Stack and magnetic device

A stack of the embodiment includes: a first magnetic substance; a second magnetic substance; and a first nonmagnetic substance which is disposed between the first magnetic substance and the second magnetic substance and contains at least one first metal element (M1) selected from the group consisting of ruthenium (Ru) and osmium (Os) and at least one second metal element (M2) selected from the group consisting of rhodium (Rh) and iridium (Ir). A magnetic device of the embodiment includes: a third magnetic substance; the stack; and a second nonmagnetic substance which is disposed between the third magnetic substance and the stack.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-114578, filed on Jun. 20, 2019; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a stack and a magnetic device.

BACKGROUND

In a magnetic random access memory (MRAM) which attracts attention as a nonvolatile memory, write operation and read operation of information are performed by using a magnetic tunnel junction (MTJ) element. The MTJ element generally has a structure where three layers of a magnetic layer as a free layer, a barrier layer, and a magnetic layer as a reference layer are stacked, and the writing of information is performed by flipping a spin of the free layer while fixing a direction of a spin of the reference layer, and the reading of information is performed by using a tunnel magnetoresistance ratio.

In advancing development of the MRAM, it has been a problem to reduce a write error rate (WER). The simple WER decreases as a write current increases. However, it has been reported that the direction of the spin of the reference layer is likely to be flipped when the current becomes a certain amount or more to increase the WER again. The spin direction of the reference layer is required to be sufficiently and strongly kept to reduce the WER due to the spin flipping of the reference layer. Further, it is also required to suppress a leakage magnetic field to prevent interference to an adjacent cell and the free layer. It has been studied to use a synthetic antiferromagnet (SAF) using antiferromagnetic coupling as the reference layer satisfying the above requirements.

A structure where a nonmagnetic metal spacer layer is inserted between two ferromagnetic metal layers is known as the SAF, and nonmagnetic metals such as ruthenium (Ru), iridium (Ir), or rhodium (Rh) are used for the spacer layer. In the SAF, it is important to control spin-coupling between the ferromagnetic metal layers. This coupling is called interlayer exchange coupling, and it is known that a sign and a size of the coupling depend on a kind and a film thickness of the nonmagnetic metal. A system enabling antiferromagnetic interlayer exchange coupling is called the SAF. By using the SAF for the reference layer, the spin is difficult to be flipped due to the interlayer exchange coupling, further, the leakage magnetic field becomes small due to the antiferromagnetic structure, and the reduction of the WER is expected.

It is required to further reduce the WER to advance miniaturization of the MTJ element. Strength of the interlayer exchange coupling of the SAF is necessary to be increased to further reduce the WER. However, it has been recognized that there is a limit to increase the strength of the interlayer exchange coupling in a case of the nonmagnetic metal layer used as the spacer layer of the conventional SAF. Accordingly, the SAF where the strength of the interlayer exchange coupling is further increased has been demanded to further increase the difficulty in the spin flipping and further decrease the leakage magnetic field.

SUMMARY

A stack of an embodiment includes: a first magnetic substance; a second magnetic substance; and a first nonmagnetic substance which is disposed between the first magnetic substance and the second magnetic substance, and contains at least one first metal element (M1) selected from the group consisting of ruthenium (Ru) and osmium (Os) and at least one second metal element (M2) selected from the group consisting of rhodium (Rh) and iridium (Ir).

A magnetic device of the embodiment includes: a third magnetic substance; the stack of the embodiment; and a second nonmagnetic substance which is disposed between the third magnetic substance and the stack.

DETAILED DESCRIPTION

Hereinafter, a stack and a magnetic device according to embodiments are described with reference to the drawings. In each embodiment presented below, substantially the same components are denoted by the same reference signs, and a description thereof is sometimes partially omitted. The drawings are schematic, and a relationship between a thickness and a planar size, thickness proportions of the respective portions, and the like are sometimes different from actual ones.

FIG. 1is a diagram illustrating a constitution of a stack and a magnetic device of the embodiment. A magnetic device1illustrated inFIG. 1includes a stack7including a first magnetic substance2, a first nonmagnetic substance3, and a second magnetic substance4. A second nonmagnetic substance5and a third magnetic substance6are further stacked on the stack7, and the magnetic device1has a multilayer structure where respective constituting layers are stacked in this order. The first magnetic substance2, the first nonmagnetic substance3, and the second magnetic substance4form a synthetic antiferromagnet. That is, the synthetic antiferromagnet as the stack7has the first nonmagnetic substance3disposed between the first magnetic substance2and the second magnetic substance4. The magnetic device1may include a third nonmagnetic substance8and a fourth magnetic substance9disposed between the second magnetic substance4and the second nonmagnetic substance5as illustrated inFIG. 2. The magnetic devices1illustrated inFIG. 1andFIG. 2each form, for example, a magnetic tunnel junction (MTJ) element, but it is not limited thereto.

The stack (the synthetic antiferromagnet)7has a stacked film structure where the first magnetic substance2, the first nonmagnetic substance3, and the second magnetic substance4are stacked in this order as mentioned above. The first magnetic substance2and the first nonmagnetic substance3, the first nonmagnetic substance3and the second magnetic substance4are respectively stacked under a state where they are directly in contact, and the synthetic antiferromagnet7is formed by the stacked film structure as stated above. Ferromagnetic substances such as, for example, cobalt (Co), iron (Fe), a cobalt-iron alloy (a CoFe alloy), a cobalt-iron-boron alloy (a CoFeB alloy), and a cobalt-platinum alloy (a CoPt alloy) can be used for the first and second magnetic substances2,4in the synthetic antiferromagnet7. A thickness of each of the first and second magnetic substances2,4is not particularly limited, but it is preferably, for example, 0.2 nm or more and 5 nm or less.

The first nonmagnetic substance3in the synthetic antiferromagnet7is an antiferromagnetic coupling film (an antiparallel coupling film) which antiferromagnetically couples between the first magnetic substance2and the second magnetic substance4due to an indirect exchange interaction, and a nonmagnetic metal is generally used. In the synthetic antiferromagnet7of the embodiment, a nonmagnetic metal material containing at least one first metal element (M1) selected from the group consisting of ruthenium (Ru) and osmium (Os) and at least one second metal element (M2) selected from the group consisting of rhodium (Rh) and iridium (Ir) is applied to the first nonmagnetic substance3. This nonmagnetic metal material is preferably an alloy of M1 and M2, more preferably a compound of M1 and M2, and the compound preferably has a composition represented by, for example, a composition formula: M11-xM2x, where x is an atomic ratio, and the details will be described later. M1 and M2 in the nonmagnetic metal material are each preferably one kind of metal, but it is not excluded to contain two kinds of metals.

In the synthetic antiferromagnet7, a spin direction D1of the first magnetic substance2and the spin direction D2of the second magnetic substance4become an antiparallel state because the first magnetic substance2and the second magnetic substance4are antiferromagnetically coupled with the first nonmagnetic substance3therebetween.FIG. 1illustrates a state where the first and second magnetic substances2,4are magnetized in a film surface vertical direction thereof, and thereby, the spin direction D1of the first magnetic substance2and the spin direction D2of the second magnetic substance4face reverse directions. The first nonmagnetic substance3preferably has a thickness of, for example, 0.3 nm or more and 2 nm or less to effectively exhibit a function as the antiferromagnetic coupling film as stated above. When the thickness of the first nonmagnetic substance3is over 2 nm, there is a possibility that antiferromagnetic coupling force between the first magnetic substance2and the second magnetic substance4is weakened. When the thickness of the first nonmagnetic substance3is less than 0.3 nm, uniform formability of the first nonmagnetic substance3is lowered. The first magnetic substance2, the first nonmagnetic substance3, and the second magnetic substance4can be each formed by using a film-forming technology such as a sputtering method, but the method is not limited to the sputtering method.

Conventionally, element metals (metal simple substances) such as ruthenium (Ru), osmium (Os), rhodium (Rh), or iridium (Ir) have been used as the first nonmagnetic substance3functioning as the above-stated antiferromagnetic coupling film, but there is a limit for the element metal layer in increasing the strength of the interlayer exchange coupling, and for example, when the magnetic device1is used as the MTJ element for the MRAM which is required to be miniaturized, it becomes difficult to reduce the write error rate (WER). The WER is likely to occur as a cell area of the MRAM is miniaturized. In the synthetic antiferromagnet7of the embodiment, the first nonmagnetic substance3made of the nonmagnetic metal material containing M1 and M2 is therefore applied.

By applying the first nonmagnetic substance3made of the alloy, the compound, or the like of at least one first metal element (M1) selected from ruthenium (Ru) and osmium (Os) being a group 8 element and at least one second metal element (M2) selected from rhodium (Rh) and iridium (Ir) being a group 9 element, the interlayer exchange coupling between the first magnetic substance2and the second magnetic substance4can be increased compared to the case when the element metal layer (simple metal layer) of Ru, Os, Rh, or Ir is used. A relationship between a composition of a compound and strength of interlayer exchange coupling in the synthetic antiferromagnet7using the first nonmagnetic substance3of the embodiment is illustrated in each ofFIG. 3toFIG. 6.FIG. 3illustrates the relationship between the composition and the strength of the interlayer exchange coupling when a compound of Ru and Rh is used.FIG. 4illustrates the relationship between the composition and the strength of the interlayer exchange coupling when a compound of Ru and Ir is used.FIG. 5illustrates the relationship between the composition and the strength of the interlayer exchange coupling when a compound of Os and Rh is used.FIG. 6illustrates the relationship between the composition and the strength of the interlayer exchange coupling when a compound of Os and Ir is used.

In a relationship diagram between the composition of the compound and the strength of the interlayer exchange coupling illustrated in each ofFIG. 3toFIG. 6, the composition of the compound of a horizontal axis represents the atomic ratio x in the composition formula: M11-xM2x. Concretely,FIG. 3illustrates the atomic ratio x in the composition formula: Ru1-xRhx.FIG. 4illustrates the atomic ratio x in the composition formula: Ru1-xIrx.FIG. 5illustrates the atomic ratio x in the composition formula: Os1-xRhx.FIG. 6illustrates the atomic ratio x in the composition formula: Os1-xIrx. A vertical axis in the relationship diagram illustrated in each ofFIG. 3toFIG. 6represents a value simulating interlayer exchange coupling in a stack of a Co3 atomic layer (the first magnetic substance2), one atomic layer of the compound (the first nonmagnetic substance3), and a Co3 atomic layer (the second magnetic substance4), and it is a relative value where the interlayer exchange coupling (J) using the compound is divided by each of the interlayer exchange couplings (JRu, JOs) using the element metal (Ru inFIG. 3andFIG. 4, and Os inFIG. 5andFIG. 6).

As illustrated inFIG. 3, it can be seen that the strength (J/JRu) of the interlayer exchange coupling is improved by using the first nonmagnetic substance3made of a compound1having the composition represented by Ru1-xRhxcompared to cases when a nonmagnetic substance made of an element substance of Ru (a case when x in Ru1-xRhxis “0” (zero)) and a nonmagnetic substance made of an element substance of Rh (a case when x in Ru1-xRhxis 1) are used. Since the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ru is used is larger than the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Rh is used, the interlayer exchange coupling of the first nonmagnetic substance3made of the compound1becomes smaller than the interlayer exchange coupling when the nonmagnetic substance formed of the element substance of Ru is used depending on the value of the composition x in Ru1-xRhx. As illustrated inFIG. 3, the strength (J/JRu) of the interlayer exchange coupling increases as the value of the composition x in Ru1-xRhxis increased, then decreases after a peak, and when the composition x in Ru1-xRhxis 0.54, the strength becomes an equivalent value as the strength (J/JRu) of the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ru is used. Accordingly, the composition x in Ru1-xRhxis preferably over “0” (zero) and less than 0.54 (0<x<0.54). The composition x in Ru1-xRhxis more preferably in a range of 0.05≤x≤0.45, and further desirably in a range of 0.15≤x≤0.35 to more increase the strength (J/JRu) of the interlayer exchange coupling.

As illustrated inFIG. 4, it can be seen that the strength (J/JRu) of the interlayer exchange coupling is improved also when the first nonmagnetic substance3made of a compound2having the composition represented by Ru1-xIrxis used compared to cases when the nonmagnetic substance made of the element substance of Ru (a case when x in Ru1-xIrxis “0” (zero)) and a nonmagnetic substance formed of an element substance of Ir (a case when x in Ru1-xIrxis 1) are used. Since the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ru is used is larger than the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ir is used, the interlayer exchange coupling of the first nonmagnetic substance3made of the compound2becomes smaller than the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ru is used depending on the value of the composition x in Ru1-xIrx. As illustrated inFIG. 4, the strength (J/JRu) of the interlayer exchange coupling increases as the value of the composition x in Ru1-xIrxis increased, then decreases after a peak, and when the composition x in Ru1-xIrxis 0.76, the strength becomes an equivalent value as the strength (J/JRu) of the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ru is used. Accordingly, the composition x in Ru1-xIrxis preferably over “0” (zero) and less than 0.76 (0<x<0.76). The composition x in Ru1-xIrxis more preferably in a range of 0.3≤x≤0.7, and further desirably in a range of 0.4≤x≤0.6 to more increase the strength (J/JRu) of the interlayer exchange coupling.

As illustrated inFIG. 5, it can be seen that the strength (J/JOs) of the interlayer exchange coupling is improved also when the first nonmagnetic substance3made of a compound3having the composition represented by Os1-xRhxis used compared to cases when a nonmagnetic substance made of an element substance of Os (a case when x in Os1-xRhxis “0” (zero)) and the nonmagnetic substance made of the element substance of Rh (a case when x in Os1-xRhxis 1) are used. Since the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Os is used is larger than the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Rh is used, the interlayer exchange coupling of the first nonmagnetic substance3made of the compound3becomes smaller than the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Os is used depending on the value of the composition x in Os1-xRhx. As illustrated inFIG. 5, the strength (J/JOs) of the interlayer exchange coupling increases as the value of the composition x in Os1-xRhxis increased, then decreases after a peak, and when the composition x in Os1-xRhxis 0.67, the strength becomes an equivalent value as the strength (J/JOs) of the interlayer exchange coupling when the nonmagnetic substance formed of the element substance of Rh is used. Accordingly, the composition x in Os1-xRhxis preferably over “0” (zero) and less than 0.67 (0<x<0.67). The composition x in Os1-xRhxis more preferably in a range of 0.2≤x≤0.6, and further desirably in a range of 0.4≤x≤0.6 to more increase the strength (J/JOs) of the interlayer exchange coupling.

As illustrated inFIG. 6, it can be seen that the strength (J/JOs) of the interlayer exchange coupling is improved also when the first nonmagnetic substance3made of a compound4having the composition represented by Os1-xIrxis used compared to cases when the nonmagnetic substance made of the element substance of Os (a case when x in Os1-xIrxis “0” (zero)) and the nonmagnetic substance formed of the element substance of Ir (a case when x in Os1-xIrxis 1) are used. Since the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ir is used is larger than the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Os is used, the interlayer exchange coupling of the first nonmagnetic substance3made of the compound4becomes smaller than the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ir is used depending on the value of the composition x in Os1-xIrx. As illustrated inFIG. 6, the strength (J/JOs) of the interlayer exchange coupling increases as the value of the composition x in Os1-xIrxis increased, then decreases after a peak, and when the composition x in Os1-xIrxis 0.07, the strength becomes an equivalent value as the strength (J/JOs) of the interlayer exchange coupling when the nonmagnetic substance made of the element substance of Ir is used. Accordingly, the composition x of Os1-xIrxis preferably over 0.07 and less than 1 (0.07<x<1). The composition x in Os1-xIrxis more preferably in a range of 0.3≤x≤0.7, and further desirably in a range of 0.4≤x≤0.6 to more increase the strength (J/JOs) of the interlayer exchange coupling.

As mentioned above, by applying the first nonmagnetic substance3made of the alloy, the compound, or the like of the first metal element (M1) being the group 8 element and the second metal element (M2) being the group 9 element, the interlayer exchange coupling between the first magnetic substance2and the second magnetic substance4can be increased. Here, an improvement effect of the strength of the interlayer exchange coupling enabled by making the substance into the alloy or the compound is only obtained based on a combination between the first metal element (M1) being the group 8 element and the second metal element (M2) being the group 9 element, and the improvement effect of the strength of the interlayer exchange coupling cannot be obtained by a combination of the group 8 elements with each other and a combination of the group 9 elements with each other.FIG. 7illustrates a relationship between the composition x in a compound (Ru1-xOsx) of Ru and Os and the strength of the interlayer exchange coupling.FIG. 8illustrates a relationship between the composition x in a compound (Ir1-xRhx) of Ir and Rh and the strength of the interlayer exchange coupling. InFIG. 7andFIG. 8, a vertical axis is a relative value where the interlayer exchange coupling (J) using the compound is divided by each of the interlayer exchange couplings (JRu, JIr) using the element metal (Ru inFIG. 7, Ir inFIG. 8). As illustrated inFIG. 7andFIG. 8, the interlayer exchange coupling changes monotonously in cases of the combination of the group 8 elements with each other and the combination of the group 9 elements with each other, and the strength cannot be increased compared to the interlayer exchange coupling using the element substance.

In the first nonmagnetic substance3formed of the alloy, the compound, or the like of the first metal element (M1) being the group 8 element and the second metal element (M2) being the group 9 element, the compositions of the alloys and the compounds using respective elements are as described above. Accordingly, when the composition ratio of the alloy, the compound, or the like of M1 and M2 is set as M11-xM2x, the interlayer exchange coupling of M11-xM2xis set as JM1M2, the interlayer exchange coupling of M1 alone is set as JM1, and the interlayer exchange coupling of M2 alone is set as JM2, where compositions when JM1>JM2and JM1M2=JM1are set as “0” (zero) and the composition x in M11-xM2xis preferably set in a range of 0<x<x1 where JM1M2>JM1. When compositions when JM2>JM1and JM1M2=JM2are set as x2 and 1, the composition x in M11-xM2xis preferably set in a range of x2<x<1 where JM1M2>JM2. The interlayer exchange coupling can be increased by applying M11-xM2xhaving the composition x as stated above to the first nonmagnetic substance3.

As mentioned above, when the magnetic device1of the embodiment is applied to the MTJ element, properties of the MRAM can be improved even when the MRAM using the MTJ element is miniaturized because the interlayer exchange coupling of the synthetic antiferromagnet7functioning as the reference layer can be increased. That is, when the magnetic device1illustrated inFIG. 1is used as the MTJ element, the synthetic antiferromagnet7functions as the reference layer (a magnetization fixed layer/a pinned layer) where a spin direction (a magnetization direction) is fixed. The second nonmagnetic substance5functioning as a tunnel barrier layer is provided on the synthetic antiferromagnet7, and the third magnetic substance6functioning as a storage layer (a free layer) where the spin direction (the magnetization direction) changes in accordance with storage contents is provided thereon. As illustrated inFIG. 2, the third nonmagnetic substance8and the fourth magnetic substance9may be provided between the second magnetic substance4and the second nonmagnetic substance5. The fourth magnetic substance9functions as the reference layer (the magnetization fixed layer/the pinned layer) where the magnetization direction is fixed by the synthetic antiferromagnet7.

For example, magnesium oxide (MgO) is used for the second nonmagnetic substance5functioning as the tunnel barrier layer. Besides, insulators such as aluminum oxide (Al—O), titanium oxide (Ti—O), and aluminum nitride (Al—N) may be used for the second nonmagnetic substance5. A thickness of the second nonmagnetic substance5is preferably, for example, 0.5 nm or more and 5 nm or less. When the thickness of the second nonmagnetic substance5is over 5 nm, there is a possibility that the function as the tunnel barrier layer is lowered. When the thickness is less than 0.5 nm, there is a possibility that it becomes difficult to uniformly form the second nonmagnetic substance5. Nonmagnetic metals such as molybdenum (Mo), tantalum (Ta), tungsten (W), and vanadium (V) are used for the third nonmagnetic substance8, and a thickness thereof is preferably, for example, 0.2 nm or more and 5 nm or less.

For example, ferromagnetic substances such as a cobalt-iron-boron alloy (a CoFeB alloy), a cobalt-iron alloy (a CoFe alloy), a cobalt-platinum alloy (a CoPt alloy) are used for the third magnetic substance6functioning as the storage layer (the free layer) and the fourth magnetic substance9functioning as the magnetization fixed layer (the pinned layer). Thicknesses of the third and fourth magnetic substances6,9are not particularly limited, but for example, they are each preferably 0.2 nm or more and 5 nm or less. The third and fourth magnetic substances6,9and the second and third nonmagnetic substances5,8can be each formed by using the film-forming technology such as the sputtering method, but the method is not limited to the sputtering method.

In the magnetic device1as the MTJ element, writing of information can be performed by, for example, injecting the spin by applying the current to the third magnetic substance6as the storage layer, and flipping the magnetization direction of the third magnetic substance6. Further, the writing can be also performed by using a current magnetic field generated by the current flowing through wirings provided in the vicinity thereof. Reading of information can be performed by using a tunnel magnetoresistance ratio which changes depending on a direction (in parallel or antiparallel to the reference layer (the magnetization fixed layer)) of the magnetization direction of the third magnetic substance6. In the MTJ element1as stated above, the spin of the synthetic antiferromagnet7can be made to be further difficult to be flipped by increasing the interlayer exchange coupling of the synthetic antiferromagnet7. That is, the magnetization direction of the synthetic antiferromagnet7as the reference layer can be further stabilized. Further, the leakage magnetic field can be made small, and interference to the adjacent cell and the free layer can be prevented by using the synthetic antiferromagnet7having the strong interlayer exchange coupling for the reference layer. It is thereby possible to reduce the write error rate (WER) even when, for example, the MTJ element1in the MRAM is miniaturized.

The stack7in the embodiment is not limited to the reference layer of the MTJ element1and can be used as the magnetization fixed layer or the like of various magnetic devices. The magnetic device1is not limited to the MTJ element and can be applied to various magnetoresistive effect elements. Further, the constitution of the magnetic device1is not limited to the above-stated components, and for example, a substrate, a base layer, a protection layer, and so on applied to a general magnetic device may be included.