Demand for various magnetic recording devices, including hard disk drives (HDDs), to have high recording densities is increasing more and more. With the increase in demand for high recording density, a perpendicular magnetic recording method has been actively studied as a method that overcomes the limit of an in-plane magnetic recording method, and HDDs using the perpendicular magnetic recording method have already been commercialized. In order to further increase the recording density of a HDD using the perpendicular magnetic recording method, high track density and higher linear recording density need to be achieved. Among various attempts for increasing the recording density of HDDs in the past, introduction of a reproducing head using a magneto-resistive effect has greatly contributed to improvements. A reproducing head using giant magneto-resistance (GMR) or tunnel magneto-resistance (TMR) is primarily used in such a head. A summary of a current reproducing method using a GMR or a TMR element is described using FIG. 1. The GMR or TMR element has a spin valve structure where a pinning layer 32, in which magnetization is pinned by an antiferromagnetic material, and a free layer 31, in which magnetization may be freely rotated by a magnetic field 37 applied from a medium 30, are provided with an intermediate layer 33 between them. Electric resistance of an element as a whole is changed depending on a relative angle θ formed by magnetization 32a of the pinning layer and magnetization 31a of the free layer. This is a fundamental principle of the spin-valve structure element.
In a current GMR or TMR element, an in-plane magnetization film is used for each of the pinning layer and the free layer. Magnetization of each of the pinning layer and the free layer is stabilized in a plane placed in a direction perpendicular to a down-track direction due to an effect of magnetic anisotropy in an in-plane direction of an element. Magnetization of the pinning layer is stabilized in a cross-track direction due to an effect of a hard bias field applied in the cross-track direction. Electric resistance of the element varies depending on whether magnetization of the free layer is parallel or antiparallel to a magnetization direction of the pinning layer. However, track width 36 is reduced with an increase in TPI (tracks per inch), and therefore in order to prevent information at a track edge or on an adjacent track from being read, reproducing element width 35 is reduced also. However, if an effect of shape magnetic anisotropy in an in-plane direction is strong, and therefore an aspect ratio (element width in the cross-track direction to element height) is reduced, in-plane alignment of magnetization becomes unstable, and a circular magnetization condition is most stable in a film plane. In this condition, sensitivity of a magnetic reproducing element is reduced, and SNR (Signal to Noise Ratio) of a reproducing head is drastically decreased.
As a method of solving the problem of the current memory element (MRAM) using the in-plane magnetization film, N. Nishimura et al. “Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory”, Journal of Applied Physics, Vol. 91, No. 8, pp. 5246-5249, 2002, describes a method where a perpendicular magnetization film is used so that a TMR effect of an element is used to read data. On the other hand, Jap. Pat. Appl. Nos. JP-A-2000-306376 and JP-A-4-351708 propose data reading using an anomalous Hall effect exhibited by the perpendicular magnetization film. Jap. Pat. Appl. No. JP-A-2000-306376 describes a method where current is flowed only through an upper layer in reading information recorded in two perpendicular magnetization films isolated by an insulating layer, so that anomalous Hall voltage appearing in the upper layer is extracted. Jap. Pat. Appl. No. JP-A-4-351708 describes a method that uses difference in polarity of generated anomalous Hall voltage depending on whether magnetization of a sensor portion is upward or downward when an external magnetic field is applied in a condition that a bias magnetic field is horizontally applied to tilt the magnetization of the sensor portion.
On the other hand, in a current TMR element, quality of an insulating layer used between the pinning layer and the free layer is a significant factor in determining performance of a magnetic reproducing element. How a high-quality insulating film is formed is important, and therefore a formation process of the film is not easy. Alternatively, what is called a CPP (Current Perpendicular to Plane) type GMR head, in which current is flowed in a direction perpendicular to a film plane, could possibly avoid a problem of high TMR element resistance in the CPP-GMR head, when current perpendicularly passes through the pinning layer and the free layer, carrier electrons are magnetically spin-polarized, causing disturbance in relative alignment of internal magnetization between the pinning layer and the free layer. As a result, as described in Jian-Gang Zhu et al. “Spin transfer induced noise in CPP read Heads”, IEEE Transactions on Magnetics, Vol. 40, No. 1, pp. 182-188, January, 2004, what is called spin torque noise occurs, causing degradation in SNR of a head. The spin torque noise tends to increase with increase in density of current flowing through an element. The CPP-GMR element structure, which has a merit that element resistance is small compared with the TMR element structure, is considered to be effective for achieving high density. As described in Hiroyuki Katada et al. “Spin-torque noise in CPP-GMR heads with current screen layer”, IEEE Transactions on Magnetics, Vol. 42, No. 10, pp. 2450-2452, October, 2006, in the CPP-GMR element structure, an insulating layer having pin holes is provided between the pinning layer and the free layer, so that current density is controlled to improve SNR of the CPP-GMR element.
Therefore, a magnetic reproducing element which has high sensitivity, and thus may reproduce a magnetization pattern recorded on a narrow track, would be beneficial to enable higher track density and higher linear recording density.