Multilayer thin film and method for producing the same

Abstract

To provide a new multilayer thin film exhibiting a tunnel magnetoresistance effect.SOLUTION: The present invention is a multilayer thin film having an insulating layer and a magnetic layer sandwiching the insulating layer. At least one of the magnetic layers is made of a Co-based alloy containing Al and / or Ga. For example, Co is contained in an amount of 58.5 to 63 at% with respect to the entire Co-based alloy. The Co-based alloy constituting a magnetic layer may be a β phase. The insulating layer is, for example, MgO or MgAl2O4. The magnetic layer on the other side of the insulating layer may be CoFeB (alloy) in addition to CoAl. Each layer is formed by sputtering, and planarization, crystallization, and the like are performed by heat treatment. A magnetic tunnel junction (MTJ) having the multilayer thin film of the present invention exhibits a high magnetoresistance change ratio (MR ratio), and can be applied to a next-generation magnetoresistance memory (MRAM) or the like.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to multilayer thin films and the like. [Background technology]

[0002] In the field of spintronics, which utilizes both the charge and spin properties of electrons, research and development is primarily focused on magnetic materials that exhibit the magnetoresistance effect, whereby electrical resistance changes in response to an external magnetic field.

[0003] A typical example is a magnetic tunnel junction (MTJ) with a stacked structure (magnetic layer / insulating layer / ferromagnetic layer) in which one insulating layer is sandwiched between two (ferro)magnetic layers. In an MTJ, the probability (tunnel probability) of electrons flowing perpendicular to the surface of the stacked structure passing through the insulating layer changes with an external magnetic field, resulting in a tunnel magnetoresistance (TMR) effect in which the electrical resistance (tunnel resistance) is greater when the magnetization directions of the two magnetic layers are not parallel than when they are parallel.

[0004] MTJs are expected to be used in next-generation energy-saving magnetoresistive memories (MRAM: Magnetic Random Access Memory) and highly sensitive magnetic sensors, and for example, there are relevant descriptions (research reports) in the following Non-Patent Documents 1 to 6. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] S. Yuasa et al., Jpn. J. Appl. Phys., 43, L588 (2004). [Non-patent document 2] H. Sukegawa et al., Appl. Phys. Lett., 96, 212505 (2010). [Non-patent document 3] DD Djayaprawira et al., Appl. Phys. Lett., 86, 092502 (2005).

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[0006] Non-Patent Documents 1 to 6 merely report on MTJ (elements) in which the insulating layer is made of MgO or MgAl2O4 and the magnetic layer is made of CoFeB or Co2MnSi.

[0007] Non-patent documents 7 to 18 report on CoAl, but these documents contain no description or suggestion whatsoever regarding the use of CoAl in MTJs or its electron spin transport properties.

[0008] The present invention has been made in view of the above circumstances, and has as its object to provide a new multilayer thin film suitable for magnetic tunnel junctions and the like. [Means for solving the problem]

[0009] As a result of intensive research by the inventors to solve this problem, they discovered that magnetic tunnel junctions (MTJs) using CoAl for the magnetic layer exhibit a high magnetoresistance ratio (MR ratio).By expanding on this finding, they have completed the present invention, as described below.

[0010] <<Multilayer thin film>> (1) The present invention is a multilayer thin film having an insulating layer and magnetic layers sandwiching the insulating layer, at least one of the magnetic layers being made of a Co-based alloy containing Al and / or Ga.

[0011] (2) By using the multilayer thin film of the present invention, deterioration of the laminated structure is suppressed, and an MTJ element or the like that can stably exhibit a high MR ratio can be realized.

[0012] <<Method for manufacturing multilayer thin films>> The present invention can also be understood as a method for manufacturing a multilayer thin film. For example, the present invention may be a method for manufacturing a multilayer thin film comprising a layering step of forming a magnetic layer made of a Co-based alloy and a heating step of heating the magnetic layer at 500 to 675°C.

[0013] "others" (1) In this specification, unless otherwise specified, compositions are expressed in atomic ratios (at%). Alloys and compounds expressed as AB or ABC simply indicate the constituent elements (A, B, C, etc.), and do not indicate the atomic ratio of each element (e.g., 1:1) unless otherwise specified.

[0014] Unless otherwise specified, the terms "upper" (side, direction) and "lower" (side, direction) in this specification refer to the order in which layers are formed (the order in which they are formed). For example, the lower layer is formed before the upper layer. The lower side means that it was formed before the upper side. Furthermore, "side" and "way" do not require whether they are adjacent (contacting) or not. When we say "a magnetic layer above an insulating layer," we mean that the magnetic layer was formed after the insulating layer, and do not necessarily mean that the two layers are in contact. For example, another layer may be interposed, such as insulating layer / intermediate layer / magnetic layer. The same applies to "a magnetic layer below an insulating layer."

[0015] (2) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b." Also, "x to ynm" in this specification means xnm to ynm. The same applies to other unit systems. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing the layered structure of the fabricated sample. [Figure 2] 1 is an electron microscope image of a cross section of the laminated layer of Sample 3. [Figure 3] 1 is a graph showing the relationship between the amount of Co contained in the magnetic layer and the MR ratio of the multilayer thin film. DETAILED DESCRIPTION OF THE INVENTION

[0017] The contents described in this specification may apply not only to the multilayer thin film but also to the manufacturing method thereof. One or more components arbitrarily selected from this specification may be added as components of the present invention. Components related to the manufacturing method may also be components related to the product. Which embodiment is best depends on the target, required performance, etc.

[0018] "background" The process by which the present invention was completed is as follows. The tunneling magnetoresistance (TMR) effect is evaluated by the rate of change in the electrical resistance of an element due to an external magnetic field (magnetoresistance change rate: MR ratio). The MR ratio improves as the spin polarization (the ratio of up-spin electrons to down-spin electrons) in the magnetic tunnel junction (MTJ) increases. To improve the MR ratio, the following research and studies have been conducted on the insulating layer and (ferro)magnetic layer, which are the main components of an MTJ.

[0019] First, Al2O3 was considered as an insulating layer, but MgO and MgAl2O4 are currently considered to be optimal. When current flows in the

[0001] direction of these crystals, only up-spin electrons flow in the Δ1 band, which is primarily responsible for coherent tunneling conduction. Therefore, an insulating layer that contributes to improving the MR ratio can be obtained by controlling the orientation of MgO or MgAl2O4 crystals in the

[0001] direction.

[0020] Next, CoFe was first considered for the magnetic layer, and currently CoFeB and half-metals (especially Heusler alloys such as Co2MnSi) are being considered as promising candidates. Immediately after deposition, CoFeB is in an amorphous state. When CoFeB is heated to 300°C or higher, the B atoms diffuse into other layers, forming epitaxially grown CoFe with a BCC structure that is lattice-matched with the insulating layer, such as MgO.

[0021] This CoFe has a Δ1 band, which allows it to exchange up-spin electrons tunneled through the Δ1 band of the insulating layer without scattering them. Therefore, in an MTJ (CoFe / MgO / CoFe or CoFe / MgAl2O4 / CoFe) in which an insulating layer made of MgO or MgAl2O4 is sandwiched between magnetic layers made of CoFe, a current flows due to almost completely spin-polarized up-spin electrons, and a high tunneling magnetoresistance effect can be achieved. However, atomic diffusion that occurs during the heating process above 300°C can affect each layer, which is about a few nanometers thick, resulting in degradation of TMR (a decrease in the MR ratio).

[0022] Co-based Heusler alloys such as Co2MnSi are half-metallic and have only up-spin electrons at their Fermi level, resulting in complete spin polarization. Therefore, Co2MnSi can pass a complete spin-polarized current without any special processing (such as heat treatment). Furthermore, because Co2MnSi also has a Δ1 band, MTJs with magnetic layers made of Co2MnSi are expected to have a high MR ratio. However, Co2MnSi can generate paramagnetic Mn ions (MnOx) at the interface with the MgO (insulating layer), which can degrade the TMR characteristics (especially its temperature dependence).

[0023] The Co-based alloy according to the present invention does not contain B or Mn, which may deteriorate the multilayer structure due to thermal diffusion. Therefore, by using the multilayer thin film according to the present invention, it is possible to realize, for example, an MTJ element that stably exhibits a high MR ratio.

[0024] 《Magnetic layer》 (1) Composition The Co-based alloy constituting the magnetic layer is, for example, a Co—Al alloy or a Co—Ga alloy. Taking the whole as 100 at%, Co is contained in an amount of, for example, 55 to 65 at%, 58.5 to 63 at%, 59 to 62.5 at%, 59.5 to 62 at%, or 60 to 61.5 at%, with the remainder being, for example, Al and / or Ga.

[0025] The Co-based alloy may be any binary alloy as described above, excluding impurities and the like. Al and Ga, both Group 13 elements, may be partially substituted for each other. That is, a Co-Al alloy may contain Ga, and a Co-Ga alloy may contain Al. The amount of substitution is not important, and the Co-based alloy may be Co(Al,Ga).

[0026] (2) Structure Co-based alloys, for example, have a β-phase crystal structure with a body-centered cubic (BCC) structure and are preferably ferromagnetic. Incidentally, the crystal structure of Co is face-centered cubic (FCC) or hexagonal close-packed (HCP). The crystal structure of Al is face-centered cubic (FCC). Ga has a low melting point (about 30°C), and its stable crystal structure varies depending on pressure and temperature.

[0027] (3) Thickness The thickness of the magnetic layer made of a Co-based alloy is, for example, 2 to 100 nm, 3 to 75 nm, 4 to 50 nm, or 5 to 25 nm.

[0028] (4) Combination The magnetic layers sandwiching the insulating layer may have the same or different component compositions. For example, both layers may be made of the Co-based alloy described above. Even in this case, the Co content may differ between layers, or one layer may be made of a CoAl alloy and the other a CoGa alloy.

[0029] Furthermore, one magnetic layer (e.g., the layer below the insulating layer) may be the above-mentioned Co-based alloy, and the other magnetic layer (e.g., the layer above the insulating layer) may be a different Co-based alloy (e.g., a CoFeB-based alloy). The CoFeB-based alloy may have any specific alloy composition, as long as it contains Co, Fe, and B as essential elements. For example, it may be a ternary alloy with Fe: 20-60 at %, B: 20-30 at %, and Co: the remainder. The CoFeB-based alloy may contain trace amounts of impurities or modifier elements in addition to the three elements. A magnetic layer made of such an alloy also preferably has a BCC crystal structure (e.g., β phase).

[0030] Insulating layer The insulating layer is, for example, an MgO layer or an MgAl2O4 layer. An MgO layer is often used as the insulating layer.

[0031] The magnetic layer below the insulating layer should preferably be made of the above-mentioned Co-based alloy (CoAl or CoGa). This allows epitaxial growth between the magnetic layer and the insulating layer, achieving crystal orientation (matching). A combination of a CoAl layer and an MgO layer is preferred.

[0032] 《Other layers》 The substrate preferably has a single crystal surface (film formation surface) made of MgO, Si, sapphire, SrTiO3, SiC, etc. The single crystal surface may be the surface of the substrate itself, or a layer (film) separately formed on the substrate.

[0033] Intermediate layers such as a buffer layer having a lattice constant that reduces lattice mismatch between layers and a barrier layer that prevents element diffusion between layers may be appropriately interposed. Furthermore, a diamagnetic layer and a protective layer may be formed above the magnetic layer formed on the insulating layer. The diamagnetic layer can make the magnetization directions of opposing magnetic layers antiparallel. The diamagnetic layer is an Mn-Ir layer, a Pt-Mn layer, or the like. The protective layer is provided to prevent oxidation of the multilayer thin film. The protective layer is an Ru layer, a Ta layer, a Cr layer, or the like. The thickness of each layer is, for example, about 1 to 20 nm, 2 to 10 nm, or 3 to 5 nm.

[0034] 《Manufacturing method》 (1) Stratification Multilayer thin films are typically obtained by sequentially depositing layers of different composition on a substrate or an underlying layer. Each layer is formed by a known thin film method, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). PVD, such as vacuum deposition (sputtering, vacuum thermal evaporation, pulsed laser deposition, etc.), allows for the easy formation of layers with desired compositions by changing or combining targets (raw materials).

[0035] Vacuum deposition is, for example, 10 -6 ~10 -10 Pa and 10 -7 ~10 -9 The deposition is preferably performed under (ultra) high vacuum of about Pa. The temperature during deposition (substrate temperature, base temperature) is, for example, from about room temperature (50°C or less, or even 30°C or less) to 600°C.

[0036] (2) Heat treatment Each layer is subjected to heat treatment as appropriate to achieve planarization, crystallization, etc. The heating temperature is, for example, 500 to 675°C or 550 to 650°C. The heating time is, for example, 0.1 to 2 hours or 0.2 to 1 hour. The heating source is electric heat, radiant heat, laser, etc. The heating atmosphere is the above-mentioned high vacuum or inert gas atmosphere. The heat treatment may be performed each time each layer is formed, or may be performed collectively after forming multiple layers.

[0037] 《Application》 The multilayer thin film is used in various spintronics devices (including elements) such as magnetic random access memories (MRAMs), magnetic sensors, and magnetic logic circuits. [Example]

[0038] Magnetic Tunnel Junction (MTJ) Magnetic layer MTJ elements (samples) having magnetic layers made of Co-based alloys were fabricated and their characteristics (MR ratios) were evaluated.The present invention will be explained in more detail based on these specific examples.

[0039] <<Sample Preparation>> (1) Overview As shown in Figure 1, a laminated sample (multilayer thin film) was fabricated using the thin film method. Specifically, the procedure is as follows: For ease of explanation, the directions indicated by the arrows in Figure 1 are the up-down direction (lamination direction) and the left-right direction.

[0040] Using an ultra-high vacuum multi-target sputtering system (ULVAC MPS-2000-C8, simply referred to as the "system"), the film was deposited on a MgO single crystal surface (100) that had been heated and cleaned under vacuum (600°C) and then cooled to near room temperature. The single crystal surface (1 cm square) was formed by polishing the surface of the MgO substrate.

[0041] Degree of vacuum achieved before film formation: 1×10 -7 The film thickness was calculated from the product of the film formation rate (0.1 nm / sec or less) and the film formation time.

[0042] As targets (raw materials), pure metals appropriate for each layer were used. For alloy layers, alloys prepared in advance to a desired composition may be used as targets.

[0043] (2) Stratification First, a Cr layer (50 nm thick) was formed on the MgO single crystal surface at room temperature, and then heated to 600°C in the equipment to flatten it.

[0044] Next, the Cr layer (underlayer) was cooled to below 50°C, and a CoAl layer (40 nm thick) was formed by simultaneous sputtering of Co and Al. The CoAl layer was heated to 600°C in the device to form a magnetic layer consisting of the β-CoAl phase. The composition (Co content) of the CoAl (Co-based alloy) was changed for each sample, as shown in Table 1. Unless otherwise specified, the composition is expressed as an atomic percentage of the total.

[0045] After the magnetic layer was cooled to below 50°C, an Mg layer (0.4 nm thick), MgO (2 nm thick), CoFeB layer (3 nm thick), Ru layer (0.95 nm thick), Co layer (2 nm thick), Mn-Ir layer (10 nm thick), Cr layer (5 nm thick), and Ru layer (7 nm thick) were stacked in this order.

[0046] The entire stack was heated (325°C) in a vacuum chamber in an aligning magnetic field (5 kOe). <100> This resulted in the crystallization of the amorphous CoFeB layer and the development of exchange magnetic anisotropy in the Co / Mn-Ir layer.

[0047] The MgO layer corresponds to the insulating layer, and the CoFeB layer above it corresponds to the other (ferro)magnetic layer. The extremely thin Mg layer was provided to recover the small amount of O remaining in the device and to stabilize the MgO layer (to prevent O from diffusing into the magnetic layer). The Mn-Ir layer corresponds to the antiferromagnetic layer, and the Ru layer corresponds to the protective layer. The combination of each layer on the insulating layer (MgO layer) fabricated in this example is known (well-known) as the stacked structure of an MTJ.

[0048] The obtained laminate was cooled to room temperature and then taken out into the atmosphere. In this way, the samples (multilayer thin films) shown in Table 1 were obtained. The compositions shown in Table 1 are the atomic ratios of Co to the total of Co and Al, and are in close agreement with the analytical results obtained using an inductively coupled plasma (ICP) optical emission spectrometer.

[0049] "observation" Sample 3 (Co 60 Al 40 The cross section of the sample was observed using a scanning transmission electron microscope (STEM), and the observed image is shown in Figure 2.

[0050] As can be seen from FIG. 2, it was confirmed that the magnetic layer (CoAl layer) and insulating layer (MgO layer) were epitaxially grown with controlled crystal orientation.

[0051] "measurement" The MR ratio of each sample was measured, which was calculated from the electrical resistance (rp) when the magnetizations of the magnetic layer (CoAl layer) and the magnetic layer (CoFeB layer) were parallel and the electrical resistance (rap) when the magnetizations were antiparallel, as follows: MR ratio = 100 × (rap - rp) / rp (%).

[0052] The electrical resistance was measured using the four-terminal method with a CIPT device. When no magnetic field was applied to the sample, the magnetization direction of each magnetic layer was antiparallel along the in-plane direction. When a magnetic field (150 Oe) was applied to the sample, the magnetization direction of each magnetic layer became parallel. The measurements were carried out at room temperature.

[0053] The MR ratio of each sample thus obtained is also shown in Table 1. The relationship between the Co amount (at %) in the magnetic layer of each sample and the MR ratio is shown in FIG.

[0054] As can be seen from Table 1 and Figure 3, all samples exhibited tunnel magnetoresistance (TMR). In particular, when the amount of Co contained in the magnetic layer below the insulating layer was 58.5 to 63.5 at %, or even 59 to 62.5 at %, the MR ratio was 100% or more.

[0055] Incidentally, when the Co content increased (Co≧64at%), a mixed phase structure of CoAl and Co phases appeared in the magnetic layer, resulting in a decrease in the MR ratio. Conversely, when the Co content in the magnetic layer decreased (Co≦58at%), the CoAl phase was well formed, but the saturation magnetization decreased (even to nearly zero) due to antiferromagnetism or paramagnetism, resulting in a deterioration of the TMR (a decrease in the MR ratio).

[0056] Thus, the present invention provides a new multilayer thin film that stably exhibits the tunnel magnetoresistance effect.

[0057]

Table 1

Claims

1. A multilayer thin film having an insulating layer and magnetic layers sandwiching the insulating layer, At least one of the magnetic layers is a multilayer thin film made of a Co-based alloy containing Al and / or Ga.

2. 2. The multilayer thin film according to claim 1, wherein the Co content of the Co-based alloy is 58.5 to 63 at %.

3. The multilayer thin film according to claim 1 , wherein the Co-based alloy is in the β phase.

4. 2. The multilayer thin film according to claim 1, wherein the magnetic layer has a thickness of 2 to 100 nm.

5. 2. The multilayer thin film according to claim 1, wherein the magnetic layer below the insulating layer is formed on an MgO single crystal or on a Cr layer on the MgO single crystal.

6. the magnetic layer below the insulating layer is made of the Co-based alloy, 2. The multilayer thin film according to claim 1, wherein the magnetic layer above the insulating layer is made of the Co-based alloy or the CoFeB-based alloy.

7. The insulating layer is made of MgO or MgAl ２ O ４ The multilayer thin film according to claim 1, comprising:

8. A method for producing a multilayer thin film according to any one of claims 1 to 7, comprising: a layering step of forming a magnetic layer made of the Co-based alloy; a heating step of heating the magnetic layer at 500 to 675°C; A method for manufacturing a multilayer thin film comprising:

9. 9. The method for producing a multilayer thin film according to claim 8, wherein the layering step is performed by a sputtering method.

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