CN105977375A - MnGa-base vertical magnetic tunnel junction taking Heusler alloy as intercalation, and preparation method - Google Patents

Abstract

A Heusler alloy is an intercalated MnGa-based vertical magnetic tunnel junction, comprising: a substrate, which is the basis for realizing multilayer film epitaxial growth; a buffer layer, which is fabricated on the substrate, for smoothing the substrate surface and reducing the Small lattice mismatch; the lower electrode, which is made on the buffer layer, and epitaxial growth; the lower intercalation layer, which is made on the lower electrode, and epitaxial growth; one barrier layer, which is made on the lower intercalation layer; one upper intercalation layer, It is fabricated on the potential barrier layer and grown epitaxially; an upper electrode is fabricated on the upper intercalation layer and grown epitaxially; a covering layer is fabricated on the upper electrode to protect the lower structure. The invention has higher tunneling magnetoresistance effect.

Description

Heusler alloy is intercalated MnGa-based vertical magnetic tunnel junction and its preparation method

technical field

The invention relates to the fields of spintronics materials and magnetoresistive random access memory with a magnetic tunnel junction as the core, in particular to a MnGa-based vertical magnetic tunnel junction with a Heusler alloy as an intercalation layer and a preparation method thereof.

technical background

The continuous development of magnetic storage devices has enabled high-density, large-capacity data storage and miniaturized electronic components, bringing great convenience to our daily lives. However, all of this is closely related to the study of spintronic materials and related devices. The core component of a magnetoresistive random access memory (MRAM) is a magnetic tunnel junction, which usually consists of a ferromagnetic metal electrode/insulator barrier/ferromagnetic metal electrode sandwich structure. The relative orientation of the magnetic moments of the two ferromagnetic electrodes (parallel arrangement or antiparallel arrangement) can be adjusted through external control, so that the tunneling probability of electrons from one electrode to the other electrode changes, so that the tunnel junction appears high and low. Two different resistance states. The magnetic tunnel junction uses this tunable tunneling magnetoresistance effect (TMR) to realize the change of signal high and low levels, so as to write and read data.

In order to improve the performance of the device, researchers have made great efforts to optimize the structure of the magnetic tunnel junction. While making progress, many new problems have been found to be further improved:

On the one hand, as the storage density increases, the distance between adjacent magnetic memory cells decreases, and the magnetic coupling between memory cells will interfere with the storage of information, resulting in a decrease in the stability of the device. Therefore, people thought of using electrode materials with perpendicular magnetization characteristics instead of in-plane magnetization materials as memory cells. The use of vertical easily magnetizable electrode materials can not only avoid the magnetic coupling between adjacent memory cells to the greatest extent, but also reduce the energy consumption required to flip the magnetic moment, improve the thermal stability of components, and facilitate the ultra-high density integration of storage elements . At this stage, the research in related fields is relatively mature, such as the interface-induced magnetic anisotropy tunnel junction represented by the CoFeB/MgO/CoFeB structure. The characteristic of this type of structure is that the perpendicular magnetic anisotropy can only be maintained when the CoFeB film is very thin. , while the magnetic damping factor of CoFeB material increases rapidly with the decrease of film thickness, and the system has very high requirements on the type of buffer layer and the quality of the interface, which to a certain extent brings inconvenience to the large-scale application of related devices. A solution to this problem is to use electrode materials with large perpendicular magnetic anisotropy (PMA) to achieve vertical easy magnetization instead of relying on the induction effect of the interface. Researchers have tried to use MnGa with high perpendicular magnetic anisotropy as the electrode material to prepare the magnetic tunnel junction of MnGa/MgO/CoFe system. However, only small tunneling magnetoresistance can be observed at low temperature. rate of change.

On the other hand, it was found that the intercalation growth at the interface between the ferromagnetic electrode and the insulator barrier is a very important work. The reason is that a suitable intercalation material can not only buffer the lattice mismatch between the ferromagnetic electrode and the insulator barrier, but also avoid the interdiffusion and interface oxidation between the electrode material and the barrier, and improve the interlayer interlayer. interface quality. Not only that, if a suitable high spin polarizability material can be found as an intercalation layer, the electronic state properties at the interface between the electrode and the barrier layer can be changed, thereby further improving the tunneling magnetoresistance change rate of the tunnel junction. At present, there have been reports about the introduction of intercalation materials such as Mg and Fe in D0 ₂₂ -MnGa/MgO/CoFe and L1 ₀ -MnGa/MgO/CoFe systems. Studies have shown that the introduction of intercalation materials can indeed improve the electrode and MgO potential. The quality of the interface between barriers. However, there is still a certain gap between the highest TMR that can be achieved by the above system and the requirements of practical devices. It can be seen that the search and optimization of suitable intercalation materials is a very meaningful work.

Contents of the invention

The technical problem mainly solved by the invention is to provide a MnGa-based vertical magnetic tunnel junction with a Heusler alloy as an intercalation layer and a preparation method thereof. In the present invention, the double-layer film structure composed of L1 ₀ -MnGa electrode and Heusler alloy intercalation is implanted into the design of the magnetic tunnel junction, and the controllable magnetic moment of the intercalation is realized by using the antiferromagnetic coupling effect between the electrode and the intercalation Flip, and can make the L1 ₀ -MnGa high perpendicular magnetic anisotropy and the semi-metallic properties of the Heusler alloy can be taken into account. Therefore, the magnetic tunnel junction device can also have a high tunneling magnetoresistance effect while ensuring the vertical easy magnetization characteristic.

The invention provides a Heusler alloy as an intercalated MnGa-based vertical magnetic tunnel junction, comprising:

A substrate, which is the basis for realizing epitaxial growth of multilayer films;

a buffer layer epitaxially grown on the substrate for smoothing the substrate surface and reducing lattice mismatch;

The lower electrode, which is epitaxially grown on the buffer layer;

The next intercalation, its epitaxial growth is on the lower electrode;

A potential barrier layer, which is epitaxially grown on the lower intercalation layer, with a thickness of 0.8-2nm;

an upper intercalation layer, whose epitaxial growth is on the barrier layer;

an upper electrode epitaxially grown on the upper intercalation layer;

A cover layer, epitaxially grown on the upper electrode, protects the underlying structure.

The present invention also provides a method for preparing a Heusler alloy as an intercalated MnGa-based perpendicular magnetic tunnel junction, comprising the following steps:

Step 1: Take a substrate;

Step 2: sequentially preparing a buffer layer, a lower electrode, a lower intercalation layer, a barrier layer, an upper intercalation layer, an upper electrode and a covering layer on the substrate;

Step 3: Vacuum magnetic field annealing treatment to complete the preparation.

Compared with prior art, advantage of the present invention is as follows:

L1 ₀ -MnGa has strong perpendicular magnetic anisotropy, high saturation magnetic moment and low magnetic damping factor, and is recognized as a good substrate for realizing perpendicular magnetization characteristics; while semi-metallic Heusler alloys (Co ₂ MnSi, Co ₂ MnAl, Co ₂ FeAl and Co ₂ FeSi) have theoretically predicted high spin polarizability and high ferromagnetic Curie temperature. In addition, experiments have confirmed that there is a strong antiferromagnetic exchange coupling between the above-mentioned semi-metallic Heusler alloy and the double-layer film structure composed of L1 ₀ -MnGa, so that the magnetization direction of the Heusler alloy intercalation can be induced by the L1 ₀ -MnGa electrode Under the action, controllable flipping is realized. Not only that, the lattice constant of the semi-metallic Heusler alloy is between L1 ₀ -MnGa and the non-magnetic insulator barrier, which can play a good buffer role between the upper and lower electrodes and the barrier layer, and has a good impact on the L1 ₀ -MnGa system. Perpendicular easily magnetizable magnetic tunnel junctions are particularly suitable. The above advantages are not available in ordinary perpendicular magnetization structures (such as [Co/Pt(Pd)]n multilayer films, Pt/CoFe, FePt) and ordinary intercalation materials (such as Fe, Co, CoFe). For the above-mentioned magnetic tunnel junction system, we have observed a tunneling magnetoresistance signal of more than 5% at room temperature, and the signal increases with the decrease of the test temperature. It can be seen that the present invention provides a feasible new method for improving the tunneling magnetoresistance characteristics of the vertical easy-magnetization magnetic tunnel junction.

Description of drawings

In order to further illustrate the technical content of the present invention, the following detailed description is as follows in conjunction with the embodiments and accompanying drawings, wherein:

Fig. 1 is a structural representation of the present invention;

Fig. 2 is the preparation flowchart of the present invention;

Fig. 3 is the hysteresis loop (M-H curve) of the vertical magnetic tunnel junction of the structure in Fig. 1;

FIG. 4 is a response curve of the tunneling magnetoresistivity (TMR) of the structure in FIG. 1 versus an external magnetic field.

detailed description

In order to set forth the specific embodiment of the present invention more clearly, a specific example of a Heusler alloy intercalated MnGa-based vertical magnetic tunnel junction will be described below:

See also shown in Fig. 1, the present invention provides a kind of Heusler alloy is the MnGa base vertical magnetic tunnel junction of intercalation, comprising:

A substrate 1, which is a semi-insulating GaAs (001) material, is the basis for realizing the epitaxial growth of a multilayer film;

A buffer layer 2, grown epitaxially on the substrate 1, consisting of a semi-metallic Heusler alloy Co ₂ MnSi film about 4 nm thick, used to smooth the substrate surface and buffer the lattice mismatch between the substrate and the long layer structure;

The next electrode 3 is epitaxially grown on the buffer layer 2, and is composed of an L1 ₀ -MnGa film of about 20nm; this electrode material has a high perpendicular magnetic anisotropy, a high saturation magnetic moment and a low damping factor, and is the most important material in this structure. The source of the vertical easy magnetization property;

The lower intercalation layer 4 is epitaxially grown on the lower electrode 3 and consists of a semi-metallic Heusler alloy Co ₂ MnSi, Co ₂ MnAl, Co ₂ FeAl or Co ₂ FeSi film with a thickness of about 0.6-1.5 nm. Theory predicts that the spin polarization of this type of material is as high as 100%, and because of the strong antiferromagnetic exchange coupling between this type of alloy and the double-layer film composed of L1 ₀ -MnGa, it can be used in L1 ₀ -MnGa Under the induction effect of , it shows the magnetization characteristics in the vertical direction, and the magnetic moment of the Heusler alloy can be controlled by the external magnetic field along with L1 ₀ -MnGa to achieve controllable reversal;

A barrier layer 5, epitaxially grown on the lower intercalation layer 4, consisting of a MgO film with a thickness of about 2nm, the barrier layer is the core part for realizing the electron tunneling effect;

An upper intercalation layer 6, epitaxially grown on the barrier layer 5, is composed of a Heusler alloy Co ₂ MnSi, Co ₂ MnAl, Co ₂ FeAl or Co ₂ FeSi film with a thickness of about 0.6-1.5 nm and has semi-metallic properties. The specific effect is the same as that of the lower intercalation layer 4. The magnetization direction of the electrode/intercalation double-layer film on the upper and lower barrier layers rotates with the external field, and the macroscopic performance shows that the resistance between the upper and lower electrodes of the tunnel junction changes. This structure can convert the external magnetic signal into an electrical signal on the tunnel junction, thereby Realize the transmission and recording of information;

An upper electrode 7, epitaxially grown on the upper intercalation layer 6, consists of an L1 ₀ -MnGa thin film of about 8-10 nm. The Mn content in the lower L1 ₀ -MnGa thin film electrode composition is slightly different from that of the upper L1 ₀ -MnGa thin film electrode, and the coercive force Hc is different. This design is to make the magnetic moment of the upper and lower electrode materials independently affected by the external Rotation occurs due to the action of the magnetic field;

A cover layer 8, epitaxially grown on the upper electrode 7, is composed of a Pd metal film of about 0.8-2nm, which protects the underlying structure.

Please refer to Fig. 2 and show in Fig. 1 in conjunction with, the present invention also provides a kind of Heusler alloy is the preparation method of the MnGa base vertical magnetic tunnel junction of intercalation, comprises the following steps:

1. Take an intrinsically semi-insulating GaAs (001) substrate 1 and put it into the MBE preparation chamber, and the vacuum degree of the chamber is on the order of 10-7Pa. After heating the substrate for degassing and deoxidation, raise the substrate temperature to 560°C and deposit a GaAs smooth layer with a growth rate of about 10nm/min and a thickness greater than 300nm. Then lower the temperature of the substrate to 150-250°C, and grow a Co ₂ MnSi buffer layer 2, which is used to smooth the substrate surface and buffer the lattice mismatch between the substrate and the long layer structure, with a growth rate of about 1nm/min, The thickness is 3-5nm, and the in-situ annealing treatment is performed at 150-250°C for 10 minutes to reduce crystal defects;

2. Keep the substrate temperature at 150-250°C, grow the L1 ₀ -MnGa bottom electrode film 3 with high perpendicular magnetic anisotropy, the growth rate is about 1nm/min, the thickness is 25nm, and keep at 150-250°C for 10min for the original Bit annealing treatment to reduce crystal defects;

3. Keep the substrate temperature at 150-250° C., grow a Heusler alloy Co ₂ MnSi upper intercalation film 4 with semi-metallic properties (only Co ₂ MnSi intercalation is selected for this example, Co ₂ MnAl can also be selected for other embodiments , Co ₂ FeAl or Co ₂ FeSi), the growth rate is about 1nm/min, the thickness is 0.6-1.5nm, and the in-situ annealing treatment is performed at 150-250°C for 10 minutes to reduce crystal defects;

4. Lower the substrate temperature to room temperature, grow the MgO barrier layer 5 using electron beam evaporation equipment, turn on the electron beam evaporation power supply, the acceleration voltage is 5kV, and the emission current is 10-15mA. At the same time, the crystal oscillator is used to monitor the thickness of the MgO layer in situ. The film deposition thickness is 0.8-2nm. The substrate temperature is raised to 300°C and kept for 20 minutes, and an in-situ annealing treatment is performed to reduce crystal defects;

5. Reduce the temperature of the substrate to 150-250°C, and grow the Heusler alloy Co ₂ MnSi upper intercalation layer 6 with semi-metallic properties (Co ₂ MnAl, Co ₂ FoAl or Co ₂ FeSi can also be selected), and the growth rate is about 1nm /min, with a thickness of 0.6-1.5nm, in-situ annealing at 150-250°C for 10 minutes to reduce crystal defects;

6. Keep the substrate temperature at 150-250°C, grow the L1 ₀ -MnGa upper electrode film 7 with high perpendicular magnetic anisotropy, the growth rate is about 1nm/min, the thickness is 8-10nm, keep at 150-250°C for 10min In-situ annealing treatment to reduce crystal defects;

7. The temperature of the substrate is lowered to room temperature, and a layer of Pd covering layer 8 (Pt, Ta or Al can also be selected) is grown on the above-mentioned multilayer film structure, with a thickness of about 0.8-2 nm. Since Pd is a chemically stable noble metal material, it can protect the surface of the device; the final structure is GaAs/Co ₂ MnSi buffer layer/L1 ₀ -MnGa electrode/Co ₂ MnSi intercalation layer/MgO barrier layer/Co ₂ MnSi intercalation/L1 ₀ -MnGa electrode/P cap layer intercalated L1 ₀ -MnGa-based vertical easily magnetizable magnetic tunnel junction multilayer film with semi-metallic Heusler alloy;

8. Perform vacuum magnetic field annealing treatment on the L1 ₀ -MnGa-based vertically magnetizable magnetic tunnel junction multilayer film prepared in step 7 with the semi-metallic Heusler alloy as the intercalation layer to optimize its performance.

Referring to Figure 3, the hysteresis loop (MH curve) of the L1 ₀ -MnGa-based vertically magnetizable magnetic tunnel junction with a semi-metallic Heusler alloy as the intercalation layer (MH curve), the direction of the applied magnetic field and the direction of the detected magnetic field are both perpendicular to the substrate surface. The steep jump of the magnetic moment in the low-field area in the figure corresponds to the reversal process of the magnetic moment of the lower electrode and the lower intercalation layer with the external magnetic field, indicating that the L1 ₀ -MnGa film on the lower electrode has good vertical easy magnetization characteristics; the magnetic moment in the high-field area in the figure The change of the moment corresponds to the reversal process of the upper electrode and the upper intercalation magnetic moment with the external magnetic field, which shows that the L1 ₀ -MnGa thin film of the upper electrode also has the characteristic of vertical easy magnetization.

Referring to Figure 4, the response curve of the tunneling magnetoresistivity (TMR) of the L1 ₀ -MnGa-based vertically magnetizable magnetic tunnel junction with the semi-metallic Heusler alloy as the intercalation layer versus the external magnetic field is shown, and the direction of the applied magnetic field is perpendicular to the substrate surface. The obvious tunneling magnetoresistance signal can be seen from the figure, indicating that the structure has good performance.

The above embodiments are only used to illustrate the technical solution of the present invention, not to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that: they can still modify the technical solutions described in the aforementioned embodiments, or perform equivalent replacements for some of the technical features; and these The modification or replacement does not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims (10)

1. A Heusler alloy is an intercalated MnGa-based vertical magnetic tunnel junction, comprising: A substrate, which is the basis for realizing epitaxial growth of multilayer films; A buffer layer, which is fabricated on the substrate, is used to smooth the surface of the substrate and reduce the degree of lattice mismatch; The lower electrode, which is epitaxially grown on the buffer layer; The next intercalation, its epitaxial growth is on the lower electrode; a barrier layer epitaxially grown on the lower intercalation layer; an upper intercalation layer, whose epitaxial growth is on the barrier layer; an upper electrode epitaxially grown on the upper intercalation layer; A cover layer, epitaxially grown on the upper electrode, protects the underlying structure. 2. The Heusler alloy according to claim 1 is an intercalated MnGa-based perpendicular magnetic tunnel junction, wherein the material of the upper and lower electrodes is L1 ₀ -MnGa film with high perpendicular magnetic anisotropy. 3. Heusler alloy according to claim 1 is the MnGa base vertical magnetic tunnel junction of intercalation, wherein the material of upper and lower intercalation is Co ₂ MnSi, Co ₂ MnAl, Co ₂ FeAl or Co ₂ FeSi film, its thickness 0.6-1.5nm. 4. The Heusler alloy according to claim 1 is an intercalated MnGa-based vertical magnetic tunnel junction, wherein the material of the barrier layer is Al ₂ O ₃ or MgO, and its thickness is 0.8-2 nm. 5. The Heusler alloy according to claim 1 is an intercalated MnGa-based perpendicular magnetic tunnel junction, wherein the material of the covering layer is Pd, Pt, Ta or Al, and its thickness is 0.8-2nm. 6. a Heusler alloy is the preparation method of the MnGa base vertical magnetic tunnel junction of intercalation, comprising the following steps: Step 1: Take a substrate; Step 2: sequentially preparing a buffer layer, a lower electrode, a lower intercalation layer, a barrier layer, an upper intercalation layer, an upper electrode and a covering layer on the substrate; Step 3: Vacuum magnetic field annealing treatment to complete the preparation. 7 . The method for preparing the MnGa-based vertical magnetic tunnel junction in which the Heusler alloy is intercalated according to claim 6 , wherein the material of the upper and lower electrodes is L1 ₀ -MnGa thin film. 8. Heusler alloy according to claim 6 is the preparation method of the MnGa base vertical magnetic tunnel junction of intercalation, wherein the material of upper and lower intercalation is Co ₂ MnSi, Co ₂ MnAl, Co ₂ FeAl or Co ₂ FeSi film , and its thickness is 0.6-1.5nm. 9. The method for preparing the MnGa-based vertical magnetic tunnel junction in which the Heusler alloy is intercalated according to claim 6, wherein the material of the barrier layer is Al ₂ O ₃ or MgO, and its thickness is 0.8-2nm. 10. The method for preparing a Heusler alloy intercalated MnGa-based perpendicular magnetic tunnel junction according to claim 6, wherein the material of the covering layer is Pd, Pt, Ta or Al, and its thickness is 0.8-2nm.