In the field of magnetic recording using a head and a medium, further performance improvement of thin film magnetic heads and magnetic recording media is in demand in association with the high recording density of magnetic disc devices. As a thin film magnetic head, at present, a composite type thin film magnetic head made of a structure where a magnetoresistant (MR) element for reading and an electromagnetic transducer element for writing are laminated is widely used.
The magnetic recording medium is a discontinuous medium in which magnetic grains are aggregated, and each magnetic grain has a single magnetic domain structure. In the magnetic recording medium, one recording bit is configured by a plurality of magnetic grains. Consequently, in order to enhance the recording density, asperities at the border between adjacent recording bits need to be reduced by decreasing the size of the magnetic grains. However, if the magnetic grains are reduced in size, there is the problem that the thermal stability of magnetization of the magnetic grains is reduced in association with a decrease in the volume of the magnetic grain.
As a countermeasure against this problem, an increase of magneto anisotropy energy Ku of magnetic grains may be considered; however, the increase of Ku results in an increase in an anisotropy field (coercive force) of a magnetic recording medium. On the other hand, the upper limit of the recording magnetic field intensity of the thin film magnetic head is substantially determined by saturation magnetic flux density of a soft magnetic material configuring a magnetic core within the head. Consequently, if the anisotropy field of the magnetic recording medium exceeds the acceptable value determined by the upper limit of the recording magnetic field intensity, it becomes impossible to write to the magnetic recording medium. Currently, as a method to solve such a thermal stability problem, a so-called thermally assisted magnetic recording method is proposed in which a recording magnetic field is applied to record information under a state where the anisotropy field is reduced by heating the magnetic recording medium while a magnetic recording medium made from a magnetic material with large Ku is used.
In this thermally assisted magnetic recording method, a method using a near-field light probe, or so-called plasmon-generator, comprising a metal piece that generates NF light from plasmon excited by laser light is generally known, and as a magnetic head including such a plasmon-generator, a magnetic head including a magnetic pole, a waveguide and a plasmon-generator facing the waveguide is proposed.
In a thermally-assisted magnetic recording head, light propagated through the waveguide is coupled with a plasmon-generator in a surface plasmon mode, and excites the surface plasmon. Propagation of such surface plasmon in the plasmon-generator causes the generation of near-field light at the near-field light generating portion positioned at the end portion of the plasmon-generator at the medium opposed surface side. Then, a magnetic recording medium is heated by irradiating the magnetic recording medium with the near-field light generated at the near-field light generating portion of the plasmon-generator, and information is recorded by applying a magnetic field under a state where the anisotropy field of the magnetic recording field is decreased.
In a thermally-assisted magnetic disk device including such a thermally-assisted magnetic recording head, the distance between the thermally-assisted magnetic recording head and the magnetic recording medium, i.e., the flying height of the thermally-assisted magnetic recording head relative to the magnetic recording medium, is an important parameter to satisfy demands, such as reduction of the bit error rate (BER) or high recording density.
Recently, in order to respond to demands, such as high recording density, a technology is proposed in which a medium opposed surface of the thermally-assisted magnetic recording head toward the magnetic recording medium side is protruded by heat-expanding the surface due to the heat generation of a heater and the like, and the flying height of the thermally-assisted magnetic recording head relative to the magnetic recording medium is reduced (see for example, JP 2010-79978). In the patent literature (JP 2010-79978), it is disclosed that the protrusion amount of the medium opposed surface is controlled by controlling the power supply to the heater, and the optimum flying height (the target flying height) can be realized. However, in a thermally-assisted magnetic disk device, in addition to the protrusion of the medium opposed surface due to supply of power to the heater and the application of writing current, since the medium opposed surface also protrudes due to the generation of near-field light, it is actually difficult to reduce the target flying height to approximately several nm (approximately 2 nm).
In the technology disclosed in the patent literature, the supply of power to the heater that can accomplish the target flying height can be set by a touch down test of the thermally-assisted magnetic recording head. In other words, the medium opposed surface gradually protrudes while the supply of power to the heater is increased, contact (touch down) between the thermally-assisted magnetic recording head and the magnetic recording medium is detected, and the supply of power to the heater that can accomplish the target flying height is determined.
In the meantime, in a thermally-assisted magnetic recording head, when a signal is recorded, the near-field light generated at the plasmon-generator is irradiated to the magnetic recording medium. In association with the generation of the near-field light, the vicinity of the medium opposed surface of the thermally-assisted magnetic recording head is heated, and the medium opposed surface protrudes toward the magnetic recording medium side. Therefore, taking into consideration the protrusion of the medium opposed surface of the thermally-assisted magnetic recording head due to the near-field light, it is necessary to set a supply of power to the heater that can accomplish the target flying height.