1. Field of the Invention
The present invention relates to a magnetoresistive effect sensor, a thin-film magnetic head, a head gimbal assembly, and a hard disk device, and more particularly to a magnetoresistive effect sensor for use in a thin-film magnetic head of a magnetic recording device such as a hard disk device etc.
2. Description of the Related Art
Magnetic heads employing a GMR (Giant Magneto-Resistive) sensor as a reproducing sensor have been widely used in high-density magnetic recording applications. In particular, a GMR sensor which employs a spin valve film (hereinafter referred to as an SV film) exhibits a high ratio of resistance change to sense current that flows through the GMR sensor in order to read information recorded in a recording medium. It is therefore possible for a GMR sensor to provide a magnetic head of higher sensitivity. An SV film is a stacked layer structure comprising a ferromagnetic layer having a fixed magnetic orientation (hereinafter referred to as pinned layer), a ferromagnetic layer having a variable magnetic orientation depending on an external magnetic field that is generated by a recording medium (hereinafter referred to as a free layer), and a nonmagnetic spacer layer sandwiched between the pinned layer and the free layer. The external magnetic field is also called a signal magnetic field hereinafter.
In general, the SV film utilizes the characteristic that an electron having a spin in the same direction as the magnetic orientation freely moves through the SV film, while an electron having a spin in the opposite direction is scattered and cannot move through the SV film. The relative angle of magnetic orientation between the free layer and the pinned layer changes due to the change in magnetic orientation of the free layer that is induced by a signal magnetic field applied from a recording medium. As a result, the spin-dependent scattering of conductive electrons changes depending on the relative angle, causing change in magnetoresistance. The magnetic head detects this change in magnetoresistance so that it reads the magnetic information from the recording medium. Therefore, it is important to provide a sufficiently high ratio of change in magnetoresistance in order to enhance the reproduction sensitivity of a magnetic head.
A magnetic field (biasing magnetic field) is applied to the free layer in one direction at all times from hard magnetic films which are disposed on lateral sides of the free layer. The intensity of the magnetic field is set such that when the free layer is not subjected to a signal magnetic field, it is magnetized in a certain orientation or in a single magnetic domain, and when the free layer is subjected to a signal magnetic field, the magnetic orientation is rotated at certain angles. This allows a simultaneous change in the magnetic orientation of the entire free layer and a resultant high ratio of change in magnetoresistance. Setting the biasing magnetic field to an appropriate level also contributes to the harmonization between a high ratio of change in magnetoresistance and linearity/low-noise characteristics of the change in magnetoresistance with respect to the change in the external magnetic field. The latter is the characteristics to suppress Barkhausen noise, or the change in magnetization in saw-teeth steps with respect to the magnetic field intensity. To maximize such an effect, the hard magnetic films are preferably disposed as closely and laterally to the free layer as possible, thereby applying the magnetic field efficiently to the free layer from the hard magnetic films. Such structures are disclosed in Japanese Patent Laid-open Publication No. 2002-329905 (Document 1), Japanese Patent Laid-open Publication No. 2002-151756 (Document 2), and Japanese Patent Laid-open Publication No. 2003-86860.
Conventionally, CIP (Current In Plane)—GMR sensors, which employ an SV film in which sense current flows parallel to the film planes, have been commonly used as MR sensors. Recently, efforts have been made to develop CPP (Current Perpendicular to the Plane)—GMR sensors in which sense current flows perpendicularly to the film planes to realize higher-density magnetic recording. CPP-type sensors include a TMR sensor employing a TMR (Tunnel Magneto Resistive) sensor. However, a CPP-GMR sensor is thought to be highly promising due to its potential, because it has lower resistance than a TMR sensor, and higher output than a CIP-GMR sensor for narrow track widths.
However, since the sense current flows perpendicularly to the film planes, i.e., the film boundaries, it is difficult for CPP-GMR sensors to obtain sufficient spin-dependent scattering at the film boundaries, which may lead to insufficient change in magnetoresistance. To cope with this drawback, attempts have been made to increase the thicknesses of the free layer and the pinned layer in order to increase the resistance due to the scattering of conductive electrons in each layer, i.e., bulk scattering, and in order to increase the absolute value of magnetoresistance, thereby obtaining a high ratio of change in magnetoresistance.
FIG. 1 shows the conventional magnetoresistive effect sensor of a CPP-GMR type in a partial cross section. CPP-GMR sensor 102 has buffer layer 5, anti-ferromagnetic layer 6, pinned layer 7, nonmagnetic spacer layer 8, free layer 9, and cap layer 10 which are deposited in this order on lower electrode/shield layer 4. The layers from buffer layer 5 to cap layer 10 are collectively called SV film 18. Pinned layer 7, nonmagnetic spacer layer 8, and free layer 9 preferably have a thickness of at least 10 nm (100 Å) in total for the reason described above. Both sides of SV film 18 are covered with insulating films 111. Hard magnetic films 112 which apply a biasing magnetic field to free layer 9 are disposed on the outer sides of insulating films 111. Upper electrode/shield layer 3 is disposed such that it fully covers hard magnetic films 112 and SV film 18.
The steps for fabricating a magnetoresistive effect sensor which includes a SV film will be now briefly described. SV film 18 is first deposited, then a resist (not shown) is formed over SV film 18. Insulating films 111 and hard magnetic films 112 are formed on both sidewalls of SV film 18. Since a CPP-GMR sensor has a large film thickness, hard magnetic films 112 are formed such that they rise along the sidewalls of SV film 18 toward end the portions that project from SV film 18. Such a configuration is rarely formed for a thin SV film. Instead, as shown in FIG. 2, hard magnetic films 112b are formed substantially flush with SV film 18, and the end portions do not project from SV film 18.
In a CPP-GMR sensor, however, it is difficult to form such a SV film configuration as shown in FIG. 2, because the SV film needs to be formed in a rather larger thickness. The hard magnetic films, on the contrary, are usually formed such that the end portions project upwardly as shown in FIG. 1. In hard magnetic films that have such a structure, the magnetic field generated by the hard magnetic films is not directed toward the free layer, but towards the upper electrode/shield layer, and fails to the apply biasing magnetic field effectively to the free layer. This makes it difficult to increase the reproduced output level and to obtain a linear magnetic response to the signal magnetic field. In addition, the magnetic characteristics of the upper electrode/shield layer are adversely affected.
These drawbacks cannot be resolved by the above-disclosed conventional techniques. According to Documents 1 and 2, block-like hard magnetic films are disposed on lateral sides of a free layer. However, it becomes more difficult to form such a structure as the thickness of the SV film increases. It may be effective to increase the thickness of the hard magnetic films in order to reduce the projection above the SV film, which, however, tends to apply an excessive biasing magnetic field, resulting in a reduction in the reproduced output level. Disposing hard magnetic films only around a free layer, as disclosed in Document 3, may result in an insufficient biasing magnetic field due to the insufficient thickness.