dielectric layer 24 decreases with decreasing distance from the arbitrary point to the top surface 1a of the substrate 1. The maximum thickness of the dielectric layer 24 is in the range of 500 to 800 nm, for example.
The main pole 26 is disposed on the dielectric layer 25 so as to lie above part of each of the top surface of the plasmon generator 40, the end face of the dielectric layer 24 and the top surface of the dielectric layer 24. The dielectric layer 25 has a thickness in the range of, for example, 10 to 40 nm, preferably in the range of 15 to 25 nm.
The third layers of the coupling portions 17A and 17B are embedded in the cladding layer 21 and the dielectric layers 22, 24, and 25. The dielectric layers 22, 24, and 25 may be formed of SiO2 or alumina, for example. The heat sink 23 is formed of a material having a high thermal conductivity, such as Au, Ag, Al, or Cu. The heat sink 23 has the function of dissipating heat generated at the plasmon generator 40. The heat sink 23 is not an essential component of the thermally-assisted magnetic recording head, and can be dispensed with.
The thermally-assisted magnetic recording head further includes a coupling layer 27 formed of a magnetic material and disposed over the third layers of the coupling portions 17A and 17B and the dielectric layer 25, and a dielectric layer 28 disposed around the main pole 26 and the coupling layer 27. The top surfaces of the main pole 26, the coupling layer 27, and the dielectric layer 28 are even with each other. The dielectric layer 28 is formed of alumina, for example.
The thermally-assisted magnetic recording head further includes a coil 29 disposed on the dielectric layer 28, an insulating layer 30 disposed to cover the coil 29, and a yoke layer 31 formed of a magnetic material and disposed over the main pole 26, the coupling layer 27 and the insulating layer 30. The yoke layer 31 magnetically couples the main pole 26 and the coupling layer 27 to each other. The coil 29 is planar spiral-shaped and wound around a part of the yoke layer 31 that lies on the coupling layer 27. The coil 29 is formed of a conductive material such as copper. The insulating layer 30 is formed of alumina, for example.
The thermally-assisted magnetic recording head further includes a protective layer 32 disposed to cover the yoke layer 31. The protective layer 32 is formed of alumina, for example.
The parts from the return pole layer 11 to the yoke layer 31 constitute a write head section. The coils 15 and 29 produce magnetic fields corresponding to data to be written on the recording medium 80. The shield layer 12, the return pole layer 11, the coupling layer 13, the coupling portions 17A and 17B, the coupling layer 27, the yoke layer 31, and the main pole 26 form a magnetic path for passing magnetic fluxes corresponding to the magnetic fields produced by the coils 15 and 29. The coils 15 and 29 are connected in series or in parallel so that the magnetic flux corresponding to the magnetic field produced by the coil 15 and the magnetic flux corresponding to the magnetic field produced by the coil 29 flow in the same direction through the main pole 26. The main pole 26 allows the magnetic flux corresponding to the magnetic field produced by the coil 15 and the magnetic flux corresponding to the magnetic field produced by the coil 29 to pass, and produces a write magnetic field for writing data on the recording medium 80 by means of a perpendicular magnetic recording system.
As has been described, the thermally-assisted magnetic recording head according to the present embodiment includes the medium facing surface 60, the read head section, and the write head section. The read head section and the write head section are stacked on the substrate 1. The write head section is located on the front side in the direction of travel of the recording medium 80 (the Z direction), i.e., on the trailing side, relative to the read head section.
The thermally-assisted magnetic recording head may include a protective film covering the medium facing surface 60. The protective film is formed of diamond-like-carbon (DLC) or Ta2O5, for example. The protective film is not an essential component of the thermally-assisted magnetic recording head and can be dispensed with.
The write head section includes the coils 15 and 29, the main pole 26, the waveguide, and the plasmon generator 40. The waveguide includes the core 20 and the cladding. The cladding includes the cladding layers 18, 19, and 21. The main pole 26 is located on the front side in the direction of travel of the recording medium 80 (the Z direction) relative to the core 20. The plasmon generator 40 is interposed between the core 20 and the main pole 26.
The core 20 and the plasmon generator 40 will now be described in detail with reference to FIG. 1 to FIG. 4. FIG. 1 is a perspective view showing the main part of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 2 is a cross-sectional view showing the main part of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 3 is a front view showing part of the medium facing surface 60 of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 4 is a plan view showing the positional relationship between the plasmon generator 40 and the core 20 of the waveguide shown in FIG. 1.
As shown in FIG. 1, the core 20 has an end face 20a located in the medium facing surface 60, an evanescent light generating surface 20e or a top surface, a bottom surface 20c, and two side surfaces. The evanescent light generating surface 20e generates evanescent light based on the light propagating through the core 20. The cladding layer 21 covers the evanescent light generating surface 20e. 
As shown in FIG. 1 and FIG. 2, the plasmon generator 40 has a front end face 40a and a core facing surface 40e. The front end face 40a is located in the medium facing surface 60. The front end face 40a generates near-field light on the principle to be described later. The core facing surface 40e faces a surface of the core 20, particularly the evanescent light generating surface 20e. 
The plasmon generator 40 according to the present embodiment is a multilayer plasmon generator including three or more layers. The plasmon generator 40 includes at least a first metal layer M1, a second metal layer M2, and an intermediate layer N1. The intermediate layer N1 is interposed between the first metal layer M1 and the second metal layer M2. Each of the first metal layer M1, the second metal layer M2 and the intermediate layer N1 has an end located in the front end face 40a. Each of the first and second metal layers M1 and M2 is formed of a metal material. The intermediate layer N1 is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer M1 and the metal material used to form the second metal layer M2. In the present embodiment, the material used to form the intermediate layer N1 is particularly a metal material different from the metal material used to form the first metal layer M1 and the metal material used to form the second metal layer M2. Hereinafter, the metal material used to form the first metal layer M1 and the metal material used to form the second metal layer M2 will each be referred to as a first-type metal material, and the metal material used to form the intermediate layer N1 will be referred to as a second-type metal material. The second-type metal material is higher in Vickers hardness than the first-type metal material. The first-type metal material is preferably higher in electrical conductivity than the second-type metal material.
In the example shown in FIG. 1 to FIG. 3, the intermediate layer N1 and the second metal layer M2 are stacked in this order on the first metal layer M1. In this example, the plasmon generator 40 further includes a second intermediate layer N2, a third metal layer M3, a third intermediate layer N3, a fourth metal layer M4, a fourth intermediate layer N4, a fifth metal layer M5, a fifth intermediate layer N5, a sixth metal layer M6, and a protective layer N6 stacked in this order on the second metal layer M2. The metal layers M3 to M6 are each formed of the first-type metal material. The intermediate layers N2 to N5 and the protective layer N6 are each formed of the second-type metal material. The protective layer N6 is provided for preventing diffusion of the material forming the metal layer M6. The protective layer N6 is not an essential component of the plasmon generator 40, and can be dispensed with.
The first-type metal material may be one of Au, Ag, Al, and Cu, for example. The second-type metal material may be one of Ru, Pt, Pd, Zr, Ti, Ta, Ni, W, Cr, NiCr, Cu, TiW, TiN, Mo, and Hf, for example. When the first-type metal material is Cu, the second-type metal material is other than Cu.
As far as the requirement that the second-type metal material be higher in Vickers hardness than the first-type metal material is satisfied, the materials used to form the metal layers M1 to M6 may all be the same or may be different from each other, or some of them may be the same. Likewise, the materials used to form the intermediate layers N1 to N5 and the protective layer N6 may all be the same or may be different from each other, or some of them may be the same.
For example, the first metal layer M1, the intermediate layer N1, and the second metal layer M2 may be formed of Au, Ru, and Au, respectively, or of Au, Ru, and Cu, respectively.
The first metal layer M1 is located closer to the core facing surface 40e than is the second metal layer M2. In the present embodiment, the core facing surface 40e is formed by the bottom surface of the first metal layer M1. The first metal layer M1 may be greater in thickness than the second metal layer M2. The intermediate layers N1 to N5 may be smaller in thickness than the metal layers M1 to M6. The thickness of the first metal layer M1 falls within the range of 5 to 20 nm, for example. The thickness of each of the metal layers M2 to M6 falls within the range of 5 to 10 nm, for example. The thickness of each of the intermediate layers N1 to N5 falls within the range of 0.3 to 5 nm, for example.
As shown in FIG. 2, the core facing surface 40e faces the evanescent light generating surface 20e with a predetermined distance therebetween. The cladding layer 21 includes an interposition part 21a interposed between the evanescent light generating surface 20e and the core facing surface 40e. Since the cladding layer 21 is part of the cladding, the cladding can be said to include the interposition part 21a. The interposition part 21a has a thickness in the range of, for example, 10 to 100 nm, preferably in the range of 15 to 50 nm.
As shown in FIG. 1 to FIG. 3, the thermally-assisted magnetic recording head may further include an adhesion layer 39 interposed between the cladding layer 21 and the plasmon generator 40. The adhesion layer 39 is to prevent the plasmon generator 40 from peeling away from the cladding layer 21. The adhesion layer 39 may be formed of one of Zr, Ti, Cr, Ta, and W, for example. The adhesion layer 39 has a thickness in the range of 0.3 to 1 nm, for example. The adhesion layer 39 is not an essential component of the thermally-assisted magnetic recording head, and can be dispensed with.
As shown in FIG. 1 and FIG. 4, the plasmon generator 40 may include a portion whose width in the X direction decreases toward the front end face 40a. 
Now, the principle of generation of near-field light in the present embodiment and the principle of thermally-assisted magnetic recording using the near-field light will be described in detail. Laser light emitted from a laser diode (not illustrated) enters the core 20. As shown in FIG. 5, the laser light 50 propagates through the core 20 toward the medium facing surface 60, and reaches the vicinity of the plasmon generator 40. In the core 20, the laser light 50 is totally reflected at the evanescent light generating surface 20e shown in FIG. 2 to generate evanescent light permeating into the interposition part 21a. In the plasmon generator 40, surface plasmons are excited on the core facing surface 40e through coupling with the aforementioned evanescent light. The surface plasmons propagate to the front end face 40a. As a result, the surface plasmons concentrate at the front end face 40a, and the front end face 40a generates near-field light based on the surface plasmons.
The surface plasmons excited on the core facing surface 40e propagate to at least the end of the first metal layer M1 located in the front end face 40a. In the present embodiment, the intermediate layers N1 to N5 are each formed of a metal material. Therefore, the surface plasmons having propagated from the core facing surface 40e to the end of the first metal layer M1 can also propagate to the respective ends of the metal layers M2 to M6 and the intermediate layers N1 to N5 located in the front end face 40a. To allow the front end face 40a to generate near-field light of sufficient intensity, the surface plasmons having propagated from the core facing surface 40e to the end of the first metal layer M1 preferably propagate to at least the end of the second metal layer M2.
The near-field light generated from the front end face 40a is projected toward the recording medium 80, reaches the surface of the recording medium 80 and heats a part of the magnetic recording layer of the recording medium 80. This lowers the coercivity of the part of the magnetic recording layer. In thermally-assisted magnetic recording, the part of the magnetic recording layer with the lowered coercivity is subjected to a write magnetic field produced by the main pole 26 for data writing.
A method of manufacturing the thermally-assisted magnetic recording head according to the present embodiment will now be described. The method of manufacturing the thermally-assisted magnetic recording head includes the steps of forming components of a plurality of thermally-assisted magnetic recording heads, except the substrates 1, on a wafer that includes portions to become the substrates 1 of the plurality of thermally-assisted magnetic recording heads, thereby fabricating a substructure including rows of a plurality pre-head portions, the plurality of pre-head portions becoming individual thermally-assisted magnetic recording heads later; and separating the plurality of pre-head portions from each other by cutting the substructure and forming the medium facing surface 60 for each of the plurality of pre-head portions (this step will be referred to as the step of forming the medium facing surface 60). A plurality of thermally-assisted magnetic recording heads are produced in this manner.
The method of manufacturing the thermally-assisted magnetic recording head according to the present embodiment will now be described in more detail with attention focused on a single thermally-assisted magnetic recording head. The method of manufacturing the thermally-assisted magnetic recording head forms the insulating layer 2, the bottom shield layer 3, and the bottom shield gap film 4 in this order on the substrate 1 first. Next, the MR element 5 and two leads (not illustrated) connected to the MR element 5 are formed on the bottom shield gap film 4. The top shield gap film 6 is then formed to cover the MR element 5 and the leads. Next, the top shield layer 7, the insulating layer 8, the middle shield layer 9, and the nonmagnetic layer 10 are formed in this order on the top shield gap film 6.
Then, the return pole layer 11 is formed on the nonmagnetic layer 10. Next, an insulating layer (not illustrated) is formed to cover the return pole layer 11. The non-illustrated insulating layer is then polished by, for example, chemical mechanical polishing (hereinafter referred to as CMP), until the return pole layer 11 is exposed. Next, the insulating layer 14 is formed over the return pole layer 11 and the non-illustrated insulating layer. The insulating layer 14 is then selectively etched to form therein two openings for exposing the top surface of the return pole layer 11. In the positions of these two openings, the shield layer 12 and the coupling layer 13 are then formed on the return pole layer 11. Next, the coil 15 is formed on the insulating layer 14.
Next, the insulating layer 16 is formed over the entire top surface of the stack. The insulating layer 16 is then polished by, for example, CMP, until the shield layer 12, the coupling layer 13 and the coil 15 are exposed. Next, although not illustrated, the first layers of the coupling portions 17A and 17B are formed on the coupling layer 13. Then, the cladding layer 18 is formed over the entire top surface of the stack. The cladding layer 18 is then polished by, for example, CMP, until the first layers of the coupling portions 17A and 17B are exposed.
Next, a dielectric layer to become the core 20 is formed over the entire top surface of the stack. The dielectric layer is then etched in part by, for example, reactive ion etching (hereinafter referred to as RIE), and thereby patterned into the core 20. Next, although not illustrated, the second layers of the coupling portions 17A and 17B are formed on the first layers of the coupling portions 17A and 17B.
Next, the cladding layer 19 is formed over the entire top surface of the stack. The cladding layer 19 is then polished by, for example, CMP, until the dielectric layer and the second layers of the coupling portions 17A and 17B are exposed.
Reference is now made to FIG. 7A through FIG. 13B to describe steps to be performed after the polishing of the cladding layer 19 up to the formation of the dielectric layer 28. FIG. 7A through FIG. 13B each show a stack of layers formed in the process of manufacturing the thermally-assisted magnetic recording head. Note that portions located below the core 20 are omitted from FIG. 7A through FIG. 13B. FIG. 7A to FIG. 13A each show a cross section that intersects the front end face 26a of the main pole 26 and that is perpendicular to the medium facing surface 60 and the top surface 1a of the substrate 1. FIG. 7B to FIG. 13B each show a cross section of the stack taken in the position at which the medium facing surface 60 is to be formed. In FIG. 7A to FIG. 13A, the symbol “ABS” indicates the position at which the medium facing surface 60 is to be formed.
FIG. 7A and FIG. 7B show a step that follows the polishing of the cladding layer 19. In this step, first, the cladding layer 21 is formed over the core 20 and the cladding layer 19. The adhesion layer 39 (not illustrated) is then formed on the cladding layer 21 by sputtering, for example. On the adhesion layer 39, a plurality of films that later become the layers of the plasmon generator 40 are formed in succession by sputtering, for example. A multilayer film 40P composed of the plurality of films is thereby formed.
FIG. 8A and FIG. 8B show the next step. In this step, first, an etching mask, not illustrated, is formed on the multilayer film 40P. Using this etching mask, the multilayer film 40P is then patterned by RIE or ion beam etching (hereinafter referred to as IBE), for example. The multilayer film 40P having been patterned includes a portion to become the plasmon generator 40 and a portion to be removed.
FIG. 9A and FIG. 9B show the next step. In this step, first, the dielectric layer 22 is formed over the entire top surface of the stack. The dielectric layer 22 is then polished by, for example, CMP, until the multilayer film 40P is exposed.
FIG. 10A and FIG. 10B show the next step. In this step, the heat sink 23 is formed to lie astride part of the multilayer film 40P and part of the dielectric layer 22.
FIG. 11A and FIG. 11B show the next step. In this step, first, the dielectric layer 24 is formed to cover the heat sink 23. Then, the dielectric layer 25 is formed to cover the dielectric layer 24.
FIG. 12A and FIG. 12B show the next step. In this step, first, the cladding layer 21 and the dielectric layers 22, 24 and 25 are selectively etched to form therein two openings for exposing the respective top surfaces of the second layers of the coupling portions 17A and 17B. Next, the third layers of the coupling portions 17A and 17B are formed on the second layers of the coupling portions 17A and 17B, respectively. Then, the main pole 26 is formed on the dielectric layer 25, and the coupling layer 27 is formed on the third layers of the coupling portions 17A and 17B and the dielectric layer 25.
FIG. 13A and FIG. 13B show the next step. In this step, first, the dielectric layer 28 is formed over the entire top surface of the stack. The dielectric layer 28 is then polished by, for example, CMP, until the main pole 26 and the coupling layer 27 are exposed. The top surfaces of the main pole 26, the coupling layer 27 and the dielectric layer 28 are thereby made even with each other.
Now, steps to follow the step of FIG. 13A and FIG. 13B will be described with reference to FIG. 5 and FIG. 6. First, the coil 29 is formed on the dielectric layer 28. The insulating layer 30 is then formed to cover the coil 29. Next, the yoke layer 31 is formed over the main pole 26, the coupling layer 27 and the insulating layer 30. Then, the protective layer 32 is formed to cover the yoke layer 31. Wiring, terminals, and other components are then formed on the top surface of the protective layer 32. When the substructure is completed thus, the step of forming the medium facing surface 60 is performed. In the step of forming the medium facing surface 60, the portion to be removed of the multilayer film 40P is removed and the multilayer film 40P is provided with the front end face 40a, whereby the multilayer film 40P becomes the plasmon generator 40. A protective film for covering the medium facing surface 60 may be formed thereafter. Being provided with the medium facing surface 60, each pre-head portion becomes a thermally-assisted magnetic recording head.
The step of forming the medium facing surface 60 includes the step of polishing the surface that is formed for each pre-head portion by cutting the substructure, and the step of forming a rail on the polished surface for allowing the slider to fly.
In the aforementioned polishing step, the layers exposed in the medium facing surface 60 may be polished in different amounts due to differences between materials used for those layers, and this may cause irregularities on the medium facing surface 60.
Further, in the aforementioned polishing step, polishing residues of the metal materials may grow to cause smears. In order to remove the smears, the step of forming the medium facing surface 60 may include the step of etching the polished surface slightly by, for example, ion beam etching, after the polishing step.
The effects of the plasmon generator 40 and the thermally-assisted magnetic recording head according to the present embodiment will now be described. The plasmon generator 40 according to the present embodiment includes at least the first metal layer M1, the second metal layer M2, and the intermediate layer N1. The intermediate layer N1 is interposed between the first metal layer M1 and the second metal layer M2. The intermediate layer N1 is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer M1 and the metal material used to form the second metal layer M2. This makes it possible to prevent the first metal layer M1 and the second metal layer M2 sandwiching the intermediate layer N1 from being deformed. Further, the plasmon generator 40 according to the present embodiment achieves higher mechanical strength as a whole when compared with a plasmon generator formed of a single metal layer of the first-type metal material. The foregoing features of the plasmon generator 40 according to the present embodiment make it possible to prevent the plasmon generator 40 from being deformed or damaged, and the front end face 40a from being significantly recessed relative to the other parts of the medium facing surface 60 in the step of forming the medium facing surface 60 or due to a temperature change of the plasmon generator 40. Consequently, according to the present embodiment, it is possible to prevent the plasmon generator 40 from being degraded in heating performance.
Further, in the plasmon generator 40 according to the present embodiment, each of the first metal layer M1, the second metal layer M2 and the intermediate layer N1 has an end located in the front end face 40a. This makes it possible for surface plasmons excited on the core facing surface 40e to propagate to the front end face 40a efficiently.
As can be seen from the foregoing, according to the present embodiment, it is possible to provide the plasmon generator 40 that has high reliability and allows surface plasmons to propagate to the front end face 40a efficiently, and to provide a thermally-assisted magnetic recording head having the plasmon generator 40.
The above-described effects become noticeable when the plasmon generator 40 includes one or more pairs of an intermediate layer and a metal layer in addition to the first metal layer M1, the second metal layer M2, and the intermediate layer N1.
Where the first-type metal material is higher in electrical conductivity than the second-type metal material, the intermediate layers N1 to N5 are preferably smaller in thickness than the metal layers M1 to M6. In this case, it is possible to reduce loss of surface plasmons when the surface plasmons propagate from the end of the metal layer M1 located in the front end face 40a to the respective ends of the metal layers M2 to M6 located in the front end face 40a. 
It is not necessarily required that the surface plasmons propagate to the respective ends of the metal layers M3 to M6 located in the front end face 40a and to the respective ends of the intermediate layers N2 to N5 located in the front end face 40a. The metal layers M3 to M6 and the intermediate layers N2 to N5, however, contribute to the enhancement of the mechanical strength of the plasmon generator 40 as a whole.
Further, in the present embodiment, the first metal layer M1 is located closer to the core facing surface 40e than is the second metal layer M2, and the core facing surface 40e is formed by the bottom surface of the first metal layer M1. In this case, in order to allow appropriate excitation and propagation of surface plasmons on the first metal layer M1, the first metal layer M1 is preferably greater in thickness than the second metal layer M2.
First and second modification examples of the plasmon generator 40 according to the present embodiment will now be described. FIG. 14 is a perspective view showing the first modification example of the plasmon generator 40. FIG. 15 is a perspective view showing the second modification example of the plasmon generator 40.
In the first modification example of the plasmon generator 40 shown in FIG. 14, the protective layer N6 is greater in thickness than the intermediate layers N1 to N5.
The second modification example of the plasmon generator 40 shown in FIG. 15 includes the metal layers M1 to M6, the intermediate layers N1 to N5 and the protective layer N6, and further includes a metal layer M7, an intermediate layer N7, a metal layer M8, and a protective layer N8 stacked in this order on the protective layer N6. The protective layer N6, the metal layer M7, the intermediate layer N7, the metal layer M8, and the protective layer N8 have their respective ends that are closest to the front end face 40a and are each located at a distance from the front end face 40a. The second modification example of the plasmon generator 40 further includes a protective layer N9 located between the front end face 40a and the respective ends of the protective layer N6, the metal layer M7, the intermediate layer N7, the metal layer M8, and the protective layer N8. The protective layer N9 has a thickness equal to the total thickness of the protective layer N6, the metal layer M7, the intermediate layer N7, the metal layer M8, and the protective layer N8. Further, the protective layer N9 has an end face located in the front end face 40a. The metal layers M7 and M8 are each formed of the first-type metal material. The intermediate layer N7 and the protective layers N8 and N9 are each formed of the second-type metal material.
The first and second modification examples of the plasmon generator 40 allow a further increase in mechanical strength of a portion of the plasmon generator 40 in the vicinity of the front end face 40a. 
[Second Embodiment]
A plasmon generator and a thermally-assisted magnetic recording head according to a second embodiment of the invention will now be described with reference to FIG. 16 to FIG. 18. FIG. 16 is a perspective view showing the main part of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 17 is a cross-sectional view showing the main part of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 18 is a front view showing part of the medium facing surface 60 of the thermally-assisted magnetic recording head according to the present embodiment.
The plasmon generator 40 according to the present embodiment includes at least a first metal layer M1, a second metal layer M2, and an intermediate layer D1. The intermediate layer D1 is interposed between the first metal layer M1 and the second metal layer M2. Each of the first metal layer M1, the second metal layer M2 and the intermediate layer D1 has an end located in the front end face 40a. Each of the first and second metal layers M1 and M2 is formed of a metal material. The intermediate layer D1 is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer M1 and the metal material used to form the second metal layer M2. In the present embodiment, the intermediate layer D1 is formed of a dielectric material, in particular.
In the example shown in FIG. 16 to FIG. 18, the intermediate layer D1 and the second metal layer M2 are stacked in this order on the first metal layer M1. In this example, the plasmon generator 40 further includes a second intermediate layer D2, a third metal layer M3, and a protective layer D3 stacked in this order on the second metal layer M2. The metal layer M3 is formed of a metal material. The intermediate layer D2 and the protective layer D3 are each formed of a dielectric material. The protective layer D3 is provided for preventing diffusion of the material forming the metal layer M3. The protective layer D3 is not an essential component of the plasmon generator 40, and can be dispensed with.
Each of the metal materials used to form the metal layers M1, M2, and M3 may be one of Au, Ag, Al, and Cu, for example. Each of the dielectric materials used to form the intermediate layers D1 and D2 and the protective layer D3 may be one of SiO2, alumina, MgO, ZrO2, and amorphous SiC, for example.
The materials used to form the metal layers M1, M2 and M3 may all be the same or may be different from each other. Alternatively, two of them may be the same with the remainder different from the two. Likewise, the materials used to form the intermediate layers D1 and D2 and the protective layer D3 may all be the same or may be different from each other. Alternatively, two of them may be the same with the remainder different from the two.
The first metal layer M1 is located closer to the core facing surface 40e than is the second metal layer M2. In the present embodiment, the core facing surface 40e is formed by the bottom surface of the first metal layer M1 . The first metal layer M1 may be greater in thickness than the second metal layer M2. The intermediate layers D1 and D2 may be smaller in thickness than the metal layers M1, M2 and M3. The thickness of each of the metal layers M1, M2 and M3 falls within the range of 5 to 10 nm, for example. The thickness of each of the intermediate layers D1 and D2 falls within the range of 0.3 to 5 nm, for example.
In the plasmon generator 40 according to the present embodiment, as in the first embodiment, surface plasmons are excited on the core facing surface 40e, i.e., a surface (the bottom surface) of the first metal layer M1, through coupling with the evanescent light generated from the evanescent light generating surface 20e of the core 20. The thicknesses of the intermediate layers D1 and D2 are sufficiently smaller than the wavelength of light propagating through the core 20. Consequently, a transfer of energy occurs between the metal layers M1 and M2 which are adjacent to each other with the intermediate layer D1 interposed therebetween, and between the metal layers M2 and M3 which are adjacent to each other with the intermediate layer D2 interposed therebetween, so that surface plasmons are excited also on the surfaces of the metal layers M2 and M3. However, the energy of the surface plasmons excited on the surfaces of the metal layers M2 and M3 is lower than the energy of the surface plasmons excited on the core facing surface 40e. 
The surface plasmons excited on the respective surfaces of the metal layers M1, M2 and M3 propagate to the front end face 40a. As a result, the surface plasmons concentrate at the front end face 40a, and the front end face 40a generates near-field light based on the surface plasmons.
The remainder of configuration, function and effects of the present embodiment are similar to those of the first embodiment.
[Third Embodiment]
Reference is now made to FIG. 19 and FIG. 20 to describe a plasmon generator and a thermally-assisted magnetic recording head according to a third embodiment of the invention. FIG. 19 is a perspective view showing the main part of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 20 is a cross-sectional view showing the main part of the thermally-assisted magnetic recording head according to the present embodiment.
How the plasmon generator 40 according to the present embodiment differs from the first embodiment will now be described. In the plasmon generator 40 according to the present embodiment, the intermediate layer N1 lies on a portion of the top surface of the first metal layer M1, the portion being located in the vicinity of the front end face 40a. The metal layer M2, the intermediate layer N2, the metal layer M3, the intermediate layer N3, the metal layer M4, the intermediate layer N4, the metal layer M5, the intermediate layer N5, the metal layer M6, and the protective layer N6 are stacked in this order on the intermediate layer N1. The metal layers M2 to M6, the intermediate layers N1 to N5 and the protective layer N6 are thus located only in the vicinity of the front end face 40a. Each of the metal layers M1 to M6, the intermediate layers N1 to N5 and the protective layer N6 has an end located in the front end face 40a. 
The plasmon generator 40 according to the present embodiment includes a metal portion 401 in addition to the metal layers M1 to M6, the intermediate layers N1 to N5 and the protective layer N6. The metal portion 401 lies on a portion of the top surface of the first metal layer M1 other than the portion on which the intermediate layer N1 lies, and contacts the first metal layer M1. The metal layers M2 to M6, the intermediate layers N1 to N5 and the protective layer N6 are sandwiched between the metal portion 401 and the front end face 40a. The metal portion 401 has an end face facing toward the front end face 40a. This end face of the metal portion 401 is in contact with respective ends of the metal layers M2 to M6, the intermediate layers N1 to N5 and the protective layer N6 that are opposite from the front end face 40a. No part of the metal portion 401 constitutes part of the front end face 40a. The metal portion 401 has a thickness equal to the total thickness of the metal layers M2 to M6, the intermediate layers N1 to N5 and the protective layer N6. The metal portion 401 is formed of, for example, the first-type metal material described in the first embodiment section. The overall shape of the plasmon generator 40 according to the present embodiment is the same as that in the first embodiment.
In the example shown in FIG. 19 and FIG. 20, the end face of the metal portion 401 facing toward the front end face 40a is inclined relative to the front end face 40a. Alternatively, however, this end face may be parallel to the front end face 40a. 
The same effects as those provided by the first embodiment can also be obtained even when only a portion of the plasmon generator 40 in the vicinity of the front end face 40a has a multilayer structure as in the present embodiment.
The remainder of configuration, function and effects of the present embodiment are similar to those of the first embodiment. In the present embodiment, however, the plurality of intermediate layers and the protective layer each formed of a metal material may be replaced with those that are each formed of a dielectric material as in the second embodiment.
[Fourth Embodiment]
Reference is now made to FIG. 21 and FIG. 22 to describe a plasmon generator and a thermally-assisted magnetic recording head according to a fourth embodiment of the invention. FIG. 21 is a cross-sectional view showing the main part of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 22 is a front view showing part of the medium facing surface 60 of the thermally-assisted magnetic recording head according to the present embodiment.
How the plasmon generator 40 and the thermally-assisted magnetic recording head according to the present embodiment differ from the first embodiment will now be described. In the present embodiment, as shown in FIG. 21, the core 20 has an end face 20a located in the medium facing surface 60, a top surface 20b, a bottom surface 20c (see FIG. 5), and two side surfaces. Further, the core 20 has a recess 201 that opens in the top surface 20b. The recess 201 is located in the vicinity of the end face 20a. The cladding layer 21 covers the top surface 20b and the recess 201 of the core 20. In the present embodiment, the evanescent light generating surface 20e is formed by the bottom surface of the recess 201. The evanescent light generating surface 20e is perpendicular to the Z direction, for example.
The plasmon generator 40 according to the present embodiment includes a metal portion 402 formed of a metal material, and a first metal layer M1, an intermediate layer N1, a second metal layer M2, an intermediate layer N2, a metal layer M3, an intermediate layer N3, a metal layer M4 and a protective layer N4 stacked in this order on the metal portion 402.
At least part of the metal portion 402 is accommodated in the recess 201 of the core 20 with the cladding layer 21 interposed between the metal portion 402 and the core 20. In the present embodiment, the major part of the metal portion 402 is accommodated in the recess 201, and the top surface of the metal portion 402 projects to be higher than the top surface 20b of the core 20 by the thickness of the cladding layer 21. The top surface of the metal portion 402 and the top surface of a portion of the cladding layer 21 lying on the top surface 20b of the core 20 are even with each other. The first metal layer M1 lies astride part of the top surface of the metal portion 402 and part of the top surface of the cladding layer 21. The metal portion 402 is thus in contact with the first metal layer M1. No part of the metal portion 402 constitutes part of the front end face 40a. The metal portion 402 has a thickness in the range of, for example, 40 to 300 nm, preferably in the range of 100 to 200 nm. In the present embodiment, the core facing surface 40e is formed by the bottom surface of the metal portion 402. The cladding layer 21 includes an interposition part 21a interposed between the evanescent light generating surface 20e and the core facing surface 40e. 
The metal portion 402 has an end face 402a lying at an end in the Y direction and closest to the medium facing surface 60 and the front end face 40a. The end face 402a may be perpendicular to the Y direction or inclined relative to the XZ plane. Where the end face 402a is inclined relative to the XZ plane, the end face 402a forms an angle θ of smaller than 90° relative to the interface between the metal portion 402 and the first metal layer M1. In this case, the distance from the medium facing surface 60 to an arbitrary point on the end face 402a decreases with decreasing distance from the arbitrary point to the first metal layer M1. FIG. 21 shows an example in which the angle θ is smaller than 90°.
Each of the metal layers M1 to M4, the intermediate layers N1 to N3 and the protective layer N4 has an end located in the front end face 40a. The metal portion 402 and the metal layers M1 to M4 are each formed of the first-type metal material described in the first embodiment section. The intermediate layers N1 to N3 and the protective layer N4 are each formed of the second-type metal material described in the first embodiment section.
The materials used to form the metal layers M1 to M4 may all be the same or may be different from each other, or some of them may be the same. Likewise, the materials used to form the intermediate layers N1 to N3 and the protective layer N4 may all be the same or may be different from each other, or some of them may be the same.
The thickness of each of the metal layers M1 to M4 falls within the range of 5 to 10 nm, for example. The thickness of each of the intermediate layers N1 to N3 falls within the range of 0.3 to 5 nm, for example.
In the plasmon generator 40 according to the present embodiment, surface plasmons are excited on the core facing surface 40e, i.e., the bottom surface of the metal portion 402, through coupling with the evanescent light generated from the evanescent light generating surface 20e of the core 20. The surface plasmons propagate along the surface of the metal portion 402 to the first metal layer M1, and further propagate along the surface of the first metal layer M1 to the front end face 40a. As a result, the surface plasmons concentrate at the front end face 40a, and the front end face 40a generates near-field light based on the surface plasmons.
As mentioned above, the plasmon generator 40 according to the present embodiment includes the metal portion 402, and no part of the metal portion 402 constitutes part of the front end face 40a. Thus, no part of the metal portion 402 is exposed in the medium facing surface 60. Accordingly, the material for the metal portion 402 can be selected from any metal materials that have high electrical conductivities and are suitable for excitation and propagation of surface plasmons, without the need for considering mechanical strength. This allows appropriate excitation and propagation of surface plasmons on the metal portion 402.
Where the end face 402a of the metal portion 402 forms an angle θ of smaller than 90° relative to the interface between the metal portion 402 and the first metal layer M1, the distance from the medium facing surface 60 to an arbitrary point on the end face 402a decreases with decreasing distance from the arbitrary point to the first metal layer M1. In this case, surface plasmons excited on the core facing surface 40e are able to propagate through the end face 402a to the vicinity of the end of the first metal layer M1 located in the front end face 40a. Further, where the angle θ is smaller than 90°, it is possible to reduce the loss of the surface plasmons as they propagate from the end face 402a to the first metal layer M1 when compared with the case where the angle θ is 90° or greater. Thus, where the angle θ is smaller than 90°, the surface plasmons excited on the core facing surface 40e are able to propagate to the first metal layer M1 more efficiently. The above-described effect obtained where the angle θ is smaller than 90° becomes smaller as the angle θ approaches 90°. On the other hand, if the angle θ is excessively small, it becomes difficult to form the end face 402a. In view of this, the angle θ is preferably in the range of 45° to 85°, and more preferably in the range of 60° to 75°.
A modification example of the thermally-assisted magnetic recording head according to the present embodiment will now be described. FIG. 23 is a cross-sectional view showing the main part of the modification example of the thermally-assisted magnetic recording head according to the present embodiment. FIG. 24 is a front view showing part of the medium facing surface 60 of the modification example of the thermally-assisted magnetic recording head according to the present embodiment.
In the modification example of the thermally-assisted magnetic recording head, the core 20 does not have the recess 201, and thus has a flat top surface. In the modification example, this top surface serves as the evanescent light generating surface 20e. 
In the modification example of the thermally-assisted magnetic recording head, the cladding includes a cladding layer 202 in addition to the cladding layers 18, 19, and 21. The cladding layer 202 lies on the evanescent light generating surface 20e. The cladding layer 202 has an opening for exposing a part of the evanescent light generating surface 20e. The cladding layer 21 covers the part of the evanescent light generating surface 20e exposed from this opening and also the cladding layer 202.
In the modification example of the thermally-assisted magnetic recording head, at least part of the metal portion 402 is accommodated in the opening of the cladding layer 202 with the cladding layer 21 interposed between the metal portion 402 and the core 20. The top surface of the metal portion 402 projects to be higher than the top surface of the cladding layer 202 by the thickness of the cladding layer 21. The top surface of the metal portion 402 and the top surface of a portion of the cladding layer 21 lying on the top surface of the cladding layer 202 are even with each other. The first metal layer M1 lies astride part of the top surface of the metal portion 402 and part of the top surface of the cladding layer 21. The remainder of configuration of the modification example of the thermally-assisted magnetic recording head is the same as the configuration shown in FIG. 21 and FIG. 22.
The remainder of configuration, function and effects of the present embodiment are similar to those of the first embodiment. In the present embodiment, however, the plurality of intermediate layers and the protective layer each formed of a metal material may be replaced with those that are each formed of a dielectric material as in the second embodiment.
The present invention is not limited to the foregoing embodiments, and various modifications may be made thereto. For example, as far as the requirements of the appended claims are met, the shape of the plasmon generator and the locations of the plasmon generator, the core, and the main pole can be chosen as desired, without being limited to the examples illustrated in the foregoing embodiments. For example, the core may be located above the plasmon generator. In this case, the bottom surface of the core serves as the evanescent light generating surface, and the top surface of the plasmon generator serves as the core facing surface.
It is apparent that the present invention can be carried out in various forms and modifications in the light of the foregoing descriptions. Accordingly, within the scope of the following claims and equivalents thereof, the present invention can be carried out in forms other than the foregoing most preferable embodiments.

## Claims

1. A plasmon generator configured to excite a surface plasmon, comprising a front end face, a first metal layer, a second metal layer, and an intermediate layer, wherein
the front end face generates near-field light based on the surface plasmon,
the intermediate layer is interposed between the first metal layer and the second metal layer,
each of the first metal layer, the second metal layer and the intermediate layer has an end located in the front end face,
each of the first and second metal layers is formed of a metal material,
the intermediate layer is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer and the metal material used to form the second metal layer, and
the material used to form the intermediate layer is a metal material different from the metal material used to form the first metal layer and the metal material used to form the second metal layer.

2. A plasmon generator configured to excite a surface plasmon, comprising a front end face, a first metal layer, a second metal layer, and an intermediate layer, wherein
the front end face generates near-field light based on the surface plasmon,
the intermediate layer is interposed between the first metal layer and the second metal layer,
each of the first metal layer, the second metal layer and the intermediate layer has an end located in the front end face,
each of the first and second metal layers is formed of a metal material,
the intermediate layer is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer and the metal material used to form the second metal layer, and
the material used to form the intermediate layer is a dielectric material.

3. A plasmon generator configured to excite a surface plasmon, comprising a front end face, a first metal layer, a second metal layer, and an intermediate layer, wherein
the front end face generates near-field light based on the surface plasmon,
the intermediate layer is interposed between the first metal layer and the second metal layer,
each of the first metal layer, the second metal layer and the intermediate layer has an end located in the front end face,
each of the first and second metal layers is formed of a metal material,
the intermediate layer is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer and the metal material used to form the second metal layer,
the plasmon generator further comprises a core facing surface that faces a surface of the core through which light propagates,
the first metal layer is located closer to the core facing surface than is the second metal layer, and
the first metal layer is greater in thickness than the second metal layer.

4. A thermally-assisted magnetic recording head comprising:
a medium facing surface facing a recording medium;
a main pole that produces a write magnetic field for writing data on the recording medium;
a waveguide including a core through which light propagates, and a cladding provided around the core; and
a plasmon generator configured to excite a surface plasmon based on the light propagating through the core, wherein
the plasmon generator has a front end face located in the medium facing surface, a first metal layer, a second metal layer, and an intermediate layer,
the front end face generates near-field light based on the surface plasmon,
the intermediate layer is interposed between the first metal layer and the second metal layer,
each of the first metal layer, the second metal layer and the intermediate layer has an end located in the front end face,
each of the first and second metal layers is formed of a metal material, and
the intermediate layer is formed of a material that is higher in Vickers hardness than the metal material used to form the first metal layer and the metal material used to form the second metal layer.

5. The thermally-assisted magnetic recording head according to claim 4, wherein the intermediate layer is smaller in thickness than the first and second metal layers.

6. The thermally-assisted magnetic recording head according to claim 4, wherein the material used to form the intermediate layer is a metal material different from the metal material used to form the first metal layer and the metal material used to form the second metal layer.

7. The thermally-assisted magnetic recording head according to claim 4, wherein the material used to form the intermediate layer is a dielectric material.

8. The thermally-assisted magnetic recording head according to claim 4, wherein
the core has an evanescent light generating surface that generates evanescent light based on the light propagating through the core,
the plasmon generator further has a core facing surface that faces the evanescent light generating surface, and
the cladding includes an interposition part interposed between the evanescent light generating surface and the core facing surface.

9. The thermally-assisted magnetic recording head according to claim 8, wherein
the first metal layer is located closer to the core facing surface than is the second metal layer, and
the first metal layer is greater in thickness than the second metal layer.

10. The thermally-assisted magnetic recording head according to claim 4, wherein the plasmon generator further has a metal portion formed of a metal material and in contact with the first metal layer, and no part of the metal portion constitutes part of the front end face.