Magnetic tunnel junction, magnetoresistive element and spintronics device in which said magnetic tunnel junction is used, and method of manufacturing magnetic tunnel junction

Abstract

The object of the present invention is to attain an unconventionally high tunnel magnetoresistance (TMR) ratio by using a barrier layer made of an MgAl.sub.2O.sub.4 type insulator material with a spinel structure. The problem can be solved by a magnetic tunnel junction in which a barrier layer is made of a cubic nonmagnetic material having a spinel structure, and both of two ferromagnetic layers that are adjacently on and below the barrier layer are made of a Co.sub.2FeAl Heusler alloy. Preferably, the nonmagnetic material is made of oxide of an Mg.sub.1−xAl.sub.x (0<x≤1) alloy, and exhibits tunnel magnetoresistance of 250% or more and 34000% or less at a room temperature.

Claims

1. A magnetic tunnel junction comprising a tunnel barrier layer and two ferromagnetic layers that are each adjacent to the tunnel barrier layer, wherein a first of the two ferromagnetic layers is above the tunnel barrier layer and a second of the two ferromagnetic layers is below the tunnel barrier layer, wherein the tunnel barrier layer is made of a cubic nonmagnetic material having a spinel structure, and wherein both of the ferromagnetic layers are made of Co.sub.2FeAl Heusler alloy.

2. The magnetic tunnel junction according to claim 1, wherein lattice mismatch between the nonmagnetic material and the Co.sub.2FeAl is 3% or less.

3. The magnetic tunnel junction according to claim 1, wherein the nonmagnetic material is made of oxide of Mg.sub.1−xAl.sub.x (0<x≤1) alloy.

4. The magnetic tunnel junction according to claim 1, wherein the nonmagnetic material is made of MgAl.sub.2O.sub.4.

5. The magnetic tunnel junction according to claim 1, exhibiting tunnel magnetoresistance of 250% or more and 34000% or less at a room temperature.

6. A spintronics device, which comprises the magnetic tunnel junction according to claim 1.

7. A spin injection element comprising a laminated film of a cubic nonmagnetic material having a spinel structure and a ferromagnetic body, wherein the ferromagnetic body is made of Co.sub.2FeAl Heusler alloy.

8. The magnetic tunnel junction according to claim 1, wherein lattice mismatch between the nonmagnetic material and the Co.sub.2FeAl is 1% or less.

9. The magnetic tunnel junction according to claim 1, wherein lattice mismatch between the nonmagnetic material and the Co.sub.2FeAl is 0.5% or less.

10. A method of manufacturing a magnetic tunnel junction, comprising: forming an underlayer on a single crystal substrate; forming a first ferromagnetic layer on the underlayer; forming a tunnel barrier layer on the first ferromagnetic layer, which includes forming an MgAl alloy layer on the substrate, on which the first ferromagnetic layer is formed, and performing oxidation treatment of the substrate on which the MgAl alloy layer is formed; forming a second ferromagnetic layer on the substrate on which the tunnel barrier layer is formed; changing a crystal structure of the tunnel barrier layer from a disordered structure into a spinel structure by performing heat treatment of the second ferromagnetic layer under predetermined heat treatment conditions; and forming at least one of an upper electrode layer and a cap layer on the substrate on which the second ferromagnetic layer is formed.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view that illustrates a basic structure of a magnetic tunnel junction (MTJ element) film according to an embodiment of the present invention.

(2) FIG. 2 is a flowchart that explains a method of manufacturing a magnetic tunnel junction (MTJ element) film according to an embodiment of the present invention.

(3) FIG. 3 is a flowchart that explains a detailed part of a process for forming a barrier layer.

(4) FIG. 4 is an X-ray diffraction profile showing that a Co.sub.2FeAl layer can be manufactured on an MgAl.sub.2O.sub.4 layer. A result obtained by forming a Co.sub.2FeAl layer with a thickness of 10 nm on a Cr/CoFe/MgAl.sub.2O.sub.4 and changing a post-anneal temperature. FIG. 4(a) is an out-of-plane scanning view, and FIG. 4(b) is an in-plane scanning view.

(5) FIG. 5 is a view that illustrates a relation of a TMR ratio and a Resistance-Area (RA) value of an upper Co.sub.2FeAl layer in a magnetic tunnel junction (MTJ element) at a post-anneal temperature (T.sub.CFA).

(6) FIG. 6 is a view that illustrates a TMR curve of a sample that has a structure of a lower Co.sub.2FeAl/a CoFe insertion layer (0.5 nm)/MgAl.sub.2O.sub.4/an upper Co.sub.2FeAl and a post-anneal temperature of T.sub.CFA=550° C.

(7) FIG. 7 is a view showing differential conductance curves of the sample shown in FIG. 6 at 300 K and 4 K.

(8) FIG. 8(a) illustrates an image (ADF-STEM image) of a cross-section of the sample shown in FIG. 6, which is observed by an annular dark-field method. FIG. 8(b) is a view that illustrates an atom ratio profile of the region corresponding to FIG. 8(a), which is identified by energy dispersive X-ray spectroscopy (EDS).

(9) FIG. 9 is a view that illustrates nano electron beam diffraction (NBD) images corresponding to respective parts in the ADF-STEM image of the magnetic tunnel junction (MTJ element) shown in FIG. 8.

(10) FIG. 10(a) illustrates an ADF-STEM image of a cross-section of a magnetic tunnel junction (MTJ element) having an MgAl.sub.2O.sub.4 layer with a disordered structure. FIG. 10(b) is a view that illustrates an NBD image obtained from MgAl.sub.2O.sub.4.

DESCRIPTION OF EMBODIMENTS

(11) Hereinafter, a magnetic tunnel junction (MTJ) element film 1 according to each embodiment of the present invention will be described in detail with reference to FIG. 1. As shown in FIG. 1, the MTJ element film 1 in an embodiment of the present invention includes a substrate 2, an underlayer 3, a first ferromagnetic layer 4, a barrier layer 5, a second ferromagnetic layer 6 and an upper electrode 7.

(12) The substrate 2 is a magnesium oxide (MgO) single crystal in (001) orientation, a spinel (MgAl.sub.2O.sub.4) single crystal having a spinel structure, a strontium titanate (SrTiO.sub.3) single crystal, an Si single crystal or a Ge single crystal. Further, the substrate 2 may be a textured polycrystalline MgO film aligned in (001) orientation, and a magnesium-titanium oxide (MgTiO.sub.x) that has a rock-salt structure as a replace of MgO and an equivalent lattice constant may be used.

(13) The underlayer 3 is adjacently on the substrate 2, and serves as an underlayer for growing crystals of the first ferromagnetic layer 4 and also as a conductive electrode. The underlayer 3 is made of: BCC-type metal such as chromium (Cr), vanadium (V) and tungsten (W), which is grown with crystal orientation controlled; B2-type alloy such as nonmagnetic NiAl, CoAl and FeAl; or a noble metal layer of ruthenium (Ru), platinum (Pt), palladium (Pd), rhenium (Re), silver (Ag), gold (Au) or the like. Needless to say, the underlayer 3 may be a multilayer film combining them. Further, Co.sub.1−xFe.sub.x (0≤x≤1) alloy, which is ferromagnetic, may be used. A thickness of the underlayer 3 typically ranges from about 5 nm to about 100 nm. Moreover, for controlling the growth of the underlayer, an oxide layer made of MgO, a spinel or the like having a thickness of about 5 nm to about 50 nm may be inserted between the substrate 2 and the underlayer 3.

(14) The first ferromagnetic layer 4 is a Co-based Heusler ferromagnetic alloy layer that is formed adjacently on the underlayer 3, and is a layer having a B2 structure (CsCl-type structure) here. Herein, the Co-based Heusler alloy is alloy that essentially has an L2.sub.1 structure (Cu.sub.2MnAl-type structure) and is composed of a Co.sub.2XY (X=Fe, Mn, Ti, Cr or the like, Y=Si, Al, Ga, Ge, Sn or the like) composition. The Co-based Heusler is ferromagnetic, and exhibits a magnetic transition temperature (Curie temperature) that is higher than a room temperature, and many of alloy compositions have very high spin polarization. Some of the compositions have completely spin-polarized band structures (spin polarization P=1), and are called as a half metal. The B2 structure is a disordered structure in which X-Y sites in the L2.sub.1 structure are randomly occupied, but it is known that its band structure is almost held. Thus, even the B2 structure exhibits P=1 that means a half metal or a high value equivalent thereto. A thickness of the first ferromagnetic layer is, for example, 5 nm, but may be smaller. A composition of the Co-based Heusler alloy may be within a composition range that can provide the B2 structure, and may have discrepancy of, for example, about 5% from a stoichiometric composition.

(15) Next, the barrier layer 5 is provided on the first ferromagnetic layer 4. This layer is made of a nonmagnetic body that is oxide having a spinel structure, and has (001) orientation. Further, a film thickness thereof ranges from about 0.8 nm to about 3 nm. The spinel structure is a crystal structure of MgAl.sub.2O.sub.4, and belongs to a space group Fd-3m (No. 227). A lattice constant of the spinel is twice as high as that of alloy with a BCC structure or a B2 structure (in the case of rotating by 45° within a plane). In this case, in general, spin polarization is effectively decreased due to the band folding effect, thereby causing a problem of decreasing a TMR ratio. Whereas, in the case where the B2-type Co-based Heusler alloy layer that is used in the present embodiment is utilized as both ferromagnetic layers of the first ferromagnetic layer 4 and the second ferromagnetic layer 6, this problem is not caused, so that a high TMR ratio can be obtained. That is, since the Co-based Heusler alloy layer has the substantially high spin polarization, influence of the band folding effect becomes very small.

(16) The cubic nonmagnetic materials having a spinel structure include ZnAl.sub.2O.sub.4, MgCr.sub.2O.sub.4, MgMn.sub.2O.sub.4, CuCr.sub.2O.sub.4, NiCr.sub.2O.sub.4, GeMg.sub.2O.sub.4, SnMg.sub.2O.sub.4, TiMg.sub.2O.sub.4, SiMg.sub.2O.sub.4, CuAl.sub.2O.sub.4, Li.sub.0.5Al.sub.2.5O.sub.4, γ-Al.sub.2O.sub.3, γ-Ga.sub.2O.sub.3, MgGa.sub.2O.sub.4, ZnGa.sub.2O.sub.4, GeZn.sub.2O.sub.4 and their solid solution.

(17) More generally, the cubic nonmagnetic material corresponds to a spinel oxide material having a composition of AB.sub.2O.sub.4, where A and B are metal elements. Herein, each of A and B is generally Li, Mg, Al, Si, Sc, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Cd, In, Sn or the like. The spinel structure includes a positive spinel structure and a negative spinel structure, and either of them and their mixture exhibit the similar effects. This fact is applicable to the above-described γ-Al.sub.2O.sub.3 and γ-Ga.sub.2O.sub.3, because they also have spinel-like structures. Further, LiAl.sub.5O.sub.8 (Li.sub.0.5Al.sub.2.5O.sub.4) or LiGa.sub.5O.sub.8 (Li.sub.0.5Ga.sub.2.5O.sub.4) having P4.sub.332 (No. 212) or P4.sub.132 (No. 213) that is a structure in which A atoms and B atoms are highly ordered is also included in this layer. Moreover, they are not required to satisfy an atom ratio of A:B=1:2 that is a stoichiometric composition, and may have the above-described spinel structure or a symmetric property of a spinel-like structure as a long periodic structure.

(18) The second ferromagnetic layer 6 is provided adjacently on the barrier layer 5. This layer is a ferromagnetic Co-based Heusler alloy layer having a B2 structure similarly to the first ferromagnetic layer 4. A film thickness thereof ranges from about 1 nm to 8 nm. Between the first ferromagnetic layer 4 and the barrier layer 5, and between the barrier layer 5 and the second ferromagnetic layer 6, a layer having a thickness of about 1 nm or less can also be inserted for decreasing interfacial damage and improving an interface state. At this time, the layer to be inserted is limited to a layer that does not decrease effective spin polarization of the first ferromagnetic layer 4 and the second ferromagnetic layer 6. For this purpose, for example, Fe, CoFe alloys, Co-based Heusler alloys, Mg, MgAl alloys, Al and MgO are applicable.

(19) The upper electrode 7 is a layer that is provided adjacently on the second ferromagnetic layer 6. Typically, the upper electrode 7 is Ru. Also, Ta, Pt and the like can be used. Further, for applying unidirectional magnetic anisotropy into a film surface of the second ferromagnetic layer 6, an IrMn layer of an antiferromagnetic layer having a thickness of about 8 nm to about 15 nm can be inserted between the second ferromagnetic layer 6 and the upper electrode 7.

(20) The barrier layer 5 and its upper layers in the MTJ element film structure of the embodiment of the present invention can be used directly as a spin injection/spin detection part with respect to a semiconductor layer of the spin MOSFET. If using them for this purpose, on the semiconductor layer of Si or Ge, a barrier layer having a spinel structure, a Co-based Heusler alloy layer having a B2 structure and an upper electrode layer can be formed in this order.

(21) Hereinafter, a process for manufacturing an MTJ element film will be explained with reference to flowcharts. FIG. 2 is a flowchart that explains a method of manufacturing an MTJ element film according to an embodiment of the present invention. FIG. 3 is a flowchart that explains a detailed part of a process for forming a barrier layer.

(22) The underlayer is formed on a single crystal substrate, for example, MgO (001) (S100). For this film formation, for example, an ultrahigh vacuum direct current magnetron sputtering apparatus is used. As the underlayer, for example, Cr is used, and a thickness thereof preferably ranges from, for example, 20 nm to 100 nm, and particularly preferably ranges from 30 nm to 50 nm.

(23) For improving crystallinity and flatness of the underlayer, a Cr layer that is the underlayer is treated by heat after it is formed (S102). A heat treatment temperature has an appropriate range depending on an underlayer material, and, for example, the Cr underlayer is preferably heated at 400° C. to 900° C., and is particularly preferably heated at 600° C. to 800° C. The heat treatment is preferably carried out in a vacuum chamber, and a heat treatment time preferably ranges from about 3 minutes to about 3 hours, and particularly preferably ranges from about 30 minutes to about 1 hour. After the heat treatment, the underlayer is preferably cooled to a room temperature. In the case where the underlayer is made of a bi-layer film of Cr/CoFe, a Cr layer is formed by the above-described method, and a CoFe layer with a film thickness of, for example, 10 nm is thereafter manufactured from a Co.sub.50Fe.sub.50 composition target by sputtering at a room temperature.

(24) Subsequently, a first ferromagnetic layer is formed on the underlayer (S104). In the case of using a Co.sub.2FeAl layer as the first ferromagnetic layer, a Co.sub.2FeAl layer having a film thickness of, for example, 5 nm is manufactured from a Co.sub.50Fe.sub.25Al.sub.25 composition target by sputtering. Thereafter, the substrate on which the first ferromagnetic layer is formed is treated by heat (S104). For example, the heat treatment is carried out at 250° C. to 400° C. in a vacuum chamber for about 10 minutes to about 30 minutes, and then is cooled to a room temperature.

(25) Next, a barrier layer is formed (S106).

(26) Detail of this formation process will be described with reference to FIG. 3. Preferably, an Mg layer is firstly formed on the substrate, which is treated by heat in S104, at a room temperature (S120). This Mg layer is preferably formed by using, for example, direct current (DC) sputtering so as to have a film thickness of about a half of a single layer to about five layers of atoms (0.1 nm to 1.0 nm). Next, this laminated film is moved into a chamber for oxidation so as to form an MgAl alloy layer (S122). In this film formation, the MgAl alloy layer is formed from an Mg.sub.17Al.sub.83 alloy composition target by, for example, high frequency (RF) sputtering so as to have a film thickness of, for example, about three layers to about ten layers of atoms (0.5 nm to 2.5 nm). Incidentally, in the method of manufacturing the MTJ element film according to the present embodiment, the step S120 can be omitted. Further, in the step S120, as a replace of the Mg layer, an MgAl layer including a higher Mg content than that of the MgAl alloy layer, which is formed in the step S122, may be formed. Moreover, these Mg layer and MgAl layer may contain oxygen.

(27) Subsequently, the substrate on which this MgAl alloy layer is formed is treated by oxidation (S124). Firstly, mixed gas of argon and oxygen is introduced into a chamber for oxidation. Next, oxidation treatment is performed by inductively coupled plasma (ICP). Then, the Mg layer and the MgAl layer are made into combined oxide so as to form a barrier layer (S126). The oxidation treatment may be carried out also by a method of introducing pure oxygen at, for example, 5 Pa into a vacuum chamber and holding it for 1 minute to 1 hour so as to obtain the barrier layer as a natural oxidation film. When completing this oxidation treatment, the barrier layer has not a spinel structure but a disordered structure. After this oxidation treatment, heat treatment is preferably conducted at 250° C. to 300° C. in vacuum for about 10 minutes to about 30 minutes for promoting crystallization. After the heat treatment, the substrate is cooled to a room temperature (S128).

(28) Next, as shown in FIG. 2, a second ferromagnetic layer is formed on the substrate on which the barrier layer is formed (S108). As a layer that corresponds to the second ferromagnetic layer, for example, Co.sub.2FeAl (5 nm to 20 nm) is manufactured from an alloy target having a composition of Co.sub.2FeAl by direct current sputtering.

(29) Thereafter, this layer is treated by heat so as to change a structure of the barrier layer into a spinel structure. For this, the heat treatment is preferably carried out at a temperature of T.sub.CFA=450° C. to 600° C. for about 15 minutes to about 60 minutes (S110).

(30) Subsequently, after cooling the substrate to the room temperature, a protective film of IrMn (8 nm to 15 nm) or Ru (2 nm to 20 nm) is formed as a cap layer at a room temperature (S112). Incidentally, an upper electrode layer may be formed in addition.

(31) Hereinafter, with reference to FIGS. 4 to 9, characteristics of the barrier layer of the present embodiment and the MTJ element using the same will be described by way of following examples.

EXAMPLE 1

(32) Firstly, a method of manufacturing a Co-based Heusler alloy layer, which is the second ferromagnetic layer 6, adjacently on a spinel MgAl.sub.2O.sub.4 layer, which is the barrier layer 5, will be shown with reference to FIG. 4. As an example of the Co-based Heusler alloy layer, a case of a Co.sub.2FeAl layer will be described. Using an ultrahigh vacuum direct current magnetron sputtering apparatus, a Cr layer (40 nm) was manufactured as an underlayer on a single crystal MgO (001) substrate at a room temperature. For improving crystallinity and flatness, the Cr layer was treated by heat at 500° C. in a vacuum chamber for 1 hour after it was formed, and then was cooled to a room temperature. Thereon, a CoFe layer (10 nm) was manufactured from a Co.sub.50Fe.sub.50 composition target by sputtering, was subsequently treated by heat at 250° C. in a vacuum chamber for 15 minutes, and was cooled to a room temperature.

(33) Subsequently, Mg (0.45 nm) was formed at a room temperature, and this laminated film was moved into a chamber for oxidation. An MgAl alloy layer (0.75 nm) was formed from an Mg.sub.17Al.sub.83 alloy composition target by high frequency (RF) sputtering. A composition of the formed MgAl film was Mg.sub.19Al.sub.81. Thereafter, mixed gas of argon and oxygen was introduced into the chamber for oxidation so as to perform oxidation treatment by inductively coupled plasma (ICP), whereby the Mg layer and the MgAl layer are made into combined oxide so as to form a barrier layer. The gas introduced at this time was 5 Pa of argon gas and 1 Pa of oxygen gas. For forming the ICP, 5 W and 100 W of RF power were applied respectively to a 2-inch diameter sputtering target and a single coil disposed near the sputtering target. During the oxidation, a shutter is provided between the generated ICP and the sample, whereby influence of the ICP was weaken so as to adjust oxidation power. An oxidation time was 50 seconds. At this time, the barrier layer had not a spinel structure but a disordered structure.

(34) The laminated film was moved into the film formation chamber again, and was treated by heat at a temperature of 250° C. for 15 minutes. Subsequently, as a layer that corresponds to the second ferromagnetic layer, a Co.sub.2FeAl (10 nm) was manufactured from an alloy target having a composition of Co.sub.47.4Fe.sub.24.4Al.sub.28.2 by direct current sputtering. A composition of the manufactured Co.sub.2FeAl layer was found as Co.sub.49Fe.sub.26Al.sub.25 by composition analysis, and this composition was confirmed to be a substantial stoichiometric composition of Co.sub.2FeAl. Thereafter, this layer was treated by heat at a temperature of T.sub.CFA=250° C. to 600° C. for 15 minutes. Incidentally, the barrier layer is changed into a spinel structure at T.sub.CFA=450° C. to 550° C. approximately. Subsequently, after cooling the substrate to a room temperature, an Ru (2 nm) protective film was formed as a cap layer at a room temperature.

(35) FIG. 4 shows X-ray diffraction (XRD) patterns of the films manufactured at T.sub.CFA=250° C., 450° C. and 600° C., respectively. Incidentally, for easier visibility, the patterns of T.sub.CFA=450° C. and 600° C. were displayed to be shifted vertically upwards. Further, the MgAl.sub.2O.sub.4 layer and the Ru layer are as thin as 2 nm or less, and thus are not observed in this measurement. FIG. 4(a) shows XRD patterns in an out-of-plane direction. At each of the temperatures, a Co.sub.2FeAl (referred to as CFA) (002) peak is obtained, and this peak is a superlattice peak of a B2 structure. Thus, either of the Co.sub.2FeAl films have at least B2 structures. FIG. 4(b) illustrates a result of in-plane XRD scan, which is obtained by allowing X-ray to enter the MgO substrate in [110] orientation.

(36) Either of FIGS. 4(a) and 4(b) show that, since (002) and (200) peak intensity is increased by the temperature increase, B2 order is increased. Further, only a (00L) peak (L is an even number) is observed by the in-plane scan (FIG. 4(a)) and only an (L00) peak is observed from the formed layer by the out-of-plane scan (FIG. 4(b)), which suggests that all of the Cr layer, the CoFe layer, the MgAl.sub.2O.sub.4 layer, and the Co.sub.2FeAl layer are grown epitaxially in (001) orientation. Moreover, on a high angle region of the in-plane scan, a peak of CoFe (002) and CFA (004) that are overlapped, in addition to the Cr (002) peak, can be obtained. This corresponds to the fact that lattice constants of CoFe and Co.sub.2FeAl are almost equal.

(37) Further, it can be found that, in the scan at a higher temperature of T.sub.CFA=600° C., positions of these peaks become closer. This fact suggests that the Cr layer and the CoFe layer and others are interdiffused, and crystallinity of interfaces between the respective layers are decreased. Near 65° in FIG. 4(b), peaks of the three layers of Cr (200), CoFe (200) and Co.sub.2FeAl (400) appear being overlapped with each other. Thus, this fact suggests that lattice spacing within film surfaces of these layers are quite equal. Therefore, this suggest that in-plane lattice spacing of the MgAl.sub.2O.sub.4 layer formed between the CoFe layer and the Co.sub.2FeAl layer is also equal to them, and a lattice-matched structure can be obtained. Accordingly, it can be found that high crystallinity is exhibited consistently throughout from the Cr layer to the Co.sub.2FeAl layer. From the above result, it can be found that the Co.sub.2FeAl layer can be formed directly on the MgAl.sub.2O.sub.4 by the manufacturing method of the present invention, and is effective because it can be obtained as a flat layer with the high B2 order.

EXAMPLE 2

(38) Next, an example of forming an MTJ element by applying the method shown in Example 1 will be described with reference to FIG. 5.

(39) Using the same ultrahigh vacuum direct current magnetron sputtering apparatus as that in Example 1, a film having two layers of Cr (40 nm)/CoFe (5 nm) was manufactured as an underlayer on a single crystal MgO (001) substrate at a room temperature. After the formation of the Cr layer, heat treatment was carried out at 500° C. in vacuum. Thereon, Co.sub.2FeAl (5 nm) was manufactured as the first ferromagnetic layer by sputtering in the same method as that in Example 1. Thereafter, the Co.sub.2FeAl film was treated by heat at 250° C. similarly to the underlayer in order to improve the crystallinity of the Co.sub.2FeAl film, and was cooled to a room temperature. Subsequently, Mg (0.45 nm) was formed at a room temperature, and this laminated film was moved into a chamber for oxidation. Subsequently, a layer of Mg (0.45 nm)/MgAl alloy (0.75 nm) was formed in the same method as that in Example 1, and was thereafter treated by oxidation so as to form a barrier layer. Oxidizing conditions at this time were also the same as those in Example 1. Next, the above-described laminated film was moved into the film formation chamber again so as to be treated by heat at 250° C. for 15 minutes.

(40) Subsequently, Co.sub.2FeAl (6 nm) was formed as the second ferromagnetic layer, and this layer was treated by heat at a temperature of T.sub.CFA=250° C. to 500° C. for 15 minutes. Thereafter, as an upper electrode layer, a laminated film of CoFe (0.5 nm)/IrMn (12 nm)/Ru (10 nm) was formed at a room temperature, thereby manufacturing a spin valve-type MTJ element. At this time, the CoFe layer had a role of reinforcing a magnetic couple between the upper Co.sub.2FeAl layer and the IrMn layer. Further, the Ru was a protective film and served as a mask in microfabrication.

(41) Next, the manufactured multilayer film was taken out from the vacuum chamber, and a whole of the laminated film was treated by heat at a temperature of 175° C. in another vacuum chamber for 30 minutes, while a magnetic field of 5 kOe was applied thereto, whereby unidirectional anisotropy was added to the upper Co.sub.2FeAl layer. Thereafter, the laminated film was micromachined into a size of about 10 μm×5 μm by photolithography and Ar ion milling. In the micromachining, after forming a mask of a photoresist into a shape of the element, the laminated film from the upper Ru layer to the MgAl.sub.2O.sub.4 layer was etched by ion milling, thereby manufacturing a cylindrical element. Thereafter, a silicon oxide layer was formed as an interlayer insulation film, thereby electrically insulating the upper electrode and the lower electrode from each other. Subsequently, after lifting off the photoresist, electric wirings made of gold were formed respectively from the lower Cr layer and the upper Ru layer. Transport characteristics of the manufactured microfabricated element were evaluated by a direct current four-terminal method while changing an external magnetic field. At this time, a change in resistance value due to the external magnetic field was measured so as to determine a TMR ratio. The TMR ratio was defined as TMR ratio (%)=100×(R.sub.AP−R.sub.p)/R.sub.p, where resistance while magnetization of the two layers of Co.sub.2FeAl layer was parallel and resistance while the magnetization was antiparallel were denoted as R.sub.p and R.sub.AP, respectively.

(42) FIG. 5 illustrates heat treatment temperature T.sub.CFA dependence of a TMR ratio (upper figure) and a resistance-area (RA) value (lower figure), which is measured at a room temperature, after the formation of the upper Co.sub.2FeAl layer. A bias voltage used at this time was 10 mV or less. In the figure of the TMR ratio, values of Co.sub.2FeAl/MgO/Co.sub.2FeAl (Non-Patent Literature 2) and Co.sub.2FeAl/MgAl.sub.2O.sub.4/CoFe (Non-Patent Literature 3) are illustrated respectively by a dotted line and a dashed line as comparative examples. From these figures, it can be realized that the TMR ratio of the manufactured MTJ element was increased as T.sub.CFA increased. Further, the TMR ratio becomes higher than that of Co.sub.2FeAl/MgO/Co.sub.2FeAl at T.sub.CFA=400° C. or more (that of Co.sub.2FeAl/MgAl.sub.2O.sub.4/CoFe at T.sub.CFA=450° C. or more). The higher value was obtained due to the contribution of the two Co.sub.2FeAl layers that are adjacently on and below the MgAl.sub.2O.sub.4 layer. Thus, it shows that the use of the structure of Co.sub.2FeAl/MgAl.sub.2O.sub.4/Co.sub.2FeAl is effective. Further, the RA value is also increased according to the increase of T.sub.CFA, thereby showing that the interface structure between Co.sub.2FeAl and MgAl.sub.2O.sub.4 was changed at the same time.

EXAMPLE 3

(43) Next, an example of forming an MTJ element by applying the method described in Example 2 so as to improve a TMR ratio will be described with reference to FIGS. 6 to 9. Using the same apparatus and method as those in Example 2, a multilayer film of Cr (40 nm)/CoFe (5 nm)/lower Co.sub.2FeAl (5 nm) was formed on an MgO (001) substrate. Thereafter, a CoFe layer of 0.5 nm was formed by a direct current sputtering method. Immediately thereafter, the film was treated by heat at 250° C. in vacuum for 15 minutes, and was cooled to a room temperature. Subsequently, an MgAl.sub.2O.sub.4 barrier layer and an upper Co.sub.2FeAl layer were formed by the same method as those in Examples 1 and 2, and were treated by heat at a temperature of T.sub.CFA=550° C. in vacuum. After cooling to a room temperature, a laminated film of CoFe (0.5 nm)/IrMn (12 nm)/Ru (10 nm) was formed as an upper electrode layer by the same method as that in Example 2, thereby manufacturing a spin valve-type MTJ element. Then, after carrying out heat treatment at a temperature of 175° C. in a magnetic field for 30 minutes, a microfabricated element was manufactured, and TMR ratios were measured. Further, current-voltage characters were measured, and were then differentiated, thereby obtaining a differential conductance curve (dI/dV curve).

(44) FIG. 6 is a view that illustrates magnetic field dependence of TMR ratio of the MTJ element having the above-described structure of Co.sub.2FeAl/a CoFe insertion layer/MgAl.sub.2O.sub.4/Co.sub.2FeAl, which was measured at a room temperature (TMR curve). As the highest TMR ratio of this sample, 342% was obtained. Further, the TMR ratio measured at 4 K was 616%. This result indicates that, by inserting the very thin CoFe layer between the lower Co.sub.2FeAl layer and the MgAl.sub.2O.sub.4 layer, damage by the oxidation process can be decreased, so that the TMR ratio can be substantially improved. Since this TMR ratio is significantly higher than 308% that is the value obtained from the CoFe/MgAl.sub.2O.sub.4 (having a disordered spinel structure)/CoFe at a room temperature, the effectiveness of the MTJ element structure and the manufacturing method described in the embodiments of the present invention can be proved.

(45) FIG. 7 is a view showing a change of a differential conductance curve of the MTJ element having the above-described structure of Co.sub.2FeAl/CoFe insertion layer/MgAl.sub.2O.sub.4/Co.sub.2FeAl at a room temperature (300 K) and a low temperature (4 K) (a parallel magnetic configuration state). In this figure, a value of a zero bias voltage is normalized to be 1. Incidentally, for easier visibility, the curve at 4 K is displayed to be shifted vertically upwards. Such a differential conductance curve is known to reflect an electronic state between a barrier layer and a ferromagnetic layer, and a transport mechanism and crystal quality can be judged thereby. Both of the differential conductance curves at 300 K and 4 K are asymmetry with respect to a voltage direction. Further, in the curve at 4 K, minimal structures are observed near ±0.35 V as marked with solid line arrows, and maximal structures are observed near ±0.2 V as marked with dotted line arrows. Their asymmetricity means that states of the interfaces of the upper and lower barrier layers are different. Whereas, from the fact that the maximal and minimal structures are observed at substantially the equal positive and negative voltage positions, it can be considered that a specific electric structure is reflected to Co.sub.2FeAl, to a greater or lesser extent. Further, since a flat region is formed in a negative bias direction (a direction of an electron e flowing from the upper electrode to the lower electrode), it can be suggested that an electronic structure of the upper Co.sub.2FeAl layer is specific. Since such behavior of the differential conductance curve has been reported also in the case where an electrode is tunneled from a half metal layer having a bandgap in a minority-spin band, it is expected that the upper Co.sub.2FeAl layer is half-metallic. Whereas, since such a clear flat structure cannot be observed in a positive voltage direction, it can be considered that the lower Co.sub.2FeAl layer is not a half metal, and spin polarization thereof is lower than that of the upper layer. This is considered to indicate that the insertion of the CoFe layer weakened the characteristics of the lower Co.sub.2FeAl layer, and reduction of damage of the lower Co.sub.2FeAl interface by the oxidation process was not sufficient. The fact that the high TMR ratio exceeding 340% was nevertheless obtained at a room temperature also indicates a further room for improvement.

(46) In order to check the detailed crystal structures of the respective layers, a cross section of the above-described MTJ element film was observed by a scanning transmission electron microscope (STEM). FIG. 8(a) shows an image of a cross-sectional structure observed by an annular dark-field method (ADF-STEM), which is cut out from the MgO substrate of the above-described laminated film in [110] orientation, near the MgAl.sub.2O.sub.4 layer (referred to as a MAO layer in the figure). Epitaxial growth with completely matched lattice can be found from the lower Co.sub.2FeAl layer to the upper Co.sub.2FeAl layer. Further, both of the upper and lower interfaces of the MgAl.sub.2O.sub.4 layer are comparatively flat, and no clear defect can be found. Thus, lattice mismatch between the upper and lower Co.sub.2FeAl layers and the MgAl.sub.2O.sub.4 layer is smaller than 1%, which indicates that their lattices are matched.

(47) Next, FIG. 8(b) illustrates concentration profiles of respective elements in a region corresponding to FIG. 8(a), which are determined by energy dispersive X-ray spectroscopy (EDS). In the MgAl.sub.2O.sub.4 layer, the respective elements of Mg, Al and O can be observed, and a composition close to that of MgAl.sub.2O.sub.4 was obtained. Further, although Al in the upper and lower Co.sub.2FeAl layers was slightly decreased from their original compositions, degrees of interdiffusion in the respective layers were small, and it can be recognized that the interfaces between the respective layers can be defined. Moreover, since presence of the Al elements can be observed also in the lower CoFe insertion layer, which is shown as Int-CoFe in FIG. 8(a), it can be considered that a small amount of Al atoms were diffused and moved into the CoFe insertion layer due to the high heat treatment temperature (550° C.). Thus, it can be considered that the CoFe insertion layer on the lower interface contributed not only to a high oxidation damage suppressing effect but also to the TMR ratio which exhibited a higher effect as the composition shifted from CoFe to Co.sub.2FeAl.

(48) FIG. 9 illustrates a nano electron beam diffraction image. In the figure, FIG. 9(a) is an image inside the upper Co.sub.2FeAl layer, FIG. 9(b) is an image inside the MgAl.sub.2O.sub.4 layer, FIG. 9(c) is an image inside the lower Co.sub.2FeAl layer, FIG. 9(d) is an image near the interface between the MgAl.sub.2O.sub.4 layer/the upper Co.sub.2FeAl layer, and FIG. 9(e) is an image near the interface between the CoFe insertion layer/the MgAl.sub.2O.sub.4 layer. Further, in the figures, “C” and “M” described in spots denote diffraction spots corresponding to Co.sub.2FeAl and MgAl.sub.2O.sub.4, respectively. Firstly, from the result that a spot in a {022} plane meaning the formation of a spinel ordered structure (referred to as M022 and the like) can be observed in FIG. 9(b), it can be realized that the barrier layer has a spinel structure. Thus, in the case of using an ordinary CoFe ferromagnetic layer, the layer has a crystal structure, from which a high TMR ratio much more than 100% cannot be obtained easily. In FIGS. 9(a) and 9(c), {002} and {222} B2 superlattice spots can be observed in both of the upper and lower Co.sub.2FeAl layers (referred to as C002 and the like). Whereas, from the result that no {111} superlattice spot which represents an L2.sub.1 ordered structure can be observed at all in the Co.sub.2FeAl layers, it can be realized that the layer has a B2 structure. Further, from the result that B2 superlattice spots can be clearly observed also in the upper interface (d), it can be resulted in that the upper Co.sub.2FeAl interface had significantly high quality, and the high TMR ratio was able to be obtained. From the result that a B2 superlattice spot, which is relatively weak due to the insertion of the CoFe layer, can be observed in the lower interface (e), it can be considered that the lower Co.sub.2FeAl layer also contributed to realize the high TMR ratio.

(49) Also in the B2 alloy, use of the MgAl.sub.2O.sub.4 barrier having the spinel ordered structure causes the band folding effect similarly to the BCC-based alloy due to the difference of their lattice unit sizes, so that the spin polarization may be decreased significantly. However, if the alloy is a half metal, and can be used as ferromagnetic bodies for both of the upper and lower barrier layers, such decrease of the spin polarization cannot occur. Thus, the MTJ element structure of the present invention can solve the problem of the decrease in TMR ratio, which is caused by the use of the barriers having the spinel structure with the large lattice unit size and its similar structure, by sandwiching the barrier layer by the B2-type Co-based Heusler alloy layers that have the high spin polarization. Further, the MTJ element shown in Examples 2 and 3 exhibited the high TMR ratios, regardless of the suggestion that the lower Co.sub.2FeAl layer did not exhibit P=1, which was expected as a half metal, was obtained. Thus, it can be recognized that, even if one of the B2 alloy layers is not in the perfectly spin-polarized state, the MTJ element structure and the manufacturing method shown in the embodiments of the present invention are effective.

(50) Incidentally, Examples 1 to 3 have provided the example of forming the layer of Mg (0.45 nm)/MgAl alloy (0.75 nm) and subsequently treating the layer by oxidation so as to form the barrier layer, but it has been confirmed that similar results as those in the above-described examples can be obtained also in the case of omitting to form the Mg layer and forming an MgAl layer of 1.2 nm instead, and in the case of forming an Mg.sub.40Al.sub.60 layer with a higher Mg content than that of Mg.sub.19Al.sub.81 after the film formation as a replace of the Mg layer.

Comparative Example 1

(51) Next, an MgAl.sub.2O.sub.4 barrier in the case of limiting a manufacturing temperature of an MTJ element up to 375° C. will be described as a comparative example. After manufacturing Cr (40 nm)/CoFe (25 nm)/Co.sub.2FeAl (5 nm)/a CoFe insertion layer (1 nm) on an MgO (001) substrate by the same method as that in Example 3, a structure of Mg (0.45 nm)/MgAl (0.9 nm) was subsequently manufactured, and then, a barrier layer was manufactured by the same method as those in Examples 1 to 3. Thereafter, a CoFe (5 nm) layer was manufactured by sputtering, and was subsequently treated by heat at 300° C. in vacuum. Then, after forming IrMn (12 nm)/Ru (7 nm) as an upper electrode by the same method as those in Examples 2 and 3, the sample was taken out. Subsequently, the sample was treated by heat at 375° C. in vacuum in a magnetic field for 30 minutes.

(52) FIG. 10(a) illustrates an ADF-STEM image of the formed MTJ element. Observation direction was the same as that of FIG. 8(a). Further, FIG. 10(b) shows an NBD image of MgAl.sub.2O.sub.4. In FIG. 10(a), an MgAl.sub.2O.sub.4 interface with high-quality epitaxial growth and completely matched lattices can be observed. Whereas, in FIG. 10(b), no {022} spot that shows a spinel ordered structure can be observed at all. Thus, it can be realized that the MgAl.sub.2O.sub.4 barrier manufactured at the low temperature had a disordered structure. Further, this element exhibited a TMR ratio as high as 280% at room temperature, and this is because the barrier layer thereof had such a disordered structure. Since spinel ordering is promoted at a temperature higher than about 400° C., for obtaining a TMR ratio exceeding 250% at a room temperature under such manufacturing conditions, B2-type Heusler alloy having high spin polarization is necessary to be formed not only for the lower ferromagnetic layer but also for the upper ferromagnetic layer.

Comparative Example 2

(53) Next, an example of using MgO instead of MgAl.sub.2O.sub.4 as the barrier layer of the MTJ element will be described as a comparative example (Non Patent Literature 4). After manufacturing a Cr (40 nm) underlayer on an MgO (001) substrate, a Co.sub.2FeAl layer of 30 nm was manufactured by an RF sputtering method. Thereafter, heat treatment was carried out at 480° C. in a vacuum chamber for 15 minutes, and was subsequently cooled to a room temperature. Then, an MgO layer of 1.8 nm was formed by an RF sputtering method using MgO as a target. Subsequently, a Co.sub.2FeAl layer of 5 nm was formed by the above described method. Then, after forming IrMn (12 nm)/Ru (7 nm) as the upper electrode, the sample was taken out, and was treated by heat at 450° C. in vacuum in a magnetic field for 60 minutes. After microfabrication, TMR ratios were measured at a room temperature and at a low temperature (15 K), and were 223% and 415%, respectively. These values are comparatively smaller than 342% at room temperature and 616% at the low temperature (4 K), which were measured in the case of using the MgAl.sub.2O.sub.4 barrier in Example 3. Only the smaller values were obtained because lattice mismatch of 3.8% between the Co.sub.2FeAl and the MgO inhibited the growth of the Co.sub.2FeAl layer with high crystallinity directly on the MgO.

(54) Incidentally, the above explanation has been provided as the embodiments of the present invention, but the present invention is not limited to them. For example, in the case of stacking the barrier layer and the high spin-polarized magnetic layer of the present invention on an Si substrate or a Ge substrate particularly for the application to spin MOSFET, if using the nonmagnetic material having the spinel structure of the present invention as a barrier, a magnetic layer with less lattice distortion can be grown, so that it is consequently possible to structure a spin injection element that can inject spin from the magnetic layer into Si or Ge efficiently.

INDUSTRIAL APPLICABILITY

(55) The MTJ element of the present invention can be applied not only to a magnetic head for a hard disk and an MRAM, but also to many kinds of spintronics devices including a spin resonance tunnel device made of a magnetic double tunnel junction, a spin logic device using a spin MOSFET and the like that requires efficient spin injection into a semiconductor, and high performance magnetic sensors.

(56) Further, the MTJ element of the present invention can be utilized as a head sensor of a hard disk drive apparatus due to its high TMR ratio, whereby development of a hard disk drive apparatus with high capacity is made possible. Moreover, since the present invention can provide a layer of a barrier/half metal that can inject spin into a semiconductor layer with high efficiency, the MTJ element of the present invention can be used as a core of a spin MOSFET, whereby development of a non-volatile logic LSI circuit is made possible.

REFERENCE SIGNS LIST

(57) 1 MTJ element film

(58) 2 substrate

(59) 3 underlayer

(60) 4 first ferromagnetic layer

(61) 5 barrier layer

(62) 6 second ferromagnetic layer

(63) 7 upper electrode