JP6740551B2 - Method for manufacturing ferromagnetic tunnel junction, ferromagnetic tunnel junction, and magnetoresistive effect element - Google Patents

Description

The present invention relates to a method for manufacturing a ferromagnetic tunnel junction, a ferromagnetic tunnel junction, and a magnetoresistive effect element.

A giant magnetoresistive (GMR) element composed of a multilayer film of a ferromagnetic layer and a nonmagnetic layer, and a tunnel magnetoresistive (TMR) element using an insulating layer (tunnel barrier layer, barrier layer) for the nonmagnetic layer are known. There is. Generally, a TMR element has a higher element resistance than a GMR element, but the magnetoresistive (MR) ratio of the TMR element is larger than the MR ratio of the GMR element. Therefore, attention is focused on the TMR element as an element for a magnetic sensor, a high frequency component, a magnetic head, and a nonvolatile random access memory (MRAM).

TMR elements can be classified into two types depending on the difference in the mechanism of electron tunnel conduction. One is a TMR element that uses only the seepage effect (tunnel effect) of the wave function between the ferromagnetic layers. The other is TMR in which a coherent tunnel (only electrons having a specific wave function symmetry tunnel) that utilizes conduction of a specific orbit of a non-magnetic insulating layer that tunnels when a tunnel effect occurs It is an element. It is known that the TMR element in which the coherent tunnel is dominant has a larger MR ratio than the TMR element using only the tunnel effect.

In order to induce the coherent tunnel effect, 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. is there.

MgO is known as a tunnel barrier layer capable of obtaining a coherent tunnel effect. Moreover, Mg-Al-O is used for the tunnel barrier layer in order to further reduce the lattice mismatch between the two ferromagnetic metal layers and the tunnel barrier layer (for example, Patent Document 1, Patent Document 2, and Non-Patent Document 1). Reference 1). The Mg-Al-O layer described in Non-Patent Document 1 is produced by stacking a MgAl alloy and then oxidizing the stacked outer surface using inductively coupled plasma (ICP) or natural oxidation.

Japanese Patent No. 5586028 JP, 2013-175615, A

Applied Physics Letters, 105, 092403 (2014).

However, with the method described in Non-Patent Document 1, it is difficult to obtain consistency between the film thickness of the laminated MgAl alloy and the oxide film thickness that is oxidized from the outer surface. Therefore, there is a problem that a sufficient oxide film thickness cannot be obtained and a high MR ratio cannot be obtained.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a ferromagnetic tunnel junction having a sufficient oxide film thickness and a method for manufacturing the same.

The inventors of the present invention can obtain a sufficient oxide film thickness by diffusing oxygen from both surfaces of the stacked metal layers by stacking the MgAl alloy in a plurality of times and performing an oxidation treatment for each stack. Then, the present invention has been completed.
That is, the present invention provides the following means in order to solve the above problems.

(1) A method of manufacturing a ferromagnetic tunnel junction body according to one aspect of the present invention includes a step of sequentially stacking a first ferromagnetic metal layer, a tunnel barrier layer, and a second ferromagnetic metal layer, The step of stacking the tunnel barrier layer includes a step of stacking a plurality of metal layers represented by a composition formula Mg _1-x Al _x (0<x<1), and a step of stacking the metal layer. An oxidation step of oxidizing at least one metal layer.

(2) In the method of manufacturing a ferromagnetic tunnel junction structure according to (1), the tunnel barrier layer may be heat-treated after the tunnel barrier layers are stacked.

(3) In the metal layer laminating step of the method for manufacturing a ferromagnetic tunnel junction according to (1) or (2) above, the magnesium concentration of the metal layer laminated first is set to the metal layer laminated later. It may be thicker than the magnesium concentration of the layer.

(4) In the method of manufacturing a ferromagnetic tunnel junction structure according to (3), the metal layer laminated first may be the metal layer laminated first.

(5) In the method for manufacturing a ferromagnetic tunnel junction structure according to any one of (1) to (4), any metal laminated in the metal layer laminating step before the metal layer laminating step. The method may further include a step of laminating an oxygen suction layer made of a metal represented by a composition formula Mg _1-x Al _x (0≦x<1) having a higher magnesium concentration than the layer. The oxygen suction layer is a layer for promoting diffusion of introduced oxygen toward the first ferromagnetic metal layer side.

(6) In the method for manufacturing a ferromagnetic tunnel junction according to any one of (1) to (5) above, the thickness of the metal layer may be 0.1 nm or more and 1.5 nm or less.

(7) In the method of manufacturing a ferromagnetic tunnel junction structure according to (5), the oxygen suction layer may have a thickness of 0.1 nm or more and 0.6 nm or less.

(8) A ferromagnetic tunnel junction body according to an aspect of the present invention includes a first ferromagnetic metal layer, a second ferromagnetic metal layer, and a space between the first ferromagnetic metal layer and the second ferromagnetic metal layer. And a tunnel barrier layer sandwiched between the tunnel barrier layer and the tunnel barrier layer, wherein the tunnel barrier layer is a non-magnetic material represented by a composition formula Mg _1-x Al _x O _y (0<x<1, 0.35≦y≦1.7). It is made of an oxide and has a high oxygen concentration region in the thickness direction from the surface of the tunnel barrier layer on the side of the second ferromagnetic metal layer to the surface on the side of the first ferromagnetic metal layer.

(9) A magnetoresistive effect element according to one aspect of the present invention includes the ferromagnetic tunnel junction structure according to (8).

According to the method for manufacturing a ferromagnetic tunnel junction body according to one aspect of the present invention, it is possible to easily obtain a balance between the thickness of the MgAl alloy to be stacked and the oxide film thickness.

According to the ferromagnetic tunnel junction body and the magnetoresistive effect element according to the aspect of the present invention, the thickness of the oxidized region in the tunnel barrier layer can be increased, and a high MR ratio can be realized.

It is a cross-sectional schematic diagram of the ferromagnetic tunnel junction body which concerns on 1 aspect of this invention. It is a cross-sectional schematic diagram which expanded the principal part of the ferromagnetic tunnel junction body which concerns on 1 aspect of this invention. FIG. 6 is an enlarged schematic cross-sectional view of another example of the main part of the ferromagnetic tunnel junction structure according to an aspect of the present invention. It is a plane schematic diagram of the magnetoresistive effect element which concerns on one aspect of this invention. FIG. 6 is a diagram showing changes in MR ratio with respect to the thickness of the tunnel barrier layer of the ferromagnetic tunnel junction bodies of Example 1 and Comparative Example 1.

Hereinafter, the present invention will be described in detail with reference to the drawings. In the drawings used in the following description, in order to make the features of the present invention easy to understand, there are cases in which the features that are the features are enlarged for convenience, and the dimensional ratios of the components may be different from the actual ones. is there. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited thereto and can be appropriately modified and implemented within the scope of the invention.

A method of manufacturing a ferromagnetic tunnel junction body according to an aspect of the present invention includes a step of sequentially laminating a first ferromagnetic metal layer, a tunnel barrier layer, and a second ferromagnetic metal layer. The step of laminating the barrier layer includes at least one of a metal layer laminating step of laminating a plurality of metal layers represented by a composition formula Mg _1-x Al _x (0<x<1) and at least one metal layer laminating step. And an oxidation step of oxidizing the metal layer.

Here, “stacked in order” means stacking the first ferromagnetic metal layer, the tunnel barrier layer, and the second ferromagnetic metal layer in this order, and even if other layers are stacked in between. Good.

FIG. 1 is a schematic sectional view of a ferromagnetic tunnel junction body according to one embodiment of the present invention. The ferromagnetic metal layer laminated first is the first ferromagnetic metal layer 1, and the ferromagnetic metal layer subsequently laminated is the second ferromagnetic metal layer 2.

The first ferromagnetic metal layer 1 and the second ferromagnetic metal layer 2 each have a ferromagnetic metal, and one of them has a coercive force larger than the other. The ferromagnetic metal layer having a large coercive force is called a fixed layer or a reference layer, and the ferromagnetic metal layer having a small coercive force is called a free layer or a recording layer. The magnetization of the fixed layer is fixed in one direction, and the magnetization direction of the free layer changes relative to the magnetization direction of the fixed layer.

Either of the first ferromagnetic metal layer 1 and the second ferromagnetic metal layer 2 may be a fixed layer or a free layer. Hereinafter, the second ferromagnetic metal layer 2 will be described as a fixed layer.

(Preparation of first ferromagnetic metal layer)
First, the first ferromagnetic metal layer 1 is laminated on the substrate.
The substrate preferably has excellent flatness. In order to obtain a surface having excellent flatness, for example, Si, SiGe, SiC, AlTiC, MgO single crystal, MgAl ₂ O ₄ single crystal, or the like can be used. For example, when the ferromagnetic tunnel junction 10 is used as an MRAM or a magnetic sensor, a circuit formed of a Si substrate is necessary, and it is preferable to use the Si substrate. When the ferromagnetic tunnel junction 10 is used as the magnetic head, it is preferable to use an AlTiC substrate that is easy to process. Further, when it is important to orient the (001) orientation of the ferromagnetic tunnel junction body 10, it is preferable to use a MgO single crystal substrate, a MgAl ₂ O ₄ single crystal substrate, or a Si substrate.

A known material capable of forming a coherent tunnel can be used for the first ferromagnetic metal layer 1. For example, a metal selected from the group consisting of Cr, Mn, Co, Fe and Ni, and an alloy containing one or more of these metals and exhibiting ferromagnetism can be used. Further, an alloy containing these metals and at least one element of B, C, and N can also be used. Specifically, Fe, Co-Fe and the like can be mentioned.

Further, in order to obtain higher output, it is preferable to use a Heusler alloy such as Co ₂ FeSi. The Heusler alloy includes an intermetallic compound having a chemical composition of X ₂ YZ, X is a transition metal element or a noble metal element of Co, Fe, Ni, or Cu group on the periodic table, and Y is Mn or V. , Cr or Ti group transition metal and can take the elemental species of X, and Z is a typical element of group III to group V. For _example, Co 2 _FeSi, etc. Co 2 MnSi and _{Co 2 Mn 1-a Fe a} Al b Si 1-b can be mentioned.

Further, when making the magnetization direction of the first ferromagnetic metal layer 1 perpendicular to the stacking plane, it is preferable to use a stacked film of Co and Pt. Specifically, the first ferromagnetic metal layer 1 is formed of [Co(0.24 nm)/Pt(0.16 nm)] ₆ /Ru(0.9 nm)/[Pt(0.16 nm)/Co(0.16 nm). )] _{4 /} Ta (0.2 nm)/FeB (1.0 nm).

As a method for laminating the first ferromagnetic metal layer 1, a known method can be used. For example, a film forming method using a sputtering device can be used. In addition to this, a thin film forming method such as a vapor deposition method, a laser ablation method, or an MBE method can be used.

An underlayer may be formed between the substrate and the first ferromagnetic metal layer 1. When the underlayer is provided, the crystal orientation such as the crystal orientation and the crystal grain size of each layer including the first ferromagnetic metal layer 1 laminated on the substrate can be controlled. The underlayer can be laminated in the same manner as the first ferromagnetic metal layer 1.

The underlayer may be either conductive or insulative, but it is preferable to use a conductive material when the underlayer is energized.
For example, as one example, the underlayer has a (001)-oriented NaCl structure, and at least one selected from the group consisting of Ti, Zr, Nb, V, Hf, Ta, Mo, W, B, Al, and Ce. A layer of nitride containing one element can be used.

As another example, a layer of a (002)-oriented perovskite-based conductive oxide represented by a composition formula of ABO ₃ can be used as the underlayer. Here, the site A contains at least one element selected from the group of Sr, Ce, Dy, La, K, Ca, Na, Pb, Ba, and the site B contains Ti, V, Cr, Mn, Fe, Co. , Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, Pb, and at least one element selected from the group.

As another example, an oxide layer having a (001)-oriented NaCl structure and containing at least one element selected from the group consisting of Mg, Al, and Ce can be used as the underlayer.

As another example, the underlayer has a (001)-oriented tetragonal structure or a cubic structure, and includes Al, Cr, Fe, Co, Rh, Pd, Ag, Ir, Pt, Au, Mo, and W. A layer containing at least one element selected from the group can be used.

Further, the base layer is not limited to a single layer, and a plurality of layers of the above examples may be laminated. By devising the structure of the underlayer, the crystallinity of each layer of the ferromagnetic tunnel junction body 10 can be increased and the magnetic characteristics can be improved.

(Preparation of tunnel barrier layer)
Next, the tunnel barrier layer 3 is laminated on the laminated first ferromagnetic metal layer 1.

The tunnel barrier layer 3 is made of a nonmagnetic insulating material represented by a composition formula Mg _1-x Al _x O _y (0<x<1, 0.35≦y≦1.7). The nonmagnetic insulating material forming the tunnel barrier layer 3 does not need to be a stoichiometric composition of MgAl ₂ O ₄ , and the composition may deviate within the range in which the effect of the invention is exhibited. For example, oxygen deficiency may occur or oxygen may be excessive.

The film thickness of the tunnel barrier layer 3 is generally 3 nm or less. When the tunnel barrier layer 3 is sandwiched by the metal material, the electron wave function of the atoms of the metal material spreads beyond the tunnel barrier layer 3, and a current flows despite the existence of the insulator.

There are two types of ferromagnetic tunnel junctions 10 that utilize a normal tunnel effect and one that is predominantly a coherent tunnel effect in which orbits during tunneling are limited. In the ordinary tunnel effect, the magnetoresistive effect can be obtained by the spin polarizability of the ferromagnetic material, but in the coherent tunnel, the orbit during the tunnel is limited. Therefore, in the magnetoresistive effect element in which the coherent tunnel is dominant, an effect higher than the spin polarizability of the ferromagnetic metal material can be expected. The coherent tunnel effect appears when the ferromagnetic metal material and the tunnel barrier layer 3 are crystallized and joined in a specific orientation.

It is known that Fe, CoFeAl, CoFe and Co ₂ FeSi widely used for the ferromagnetic metal layer and MgO widely used for the tunnel barrier layer have a lattice constant difference of about 4%. When the tunnel barrier layer is made of MgO, the difference in lattice constant between the ferromagnetic metal layer and the tunnel barrier layer can be alleviated to some extent by oxygen deficiency. However, the difference in lattice constant due to oxygen deficiency is small, and it is not possible to sufficiently reduce the lattice mismatch between the ferromagnetic metal layer and the tunnel barrier layer. As a result, the coherent tunnel effect is locally reduced.

On the other hand, in the tunnel barrier layer 3, some of the divalent ions (Mg ²⁺ ) are replaced with the trivalent ions (Al ³⁺ ions). When predetermined ions are replaced with ions having different ionic radii, the lattice constant of the crystal forming the tunnel barrier layer 3 changes significantly. Further, by changing the substitution amount, the lattice constant of the crystal forming the tunnel barrier layer 3 can be freely controlled. As a result, the lattice mismatch between the first ferromagnetic metal layer 1 and the tunnel barrier layer 3 can be reduced, and the coherent tunnel effect can be obtained in a wide range.

In order to obtain a more excellent coherent tunnel effect, the crystal structure of the tunnel barrier layer 3 is preferably a spinel structure. More preferably, the crystal structure of the tunnel barrier layer 3 is preferably an irregular spinel structure. Here, the disordered spinel structure has a structure in which the arrangement of O atoms is a close-packed cubic lattice almost equivalent to that of the spinel structure, but the atomic arrangement of Mg and Al is disordered, and is a cubic structure as a whole. Refers to. The crystal structure of the tunnel barrier layer 3 is preferably oriented in the (001) direction.

The tunnel barrier layer 3 includes at least one metal layer between the metal layer laminating step of laminating a plurality of metal layers represented by the composition formula Mg _1-x Al _x (0<x<1) and the metal layer laminating step. It is produced according to an oxidation step of oxidizing. In the case of oxidizing a plurality of metal layers, a multi-step oxidation step is performed in which the step of stacking the metal layers and the step of oxidizing the metal layers are performed in multiple steps.
FIG. 2 is an enlarged cross-sectional schematic view of the tunnel barrier layer of the ferromagnetic tunnel junction body according to one embodiment of the present invention.

Specifically, first, the metal layer b ₁ is laminated on the laminated first ferromagnetic metal layer 1. Similar to the first ferromagnetic metal layer 1, the metal layer b ₁ is laminated using a known thin film forming method such as a sputtering method, a vapor deposition method, a laser ablation method, an MBE method or the like.

The thickness of the metal layer b ₁ is preferably 0.1 nm to 1.5 nm, and more preferably 0.1 nm to 0.6 nm.

When the upper limit of the thickness of the metal layer b ₁ laminated in the first metal layer laminating step is set to 1.5 nm, it is possible to prevent the thickness of the tunnel barrier layer 3 produced from becoming too thick, and an excellent coherent tunnel effect can be obtained. can get. On the other hand, the atomic size of aluminum and magnesium forming the metal layer b ₁ is about 0.1 nm. By setting the lower limit of the thickness of the metal layer b ₁ laminated in the first metal layer laminating step to 0.1 nm, the uniformity of the metal layer b ₁ is enhanced.

Next, the stacked metal layer b ₁ is oxidized. Oxidation is performed using plasma oxidation or natural oxidation by introducing oxygen. The introduced oxygen diffuses from the surface of the metal layer b ₁ opposite to the first ferromagnetic metal layer 1. In order to sufficiently diffuse the introduced oxygen from the oxygen introduction surface to the opposite surface, the thickness of the metal layer b ₁ laminated in the first metal layer laminating step is preferably 0.1 nm to 0.6 nm. ..

Then, after the metal layer laminating step and the oxidizing step are performed once, these steps are repeated. As a representative of each metal layer laminating step, the metal layer b _m laminated in the arbitrary m-th metal layer laminating step will be described.

The metal layer b _m is stacked on the metal layer b _m-1 after the oxidation process has already been performed. Therefore, oxygen exists at the interface between the metal layer b _m-1 and the metal layer b _m . Further, the metal layer b _m is subjected to an oxidation process after being laminated. That is, the metal layer b _m is oxidized from both sides in the stacking direction.

By oxidizing the metal layer b _m from both sides in the stacking direction, oxygen diffuses to the center of the metal layer b _{m in} the thickness direction even when the required amount of oxygen diffusion in the thickness direction is small. As a result, the metal layer b _m can be sufficiently oxidized over the entire thickness direction.

Further, the obtained tunnel barrier layer 3 is a stack of metal layers b _{1 to} b _{n that} are sufficiently oxidized to the inside. That is, a nonmagnetic oxide film having a sufficient thickness can be obtained as the entire tunnel barrier layer 3. If the thickness of the non-magnetic oxide film is sufficient, current modes flowing due to causes other than the coherent tunnel effect are eliminated, and a high MR ratio can be obtained.

On the other hand, when the metal layer is formed at one time, oxygen can be introduced only from the surface of the metal layer opposite to the substrate. That is, oxygen cannot be sufficiently diffused to the substrate side, and a nonmagnetic oxide film having a sufficient thickness cannot be obtained.

The thickness of the metal layer b _m laminated in the optional m-th metal layer laminating step is preferably 0.1 nm to 1.5 nm. Although it depends on various conditions such as heating described later, there is a report that the oxygen diffusion length is about 0.65 nm (Non-Patent Document 1). Since the metal layer b _m is oxidized from both sides, if the thickness is about 1.5 nm, the metal layer b _m can be sufficiently oxidized to the inside. Also considering the atomic size constituting the metal layer b _m, With 0.1nm thickness lower limit of the metal layer b _m, increases the uniformity of the metal layer b _m.

It is preferable that the metal layer laminating step and the oxidizing step be performed twice or more and 30 times or less. Generally, if the tunnel barrier layer 3 having a total thickness of about 3 nm is laminated and oxidized in two steps, oxygen can be sufficiently delivered to the center in the thickness direction of the metal layer laminated each time. Further, if the number of repetitions is 30 times or less, considering that the lower limit of the thickness of the metal layer laminated at one time is 0.1 nm, it is possible to avoid the tunnel barrier layer 3 from becoming too thick.

Here, the metal layer b _m are laminated in arbitrary m-th metal layer lamination step, comparing the metal layer b _m-1 to be stacked in any m-1 th metal layer lamination step, the metal layer b it is preferred magnesium concentration of _m-1 is darker than the magnesium concentration of the metal layer b _m. That is, it is preferable to make the magnesium concentration of the metal layer that is first laminated higher than the magnesium concentration of the metal layer that is later laminated.

In addition to the comparison of the metal layers laminated in the metal layer laminating step, there may be a region in the metal layers laminated earlier than the average magnesium concentration of the metal layers laminated later. That is, the metal layer b _m are laminated in arbitrary m-th metal layer lamination step, comparing the metal layer b _m-1 to be stacked in any m-1 th metal layer lamination step, the metal layer b _m at least a portion of _-1, may include a dark space than the average concentration of magnesium metal layer b _m.

Comparing magnesium and aluminum forming the metal layer, the magnesium atom is more likely to be oxidized due to the relationship of the ionization tendency. Therefore, if the magnesium concentration of the metal layer on the first ferromagnetic metal layer 1 side is increased as a whole of the tunnel barrier layer 3, diffusion of introduced oxygen to the first ferromagnetic metal layer 1 side is further promoted. As a result, the thickness of the nonmagnetic oxide film in the tunnel barrier layer 3 can be made sufficiently thick, and a high MR ratio can be obtained.
Controlling the amount of oxygen diffusion using the difference in ionization tendency is effective when two or more kinds of metals are used in the metal layer.

From the same viewpoint, the magnesium concentration of the metal layer b ₁ laminated in the first metal layer laminating step is preferably higher than the magnesium concentration of the other metal layers b _{2 to} b _n laminated thereafter. .. By increasing the concentration of magnesium in the first layer, it is possible to further promote the diffusion of introduced oxygen to the first ferromagnetic metal layer 1 side.

Further, before laminating the metal layers b _{1 to} b _n , an oxygen suction layer b ₀ may be laminated as shown in FIG. The oxygen suction layer b ₀ is a metal represented by Mg _1-x Al _x (0≦x<1). It differs from the metal layers b _{1 to} b _{n in} that x=0 is included. The magnesium concentration of the oxygen suction layer b ₀ is higher than the magnesium concentration of the metal layers b _{1 to} b _n .

By providing the oxygen suction layer b ₀ , diffusion of introduced oxygen toward the first ferromagnetic metal layer 1 side can be further promoted.
The thickness of the oxygen suction layer b ₀ is preferably 0.1 nm to 0.6 nm in order to sufficiently diffuse the introduced oxygen from the oxygen introduction surface to the opposite surface.

Further, even when the oxygen suction layer b ₀ is x=0 (that is, Mg alone), the oxygen suction layer b ₀ has a sufficiently small thickness, so that the lattice matching with the first ferromagnetic metal layer 1 and the tunnel barrier layer 3 is performed. ..

In the above description, the oxygen suction layer b ₀ is provided before forming the tunnel barrier layer 3, but the metal layer b ₁ laminated in the first metal layer laminating step in the tunnel barrier layer 3 is replaced with the oxygen suction layer b _0. May be replaced with

Further, in the above description, the case where the oxidation process is performed every time the metal layers are laminated is explained, but the oxidation may not be performed every time all the metal layers are laminated.

(Preparation of second ferromagnetic metal layer)
After producing the tunnel barrier layer 3, the second ferromagnetic metal layer 2 is laminated.
Similar to the first ferromagnetic metal layer 1, the second ferromagnetic metal layer 2 is laminated using a known thin film forming method such as a sputtering method, a vapor deposition method, a laser ablation method, an MBE method or the like.

As the material of the second ferromagnetic metal layer 2, a ferromagnetic material, especially a soft magnetic material can be applied. For example, a metal selected from the group consisting of Cr, Mn, Co, Fe and Ni, an alloy containing one or more of these metals, and these metals and at least one or more elements of B, C and N are included. The alloy etc. which are used can be used. Specifically, Fe, Co-Fe and the like can be mentioned.

When the magnetization direction of the second ferromagnetic metal layer 2 is perpendicular to the stacking plane, the thickness of the second ferromagnetic metal layer 2 is preferably 2.5 nm or less. Perpendicular magnetic anisotropy can be added to the second ferromagnetic metal layer 2 at the interface between the second ferromagnetic metal layer 2 and the tunnel barrier layer. Further, since the effect of the perpendicular magnetic anisotropy is attenuated by increasing the film thickness of the second ferromagnetic metal layer 2, it is preferable that the film thickness of the second ferromagnetic metal layer 2 is thin.

In order to increase the coercive force of the second ferromagnetic metal layer 2 with respect to the first ferromagnetic metal layer 1, an antiferromagnetic material such as IrMn or PtMn may be used as a material in contact with the second ferromagnetic metal layer 2. .. Further, in order to prevent the leakage magnetic field of the second ferromagnetic metal layer 2 from affecting the first ferromagnetic metal layer 1, a synthetic ferromagnetic coupling structure may be used. Further, a heat treatment in a magnetic field for imparting uniaxial magnetic anisotropy to the second ferromagnetic metal layer 2 may be performed.

In order to utilize the magnetoresistive effect element as the magnetic sensor, it is preferable that the resistance change linearly changes with respect to the external magnetic field. In a general laminated film of ferromagnetic layers, the direction of magnetization tends to be in the laminated plane due to shape anisotropy. In this case, for example, by applying a magnetic field from the outside to make the magnetization directions of the first ferromagnetic metal layer and the second ferromagnetic metal layer orthogonal to each other, the resistance change linearly changes with respect to the external magnetic field. However, in this case, a mechanism for applying a magnetic field is required near the magnetoresistive effect element, which is not desirable for integration. Therefore, it is preferable that the ferromagnetic metal layer itself has perpendicular magnetic anisotropy.

(Heat treatment)
The heat treatment is performed after the tunnel barrier layer 3 is laminated. The heat treatment may be performed before or after the second ferromagnetic metal layer 2 is laminated. By performing the heat treatment, oxygen diffusion in the tunnel barrier layer 3 is promoted. The heat treatment also has the effect of promoting crystallization of the tunnel barrier layer 3. The heat treatment is not limited to the manufacturing process of the tunnel barrier layer 3, but may be each manufacturing process of the underlayer, the first ferromagnetic layer 1, the second ferromagnetic layer 2, the electrode layer 11 and the electrode layer 12 which will be described later, if necessary. Or may be carried out collectively after all the layers constituting the magnetoresistive effect element are laminated.

From the viewpoint of crystallization, it is preferable to perform the heat treatment before stacking the second ferromagnetic metal layer 2. When the second ferromagnetic metal layer 2 is stacked, the tunnel barrier layer 3 is sandwiched between the first ferromagnetic metal layer 1 and the second ferromagnetic metal layer, and the degree of freedom of crystallization in the tunnel barrier layer 3 decreases. .. By performing heat treatment before stacking the second ferromagnetic metal layer 2, it is possible to enhance the lattice matching between the first ferromagnetic metal layer 1 and the tunnel barrier layer 3.

On the other hand, it is preferable to avoid performing the heat treatment each time the oxidation step is performed. When heat treatment is performed in each oxidation step, each crystallizes in each oxidation step. If each is crystallized, nonuniformity of oxygen concentration, lattice mismatch, etc. cannot be made uniform in the tunnel barrier layer 3 as a whole. As a result, nonuniformity of oxygen concentration, lattice mismatch, etc. cannot be sufficiently eliminated, and a high MR ratio cannot be obtained.

As described above, according to the method for manufacturing a ferromagnetic tunnel junction body according to an aspect of the present invention, the inside of the tunnel barrier layer in the thickness direction can be sufficiently oxidized. Therefore, the thickness of the nonmagnetic oxide film in the tunnel barrier layer can be increased, current modes flowing due to causes other than the coherent tunnel effect can be eliminated, and a ferromagnetic tunnel junction having a high MR ratio can be manufactured.

(Ferromagnetic tunnel junction)
The ferromagnetic tunnel junction body 10 manufactured by the above manufacturing method includes a first ferromagnetic metal layer 1, a second ferromagnetic metal layer 2, a first ferromagnetic metal layer 1 and a second ferromagnetic metal layer 2. The tunnel barrier layer 3 is sandwiched between the tunnel barrier layer 3 and the tunnel barrier layer 3. The tunnel barrier layer 3 is represented by a composition formula Mg _1-x Al _x O _y (0<x<1, 0.35≦y≦1.7). It consists of a non-magnetic oxide.

The tunnel barrier layer 3 has a region where the oxygen concentration increases in the thickness direction from the surface of the tunnel barrier layer 3 on the second ferromagnetic metal layer 2 side to the surface on the first ferromagnetic metal layer 1 side.

When the metal layer is formed at one time, oxygen can be introduced only from the surface on the second ferromagnetic metal layer 2 side. That is, the oxygen concentration gradually decreases from the second ferromagnetic metal layer 2 toward the first ferromagnetic metal layer 1.

On the other hand, the tunnel barrier layer 3 manufactured by the above-mentioned process is formed by performing the metal layer laminating process and the oxidizing process. Therefore, the oxygen concentration becomes high in the portion where the oxidation step is provided. That is, when the horizontal axis represents the thickness from the second ferromagnetic metal layer 2 side surface of the tunnel barrier layer 3 to the first ferromagnetic metal layer 1 side surface, and the vertical axis represents the oxygen concentration, the oxygen concentration is convex. Draw the curve of. The area that draws this convex curve is the high oxygen concentration area. For example, the high oxygen concentration area should be between the two inflection points that replace the concave curve with the convex curve with the vertex of the convex curve sandwiched. You can

The tunnel barrier layer 3 may be manufactured by alternately repeating the metal layer laminating step and the oxidizing step. In this case, the tunnel barrier layer 3 alternately has a high oxygen concentration region and a low oxygen concentration region in which the oxygen concentration is lower than the high oxygen concentration region.

The layered atoms and the difference in oxygen concentration can be determined by atom mapping using a transmission electron microscope (TEM).

Regarding the oxygen concentration, a gradation of the oxygen concentration can be confirmed by atom mapping in the cross section in the thickness direction. Further, not only when the high oxygen concentration region and the low oxygen concentration region can be clearly detected, but when the tunnel barrier layer 3 is etched and scraped from the surface even when it cannot be clearly detected in the cross section, You may measure the oxygen concentration of the front and back surfaces. That is, the case where the oxygen concentration on the surface before and after the etching is increased when the tunnel barrier layer 3 is etched and scraped from the surface is also included in the case where the high oxygen concentration region is provided.

Further, this includes not only the case where the oxygen concentration is uniform in the in-plane direction, but also the case where the oxygen concentration differs in the entire layer even if the oxygen concentration varies in the in-plane direction.

As described above, in the ferromagnetic tunnel junction body according to one aspect of the present invention, the sufficiently oxidized tunnel barrier layer is thick and a high MR ratio can be obtained. Therefore, the magnetoresistive effect element using this ferromagnetic tunnel barrier layer exhibits excellent characteristics as an element for a magnetic sensor, a high frequency component, a magnetic head and a nonvolatile random access memory (MRAM).

(Configuration when used)
FIG. 4 is a schematic view of the magnetoresistive effect element as viewed from above in the stacking direction. The magnetoresistive effect element 20 includes a ferromagnetic tunnel junction body 10, two electrode layers 11 and 12, a power supply 13, and a voltmeter 14. The ferromagnetic tunnel junction body 10 is arranged between the two electrode layers 11 and 12. The electrode layer 11 is connected to the second ferromagnetic metal layer 2 of the ferromagnetic tunnel junction body 10 shown in FIG. 1, and the electrode layer 12 is connected to the first ferromagnetic metal layer 1 of the ferromagnetic tunnel junction body 10 shown in FIG. Has been done. The two electrode layers 11 and 12 are connected to a power supply 13 and a voltmeter 14. When the power supply 13 applies a voltage, a current flows in the stacking direction of the ferromagnetic tunnel junction body 10 (direction perpendicular to the paper surface of FIG. 3), and the applied voltage at this time is monitored by the voltmeter 14.

The electrode layer 11 can also have a function as a protective layer of the second ferromagnetic layer 2. Further, the electrode layer 12 can also have a function as a base layer of the first ferromagnetic layer 1.

As the two electrode layers 11 and 12, a conductive layer containing at least one element selected from the group of Ru, Ta, Cu, Cr and Au can be used.

Further, the two electrode layers 11 and 12 are not limited to one layer, and a plurality of the above layers may be laminated.

(Evaluation method)
A method for evaluating the magnetoresistive effect element will be described with reference to FIG. A constant current is applied to the magnetoresistive effect element from the power supply 13. By measuring the voltage while sweeping the magnetic field from the outside, the resistance change of the magnetoresistive effect element can be observed by the voltmeter 14.
As another evaluation method of the magnetoresistive effect element, a Current-In-Plane-Tunneling (CIPT) method using a 12-terminal probe can be used.

The MR ratio is generally represented by the following formula.
MR ratio _{(%) = (R AP -R} P) / R P × 100
R _P is a tunnel resistance when the magnetization directions of the first ferromagnetic metal layer 1 and the second ferromagnetic metal layer 2 are parallel to each other, and R _AP is the first ferromagnetic metal layer 1 and the second ferromagnetic metal layer 2 Is the tunnel resistance when the magnetization directions of are parallel.

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications are possible within the scope of the gist of the present invention described in the claims. Can be modified and changed.

(Example 1)
Each layer of the magnetoresistive effect element was formed on the MgO (001) substrate by film formation using a sputtering method.

First, Cr was laminated to a thickness of 40 nm as a base layer (also serving as the electrode layer 12), and Fe was laminated to a thickness of 30 nm as a first ferromagnetic metal layer.
Then, a tunnel barrier layer was formed on the obtained first ferromagnetic metal layer by the following procedure. First, a single Mg layer having a thickness of 0.2 nm was laminated as an oxygen suction layer.

Then, a tunnel barrier layer was formed on the oxygen suction layer by repeating the metal layer laminating step and the oxidizing step twice. The thickness of the tunnel barrier layer was changed variously.
In the metal layer laminating step, the metal layer was laminated by a sputtering method using a target having a MgAl ₅ composition.
The oxidation step was performed by exposing the stacked metal layers to a pure oxygen atmosphere and an environment of 5 Pa for 600 seconds to perform natural oxidation.

Then, after the metal layer laminating step and the oxidizing step were performed twice, heating was performed in a vacuum at 400° C. for 15 minutes to obtain a tunnel barrier layer.

Then, Fe was deposited to a thickness of 6 nm as a second ferromagnetic metal layer on the tunnel barrier layer to obtain a ferromagnetic tunnel junction.
Next, IrMn for the antiferromagnetic layer was formed to a thickness of 12 nm, and Ru for the electrode layer 11 was formed to a thickness of 20 nm.

Finally, a heat treatment was performed at a temperature of 175° C. while applying a magnetic field of 5 kOe to impart uniaxial magnetic anisotropy to the second ferromagnetic metal layer.

(Comparative Example 1)
The difference from Example 1 is that the tunnel barrier layer was not manufactured in two steps.
Specifically, after laminating the first ferromagnetic metal layer, a MgAl ₅ layer was formed at one time. Then, the MgAl ₅ layer after film formation was exposed to a pure oxygen atmosphere and an environment of 5 Pa for 600 seconds to perform natural oxidation. Then, heat treatment was performed under the same conditions as in Example 1 to further stack the second ferromagnetic metal layer, antiferromagnetic layer, and electrode layer 11.

The MR ratios of the ferromagnetic tunnel junctions of Example 1 and Comparative Example 1 are shown in FIG. Note that both Example 1 and Comparative Example 1 are measurement results using the CIPT method. In FIG. 5, the horizontal axis represents the thickness of the tunnel barrier layer, and the vertical axis represents the MR ratio.

As shown in FIG. 5, the MR ratios of the ferromagnetic tunnel junctions of Example 1 and Comparative Example 1 showed the same transition up to the thickness of the tunnel barrier layer of 0.5 nm, but varied over 0.5 nm. Is shown.

The ferromagnetic tunnel junction body of Comparative Example 1 showed the maximum MR ratio (130%) when the thickness of the tunnel barrier layer was around 0.5 nm to 0.6 nm. The MR ratio gradually decreased as the thickness increased.

That is, in the stage where the thickness of the tunnel barrier layer is thin, oxygen introduced in the oxidation step can diffuse to the first ferromagnetic body side. On the other hand, when the thickness of the tunnel barrier layer becomes large, oxygen cannot reach the first ferromagnetic metal side. Therefore, when the thickness of the tunnel barrier layer exceeds a certain thickness, the thickness of the nonmagnetic oxide film (Mg _1-x Al _x O _y ) capable of producing the coherent tunnel effect does not change, and the MR ratio does not improve. Furthermore, the thickness of the non-magnetic oxide film (Mg _1-x Al _x O _y ) does not change and the thickness of the tunnel barrier layer increases, which means that an unnecessary metal layer (MgAl It means that ₅ layers) remain. The MR ratio gradually decreases as the thickness of the unnecessary metal layer increases.

On the other hand, in the ferromagnetic tunnel junction body of Example 1, the maximum MR ratio (180%) was exhibited when the thickness of the tunnel barrier layer was around 0.6 nm, and thereafter the MR ratio remained high.

That is, even if the thickness of the tunnel barrier layer is increased, oxygen reaches the first ferromagnetic metal side, and an unnecessary metal layer is not formed. Therefore, it is considered that the entire tunnel barrier layer is made of a nonmagnetic oxide film (Mg _1-x Al _x O _y ) capable of producing the coherent tunnel effect, and the MR ratio does not decrease and remains high.

Further, a non-magnetic oxide film (Mg _1-x Al _x O _y ) having a film thickness that could not be realized in Comparative Example 1 could be obtained, and a high MR ratio could be realized.

10 ... ferromagnetic tunnel junction element, 1 ... first ferromagnetic metal layer, 2: Second ferromagnetic metal layer, 3 ... tunnel barrier _{layer, b} 1 ~b n _...... metal layer, b 0 _... oxygen suction layer, 20 ... Magnetoresistive element, 11, 12... Electrode layer, 13... Power supply, 14... Voltmeter

Claims (12)

A step of sequentially stacking a first ferromagnetic metal layer, a tunnel barrier layer, and a second ferromagnetic metal layer,
The step of stacking the tunnel barrier layer includes a step of stacking a plurality of metal layers represented by a composition formula Mg _1-x Al _x (0<x<1), and a step of stacking the metal layer. An oxidation step for oxidizing at least one metal layer,
In any of the adjacent metal layers of the plurality of metal layers laminated in the metal layer laminating step, the magnesium concentration of the metal layer laminated first is made higher than the magnesium concentration of the metal layer laminated later. Manufacturing method of ferromagnetic tunnel junction. A step of sequentially stacking a first ferromagnetic metal layer, a tunnel barrier layer, and a second ferromagnetic metal layer,
The step of stacking the tunnel barrier layer includes a step of stacking a plurality of metal layers represented by a composition formula Mg _1-x Al _x (0<x<1), and a step of stacking the metal layer. An oxidation step for oxidizing at least one metal layer,
At least a part of the metal layer to be stacked m-1 times among the plurality of metal layers includes a region having a higher magnesium concentration than the average magnesium concentration of the metal layer to be stacked m times. Production method. Wherein after the tunnel barrier layer are laminated, the manufacturing method of the ferromagnetic tunnel junction of claim 1 or 2 annealing the tunnel barrier layer. In any of the adjacent metal layers of the plurality of metal layers laminated in the metal layer laminating step, the magnesium concentration of the metal layer laminated first is higher than the magnesium concentration of the metal layer laminated later. Item 3. A method of manufacturing a ferromagnetic tunnel junction body according to Item 2 . Metal layer laminated on the destination, the manufacturing method of the ferromagnetic tunnel junction of claim 1 or 4 first a metal layer to be laminated. Prior to the metal layer laminating step, the metal layer has a higher magnesium concentration than any of the metal layers laminated in the metal layer laminating step and is made of a metal represented by a composition formula Mg _1-x Al _x (0≦x<1). method for producing a ferromagnetic tunnel junction element according to any one of claims 1 to 5 having further a step of laminating the oxygen suction layer. Each of the thickness of the metal layer, the manufacturing method of the ferromagnetic tunnel junction element according to any one of claims 1 to 6 at 0.1nm or 1.5nm or less. The method for manufacturing a ferromagnetic tunnel junction body according to claim 6 , wherein the oxygen suction layer has a thickness of 0.1 nm or more and 0.6 nm or less. A first ferromagnetic metal layer, a second ferromagnetic metal layer, and a tunnel barrier layer sandwiched between the first ferromagnetic metal layer and the second ferromagnetic metal layer,
The tunnel barrier layer is made of a non-magnetic oxide represented by a composition formula Mg _1-x Al _x O _y (0<x<1, 0.35≦y≦1.7),
A ferromagnetic tunnel junction having two or more high oxygen concentration regions in the thickness direction from the surface of the tunnel barrier layer on the side of the second ferromagnetic metal layer to the surface on the side of the first ferromagnetic metal layer. In the thickness direction that leads to the surface from the surface of the second ferromagnetic metal layer side of the tunnel barrier layer first ferromagnetic metal layer side, alternately having a high oxygen concentration region and the low density region, according to claim 9 Ferromagnetic tunnel junction. A first ferromagnetic metal layer, a second ferromagnetic metal layer, and a tunnel barrier layer sandwiched between the first ferromagnetic metal layer and the second ferromagnetic metal layer,
The tunnel barrier layer is made of a non-magnetic oxide represented by a composition formula Mg _1-x Al _x O _y (0<x<1, 0.35≦y≦1.7),
The tunnel barrier layer has a high oxygen concentration region in the thickness direction from the surface on the second ferromagnetic metal layer side to the surface on the first ferromagnetic metal layer side,
The tunnel barrier layer is a ferromagnetic tunnel junction having a concentration of oxygen concentration in the in-plane direction. The magnetoresistive element having a ferromagnetic tunnel junction element according to any one of claims 10 to 11.

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