JP7226710B2 - Magnetoresistive element and method for manufacturing the same - Google Patents

Description

The present invention relates to a magnetoresistive element and its manufacturing method.

Giant magnetoresistive (GMR) elements composed of multilayer films of ferromagnetic layers and nonmagnetic layers, and tunnel magnetoresistive (TMR) elements using insulating layers (tunnel barrier layers, barrier layers) as nonmagnetic layers are known. there is In general, a TMR element has a higher element resistance than a GMR element, but a larger magnetoresistance (MR) ratio. Attention is focused on TMR elements as elements for magnetic sensors, high-frequency components, magnetic heads, and non-volatile random access memories (MRAM).

TMR elements can be classified into two types according to the difference in electron tunnel conduction mechanism. One is a TMR element that utilizes only the seepage effect (tunnel effect) of the wave function between ferromagnetic layers. The other is TMR dominated by coherent tunneling (only electrons with specific symmetry of the wave function tunnel) using the conduction of specific orbitals in the non-magnetic insulating layer that tunnel when the tunnel effect occurs. element. It is known that a TMR element in which coherent tunneling is dominant provides a higher MR ratio than a TMR element using only the tunnel effect.

In order to obtain a coherent tunnel effect in a magnetoresistive element, the two ferromagnetic metal layers and the tunnel barrier layer must be crystalline with each other, and the interface between the two ferromagnetic metal layers and the tunnel barrier layer must be crystallographically continuous. must be

MgO is widely known as a tunnel barrier layer capable of obtaining a coherent tunnel effect. In addition, investigations of materials to replace MgO are underway. For example, by using a ternary oxide (Mg—Al—O) composed of Mg, Al, and O, it is possible to improve lattice matching with the ferromagnetic material compared to MgO. This is known to be more preferable in terms of application because it leads to a higher MR ratio output (lower bias voltage dependence) than conventional MgO when a higher bias voltage is applied.

For example, Patent Document 1 proposed by the applicant of the present application discloses that MgAl ₂ O ₄ having a spinel crystal structure can be used as a tunnel barrier layer. However, since the composition region in which the spinel structure is stable is extremely limited, there is a problem that the applicable range is narrow.

On the other hand, in Patent Document 2 proposed by the present applicant, a ternary system having a cubic crystal (cation disordered spinel structure) having a structure different from the spinel structure and having half the lattice constant of the spinel structure An oxide (Mg--Al--O) is described. Since the cation disordered spinel structure is a metastable structure, the tunnel barrier layer can be constructed without being limited to the stoichiometry of the spinel type structure. Therefore, the lattice constant can be changed continuously by adjusting the Mg-Al composition ratio over a wide range, and it is expected that the lattice matching of the TMR element can be further improved. As a result, for example, the bias voltage dependence of the MR ratio can be further reduced.

Furthermore, in Patent Document 2, by combining a cation disordered spinel structure barrier and a BCC type Co—Fe ferromagnetic electrode, the occurrence of a physical mechanism called a band folding effect is suppressed, and a large MR ratio is achieved. It is described that it can be stably obtained.

Japanese Patent No. 5586028 Japanese Patent No. 5988019 JP 2017-183355 A

However, no method has been established to stably obtain a metastable cationically disordered spinel structure as a very thin barrier layer on the order of nanometers. For example, regarding the oxygen composition, Patent Document 2 states that "the cationic disordered spinel structure may have oxygen deficiency or excess", and the relationship with the crystal structure is not clear. For this reason, there is a practical problem that it is necessary to distinguish the crystal structure of the tunnel barrier layer for each device using advanced fine structure analysis technology that requires a long inspection time and a high inspection cost. This challenge stems from the fact that the requirements necessary to obtain disordered structures have not been clarified.

In view of such circumstances, an object of the present invention is to stably obtain a cation disordered spinel structure as a tunnel barrier layer of a TMR element.

The present inventors have produced TMR elements having Mg--Al--O tunnel barriers under various conditions, and in the process of microstructural analysis, found that there is a strong correlation between the oxygen content and the crystal structure of the tunnel barrier layer. rice field. In particular, it was found that the cationic disordered spinel structure can be stably obtained in the case of an oxygen-deficient state, rather than the predetermined amount of oxygen expected from the amounts of Mg and Al. Further, for example, in the post-oxidation shown in Patent Document 2 proposed by the present applicant and the multi-stage post-oxidation shown in Patent Document 3 proposed by the present applicant, the Mg—Al composition ratio We have found that a cationically disordered spinel structure can be fabricated without stress.
That is, the present invention provides the following means in order to solve the above problems.

(1) A magnetoresistive element according to a first aspect includes a first ferromagnetic layer, a second ferromagnetic layer, and a tunnel barrier layer sandwiched therebetween, the tunnel barrier layer comprising: An oxide of Mg _x Al _1-x (0≦x<1), wherein the amount of oxygen in the tunnel barrier layer is the amount of oxygen in a completely oxidized state in which the oxide has an ordered spinel structure lower than

(2) In the magnetoresistive element according to the above aspect, the amount of oxygen in the tunnel barrier layer may be 67% or more of the amount of oxygen in the fully oxidized state.

(3) In the magnetoresistive element according to the aspect described above, the amount of oxygen in the tunnel barrier layer may be 67% or more and 95% or less of the amount of oxygen in the fully oxidized state.

(4) In the magnetoresistive element according to the above aspect, the amount of oxygen in the tunnel barrier layer may be 77% or more and 95% or less of the amount of oxygen in the fully oxidized state.

(5) In the magnetoresistive element according to the above aspect, the oxide may be represented by a composition formula Mg _α Al _β O _γ , where α satisfies 0≦α≦0.41.

(6) In the magnetoresistive element according to the above aspect, α in the composition formula may satisfy 0.13≦α≦0.41.

(7) In the magnetoresistive element according to the aspect described above, crystals that mainly constitute the tunnel barrier layer may be (001) oriented.

(8) In the magnetoresistive element according to the aspect described above, at least one of the first ferromagnetic layer and the second ferromagnetic layer may contain Fe element.

(9) In the magnetoresistive element according to the aspect described above, the tunnel barrier layer may have a thickness of 3 nm or less.

(10) A method of manufacturing a magnetoresistive effect element according to a second aspect is a method of manufacturing a magnetoresistive effect element according to the above aspect, comprising: preparing a target of the oxide; and laminating a barrier layer.

(11) A method for manufacturing a magnetoresistive effect element according to a third aspect is a method for manufacturing a magnetoresistive effect element according to the above aspect, wherein the composition formula is Mg _x Al _1-x (where 0≦x<1) and an oxidation step of oxidizing the alloy to form a tunnel barrier layer.

(12) In the method for manufacturing a magnetoresistive effect element according to the third aspect, the film forming step uses an alloy represented by a composition formula Mg _x Al _1-x (where 0≦x<0.5). It is a step of forming a film, and the film forming step and the subsequent oxidation step may be performed only once.
(13) In the method for manufacturing a magnetoresistive effect element according to the third aspect, the film forming step uses an alloy represented by a composition formula Mg _x Al _1-x (where 0.5≦x<1). It is a step of forming a film, and the film forming step and the subsequent oxidation step may be repeated multiple times.

(14) The method for manufacturing a magnetoresistive element according to the third aspect may include a step of forming a Mg film after performing the film forming step and the oxidizing step.

(15) A method for manufacturing a magnetoresistive effect element according to a fourth aspect is a method for manufacturing a magnetoresistive effect element according to the above aspect, wherein the composition formula is Mg _x Al _1-x (where 0≦x<1) has a step of forming a film in a metal mode or a transition mode while oxidizing the alloy represented by the reactive film formation.
(16) In the method for manufacturing a magnetoresistive element according to the fourth aspect, after the step of forming a film in a metal mode or transition mode while being oxidized by reactive film formation, a step of forming a Mg film is provided. You may have

According to the present invention, a cationic disordered spinel structure can be stably obtained.

It is a cross-sectional schematic diagram of the magnetoresistive effect element concerning this embodiment. It is a figure which shows the crystal structure of a spinel structure. FIG. 2 shows the crystal structure of a disordered spinel structure; It is the result of also performing nano electron diffraction (NBD) on the tunnel barrier layer according to Comparative Example 1 using a transmission electron microscope (TEM). It is the result of also performing nano electron diffraction (NBD) on the tunnel barrier layer according to Example 1 using a transmission electron microscope (TEM).

Hereinafter, the present invention will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, there are cases where characteristic portions are enlarged for convenience in order to make it easier to understand the features of the present invention, and the dimensional ratios of each component may differ from the actual ones. be. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be implemented with appropriate modifications without changing the gist of the invention.

"Magnetoresistive element"
FIG. 1 is a schematic cross-sectional view of a magnetoresistive element according to this embodiment. A magnetoresistive element 10 shown in FIG. 1 includes a first ferromagnetic layer 1 , a second ferromagnetic layer 2 and a tunnel barrier layer 3 . In addition to these layers, the magnetoresistive element 10 may have a cap layer, an underlying layer, and the like.

(First ferromagnetic layer, second ferromagnetic layer)
The first ferromagnetic layer 1 and the second ferromagnetic layer 2 have magnetization. The magnetoresistive element 10 outputs these relative angular changes in magnetization as resistance value changes. For example, when the magnetization direction of the second ferromagnetic layer 2 is fixed and the magnetization direction of the first ferromagnetic layer 1 is made variable with respect to the magnetization direction of the second ferromagnetic layer 2, the first ferromagnetic layer 1 The resistance value of the magnetoresistive effect element 10 is changed by changing the magnetization direction of . A layer whose magnetization direction is fixed is generally called a fixed layer, and a layer whose magnetization direction is variable is generally called a free layer. An example in which the first ferromagnetic layer 1 is a free layer and the second ferromagnetic layer is a fixed layer will be described below.

A ferromagnetic material can be used for the first ferromagnetic layer 1 and the second ferromagnetic layer 2 . For example, a metal selected from the group consisting of Cr, Mn, Co, Fe and Ni, an alloy containing one or more metals selected from these groups, or one or more metals selected from these, and B , C, and N and at least one of the elements. In particular, the first ferromagnetic layer 1 and the second ferromagnetic layer 2 preferably contain Fe elements. Since the first ferromagnetic layer 1 and the second ferromagnetic layer 2 contain the Fe element, the spin polarization can be increased, and the MR ratio of the magnetoresistance effect element 10 can be increased. For example, specific examples of the first ferromagnetic layer 1 and the second ferromagnetic layer 2 containing Fe include Fe, Co--Fe, Co--Fe--B, and Ni--Fe.

A Heusler alloy such as Co ₂ FeSi can be used for the first ferromagnetic layer 1 and the second ferromagnetic layer 2 . A Heusler alloy has a high spin polarization and can achieve a high MR ratio. A Heusler alloy comprises an intermetallic compound with a chemical composition of X ₂ YZ. X is a transition metal element or noble metal element of the Co, Fe, Ni, or Cu group on the periodic table. Y can be selected from Mn, V, Cr, Ti-group transition metals, or X element species. Z is a typical element of groups III to V. For example, Co ₂ FeSi, Co ₂ MnSi, Co ₂ Mn _1-a Fe _a Al _b Si _1-b , etc. can be used.

When the magnetization directions of the first ferromagnetic layer 1 and the second ferromagnetic layer 2 are to be perpendicular to the lamination plane, the thickness is preferably 3 nm or less. Perpendicular magnetic anisotropy is added to the first ferromagnetic layer 1 and the second ferromagnetic layer 2 at the interface with the tunnel barrier layer 3 . Since the effect of the perpendicular magnetic anisotropy is attenuated by increasing the thickness of the first ferromagnetic layer 1 and the second ferromagnetic layer 2, the thickness of the first ferromagnetic layer 1 and the second ferromagnetic layer 2 is Thinner is preferred.

In order to use the second ferromagnetic layer 2 as a pinned layer, the coercive force of the second ferromagnetic layer 2 is made larger than that of the first ferromagnetic layer 1 . If an antiferromagnetic material such as IrMn or PtMn is adjacent to the second ferromagnetic layer 2, the coercive force of the second ferromagnetic layer 2 increases, for example. Further, in order to prevent the leakage magnetic field of the second ferromagnetic layer 2 from affecting the first ferromagnetic layer 1, a synthetic ferromagnetic coupling structure may be employed.

(tunnel barrier layer)
The tunnel barrier layer 3 is an oxide of Mg _x Al _1-x . This oxide is represented by the composition formula Mg _α Al _β O _γ . When the oxide has a stoichiometric composition, it satisfies α+β+γ=1 and maintains electrical neutrality. When the oxide is fully oxidized, the tunnel barrier layer becomes an ordered spinel structure. FIG. 2 is a diagram schematically showing the crystal structure of a spinel structure.

Here, the fully oxidized state means that the oxide contains a predetermined amount or more of oxygen. For example, there is a stoichiometric composition as one of indices indicating the content of oxygen. The ideal value of γ in the stoichiometric composition is determined so as to satisfy the electroneutrality condition between ionized Mg and Al (Mg ²⁺ and Al ³⁺ respectively) and oxygen ions O ²⁻ . Therefore, γ in the stoichiometric composition changes depending on the Mg _x Al _1-x composition before oxidation treatment. For example, when the Mg ₃₃ Al ₆₇ alloy layer is completely oxidized and becomes an oxide layer with a stoichiometric composition, the composition formula is MgAl ₂ O ₄ , α=0.14, β=0.29, γ=0. 57. Also, when the pure Al layer is completely oxidized to form an oxide layer with a stoichiometric composition, the compositional formula is Al ₂ O ₃ where α=0, β=0.4, and γ=0.6.

If the oxide satisfies the stoichiometric composition, the oxide is theoretically fully oxidized. However, due to the accuracy of the analysis equipment, etc., there are cases in which even if the composition-analyzed oxide satisfies the stoichiometric composition, it is not completely oxidized. Therefore, the case where the following relationship is satisfied as an actual measurement value is defined as a completely oxidized state.
γ≧{(2×α+3×β)/2}×1.12

The tunnel barrier layer 3 according to the present embodiment has a smaller oxygen content than the oxygen content γ in the fully oxidized state of the tunnel barrier layer 3, and the tunnel barrier layer 3 is substantially devoid of oxygen. When the tunnel barrier layer 3 satisfies the relationship, the crystal structure of the tunnel barrier layer 3 becomes a cation-disordered spinel structure (hereinafter sometimes referred to as "disordered spinel structure"). Here, the term "substantially lacking" includes not only the state in which oxygen vacancies exist in the oxide crystal, but also the state in which some of the metal atoms of Mg and Al remain unoxidized. .

The amount of oxygen in the tunnel barrier layer 3 is preferably 67% or more and 95% or less, more preferably 77% or more and 95% or less, of the amount γ of oxygen in the substantially completely oxidized state. When the amount of oxygen γ is within the former range, the crystal structure of the tunnel barrier layer 3 is stabilized with a disordered spinel structure even when α=0.

The amount of Mg in the tunnel barrier layer 3 preferably satisfies 0≦α≦0.41, and more preferably satisfies 0.13≦α≦0.41. When the range of α is within this range, the crystal structure of the tunnel barrier layer 3 becomes more stable with a disordered spinel structure.

FIG. 3 is a diagram schematically showing the crystal structure of a cationic disordered spinel structure. As shown in FIG. 2, in the case of the ordered spinel structure, the site into which the Mg element is ionized and the site into which the Al element is ionized are fixed. Therefore, the arrangement of these elements becomes regular. In contrast, in the case of the disordered spinel structure, the Mg element or the Al element can exist at both the tetrahedral and octahedrally coordinated sites with respect to oxygen shown in FIG. A disordered spinel structure corresponds to a case where it can be regarded as random which site the Mg element and the Al element enter. The lattice constant (a/2) of this disordered spinel structure is half the lattice constant (a) of the ordered spinel structure. The disordered spinel structure has Fm-3m space group symmetry or F-43m space group symmetry.

When the composition of the tunnel barrier layer 3 is completely oxidized, the spinel structure in which the crystal structure of the tunnel barrier layer 3 is ordered is the most stable. On the other hand, when the oxygen content of the tunnel barrier layer 3 is less than the fully oxidized state, the crystal structure of the tunnel barrier layer 3 becomes more stable with an irregular spinel structure. The amount γ of oxygen in the tunnel barrier layer 3 is preferably 67% or more of the amount of oxygen in the fully oxidized state (substantially stoichiometric composition) of the oxide. When the tunnel barrier layer 3 satisfies this relationship, even when the amount of magnesium in the tunnel barrier layer 3 is 0, the crystal structure of the tunnel barrier layer 3 becomes more stable with an irregular spinel structure.

Although the reason why the crystal structure of the tunnel barrier layer 3 is easily stabilized in the disordered spinel structure when the composition is oxygen-deficient, it is considered as follows.

Whether the Mg element or the Al element is in the tetrahedral coordination site or the octahedral coordination site with respect to oxygen is greatly affected by the energy potential. When a sufficient amount of oxygen is supplied, the sites where each of the Mg element and the Al element are stabilized are fixed from the viewpoint of energy potential. In addition, due to the action of the ionic radii of Mg ²⁺ and Al ³⁺ and the coulombic repulsive force, the energy becomes the lowest when these ions are regularly arranged, so that the ordered spinel structure is stable. Therefore, it is difficult to obtain a disordered spinel structure as the tunnel barrier layer 3 .

On the other hand, in the oxygen-deficient tunnel barrier layer 3, oxygen elements shown in FIGS. 2 and 3 are deficient from predetermined positions. When the oxygen element is deficient, the crystal structure is disturbed because the element responsible for the crystal lattice is removed. When the crystal structure is disturbed, the energy states at the tetrahedral and octahedrally coordinated sites for oxygen are also disturbed. When the energy state is disturbed, the Mg element, which should be stabilized at the tetrahedral coordination sites with respect to oxygen, is stabilized at the octahedral coordination sites with respect to oxygen, and vice versa. Moreover, in the ordered spinel structure, tetrahedral coordination sites and octahedral coordination sites, which are vacancies, may also be occupied. In other words, which site the Mg element and the Al element enter becomes totally random, and as a result, the more disordered spinel structure is easily stabilized.

The tunnel barrier layer 3 preferably has a thickness of 3 nm or less. When the thickness of the tunnel barrier layer 3 is 3 nm or less, the wave functions of the first ferromagnetic layer 1 and the second ferromagnetic layer 2 tend to overlap beyond the tunnel barrier layer 3, and the tunnel effect of the wave function between the ferromagnetic layers and A coherent tunnel effect can be easily obtained.

Also, the crystals that mainly constitute the tunnel barrier layer 3 are preferably (001) oriented. If the tunnel barrier layer 3 is a (001)-oriented single crystal, the "crystal mainly constituting the tunnel barrier layer 3" is a polycrystal, and the polycrystal is a (001)-oriented crystal. Includes any case where it includes primarily. When the tunnel barrier layer 3 is (001) oriented, the lattice matching with the first ferromagnetic layer 1 and the second ferromagnetic layer 2 is improved, and the coherent tunnel effect is easily obtained. In particular, when the first ferromagnetic layer 1 or the second ferromagnetic layer 2 contains Fe element, such as Fe, Co--Fe, Co-based Heusler alloy, etc., the lattice matching is enhanced.

As described above, the magnetoresistive element according to this embodiment has a spinel structure in which the crystal structure of the tunnel barrier layer 3 is disordered because the tunnel barrier layer 3 has a predetermined composition. A disordered spinel structure has an effective lattice constant halved compared to an ordered spinel structure, which can result in a higher MR ratio. This is because the energy band structure of the disordered spinel structure is similar to that of the ordered spinel structure and can maintain the coherent tunneling effect. That is, by making the crystal structure of the tunnel barrier layer 3 disordered spinel structure, the coherent tunnel effect and high lattice matching can be maintained, and the MR ratio of the magnetoresistance effect element 10 is increased.

The magnetoresistive element according to this _{embodiment has a composition ratio of Mg z Al 1- z} ( It may have a barrier underlayer consisting of 0≦z≦1). The barrier underlayer becomes part of the tunnel barrier layer which is an oxide of Mg _x Al _1-x by a subsequent heat treatment.
A Mg layer may be provided on the surface of the Mg _x Al _1-x oxide layer opposite to the barrier underlayer. The thickness of the Mg layer can be, for example, 0.1-1 nm. The Mg layer becomes part of the tunnel barrier layer which is an oxide of Mg _x Al _1-x by subsequent heat treatment.

(element shape and dimensions)
A laminate composed of the first ferromagnetic layer 1, the tunnel barrier layer 3 and the second ferromagnetic layer 2 constituting the magnetoresistive element 10 has a columnar shape. The shape of the laminated body in a plan view can take various shapes such as circular, square, triangular, and polygonal, but a circular shape is preferable from the viewpoint of symmetry. In other words, it is preferable that the laminate has a columnar shape.

When the laminate has a cylindrical shape, the diameter in plan view is preferably 80 nm or less, more preferably 60 nm or less, and even more preferably 30 nm or less. When the diameter is 80 nm or less, it becomes difficult to form a domain structure in the ferromagnetism, and there is no need to consider a component different from the spin polarization in the ferromagnetic metal layer. Furthermore, when the thickness is 30 nm or less, a single domain structure is formed in the ferromagnetic layer, and the magnetization reversal speed and probability are improved. In addition, there is a strong demand for a reduction in the resistance of a miniaturized magnetoresistive element.

(others)
In this embodiment, the magnetoresistive element 10 has a top-pin structure in which the first ferromagnetic layer 1 is the free layer and the second ferromagnetic layer 2 is the fixed layer. However, the structure of the magnetoresistive element 10 is not limited to this case, and may be a bottom pin structure.

A magnetoresistive element using this embodiment can be used as a memory such as a magnetic sensor or MRAM.

"Manufacturing method of magnetoresistive element"
Next, a method for manufacturing the magnetoresistive element will be described.
The method for manufacturing a magnetoresistive element according to the present embodiment has a step of laminating a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer. As a method for forming these layers, known methods such as a sputtering method, a vapor deposition method, a laser ablation method, and a molecular beam epitaxial (MBE) method can be used.

The tunnel barrier layer can be produced, for example, by any of the following three methods.
A first method prepares an oxide target having the same composition as the tunnel barrier layer to be fabricated, and uses the target to stack the tunnel barrier layer. Since this method prepares a target with a predetermined composition from the beginning, it is easy to make the composition of the tunnel barrier layer oxygen-deficient.

In the second method, first, an alloy represented by Mg _x Al _1-x (where 0≦x<1) is laminated, and the alloy is oxidized to form a tunnel barrier layer. Oxidation is performed by plasma oxidation or oxygen introduction. At this time, the flow rate of oxygen to be introduced, the pressure in the oxidation treatment chamber, and the oxidation time are controlled so that the finally obtained tunnel barrier layer is in an oxygen-deficient state, and the amount of oxygen relative to the alloy is adjusted to perform oxidation. . By performing this treatment, the tunnel barrier layer becomes oxygen-deficient, and the crystal structure of the tunnel barrier layer becomes an irregular spinel structure.

In the second method, the step of forming an alloy represented by the composition formula Mg _x Al _1-x (where 0≦x<0.5) and then the step of oxidizing the alloy are each performed only once. you can go
In the second method, the step of forming a film of an alloy represented by the composition formula Mg _x Al _1-x (where 0.5≦x<1) and the subsequent step of oxidizing the alloy are performed by: May be repeated multiple times.

In the third method, oxygen is introduced into the chamber during film formation of the MgAl alloy, and the layers are laminated while being oxidized by reactive film formation. Also in this case, the film is formed while adjusting the oxidizing power to the alloy so that the finally obtained tunnel barrier layer is in an oxygen-deficient state. Specifically, the oxidizing power to be supplied is made smaller than the oxidizing power sufficient to oxidize the alloy to be deposited. By performing this treatment, the tunnel barrier layer becomes oxygen-deficient, and the crystal structure of the tunnel barrier layer becomes an irregular spinel structure.
When the third method is used to deposit an alloy represented by the composition formula Mg _x Al _1-x (where 0≦x<1) while being oxidized by reactive deposition, metal mode or transition mode It is preferable to form the film at .
In the reactive film formation, the film formation rate varies greatly depending on the flow rate of the introduced reactive gas (oxygen). A state in which the flow rate of oxygen is low and the target material is scattered in the state of metal and the deposition rate is high is called the metal mode. A state in which the flow rate of oxygen is large and the target material scatters as a compound while reacting with oxygen is slow is called a reactive (oxidation) mode. A region in which the deposition rate is between the metal mode and the oxidation mode is called a transition mode.

Before depositing the MgAl alloy in the second method and the third method, from the viewpoint of protection of the first ferromagnetic layer, a barrier underlayer having a composition ratio of Mg _z Al _1-z (0 ≤ z ≤ 1) may be laminated. After a subsequent heat treatment, the barrier underlayer becomes part of the tunnel barrier layer, which is an oxide of Mg _x Al _1-x .

In the first method, the second method, and the third method, the Mg layer may be formed by depositing Mg after forming the layer made of the oxide of Mg _x Al _1-x . The Mg layer becomes part of the tunnel barrier layer which is an oxide of Mg _x Al _1-x by subsequent heat treatment.

As described above, according to the method of manufacturing the magnetoresistive effect element according to the present embodiment, the tunnel barrier layer having the predetermined composition can be easily manufactured. Further, since the tunnel barrier layer has a predetermined composition, the crystal structure of the tunnel barrier layer becomes a disordered spinel structure, and a magnetoresistive element having a large MR ratio can be manufactured.

(Comparative example 1)
A magnetoresistive element 10 shown in FIG. 1 was fabricated on an MgO (001) single crystal substrate. First, a 40 nm Cr layer was deposited as an underlayer on the substrate, and heat treatment was performed at 800° C. for 1 hour. As the first ferromagnetic layer 1, 30 nm of Fe was laminated and heat-treated at 300° C. for 15 minutes.

Next, on the first ferromagnetic layer 1, a barrier underlayer made of Mg was formed to a thickness of _{0.2 nm} . Two treatments were performed. As a result, an oxide film with a total thickness of 0.8 nm was obtained. Natural oxidation was performed by exposing to air at a pressure of 5 Pa for 600 seconds. By performing this repeated oxidation, the MgAl layer was sufficiently oxidized. After oxidation, heat treatment was performed in vacuum at 400° C. for 15 minutes to obtain a homogeneous Mg—Al—O layer.

Next, on the tunnel barrier layer 3, Fe was laminated as the second ferromagnetic layer 2 with a thickness of 6 nm, and heat treatment was performed at 350° C. for 15 minutes to obtain a ferromagnetic tunnel junction. Next, a film of IrMn having a thickness of 12 nm was formed as an antiferromagnetic layer, and a film of Ru having a thickness of 20 nm was formed as a cap layer to obtain the magnetoresistive element 10 . Finally, while applying a magnetic field of 5 kOe, heat treatment was performed at a temperature of 175° C. for 30 minutes to impart uniaxial magnetic anisotropy to the second ferromagnetic layer 2 .

Then, the fabricated magnetoresistive element 10 was cut with a focused ion beam along a plane along the stacking direction to fabricate a thin sample of the tunnel barrier layer. Then, composition analysis of this flake sample was performed by energy dispersive X-ray spectroscopy (EDS) in a transmission electron microscope (TEM). As a result, the composition of the Mg--Al--O layer was Mg _0.18 Al _0.23 O _0.59 . In the case of α=0.18 and β=0.23, if γ≧0.59, it can be said that it is in a completely oxidized state. That is, Comparative Example 1 is in a completely oxidized state. The analysis method is not limited to this, and secondary ion mass spectroscopy (SIMS), atom probe method, and electron energy loss spectroscopy (EELS) can also be used.

The TEM-EDS analysis results were obtained by subtracting the background signals of the measured elements (Mg, Al, and O).

Nanoelectron diffraction (NBD) was also performed using a transmission electron microscope (TEM). Specifically, a thin sample was irradiated with an electron beam narrowed down to a diameter of about 1 nm, and an electron beam spot subjected to transmission diffraction was measured. FIG. 4 shows the results of electron beam incidence in the Mg--Al--O [100] direction.

(Example 1)
Example 1 differs from Comparative Example 1 in that oxygen deficiency was caused when the Mg--Al--O layer was produced. Specifically, first, a barrier underlayer made of Mg was formed to a thickness of 0.2 nm. Thereafter, 0.5 nm of an alloy represented by Mg ₁₇ Al ₈₃ was layered at a time, and an oxidation treatment was performed under the same conditions as in Comparative Example 1. The oxidation conditions are oxygen-deficient oxidation conditions. After oxidation, heat treatment was performed in vacuum at 400° C. for 15 minutes to obtain an Mg—Al—O layer. Other conditions and analysis method were the same as in Example 1.

The composition of the Mg--Al--O layer of Example 1 was Mg _0.21 Al _0.22 O _0.57 . In the case of α=0.21 and β=0.22, if γ≧0.60, it can be said that the state is completely oxidized. That is, Example 1 is oxygen deficient with respect to the fully oxidized state.

FIG. 5 shows the results of nano-electron diffraction (NBD) of the tunnel barrier layer according to Example 1 also performed using a transmission electron microscope (TEM). In the NBD pattern shown in FIG. 5, unlike the NBD pattern of Comparative Example 1 shown in FIG. 4, some diffraction spots were not observed. Since the electron diffraction pattern can be regarded as the Fourier transform of the crystal lattice, effective lattice constant changes and crystal symmetry changes can be observed. The additional spots seen in the area enclosed by the dotted line in Comparative Example 1 are caused by the regular array of cations in the regular spinel structure, and are reflected from the MgAl ₂ O ₄ {220} plane. show. On the other hand, in Example 1, only the fundamental reflection from the body-centered cubic (FCC) lattice is observed, so it has a disordered spinel structure. From these facts, the effective lattice constant of the tunnel barrier layer of Comparative Example 1 is twice the effective lattice constant of the tunnel barrier layer of Example 1, which is a regular spinel structure.

Further, the tunnel barrier layer according to Example 1 was produced 10 times, and the spot image shown in FIG. 5 was confirmed for all 10 times. That is, the tunnel barrier layer according to Example 1 has a stably disordered spinel structure. On the other hand, in the tunnel barrier layer according to Comparative Example 1, almost the same spot image as that shown in FIG. 4 was obtained, although the diffraction spots were hardly visible in the area surrounded by the dotted line in some cases. That is, the tunnel barrier layer according to Comparative Example 1 could not stably develop a disordered spinel structure.

Patent Document 1 also discloses that a high MR ratio is exhibited when the tunnel barrier layer has an irregular spinel structure. For example, Patent Document 1 describes that a disordered spinel structure tunnel barrier made of 1.45 nm MgAl can achieve an extremely high MR ratio of 308% at room temperature.

(Example 2)
Example 2 differs from Example 1 in that the thickness of the alloy represented by Mg ₁₇ Al ₈₃ was 0.8 nm. Other conditions and analysis method were the same as in Example 1.

The composition of the Mg--Al--O layer of Example 2 was Mg _0.19 Al _0.27 O _0.54 . In the case of α=0.19 and β=0.27, if γ≧0.67, it can be said that it is in a fully oxidized state. That is, Example 2 is oxygen deficient with respect to the fully oxidized state. The NBD pattern of the tunnel barrier layer according to Example 2 was similar to the pattern of the tunnel barrier layer according to Example 1 (FIG. 5).

(Example 3)
In Example 3, a 0.45 nm barrier underlayer made of Mg is formed, the thickness of the alloy represented by Mg ₁₇ Al ₈₃ is 0.63 nm, and a reactive film formation method is used to form an Mg—Al—O layer. is different from Example 1 in that it is oxidized. The reactive film forming method is a method of forming a film while oxidizing a Mg—Al alloy by introducing oxygen during film formation, and the gas flow rate was 20 sccm for Ar and 2 sccm for oxygen. Other conditions and analysis method were the same as in Example 1.

The composition of the Mg--Al--O layer of Example 3 was Mg _0.24 Al _0.24 O _0.52 . In the case of α=0.24 and β=0.24, if γ≧0.67, it can be said that it is in a fully oxidized state. That is, Example 3 is oxygen deficient with respect to the fully oxidized state. Also, the NBD pattern of the tunnel barrier layer according to Example 3 was the same as the pattern of the tunnel barrier layer according to Example 1 (FIG. 5).

(Example 4)
Example 4 differs from Example 3 in that the thickness of the alloy represented by Mg ₁₇ Al ₈₃ was set to 1.5 nm. Other conditions and analysis method were the same as in Example 3.

The composition of the Mg--Al--O layer of Example 4 was Mg _0.19 Al _0.24 O _0.57 . In the case of α=0.19 and β=0.24, if γ≧0.62, it can be said to be in a fully oxidized state. That is, Example 4 is oxygen deficient with respect to the fully oxidized state. This oxygen value corresponds to 96% of the fully oxidized state, which is oxygen deficient. Also, the NBD pattern of the tunnel barrier layer according to Example 4 was the same as the spot image of the tunnel barrier layer according to Example 1 (FIG. 5).

(Example 5)
Example 5 differs from Example 3 in that the thickness of the alloy represented by Mg ₁₇ Al ₈₃ was set to 1.9 nm. Other conditions and analysis method were the same as in Example 3.

The composition of the Mg--Al--O layer of Example 5 was Mg _0.13 Al _0.29 O _0.58 . In the case of α=0.13 and β=0.29, if γ≧0.63, it can be said that the state is completely oxidized. That is, Example 5 is oxygen deficient with respect to the fully oxidized state. The NBD pattern of the tunnel barrier layer according to Example 5 was similar to the pattern of the tunnel barrier layer according to Example 1 (FIG. 5).

(Example 6)
In forming the tunnel barrier layer of Example 6, a pure Al film of 1.3 nm was formed and then oxidized by oxygen plasma. As a plasma oxidation condition, a gas (6 Pa in total) in which 5 Pa of oxygen gas and Ar gas were mixed at 5:1 was used. Oxygen plasma was formed at a high frequency power density of 0.34 W/cm, and the Al layer was directly exposed to this plasma for 15 seconds. Other conditions and analysis method were the same as in Example 1.

The Al—O layer of Example 6 was obtained as a single crystal layer grown with the (001) plane, and its composition was Al _0.47 O _0.53 . In the case of α=0 and β=0.47, if γ≧0.79, it can be said that it is in a fully oxidized state. That is, Example 4 is oxygen deficient with respect to the fully oxidized state. Also, the NBD pattern of the tunnel barrier layer according to Example 6 was similar to the pattern of the tunnel barrier layer according to Example 1 (FIG. 5).

(Example 7)
In Example 7, after forming a 0.3 nm barrier underlayer made of Mg, a 0.1 nm film of an alloy represented by Mg ₆₇ Al ₃₃ was formed, and oxidation treatment was performed under the same conditions as in Example 1. After that, every time an alloy represented by Mg ₆₇ Al ₃₃ was deposited to a thickness of 0.2 nm, oxidation treatment was performed under the same conditions as in Example 1. The alloy film formation and the subsequent oxidation treatment were repeated four times. After that, Mg was deposited to a thickness of 0.2 nm. Other conditions and analysis method were the same as in Example 1.

The composition of the Mg--Al--O layer of Example 7 was Mg _0.41 Al _0.06 O _0.53 . In the case of α=0.41 and β=0.06, if γ≧0.56, it can be said that it is in a fully oxidized state. That is, Example 7 is oxygen deficient with respect to the fully oxidized state. The NBD pattern of the tunnel barrier layer according to Example 7 was similar to the pattern of the tunnel barrier layer according to Example 1 (FIG. 5).

The above results are summarized in Table 1 below.

As can be seen from Table 1, in any of the examples, a disordered spinel structure can be obtained by reducing the amount of oxygen in the tunnel barrier layer 3 having the ordered spinel structure described in Comparative Example 1.

1 first ferromagnetic layer 2 second ferromagnetic layer 3 tunnel barrier layer 10 magnetoresistive element

Claims (8)

comprising a first ferromagnetic layer, a second ferromagnetic layer, and a tunnel barrier layer sandwiched therebetween;
the tunnel barrier layer is an oxide of Mg _x Al _1-x (0.41≦x≦0.88);
the amount of oxygen in the tunnel barrier layer is lower than the amount of oxygen in a fully oxidized state in which the oxide has an ordered spinel structure;
The magnetoresistive effect element, wherein the amount of oxygen in the tunnel barrier layer is 77% or more and 95% or less of the amount of oxygen in the fully oxidized state. comprising a first ferromagnetic layer, a second ferromagnetic layer, and a tunnel barrier layer sandwiched therebetween;
The tunnel barrier layer is an Al oxide,
the amount of oxygen in the tunnel barrier layer is lower than the amount of oxygen in a fully oxidized state in which the oxide has an ordered spinel structure;
The magnetoresistive effect element, wherein the amount of oxygen in the tunnel barrier layer is 67% or more and 95% or less of the amount of oxygen in the fully oxidized state. 3. The magnetoresistive element according to claim 2, wherein the amount of oxygen in said tunnel barrier layer is 77% or more and 95% or less of the amount of oxygen in said fully oxidized state. 4. The magnetoresistive element according to claim 2, wherein crystals mainly constituting said tunnel barrier layer are (001) oriented. 5. The magnetoresistive element according to claim 1, wherein at least one of said first ferromagnetic layer and said second ferromagnetic layer contains Fe element. 6. The magnetoresistive element according to claim 1, wherein said tunnel barrier layer has a thickness of 3 nm or less. A method for manufacturing a magnetoresistance effect element according to any one of claims 1 to 6,
A method of manufacturing a magnetoresistive effect element, comprising the step of forming a film in a metal mode while oxidizing an alloy represented by the composition formula Mg _x Al _1-x (where 0≦x<1) by reactive film formation. 8. The method of manufacturing a magnetoresistive element according to claim 7, further comprising the step of forming a film of Mg after performing the step of forming a film in a metal mode while being oxidized by reactive film formation.

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