Patent Application
20080026253
MR elements in exemplary embodiments of the present invention will be described below with reference to the drawings.
With reference to
First, a description will be given of an increase in MR by the current confining effect. In a CPP-GMR element, a related art technique involves confining a current path by interposing an ultrathin oxide layer in an interface between an intermediate layer and a magnetization pinned layer (or a magnetization free layer), thereby to improve the MR ratio (refer to, for example, H. Fukuzawa et al., IEEE-Mag. Vol. 40 (2004), pp. 2236). The MR ratio may be improved because the current path through the intermediate layer is confined by micropores in the ultrathin oxide layer, and the proportions of a current flowing through the magnetization pinned layer and the intermediate layer and of a current flowing through the intermediate layer and the magnetization free layer increase, such that the effect of the parasitic resistance of an electrode layer decreases relatively. However, in the related art methods that involve using an ultrathin oxide film, the effect on an improvement in MR in a CPP-GMR element was on the order of 5% at most.
On the other hand, in general, in a barrier made of a single-crystal material (hereinafter called a single-crystal barrier), the transmission coefficient of electrons has different values depending on the crystal orientations and electronic bands. A high MR ratio is realized if electrons of a certain electronic band with a high spin polarization rate (i.e., an electronic band in which the proportions of an upward spin and a downward spin are remarkably unbalanced) are caused to be selectively transmitted. Causing only a current having a unidirectional spin to flow selectively by utilizing the electronic properties of a material like this is called the spin filter effect.
An increase in MR by various kinds of single-crystal barriers has hitherto been searched and it has been shown by theoretical research that among others, a TMR element of an Fe/MgO/Fe structure in which iron having a (001) oriented single-crystal structure (hereinafter called Fe(001)) is used in a magnetization free layer and a magnetization pinned layer has substantially a large MR ratio. Prior to the experiment, the inventors have demonstrated that in the above-described TMR element formed from a (001) oriented single crystal (hereinafter called an Fe/MgO/Fe-TMR element), an MR ratio as high as three times or more that of a conventional TMR element using an aluminum oxide barrier occurs (refer to, for example, a group of papers on MgO-TMR elements having high MR ratios: 1) S. Yuasa et al., Nature Mater. Vol. 3 (2004), pp. 868; 2) S. Yuasa et al., Appl. Phys. Lett. Vol. 87 (2005), pp. 222508.; 3) S. S. Parkin et al., Nature Mater. Vol. 3 (2004), pp. 862; and 4) D. Djayaprawira et al., Appl. Phys. Lett. Vol. 86 (2005), pp. 092502).
With reference to
By using the ultrahigh vacuum molecular beam epitaxy method (hereinafter called MBE method), the present inventors realized huge MR ratios of not less than 180% in an elaborated single-crystal Fe/MgO/Fe-TMR element that is controlled in terms of atomic accuracy. (Refer to a paper in the group of papers on MgO-TMR elements having high MR ratios: 1) S. Yuasa et al., Nature Mater. Vol. 3 (2004), pp. 868.).
Next, details of the experiment conducted by the inventors will be described. An MR thin film having a high-quality, single-crystal structure whose crystal orientation aligned with (001) direction was made and fabricated into a sub-micrometer-size CCP-CPP type MR element by use of a microfabrication technique, and the characteristics of the CCP-CPP type MR element were evaluated. The MR thin film used here refers to a multilayer film in which, as the structure of elements 14 shown in
The manufacturing process of an MR element in this exemplary embodiment will be described below with reference to the drawings.
First, chromium 23 as a seed layer and gold 25 as a buffer layer are deposited on a cleaned single-crystal MgO(001) substrate 21 (refer to
Subsequently, annealing treatment is performed at a temperature that enables the surface of the Fe(Co) layer 27 (27a or a combination of 27a and 27b) deposited in the above-described steps to be planarized at an atomic level, for example, at 350° C. However, all of the surface of the Fe(Co) layer 27 is not planarized by this annealing treatment and a terrace structure as shown in
Next, as shown in
When a dissimilar metal (gold is used here) is put into the micropores, the following step is performed further.
Next, as shown in
Subsequently, in the structure of
In the above-described structure, it is also possible to form a similar structure by causing the gold to evaporate in a state heated to 300° C. from the state of
After the fabrication of the structure of
As a method of putting another metal into the micropores formed in the ultrathin MgO(001) layer 31, it is possible to utilize a difference in surface energy between the MgO surface of this metal and the surface in the micropores (in the above-described embodiment, the surface of the Fe(Co) layer 27). For instance, in the case of the above-described example, the surface energy is lower when gold atoms are present on an iron surface than when gold atoms are present on the MgO(001) surface. Because gold atoms have high mobility at relatively low temperatures (300° C. or so), it is possible to ensure that gold is filled into the micropores 32 by performing annealing at 300° C. or so after the evaporation of a substantially small amount of gold (for example, an amount corresponding to a 0.1 atomic layer or so) or by performing evaporation under heating at 300° C. or so.
Subsequently, the above-described multilayer film with the structure F thus obtained was fabricated to form a micro CPP type MR element having a sub-micrometer-size cross-sectional area by a combination of the electron beam lithography and the argon ion milling. The depth of etching by the argon ion milling is up to a level where the ultrathin MgO(001) layer 31 is exceeded from the cap layer 37 (a level several nanometers deep into the magnetization free layer 27). The island-like region thus fabricated has two sizes, 120 nm×220 nm and 220×420 nm. For these sizes, after the etching by the argon ion milling, actually fabricated microjunctions were observed under an electron microscope and the size of the junctions was actually evaluated. For the MR element thus fabricated, the element resistance was measured by the four-terminal method and its resistance values of only the CPP portion were evaluated.
In a CCP type MR element having such an ultrathin MgO(001) layer as an intermediate layer, whether the conduction properties are tunnel-like ones or metallic ones may be important from a practical view point.
There are two kinds of MR elements depending on the material for an intermediate layer. In one kind, an insulator is used as the intermediate layer and electrons conduct by tunnel conduction. This is the TMR (tunnel magneto-resistance) element. In this element, the current-voltage characteristics become nonlinear and current increases exponentially when voltage is applied to the element. The resistance value decreases with increasing temperature. The other kind is an element in which a nonmagnetic metal is used as the intermediate layer and electrons conduct by normal metal conduction. In particular, an MR element having a perpendicular-to-plane structure is called the CPP-GMR element. In this element, the current-voltage characteristics are linear (Ohm's Law) and the resistance value increases with increasing temperature.
To ascertain which type of conduction occurs in this present element, the MR curve, resistance value and MR ratio of the element indicated by an arrow in
Although the MR ratio increases with decreasing temperature, this is due to the fact that a difference of the resistance between a high-resistance state and a low-resistance state varies little while the resistance valve decreases with decreasing temperature. These characteristics are the features of a CPP-GMR element of a metal material. That is, this shows that in this element the ultrathin MgO(001) layer functions as an intermediate layer having metallic conduction properties, and not as a tunnel barrier.
Furthermore, to ascertain that an ultrathin MgO(001) layer having a thickness in the vicinity of 0.6 nanometers shows metallic conduction, the ultrathin Mg(001) layer on single-crystal Fe(001) surface was observed by a scanning tunnel microscope (hereinafter called SEM).
Table 1 provides a summary of RA values and MR ratios in CPP type MR elements having a (001) oriented single-crystal Fe(or Co)/MgO/Fe structure obtained in the experiment. In Table 1, an element with an RA value of 0.96 Ω/square micrometer has an ultrathin MgO(001) layer of 1.0 nm and an element with an RA value of 0.14 Ω/square micrometer has an ultrathin MgO(001) layer of 0.6 nm.
As described above, on the basis of the above-described experiment, it was possible to realize a CCP-CPP type MR element having low resistance (RA value: not more than 1 Ω/square micrometer) and a high MR ratio (not less than 20%).
In an element utilizing tunnel conduction, when the current flowing through the element is increased, the resistance further decreases where the temperature has risen and the current is concentrated on the barrier that is thinnest. In general, this element is vulnerable to overcurrent. On the other hand, in an element utilizing metallic conduction, when the temperature rises, the resistance in portions where the temperature has risen increases, and naturally the current becomes uniformly distributed. For this reason, this element has resistance against heat. Therefore, in applications where a considerably large bias current is caused to flow constantly, such as the read-head of a hard disk, the element of metallic conduction type may be advantageous, but is not required to be advantageous.
Next, the second exemplary embodiment will be described with reference to the drawings.
Furthermore, for the magnetization pinned layer, it is also possible to use a multilayer film having a structure called a synthetic anti-ferromagnetic layer. Incidentally, a synthetic anti-ferromagnetic layer refers to a multilayer film that sandwiches two ferromagnetic layers having substantially the same magnitude of magnetization via an antiparallel bonding film and magnetically bonds the two ferromagnetic layers in an antiparallel direction. As an example of a synthetic anti-ferromagnetic layer, there is an iron-cobalt alloy/ruthenium thin film/iron-cobalt alloy. Although examples of a material for an antiparallel bonding film include alloys made of one kind or two kinds of substances selected from the group consisting of ruthenium, iridium, rhodium, rhenium and chromium, it is desirable to use a ruthenium thin film (film thickness: about 0.5 to 1.0 nm).
In the MR element of the second embodiment, the thickness of the MgO(001) layer, which is the intermediate film, may be about 0.5 to about 0.7 nm. As the experiment results described in the first exemplary embodiment, when the intermediate layer is a single-crystal MgO(001) layer, it is possible to ensure that in the vicinity of a thickness of 0.6 nm, low area resistance (0.14 Ω/square micrometer) and a high MR ratio (not less than 20%) are compatible with each other. Furthermore, in an MR element of the second exemplary embodiment, it is preferred that the diameter of the micropores present in the MgO(001) layer, which is the intermediate layer, be not more than about 50 nm. If the diameter of the micropores is not less than about 50 nm, i.e., sizes that cannot be neglected compared to a micro MR element (fore example, the size of an element required of a high-density magnetic head is on the order of several hundreds of nanometers square), then there is a fear that variations among elements might become great.
As described above, the thickness of 0.6 nm corresponds to the thickness of three atomic layers of the MgO(001) layer. In order to fabricate such a thin layer, it is necessary to planarize the under layer to an atomic layer level. Annealing at an appropriate temperature may be adopted as a method of planarization. In actuality, however, the whole under layer is not planarized by annealing treatment in an ultrahigh vacuum and it is planarized only to a certain macroscopic size (for example, in the shape of a terrace). In the case of an Fe(001) single crystal, it is known that a surface of an Fe(001) single crystal is planarized in a terrace-like shape having a size of several tens to several hundreds of nanometers. When an ultrathin MgO layer is formed on such a structure, portions where the MgO(001) layer becomes discontinuous (thin portion) are formed on the boundaries of the terrace (steps) because of difference in size of the iron molecule and the MgO molecule, with the result that micropores are formed. That is, when MgO(001) layer having a thickness of three atomic layers is formed, part of the MgO(001) layer provides holes and other part becomes thick (4 layers or more).
Accordingly, the structure of the under layer is considered for making regular micropores of not more than about 50 nm in the ultrathin MgO(001) layer. The under layer (magnetization free layer) is an Fe(001) single crystal formed by the MBE method. From an observation experiment using a SEM it has been ascertained that when an Fe(001) single crystal is formed on a gold buffer layer, it obtains a periodic terrace structure of about 50 nm to about 100 nm. Therefore, an Fe(001) single crystal is suitable for making an ultrathin MgO(001) layer having periodic micropores.
Next, the third exemplary embodiment will be described.
Next, the fourth exemplary embodiment will be described.
Next, the fifth exemplary embodiment will be described.
Next, the sixth exemplary embodiment will be described.
The effectiveness of the technique for interposing the ultrathin nonmagnetic layer in the interface in the CCP-CPP type MR elements of the above-described fifth and sixth exemplary embodiments has been demonstrated in the point of reducing the area resistance of the elements while maintaining high MR ratios in an MgO-TMR element. For example, by interposing a magnesium thin film in an interface between an MgO layer and a magnetization pinned layer, an M ratio of 138% was realized in an MgO-TMR element having an RA value of 2.4 Ω/square micrometer (refer to “A Paper on MgO-TMR Elements Having Low RA Values”; K. Tsunekawa et al., Appl. Phys. Lett. 87, 072503 (2005)). In contrast to this, also in the CCP-CPP type MR element having an ultrathin MgO layer, it is possible to increase the MR ratio in a lower-resistance region by interposing a buffer layer in the interface between the ultrathin MgO layer and the magnetization free layer.
Next, a CCP-CPP type MR element in the seventh exemplary embodiment will be described. The CCP-CPP type MR element in the seventh exemplary embodiment is such that a material containing iron, cobalt and nickel as main components is used as the material for the ferromagnetic material of a bcc(001) structure used in the MR elements of the second to fifth exemplary embodiments. Concretely, iron, cobalt, cobalt-iron alloys, cobalt-iron-boron alloys, cobalt-iron-boron-nickel alloys, and alloys obtained by adding molybdenum, vanadium, chromium, silicon and aluminum to these metals and alloys, or two or more kinds of ferromagnetic materials of a bcc(001) structure can be fabricated into objects that are stacked in lamellar form (laminated structures of thin films).
As already described, it is thought that a cause of the huge MR in a TMR in which an MgO(001) barrier layer is used as the intermediate layer may be the spin filter effect that comes into play when the MgO(001) barrier layer is combined with a ferromagnetic material of a bcc(001) structure. For iron, cobalt, cobalt-iron alloys, cobalt-iron-boron alloys and cobalt-iron-boron-nickel alloys among the above-described materials, it has already been ascertained that these metals and alloys are ferromagnetic materials of a bcc(001) structure, and that huge MR ratios (of not less than 100% at room temperature) are caused to occur in a TMR element in which an MgO barrier layer is used as the intermediate layer. Also in the CCP-CPP type MR element in which an ultrathin MgO layer is used as the intermediate layer, the Δ1 Bloch state having a high polarization rate due to the bcc(001) structure may be one of the causes of the high MR ratio. Therefore, the above-described group of materials is desirable as materials for the magnetization free layer and magnetization pinned layer in the MR element of the exemplary embodiment.
Next, the eighth exemplary embodiment will be described. The CCP-CPP type MR element of the eighth exemplary embodiment is such that as the ferromagnetic materials of a bcc(O) structure used in the MR elements of the second to fifth exemplary embodiments, there is used a material that has an amorphous structure in a state immediately after thin-film fabrication and becomes crystallized to form a bcc(001) structure by post annealing. Examples of the material include cobalt-iron alloys, cobalt-iron-boron alloys, cobalt-iron-boron-nickel alloys and cobalt-iron-boron-copper alloys.
It has been reported that in related art TMR elements in which an MgO barrier layer is used as the intermediate layer, the MR ratio is substantially sensitive to the crystallizability of the magnetization pinned layer and magnetization free layer. If the magnetization pinned layer and magnetization free layer have a bcc(001) structure, a high MR occurs. However, the MR ratio becomes substantially small if the crystal structure of these layers becomes irregular. The present inventors realized a bcc(001) structure in the magnetization pinned layer and magnetization free layer by fabricating an elaborated single-crystal TMR element that is controlled at an atomic level by using the ultrahigh vacuum MBE method (refer to 1): S. Yuasa et al., Nature Mater. Vol. 3 (2004), pp. 868 in a group of papers on MgO-TMR elements having high M ratios). However, this method may not be suitable for mass production. On the other hand, D. Djayaprawira et al. fabricated a TMR element in which an MgO barrier layer is used as the intermediate layer by the sputtering method and caused the structure of the magnetization pinned layer and magnetization free layer to be crystallized from an amorphous structure into a bcc(001) structure by performing post annealing (annealing after film forming), with the result that they obtained an MR ratio equivalent to that of an element fabricated by the ultrahigh vacuum MBE method (refer to 4): D. Djayaprawira et al., Appl. Phys. Lett. Vol. 86 (2005), pp. 092502 in a group of papers on MgO-TMR elements having high MR ratios). This method is highly evaluated as a technique indispensable for the mass production of TMR elements in which an MgO barrier film is used as the intermediate layer.
Also in the MR element of the exemplary embodiment, that it is possible to be able to fabricate an element by the above-described method (the magnetization pinned element and magnetization free element are fabricated by the sputtering method and caused to be crystallized into a bcc(001) structure by post annealing) is indispensable for mass production. For cobalt-iron alloys, cobalt-iron-boron alloys and cobalt-iron-boron-nickel alloys among the above-described materials, in a TMR element in which an MgO barrier layer is used as the intermediate layer, high MR ratios (not less than 100% at room temperature) are realized by the sputtering film-fabrication and the post annealing. It might be thought that also in the MR element of the exemplary embodiment, in which an ultrathin MgO(001) layer is used as the intermediate layer, high MR ratios occur by using the above-described materials as the materials for the magnetization pinned layer and magnetization free layer and adopting the sputtering film-fabrication and the post annealing.
Next, the ninth exemplary embodiment will be described. In the CCP-CPP type MR elements described in the above first to eighth exemplary embodiments, it is possible to realize low resistance and high MR ratios compared to those of conventional MR elements. For this reason, by using these MR elements, it becomes possible to provide a magnetic sensor capable of higher-accuracy, higher-density sensing. As described in connection with the first exemplary embodiment, in an experiment on a CPP type MR element made of single-crystal Fe/ultrathin MgO/Fe oriented in the (001) orientation, it is possible to realize an RA value of 0.14 Ω/square micrometer and an MR ratio of 23%. These values sufficiently exceed RA values of not more than 1 Ωl/square micrometer and MR ratios of not less than 20%, which are the specifications required of the magnetic head of a hard disk with a high recording density of about 500 Gbytes/square inch.
Incidentally, in applications to a magnetic head for reading out a high-density hard disk, it is required to lower the area resistance value rather than to raise the MR ratio. For example, area resistance of 4 Ω/square micrometer is required for 200 Gbytes/square inch, and 1 Ω/square micrometer is required for about 500 Gbytes/square inch. On the assumption that the above-described scaling holds for recording density, area resistance of 0.25 Ω/square micrometer is required for 1 Tbytes/square inch. Therefore, it is possible to cope with a recording density of 1 Tbytes/square inch by using the element of this exemplary embodiment.
As described above, it is apparent that by using the CPP type MR elements of this exemplary embodiment, it is possible to provide a magnetic head adaptable to a high recording density hard disk.
As described above, according to the CCP-CPP type GMR element having an ultrathin MgO barrier layer as described in each of the exemplary embodiments, it is possible to obtain an MR element having low resistance (RA value of not more than 1 Ω/square micrometer) and high MR values (not less than 20%) without using a complex multilayer structure.
By using this CCP-CPP type GMR element as a magnetic sensor, it becomes possible to provide an MR head adaptable to magnetic recording densities of not less than about 500 Gbytes/square inch.
The MR element of this exemplary embodiment is characterized by a substantially low impedance. Examples of the low-impedance MR element include a low-noise magnetic sensor and an output element in a magnetic theoretical circuit.
The exemplary embodiments can be applied to a magnetic sensor
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-204713 | Jul 2006 | JP | national |
