1. Field of the Invention
The present invention relates to a magnetoresistive element having a structure in which a current is supplied perpendicularly to the plane of the element, as well as a magnetic head, a magnetic recording apparatus and a magnetic memory which use the magnetoresistive element.
2. Description of the Related Art
The performance of magnetic devices has drastically been improved by discovery of a giant magnetoresistive effect (GMR) in a stacked structure of magnetic films. In particular, a spin-valve film (SV film) has a structure easily applicable to a magnetic device to enable the GMR effect to be effectively produced. Consequently, the spin-valve film has brought about marked technical improvement to magnetic heads and magnetic devices such as MRAMs (magnetic random access memories).
The “spin-valve film” is a stacked film having a structure in which a nonmagnetic metal spacer layer is sandwiched between two ferromagnetic layers. In the spin-valve film, the magnetization of one ferromagnetic layer (referred to as a “pinned layer” or “magnetization pinned layer”) is pinned by an antiferromagnetic layer or the like, whereas the magnetization of the other ferromagnetic layer (referred to as a “free layer” or “magnetization free layer”) is made rotatable in accordance with an external field (for example, a media field). In the spin-valve film, a giant magnetoresistace change can be produced by varying the relative angle between the magnetization directions of the pinned layer and free layer.
Conventional spin-valve films are CIP (current-in-plane)-GMR elements in which a sense current is supplied parallel to the plane of the element. In recent years, much attention has been paid to CPP (current-perpendicular-to-plane)-GMR elements (referred to as “CPP elements” hereinafter) in which a sense current is supplied substantially perpendicular to the plane of the element.
When such a magnetoresistive element is applied to a magnetic head, a higher element resistance poses problems in regard to shot noise and high frequency response. It is appropriate to evaluate the element resistance in terms of RA (a product of the resistance and the area). Specifically, RA must be less than 1 Ωμm2 at a recording density of 200 Gbpsi (Gigabit per square inch).
In connection with these requirements, the CPP element is advantageous in that the resistance of the element depends on its area so that reduction in the size of the element increases the change in resistance. The CPP element is thus advantageously applicable on a trend of increasingly reducing the size of the magnetic device. Under the circumstances, the CPP element and the magnetic head using the same are expected to be promising candidates to achieve a recording density of 200 Gbpsi to 1 Tbpsi (terabits per square inch). However, CPP elements using a spacer layer made of a nonmagnetic metal exhibit only a very small resistance change. The CPP elements are thus hard to provide high output signals.
To partially solve this problem, a CPP (current-confined-path) element has been proposed which uses a spacer layer comprising an insulating layer in which fine current paths (current confined paths) consisting of a nonmagnetic metal penetrating the insulating layer are formed. Such a CPP element (referred to as a CCP-CPP element hereinafter) exhibits a current confining effect and provides high output signals than a simple CPP element using a nonmagnetic metal spacer layer. However, if the CCP-CPP element were applied to a magnetic head adapted for high density recording, the MR ratio thereof might still be insufficient.
An element that realizes an MR ratio high enough to adapt to a high recording density has been proposed which has a spacer layer in which current confined paths in an oxide layer are formed of a metallic magnetic material and utilizes a ballistic magnetoresistive (BMR) effect (referred to as a BMR element hereinafter). See, for example, Jpn. Pat. Appln. Publication No. 2003-204095. However, a physical principle by which the BMR element allows to provide a high MR ratio is still unknown. Thus, a high MR ratio cannot be achieved simply by causing ballistic conduction in magnetic paths. Actually, there has not yet been reported that a BMR element in a stacked structure of thin films has achieved a higher MR ratio than that a conventional CPP element could have achieved. It is thus desired to provide an MR element that can achieve a high MR ratio based on a new mechanism.
The conventionally proposed BMR element has another problem besides the problem that there has not yet been experimentally confirmed that the element in a stacked structure of thin films can exhibit a high MR ratio. The another problem is due to a structure in which a current confined layer, comprising an insulating layer and fine current confined paths made of a metallic magnetic material penetrating the insulating layer, is provided between the pinned layer and the free layer. When the current confined layer comprising the insulating layer with the current confined paths made of the metallic magnetic material formed therein is used, magnitude of the interlayer coupling field Hin between the pinned layer and the free layer may be increased, which disadvantageously prevents the magnetization of the free layer from rotating with respect to an external field. Here, the maximum value of Hin in the conventional spin valve film is said to be limited at about 20 Oe, taking into consideration of practical use. However, for the CCP-CPP element and BMR element, it is disadvantageous to increase the thickness of the current confined layer, that is, the thickness of the insulating layer. Thus, even with the CCP-CPP element, which has current confined paths formed of a nonmagnetic metal material, the Hin value may become higher than 20 Oe if process conditions are inappropriate. With the BMR element, which has current confined paths formed of a metallic magnetic material, it is difficult to reduce Hin to a level less than 20 Oe. It is thus very difficult to put the BMR element into practical use. Accordingly, a practical element cannot be implemented easily using the so-called BMR element. Moreover, as described previously, the physical principle on the basis of the structure by which the BMR element allows to provide a high MR ratio is still unknown. Therefore, the BMR element in a stacked structure of thin films cannot experimentally realize a high MR ratio at present if only Hin could be reduced.