CN102364618B - Multilayer film material with vertical magnetic anisotropy - Google Patents

Abstract

A multi-layer film material with perpendicular magnetic anisotropy. The multi-layer film material includes from bottom to top: substrate, core buffer layer, amorphous ferromagnetic layer and oxide barrier layer. The "CoFeB/MgO" interfacial perpendicular anisotropic multilayer film material in the present invention, the material makes the perpendicular magnetic anisotropy of the system by replacing Ta in the "Ta/CoFeB/MgO" structure with Mo or Hf Anisotropy increases by approximately 22% and 37%, respectively. Moreover, for the "Mo/CoFeB/MgO" system, the thermal stability is greatly enhanced. After annealing at 400°C for two hours, its vertical anisotropy remains stable, thereby improving the application value and making this material applicable to vertical magnetism. in the tunnel.

Description

A multilayer film material with perpendicular magnetic anisotropy

technical field

The invention relates to a composite material with enhanced "CoFeB/MgO" interfacial perpendicular anisotropy performance.

Background technique

Magnetic tunnel junctions have attracted much attention because of their high magnetoresistivity. Usually, the magnetic moment of the ferromagnetic electrode of the magnetic tunnel junction is parallel to the film surface, which is called an in-plane magnetic tunnel junction. However, magnetic tunnels with vertical magnetic anisotropy ferromagnetic electrodes (hereinafter referred to as vertical tunnel junctions) have been considered to be used in the realization of next-generation high-density nonvolatile memory—magnetic random access memory (MRAM) in recent years. This is mainly because compared with the in-plane magnetic tunnel junction, the vertical tunnel junction: can overcome the edge effect at a small scale, have larger magnetoresistivity, higher signal-to-noise ratio; larger anisotropy, and stronger thermal disturbance resistance , The superparamagnetic limit size is smaller, so the density of the device can be made higher and more reliable; the critical switching current is relatively reduced by an amount related to the magnetostatic energy, so it can be smaller. Common multilayer film materials with perpendicular magnetic anisotropy include: transition-rare earth alloys (such as TbFeCo, GdFeCo, etc.), L10 phase (Co, Fe)-(Pt, Pd) alloys, and Co/(Pd, Pt, Ni) multilayer film. However, these materials cannot meet the requirements of the application. The main reasons are: poor thermal stability or harsh preparation conditions; the vertical anisotropy is not large enough; the mismatch between the crystal structure and the barrier layer MgO (001) makes the magnetoresistance value too small; And the magnetic damping coefficient is too large so that the critical switching current value is large. S. Ikeda [S. Ikeda et al, Nature Mater. 9, 721 (2010)] et al proposed a new multilayer film material Ta/CoFeB/MgO with magnetic perpendicular anisotropy. This system uses the interfacial anisotropy at the CoFeB/MgO interface to overcome the influence of demagnetization energy, so that the magnetic moment of the ferromagnetic layer CoFeB is perpendicular to the film surface; it should be emphasized that the Ta layer is considered to be beneficial to the vertical isotropy of CoFeB Anisotropy, up to now vertical anisotropic CoFeB films are inseparable from the Ta neighbor layer [D. C. Worledge et al, Appl. Phys. Lett. 98, 022501 (2011)]. Compared with the above three conventional perpendicular magnetic anisotropy films, the Ta/CoFeB/MgO structure is conveniently applied to the MgO magnetic tunnel junction, and the preparation method is simple; moreover, after a simple annealing treatment of this system, the ferromagnetic electrode CoFeB and The crystal structure of the barrier layer MgO is very matched (bcc(001) and fcc(100) respectively, and the mismatch degree is less than 4%), so the tunnel junction prepared by this system has a high magnetoresistance; at the same time, the CoFeB magnetic The damping coefficient is small so that the critical flipping current value is small. However, the disadvantage is that the perpendicular magnetic anisotropy of this system is still too small, so that when the thickness of CoFeB increases to only about 1.5 nm, its magnetic moment returns from the vertical direction to the in-plane. Not only that, W. G. Wang [W. G. Wang et al, Appl. Phys. Lett. 99, 102502 (2011)] et al. found that the Ta/CoFeB/MgO system was vertically The magnetic anisotropy energy will drop rapidly. This will undoubtedly be fatal, because in order to increase the magnetoresistance value of the magnetic tunnel junction and reduce the surface resistivity of the junction, the tunnel junction usually needs to be annealed at a temperature above 350 degrees to ensure a good crystallization of the barrier layer MgO .

Contents of the invention

Aiming at the problems existing in the multilayer film material with perpendicular magnetic anisotropy in the prior art, the present invention discloses a novel multilayer film material with perpendicular magnetic anisotropy, which uses Mo or Hf as the core buffer layer material, increasing the perpendicular magnetic anisotropy and thermal stability.

In order to achieve the above object, 1. A multilayer film material with perpendicular magnetic anisotropy of the present invention is characterized in that the multilayer film material is from bottom to top: substrate, core buffer layer, amorphous ferromagnetic layer and oxide barrier layer.

Furthermore, the material of the substrate is silicon, glass or other substances with stable chemical properties and smooth surface.

Further, the material of the core buffer layer is Mo or Hf; the thickness is 0.5-200 nm.

Further, the material of the amorphous ferromagnetic layer is an amorphous Co, Fe, B ternary alloy or other amorphous ferromagnetic materials in the prepared state. The thickness of the amorphous ferromagnetic layer is 0.5-10 nm.

Further, the material of the oxide barrier layer is MgO; the thickness of the oxide barrier layer is 0.5-10 nm.

A multi-layer film material with perpendicular magnetic anisotropy. The multi-layer film material is arranged from bottom to top: a substrate, an oxide barrier amorphous ferromagnetic layer and a core protection layer.

Furthermore, the material of the substrate is silicon, glass or other substances with stable chemical properties and smooth surface.

Further, the material of the core protection layer is Mo or Hf; the thickness is 0.5-200 nm.

Further, the material of the amorphous ferromagnetic layer is an amorphous Co, Fe, B ternary alloy or other amorphous ferromagnetic materials in the prepared state. The thickness of the amorphous ferromagnetic layer is 0.5-10 nm.

Further, the material of the oxide barrier layer is MgO; the thickness of the oxide barrier layer is 0.5-10 nm.

The "CoFeB/MgO" interfacial perpendicular anisotropic multilayer film material in the present invention, the material makes the perpendicular magnetic anisotropy of the system by replacing Ta in the "Ta/CoFeB/MgO" structure with Mo or Hf Anisotropy increases by approximately 22% and 37%, respectively. Moreover, for the "Mo/CoFeB/MgO" system, the thermal stability is greatly enhanced. After annealing at 400°C for two hours, its vertical anisotropy remains stable, thereby improving the application value and making this material applicable to vertical magnetism. in the tunnel.

Description of drawings

Fig. 1 is a kind of structural representation with magnetic perpendicular anisotropy multi-layer film material obtained by using the method provided by the present invention;

Fig. 2 is another kind of structure schematic diagram with magnetic perpendicular anisotropy multi-layer film material obtained by using the method provided by the present invention;

Fig. 3 shows the magnetization curve of the multilayer film material in embodiment 1 in the direction perpendicular to the film plane;

Fig. 4 shows the magnetization curve of the multilayer film material in embodiment 1 in the direction parallel to the film plane;

Fig. 5 shows the variation of Kt of the multilayer film material in embodiment 1 along with t;

Fig. 6 shows the magnetization curves of the samples of Example 4 in the directions perpendicular to the film plane and parallel to the film plane.

Detailed ways

Example 1:

This embodiment is the first multilayer film material with perpendicular magnetic anisotropy obtained by using the method provided by the present invention. As shown in Figure 1, its structure is arranged in order from bottom to top: thermally oxidized silicon substrate 1; Mo core buffer layer 2 with a thickness of 5 nm; Co ₄₀ Fe ₄₀ B ₂₀ amorphous ferromagnetic layer 3 with a thickness of 0.6 nm-1.8 nm; MgO oxide barrier layer 4 with a thickness of 2 nm.

The preparation method of the vertically anisotropic multilayer film material in this embodiment is as follows: the method of magnetron sputtering is adopted, the background vacuum is better than 5×10 ^-5 Pa, Ar gas is used as the sputtering gas, and the sputtering pressure is 0.5 Pa, each layer of vertically anisotropic multilayer film material is sequentially deposited on the surface oxidized Si wafer. Wherein, after the deposition is completed, the vertically anisotropic multilayer film material is vacuum annealed at 300 degrees for two hours.

Figures 3 and 4 respectively show the magnetization curves of some typical samples in the multilayer film material of this embodiment in the direction perpendicular to the film surface and parallel to the film surface. It can be seen from the figure that the multilayer film material exhibits obvious vertical anisotropy, and it increases as the thickness of Co ₄₀ Fe ₄₀ B ₂₀ decreases. When the thickness of Co ₄₀ Fe ₄₀ B ₂₀ is 1.3 nm, the perpendicular anisotropy energy is enough to overcome the demagnetization energy, so that the easy axis of the magnetic moment is perpendicular to the film plane. However, the "Ta/CoFeB/MgO" multilayer film material prepared by the same method is to replace the "Mo core buffer layer 2" in this example with Ta. As a result, Co ₄₀ Fe ₄₀ B ₂₀ is magnetic only when the thickness is less than 1.1nm. moment to be perpendicular to the membrane surface. Apparently, the multilayer film material provided in this embodiment has higher vertical anisotropy than the "Ta/CoFeB/MgO" system commonly used at present. The vertical anisotropy of "Mo/CoFeB/MgO" and "Ta/CoFeB/MgO" multilayer materials are derived from the interfacial anisotropy at the "CoFeB/MgO" interface, and the interfacial anisotropy The magnitudes can be obtained from the Y-intercepts of their Kt-t curves [M. T. Johnson et al, Rep. Prog. Phys. 59, 1409 (1996)]. Here K is the perpendicular anisotropy constant, which is positive (negative) when the magnetic moment of Co ₄₀ Fe ₄₀ B ₂₀ is perpendicular to (parallel to) the film plane, which is equal to that of Co ₄₀ Fe ₄₀ B ₂₀ parallel to (perpendicular to) the film plane 1/2 of the product of the saturation field of the magnetization curve and the saturation magnetization; t is the thickness of Co ₄₀ Fe ₄₀ B ₂₀ . Figure 5 shows the Kt-t curves of the above two multilayer film materials. It can be obtained from the figure that after replacing the currently commonly used Ta with a Mo buffer layer, the interfacial anisotropy at the "CoFeB/MgO" interface increases from 1.7 erg/cm ² to 2.08 erg/cm ² , an increase of 22%.

It should also be noted that the interfacial vertical anisotropy of CoFeB/MgO is also closely related to the composition of CoFeB. Therefore, when using CoFeB with different components to study the vertical anisotropy of "Ta/CoFeB/MgO", the results obtained by each research group The limit thickness of the CoFeB thin film magnetic moment can be different: when S. Ikeda uses Co ₂₀ Fe ₆₀ B ₂₀ , the limit thickness is 1.4 nm [S. Ikeda et al, Nature Mater. 9, 721 (2010]; D. C. Worledge et al. used Co ₆₀ Fe ₂₀ B ₂₀ to obtain a limiting thickness of 1 nm [D. C. Worledge et al, Appl. Phys. Lett. 98, 022501 (2011)]; while we obtained the limiting thickness using Co ₄₀ Fe ₄₀ B ₂₀ as In any case, in principle, the Mo layer should have a similar enhancement effect on the vertical anisotropy of CoFeB films with other components.

Example 2:

The structure of the multilayer film in this example is basically the same as that of Example 1, and its structure from bottom to top is: thermally oxidized silicon substrate 1; Mo core buffer layer 2, whose thickness is 5nm; Co ₄₀ Fe ₄₀ B ₂₀ amorphous The ferromagnetic layer 3 has a thickness of 0.6nm-1.8nm; the MgO oxide barrier layer 4 has a thickness of 2nm.

The preparation method of the vertically anisotropic multilayer film material in this embodiment is as follows: the method of magnetron sputtering is adopted, the background vacuum is better than 5×10 ^-5 Pa, Ar gas is used as the sputtering gas, and the sputtering pressure is 0.5 Pa, each layer of vertically anisotropic multilayer film material is sequentially deposited on the surface oxidized Si wafer. After the deposition, the vertically anisotropic multilayer material was annealed in vacuum at 400°C for two hours.

Compared with the multilayer film material of Example 1, the magnetic properties of the multilayer film material in this embodiment are basically the same, specifically: the thickness of the Co ₄₀ Fe ₄₀ B ₂₀ magnetic moment from in-plane to vertical direction is still 1.3 nm, "CoFeB The interfacial anisotropy energy at the /MgO” interface is still around 2.1erg/cm ² . That is to say, the "Mo/CoFeB/MgO" vertical multilayer film material exhibits good thermal stability, especially the annealing temperature in this embodiment is 400 degrees, which has exceeded the 350 degrees required by the MgO tunnel junction. For the "Ta/CoFeB/MgO" multilayer film material, after replacing the "Mo core buffer layer 2" in this embodiment with Ta, after annealing at 350 degrees for only half an hour, the vertical anisotropy of the system has dropped rapidly so that the magnetic moment of Co ₄₀ Fe ₄₀ B ₂₀ is parallel to the film surface at any thickness. This result is basically consistent with the result of W. G. Wang et al [W. G. Wang et al, Appl. Phys. Lett. 99 , 102502 (2011)].

Example 3:

The multilayer film material with perpendicular magnetic anisotropy in the present embodiment, its structure is basically the same as that of Embodiment 1, from bottom to top is: thermally oxidized silicon substrate 1; H _f core buffer layer 2, its thickness is 5nm ; Co ₄₀ Fe ₄₀ B ₂₀ amorphous ferromagnetic layer 3 with a thickness of 0.6nm-1.8nm; MgO oxide barrier layer 4 with a thickness of 2nm.

The preparation method of the vertically anisotropic multilayer film material in this embodiment is as follows: the method of magnetron sputtering is adopted, the background vacuum is better than 5×10 ^-5 Pa, _Ar gas is used as the sputtering gas, and the sputtering pressure is 0.5 Pa, each layer of vertically anisotropic multilayer film material is sequentially deposited on the surface oxidized Si wafer. After the deposition, the vertically anisotropic multilayer material was annealed in vacuum at 300°C for two hours.

Similar to Example 1, comparing the results of the vertically anisotropic multilayer film material "Hf/CoFeB/MgO" in this example with the commonly used "Ta/CoFeB/MgO" shows that the Hf buffer layer is used instead of the current After the commonly used Ta, the interfacial anisotropy at the "CoFeB/MgO" interface increased from 1.7erg/cm2 to 2.34erg/ ^cm2 , an increase of 37%, so that the Co ₄₀ Fe ₄₀ B ₂₀ magnetic moment can be perpendicular to the film The limit thickness of the surface is increased from 1.1nm to 1.5nm.

Example 4:

This embodiment is the second multilayer film material with perpendicular magnetic anisotropy obtained by using the method provided by the present invention. As shown in Figure 2, its specific structure is from bottom to top: thermally oxidized silicon substrate 1; MgO The oxide barrier layer 4 has a thickness of 5 nm; the Co ₄₀ Fe ₄₀ B ₂₀ amorphous ferromagnetic layer 3 has a thickness of 1.2 nm; the Mo core protection layer 5 has a thickness of 5 nm.

The preparation method of the vertically anisotropic multilayer film material in this embodiment is as follows: the method of magnetron sputtering is adopted, the background vacuum is better than 5×10 ^-5 Pa, Ar gas is used as the sputtering gas, and the sputtering pressure is 0.5 Pa, each layer of vertically anisotropic multilayer film material is sequentially deposited on the surface oxidized Si wafer. After the deposition, the vertically anisotropic multilayer material was annealed in vacuum at 300°C for two hours.

Fig. 6 shows the magnetization curves of the perpendicular anisotropic thin film of this embodiment in the directions perpendicular to the film plane and parallel to the film plane. It can be seen from the figure that the curve perpendicular to the film surface shows obvious easy-axis magnetization characteristics - the remanence rate of the square is basically 1, while the curve parallel to the film surface shows obvious hard-axis magnetization characteristics --The linear remanence rate is basically zero, that is to say, the easy magnetization axis of the multilayer film material is perpendicular to the film surface.

Example 5:

The multilayer film material with perpendicular magnetic anisotropy in this embodiment is basically the same as that in Embodiment 4, and the specific structure is from bottom to top: thermally oxidized silicon substrate 1; MgO oxide barrier layer 4, its thickness 5 nm; Co ₄₀ Fe ₄₀ B ₂₀ amorphous ferromagnetic layer 3 with a thickness of 1.2 nm; Hf core protective layer 5 with a thickness of 5 nm.

The preparation method of the vertically anisotropic multilayer film material in this embodiment is as follows: the method of magnetron sputtering is adopted, the background vacuum is better than 5×10 ^-5 Pa, Ar gas is used as the sputtering gas, and the sputtering pressure is 0.5 Pa, each layer of vertically anisotropic multilayer film material is sequentially deposited on the surface oxidized Si wafer. After the deposition, the vertically anisotropic multilayer material was annealed in vacuum at 300°C for two hours.

Similar to the results of Example 4, the multilayer film material provided in this example has perpendicular magnetic anisotropy, and its easy magnetization direction is perpendicular to the film surface.

Of course, the thickness of each layer and the composition of the alloy material given in all the above examples are only examples given as a preferred embodiment, and can be appropriately changed according to specific applications. The thickness of each layer of film can be in the following range according to The actual situation selection: the thickness of the core buffer layer/protection layer is 0.5-200 nm, the thickness of the amorphous ferromagnetic layer is 0.5-10 nm, and the thickness of the oxide barrier layer is 0.5-10 nm. In addition, pay special attention to: as described at the end of Implementation 1, although the components of CoFeB in the above examples are all Co ₄₀ Fe ₄₀ B ₂₀ , in principle, the Mo or Hf layer is perpendicular to the CoFeB film of other components. Anisotropy should have a similar enhancement effect.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than limit them. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all of them should be included in the scope of the present invention. within the scope of the claims.

Claims (2)

1. A multilayer film material with perpendicular magnetic anisotropy, characterized in that, the multilayer film material is from bottom to top: substrate, core buffer layer, amorphous ferromagnetic layer and oxide barrier layer; The material of the substrate is silicon or glass; the material of the core buffer layer is Mo or Hf, and the thickness is 0.5-200 nm; the material of the amorphous ferromagnetic layer is amorphous Co, Fe, B ternary alloy; the thickness of the amorphous ferromagnetic layer is 0.5-10 nm; the material of the oxide barrier layer is MgO; the thickness of the oxide barrier layer is 0.5-10 nm. 2. A multilayer film material with perpendicular magnetic anisotropy is characterized in that, the multilayer film material is provided with from bottom to top: substrate, oxide barrier layer, amorphous ferromagnetic layer and core protective layer The material of the substrate is silicon or glass; the material of the core protective layer is Mo or Hf, and the thickness is 0.5-200 nm; the material of the amorphous ferromagnetic layer is amorphous Co under the prepared state , Fe, B ternary alloy; the thickness of the amorphous ferromagnetic layer is 0.5-10 nm; the material of the oxide barrier layer is MgO; the thickness of the oxide barrier layer is 0.5-10 nm.

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