1. Field of the Invention
The present invention relates to a near-field light generating device for use in heat-assisted magnetic recording where a magnetic recording medium is irradiated with near-field light to lower the coercivity of the magnetic recording medium for data recording, and a method of manufacturing the same, and to a heat-assisted magnetic recording head, a head gimbal assembly, and a magnetic recording device each of which includes the near-field light generating device.
2. Description of the Related Art
Recently, magnetic recording devices such as a magnetic disk drive have been improved in recording density, and thin-film magnetic heads and magnetic 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 reproducing head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a recording head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. 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 recording with existing magnetic heads.
To solve the foregoing problems, there has been proposed a method so-called heat-assisted magnetic recording. This method uses a recording medium having high coercivity. When recording data, a magnetic field and heat are simultaneously applied to the area of the recording medium where to record data, so that the area rises in temperature and drops in coercivity for data recording. Hereinafter, a magnetic head for use in heat-assisted magnetic recording will be referred to as a heat-assisted magnetic recording head.
In heat-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A commonly known method for generating near-field light is to use a near-field optical probe or so-called plasmon antenna, which is a piece of metal that generates near-field light from plasmons excited by irradiation with light. U.S. Pat. Nos. 6,649,894 and 6,768,556 each disclose a method of exciting plasmons by directly irradiating the plasmon antenna with light.
However, a plasmon antenna that is directly irradiated with light to generate near-field light is known to exhibit very low efficiency of conversion of the applied light into near-field light. The energy of the light applied to the plasmon antenna is mostly reflected off the surface of the plasmon antenna, or converted into thermal energy and absorbed by the plasmon antenna. The plasmon antenna is small in volume since the size of the plasmon antenna is set to be smaller than or equal to the wavelength of the light. The plasmon antenna therefore shows a significant increase in temperature when it absorbs the thermal energy.
Such a temperature increase makes the plasmon antenna expand in volume and protrude from a medium facing surface, which is the surface of the heat-assisted magnetic recording head to face the magnetic recording medium. This causes an end of the reproducing head located in the medium facing surface to get farther from the magnetic recording medium, thereby causing the problem that a servo signal cannot be read during recording operations.
To cope with this, as described in, for example, U.S. Pat. No. 7,330,404, there has been proposed a technique in which light propagating through a waveguide is not directly applied to a plasmon antenna but is coupled with a near-field light generating element via a buffer layer in a surface plasmon polariton mode to thereby excite surface plasmons on the near-field light generating element. The near-field light generating element has a near-field light generating part which is a sharp-pointed part located in the medium facing surface to generate near-field light. According to this technique, the light propagating through the waveguide is totally reflected at the interface between the waveguide and the buffer layer to generate evanescent light permeating into the buffer layer. The evanescent light and collective oscillations of charges on the near-field light generating element, i.e., surface plasmons, are coupled with each other to excite the surface plasmons on the near-field light generating element. In the near-field light generating element, the excited surface plasmons propagate to the near-field light generating part, and near-field light occurs from the near-field light generating part. According to this technique, since the near-field light generating element is not directly irradiated with the light propagating through the waveguide, it is possible to prevent an excessive increase in temperature of the near-field light generating element.
In order to use the foregoing near-field light generating element and improve the use efficiency of light propagating through the waveguide, it is necessary to match the wave number of the evanescent light with the wave number of surface plasmons to be excited on the near-field light generating element so that the surface plasmons are resonantly excited by the evanescent light. The phenomenon that surface plasmons are resonantly excited by light will hereinafter be referred to as surface plasmon polariton coupling.
The wave number of surface plasmons excited on the near-field light generating element varies according to the material and shape of the near-field light generating element. The selection of the material and shape for the near-field light generating element is thus critical to produce the surface plasmon polariton coupling to improve the use efficiency of the light propagating through the waveguide.
Among known materials of the near-field light generating element that can produce surface plasmon polariton coupling are noble metals such as Ag and Au. For example, Michael Hochberg, Tom Baehr-Jones, Chris Walker & Axel Scherer, “Integrated Plasmon and dielectric waveguides,” OPTICS EXPRESS Vol. 12, No. 22, pp. 5481-5486 (2004), and U.S. Patent Application Publication No. 2005/0249451 A1 describe that a waveguide made of Si and a plasmon waveguide (metal waveguide) made of Ag can produce surface plasmon polariton coupling.
Given a near-field light generating device that includes a waveguide, a buffer layer and a near-field light generating element, the layout of the waveguide, the buffer layer and the near-field light generating element may be such that the near-field light generating element is disposed above the top surface of the waveguide with the buffer layer therebetween. In such a case, the near-field light generating device is manufactured by forming the buffer layer on the top surface of the waveguide, and forming the near-field light generating element on the buffer layer. Possible combinations of the materials of the waveguide and the buffer layer include Ta2O5 or other tantalum oxide as the material of the waveguide and Al2O3 or SiO2 as the material of the buffer layer.
In the case of forming the near-field light generating element made of a noble metal such as Ag or Au on the buffer layer made of Al2O3 or SiO2, however, there occurs the problem that the near-field light generating element may exfoliate in the process of manufacturing the near-field light generating device since noble metals such as Ag and Au are low in strength of adhesion to Al2O3 or SiO2.
To cope with this, an adhesion layer made of metal may be formed on the buffer layer so that the near-field light generating element is formed on the adhesion layer. In this case, the adhesion layer is formed of a metal different from that constituting the near-field light generating element, and the metal is such one that a layered structure consisting of the buffer layer, the adhesion layer and the near-field light generating element has an adhesive strength higher than that of a layered structure consisting of the buffer layer and the near-field light generating element.
If an adhesion layer made of a metal different from that constituting the near-field light generating element is interposed between the buffer layer and the near-field light generating element, however, the surface plasmon polariton coupling will not be produced satisfactorily. This causes the problem of a significant drop in the use efficiency of the light propagating through the wave guide.
JP 2009-7634 A describes the use of a film of a metal such as Cr, Mo or Ti or an oxide film that contains silver and an oxide such as ITO or TiO2 as an adhesion film to be interposed between a substrate and a silver alloy film.
Experiments made by the inventors showed, however, that the exfoliation of the near-field light generating element could not be adequately avoided even when a film of a metal oxide such as TiO2 was formed on the buffer layer and the near-field light generating element made of Ag was disposed on the film.