MAGNETIC TUNNEL JUNCTION DEVICE AND STOCHASTIC COMPUTING SYSTEM INCLUDING THE SAME

A magnetic tunnel junction device includes a pinned magnetic layer, a free magnetic layer, and a tunnel barrier layer between the pinned and free magnetic layers. The free magnetic layer includes a first free layer, a second free layer spaced apart from the tunnel barrier layer with the first free layer therebetween, and a spacer layer between the first free layer and the second free layer. The first free layer and the second free layer are antiferromagnetically coupled to each other by the spacer layer, and each of the first free layer and the second free layer has a magnetization direction substantially perpendicular to an interface between the free magnetic layer and the tunnel barrier layer. A thermal stability of the free magnetic layer is in a range of 0 to 15.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0134185, filed on Oct. 8, 2021 in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present disclosure relates to a magnetic tunnel junction device and/or a stochastic computing system including the same.

A magnetic tunnel junction device may include two magnetic layers and an insulating layer disposed between the two magnetic layers. A resistance value of the magnetic tunnel junction device may be changed according to magnetization directions of the two magnetic layers. For example, the magnetic tunnel junction device may have a high resistance value when the magnetization directions of the two magnetic layers are antiparallel to each other, and the magnetic tunnel junction device may have a low resistance value when the magnetization directions of the two magnetic layers are parallel to each other. Data may be written in and read from the magnetic tunnel junction device by using a difference between these resistance values.

Recently, a spintronics-based stochastic computing system has been suggested. Various techniques for using the magnetic tunnel junction device as a random bit-stream generator of the stochastic computing system have been studied.

SUMMARY

Embodiments of inventive concepts may provide a magnetic tunnel junction device with a reduced relaxation time.

Embodiments of inventive concepts may also provide a stochastic computing system with an improved operating speed.

In an embodiment, a magnetic tunnel junction device may include a pinned magnetic layer, a free magnetic layer, and a tunnel barrier layer between the pinned magnetic layer and the free magnetic layer. The free magnetic layer may include a first free layer, a second free layer spaced apart from the tunnel barrier layer, the first free layer interposed between the tunnel barrier layer and the second free layer, and a spacer layer between the first free layer and the second free layer. The first free layer and the second free layer may be antiferromagnetically coupled to each other by the spacer layer. A magnetization direction of the first free layer and a magnetization direction of the second free layer each may be perpendicular to an interface between the free magnetic layer and the tunnel barrier layer. A thermal stability of the free magnetic layer may be in a range of 0 to 15.

In an embodiment, a magnetic tunnel junction device may include a pinned magnetic layer, a free magnetic layer, and a tunnel barrier layer between the pinned magnetic layer and the free magnetic layer. The free magnetic layer may include a plurality of free layers stacked on the tunnel barrier layer and a plurality of spacer layers between the plurality of free layers, respectively. The plurality of free layers may be antiferromagnetically coupled to each other by the plurality of spacer layers. Each of the plurality of free layers may have a magnetization direction perpendicular to an interface between the free magnetic layer and the tunnel barrier layer. The free magnetic layer may be configured to allow a net magnetization of the free magnetic layer to be 0.

In an embodiment, a stochastic computing system may include a random bit-stream generator including a magnetic tunnel junction device; a write circuit configured to provide a current for generating a bit in the random bit-stream generator; and a read circuit configured to read the bit generated by the random bit-stream generator. The magnetic tunnel junction device may include a pinned magnetic layer, a free magnetic layer, and a tunnel barrier layer between the pinned magnetic layer and the free magnetic layer. The free magnetic layer may include a first free layer, a second free layer spaced apart from the tunnel barrier layer with the first free layer therebetween, and a spacer layer between the first free layer and the second free layer. The first free layer and the second free layer may be antiferromagnetically coupled to each other by the spacer layer. A magnetization direction of the first free layer and a magnetization direction of the second free layer each may be perpendicular to an interface between the free magnetic layer and the tunnel barrier layer. The free magnetic layer may be configured to allow a net magnetization of the free magnetic layer to be 0.

DETAILED DESCRIPTION

Example embodiments of inventive concepts will now be described more fully with reference to the accompanying drawings.

FIG.1is a cross-sectional view illustrating a magnetic tunnel junction device according to some embodiments of inventive concepts.

Referring toFIG.1, a magnetic tunnel junction device500may include a pinned magnetic layer120, a free magnetic layer130, and a tunnel barrier layer TBR between the pinned and free magnetic layers120and130. The magnetic tunnel junction device500may further include a lower electrode BE connected to a source line SL, and an upper electrode TE connected to a bit line BL. In some embodiments, the pinned magnetic layer120may be disposed between the lower electrode BE and the tunnel barrier layer TBR, and the free magnetic layer130may be disposed between the upper electrode TE and the tunnel barrier layer TBR. The magnetic tunnel junction device500may further include a seed layer110between the lower electrode BE and the pinned magnetic layer120, and a capping layer140between the upper electrode TE and the free magnetic layer130.

For example, the lower electrode BE may include a conductive metal nitride (e.g., titanium nitride or tantalum nitride). The upper electrode TE may include at least one of a metal (e.g., Ta, W, Ru, Ir, etc.) or a conductive metal nitride (e.g., TiN). The seed layer110may include a material for assisting crystal growth of the pinned magnetic layer120. For example, the seed layer110may include at least one of chromium (Cr), iridium (Ir), or ruthenium (Ru).

The pinned magnetic layer120may include a first pinned layer122, a second pinned layer126, and an exchange coupling layer124between the first and second pinned layers122and126. The first pinned layer122may be disposed between the seed layer110and the tunnel barrier layer TBR, and the second pinned layer126may be disposed between the first pinned layer122and the tunnel barrier layer TBR. The exchange coupling layer124may be disposed between the first pinned layer122and the second pinned layer126, and the first pinned layer122and the second pinned layer126may be antiferromagnetically coupled to each other by the exchange coupling layer124.

The first pinned layer122may have a magnetization direction122M fixed in one direction. The magnetization direction122M of the first pinned layer122may be substantially perpendicular to an interface between the tunnel barrier layer TBR and the free magnetic layer130. The first pinned layer122may include a magnetic element and may include at least one of, for example, iron (Fe), cobalt (Co), or nickel (Ni). The first pinned layer122may include at least one of an intrinsic perpendicular magnetic material or an extrinsic perpendicular magnetic material. The intrinsic perpendicular magnetic material may include a material which has a perpendicular magnetization property even though an external factor does not exist. The intrinsic perpendicular magnetic material may include at least one of a perpendicular magnetic material (e.g., CoFeTb, CoFeGd, or CoFeDy), a perpendicular magnetic material having a L10structure, a CoPt alloy having a hexagonal close packed (HCP) lattice structure, or a perpendicular magnetic structure. The perpendicular magnetic material having the L10structure may include at least one of FePt having the L10structure, FePd having the L10structure, CoPd having the L10structure, or CoPt having the L10structure. The perpendicular magnetic structure may include magnetic layers and non-magnetic layers, which are alternately and repeatedly stacked. For example, the perpendicular magnetic structure may include at least one of (Co/Pt)n, (CoFe/Pt)n, (CoFe/Pd)n, (Co/Pd)n, (Co/Ni)n, (CoNi/Pt)n, (CoCr/Pt)n, or (CoCr/Pd)n, where denotes the number of bilayers. The extrinsic perpendicular magnetic material may include a material which has an intrinsic horizontal magnetization property but has a perpendicular magnetization property by an external factor. For example, the extrinsic perpendicular magnetic material may have the perpendicular magnetization property by magnetic anisotropy induced by a junction of the pinned magnetic layer120and the tunnel barrier layer TBR. The extrinsic perpendicular magnetic material may include, for example, CoFeB. In certain embodiments, the first pinned layer122may include a Co-based Heusler alloy.

The second pinned layer126may have a magnetization direction126M fixed in one direction. The magnetization direction126M of the second pinned layer126may be substantially perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130. The second pinned layer126may be antiferromagnetically coupled to the first pinned layer122by the exchange coupling layer124, and thus the magnetization direction126M of the second pinned layer126may be antiparallel to the magnetization direction122M of the first pinned layer122. The second pinned layer126may include a magnetic element and may include at least one of, for example, iron (Fe), cobalt (Co), or nickel (Ni). The second pinned layer126may include at least one of the aforementioned intrinsic perpendicular magnetic material or the aforementioned extrinsic perpendicular magnetic material. In certain embodiments, the second pinned layer126may include a Co-based Heusler alloy.

The exchange coupling layer124may include a non-magnetic metal. For example, the exchange coupling layer124may include ruthenium (Ru), iridium (Ir), tungsten (W), tantalum (Ta), or any alloy thereof.

The tunnel barrier layer TBR may include a metal oxide layer. The tunnel barrier layer TBR may include at least one of, for example, a magnesium (Mg) oxide layer, a titanium (Ti) oxide layer, an aluminum (Al) oxide layer, a magnesium-zinc (Mg—Zn) oxide layer, or a magnesium-boron (Mg—B) oxide layer.

The free magnetic layer130may include a first free layer131, a second free layer133, and a spacer layer132between the first and second free layers131and133. The first free layer131may be disposed adjacent to the tunnel barrier layer TBR, and the second free layer133may be spaced apart from the tunnel barrier layer TBR with the first free layer131interposed therebetween. The first free layer131may be disposed between the tunnel barrier layer TBR and the capping layer140, and the second free layer133may be disposed between the first free layer131and the capping layer140. The spacer layer132may be disposed between the first free layer131and the second free layer133, and the first free layer131and the second free layer133may be antiferromagnetically coupled to each other by the spacer layer132.

The first free layer131may have a magnetization direction131M changeable to be parallel or antiparallel to the magnetization direction126M of the second pinned layer126. The magnetization direction131M of the first free layer131may be substantially perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The first free layer131may include a magnetic element and may include at least one of, for example, iron (Fe), cobalt (Co), or nickel (Ni). The first free layer131may include at least one of the perpendicular magnetic material (e.g., CoFeTb, CoFeGd, CoFeDy), the perpendicular magnetic material having the L10structure, the CoPt alloy having the HCP lattice structure, or the perpendicular magnetic structure. The first free layer131may include a magnetic material (e.g., CoFeB) which has a perpendicular magnetic property by magnetic anisotropy induced by a junction of the free magnetic layer130(e.g., the first free layer131) and the tunnel barrier layer TBR. In certain embodiments, the first free layer131may include a Co-based Heusler alloy.

The second free layer133may have a changeable magnetization direction133M, and the magnetization direction133M of the second free layer133may be substantially perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The second free layer133may be antiferromagnetically coupled to the first free layer131by the spacer layer132, and thus the magnetization direction133M of the second free layer133may be antiparallel to the magnetization direction131M of the first free layer131. The second free layer133may include a magnetic element and may include at least one of, for example, iron (Fe), cobalt (Co), or nickel (Ni). The second free layer133may include at least one of the perpendicular magnetic material (e.g., CoFeTb, CoFeGd, CoFeDy), the perpendicular magnetic material having the L10structure, the CoPt alloy having the HCP lattice structure, or the perpendicular magnetic structure. The second free layer133may include a magnetic material (e.g., CoFeB) which has a perpendicular magnetic property by magnetic anisotropy induced by a junction of the free magnetic layer130(e.g., the first free layer131) and the tunnel barrier layer TBR. In certain embodiments, the second free layer133may include a Co-based Heusler alloy.

The spacer layer132may include a non-magnetic metal. For example, the spacer layer132may include ruthenium (Ru), iridium (Ir), tungsten (W), tantalum (Ta), or any alloy thereof.

The pinned magnetic layer120may be configured to have a thermal stability of 40 or more(for example, ranges from 40 to 1000), and the free magnetic layer130may be configured to have a thermal stability of 15 or less. The thermal stability may be represented by the following formula 1.

Here, ‘Δ’ is the thermal stability, ‘EB’ is an energy barrier required for magnetization reversal, ‘kB’ is a Boltzmann constant, and ‘T’ is an absolute temperature.

The energy barrier EBmay be represented by the following formula 2.

Here, ‘Keff’ is an effective magnetic anisotropy energy density of a magnetic layer, and ‘V’ is a volume of the magnetic layer.

Each of the first pinned layer122and the second pinned layer126may be configured to have the thermal stability Δ of 40 or more (for example, ranges from 40 to 1000). For example, each of the first pinned layer122and the second pinned layer126may be formed of a material having a relatively great perpendicular magnetic anisotropy energy. In this case, the effective magnetic anisotropy energy density Keffof each of the first pinned layer122and the second pinned layer126may be increased, and thus the energy barrier EBof each of the first pinned layer122and the second pinned layer126may be increased. The energy barrier EBof each of the first pinned layer122and the second pinned layer126may be adjusted in such a way that the thermal stability Δ of each of the first pinned layer122and the second pinned layer126is 40 or more. For example, the adjusting of the energy barrier EBof each of the first pinned layer122and the second pinned layer126may include adjusting the effective magnetic anisotropy energy density Keffof each of the first pinned layer122and the second pinned layer126.

Each of the first free layer131and the second free layer133may be configured to have the thermal stability Δ of 15 or less. For example, each of the first free layer131and the second free layer133may be formed of a material having a relatively small perpendicular magnetic anisotropy energy. In this case, the effective magnetic anisotropy energy density Keffof each of the first free layer131and the second free layer133may be reduced, and thus the energy barrier EBof each of the first free layer131and the second free layer133may be reduced. The energy barrier EBof each of the first free layer131and the second free layer133may be adjusted in such a way that the thermal stability Δ of each of the first free layer131and the second free layer133is 15 or less. For example, the adjusting of the energy barrier EBof each of the first free layer131and the second free layer133may include adjusting the effective magnetic anisotropy energy density Keffof each of the first free layer131and the second free layer133.

When the free magnetic layer130includes the first free layer131and the second free layer133which are antiferromagnetically coupled to each other, the energy barrier EBof the free magnetic layer130may be represented by the following formula 3.

Here, ‘K1’ is a magnetic anisotropy constant of the first free layer131, ‘K2’ is a magnetic anisotropy constant of the second free layer133, ‘T1’ is a thickness of the first free layer131, ‘T2’ is a thickness of the second free layer133, ‘Ms1’ is a saturation magnetization of the first free layer131, ‘Ms2’ is a saturation magnetization of the second free layer133, ‘(Nz−Nx)’ is a demagnetizing tensor between the first free layer131and the second free layer133, and ‘A’ is an area of a top surface of the free magnetic layer130when viewed in a plan view.

The energy barrier EBof the free magnetic layer130may be adjusted in such a way that the thermal stability Δ of the free magnetic layer130is 15 or less. For example, the adjusting of the energy barrier EBof the free magnetic layer130may include adjusting the magnetic anisotropy constant, the saturation magnetization and the thickness of each of the first free layer131and the second free layer133, and adjusting the area of the top surface of the free magnetic layer130, as shown in the formula 3.

For example, the free magnetic layer130may be configured to have the thermal stability Δ in a range of 0 to 15. The thermal stability Δ of the free magnetic layer130may be greater than 0 and less than 15. For example, the thermal stability Δ of the free magnetic layer130may be equal to or greater than 1 and equal to or less than 11. For example, each of the first free layer131and the second free layer133may be configured to have the thermal stability Δ in a range of 0 to 15. The thermal stability Δ of each of the first free layer131and the second free layer133may be greater than 0 and less than 15. For example, the thermal stability Δ of each of the first free layer131and the second free layer133may be equal to or greater than 1 and equal to or less than 11.

Since the free magnetic layer130is configured to have the relatively low thermal stability Δ, a relaxation time of the magnetic tunnel junction device500may be reduced. The relaxation time may mean a time for which data written in the magnetic tunnel junction device500is retained. For example, since each of the first free layer131and the second free layer133is configured to have the relatively low thermal stability Δ, a time (e.g., the relaxation time) for which the magnetization direction131M/133M of each of the first and second free layers131and133remains in parallel or antiparallel to the magnetization direction126M of the second pinned layer126may be reduced. The relaxation time of the magnetic tunnel junction device500may determine an operating speed of a stochastic computing system to be described later. Since the relaxation time of the magnetic tunnel junction device500is reduced, the operating speed of the stochastic computing system may be increased.

The free magnetic layer130may be configured to allow a net magnetization Mst of the free magnetic layer130to be reduced. For example, the free magnetic layer130may be configured in such a way that the net magnetization Mst of the free magnetic layer130is 0 (zero). When the free magnetic layer130includes a plurality of free layers, the net magnetization Mst of the free magnetic layer130may satisfy a condition of the following formula 4.

Here, Msi′ is a saturation magnetization of each of the plurality of free layers, and ‘Ti’ is a thickness of each of the plurality of free layers, and ‘n’ is the number of the plurality of free layers.

For example, when the free magnetic layer130includes the first free layer131and the second free layer133(e.g., n=2 in the formula 4), the net magnetization Mst of the free magnetic layer130may satisfy a condition of the following formula 5.

Here, ‘Ms1’ and ‘T1’ are a saturation magnetization and a thickness of the first free layer131, respectively, and ‘Ms2’ and ‘T2’ are a saturation magnetization and a thickness of the second free layer133, respectively. The thickness T1 of the first free layer131and the thickness T2 of the second free layer133may be measured in a direction perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The saturation magnetization Ms1 and the thickness T1 of the first free layer131and the saturation magnetization Ms2 and the thickness T2 of the second free layer133may be adjusted in such a way that the net magnetization Mst of the free magnetic layer130is 0.

Since the free magnetic layer130is configured to allow the net magnetization Mst to be reduced (e.g., to allow the net magnetization Mst to be 0), a demagnetization field of the free magnetic layer130may be reduced, and thus the relaxation time of the magnetic tunnel junction device500may be reduced.

According to embodiments of inventive concepts, the free magnetic layer130may include the first free layer131and the second free layer133, which are antiferromagnetically coupled to each other, and the magnetization directions131M and133M of the first free layer131and the second free layer133may be substantially perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The energy barrier EBof each of the first and second free layers131and133or the energy barrier EBof the free magnetic layer130may be adjusted in such a way that the thermal stability Δ of the free magnetic layer130is 15 or less (e.g., 0 to 15). In addition, the saturation magnetization Ms1/Ms2 and the thickness T1/T2 of each of the first and second free layers131and133may be adjusted in such a way that the net magnetization Mst of the free magnetic layer130is 0. Thus, the relaxation time of the magnetic tunnel junction device500may be reduced, and as a result, the operating speed of the stochastic computing system including the magnetic tunnel junction device500may be increased.

FIG.2is a cross-sectional view illustrating a magnetic tunnel junction device according to some embodiments of inventive concepts. Hereinafter, differences between the present embodiment and the embodiment ofFIG.1will be mainly described for the purpose of ease and convenience in explanation.

Referring toFIG.2, a magnetic tunnel junction device510may include a pinned magnetic layer120, a free magnetic layer130, and a tunnel barrier layer TBR between the pinned and free magnetic layers120and130. The magnetic tunnel junction device510may further include a lower electrode BE connected to a source line SL, and an upper electrode TE connected to a bit line BL. In some embodiments, the pinned magnetic layer120may be disposed between the lower electrode BE and the tunnel barrier layer TBR, and the free magnetic layer130may be disposed between the upper electrode TE and the tunnel barrier layer TBR. The magnetic tunnel junction device510may further include a seed layer110between the lower electrode BE and the pinned magnetic layer120, and a capping layer140between the upper electrode TE and the free magnetic layer130.

The free magnetic layer130may include a first free layer131, a second free layer133and a third free layer135which are sequentially stacked on the tunnel barrier layer TBR; a first spacer layer132between the first free layer131and the second free layer133; and a second spacer layer134between the second free layer133and the third free layer135. The first free layer131may be disposed between the tunnel barrier layer TBR and the capping layer140, and the second free layer133may be disposed between the first free layer131and the capping layer140. The third free layer135may be disposed between the second free layer133and the capping layer140. The first spacer layer132may be disposed between the first free layer131and the second free layer133, and the first free layer131and the second free layer133may be antiferromagnetically coupled to each other by the first spacer layer132. The second spacer layer134may be disposed between the second free layer133and the third free layer135, and the second free layer133and the third free layer135may be antiferromagnetically coupled to each other by the second spacer layer134.

The first free layer131, the first spacer layer132and the second free layer133may be substantially the same as the first free layer131, the spacer layer132and the second free layer133described with reference toFIG.1, respectively.

The third free layer135may have a changeable magnetization direction135M, and the magnetization direction135M of the third free layer135may be substantially perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The third free layer135may be antiferromagnetically coupled to the second free layer133by the second spacer layer134, and thus the magnetization direction135M of the third free layer135may be antiparallel to the magnetization direction133M of the second free layer133. The third free layer135may include a magnetic element and may include at least one of, for example, iron (Fe), cobalt (Co), or nickel (Ni). The third free layer135may include at least one of the perpendicular magnetic material (e.g., CoFeTb, CoFeGd, CoFeDy), the perpendicular magnetic material having the L10structure, the CoPt alloy having the HCP lattice structure, or the perpendicular magnetic structure. The third free layer135may include a magnetic material (e.g., CoFeB) which has a perpendicular magnetic property by magnetic anisotropy induced by a junction of the free magnetic layer130(e.g., the first free layer131) and the tunnel barrier layer TBR. In certain embodiments, the third free layer135may include a Co-based Heusler alloy.

The second spacer layer134may include a non-magnetic metal. For example, the second spacer layer134may include ruthenium (Ru), iridium (Ir), tungsten (W), tantalum (Ta), or any alloy thereof.

The free magnetic layer130may be configured to have a thermal stability Δ of 15 or less (e.g., in a range of 0 to 15). For example, each of the first to third free layers131,133and135may be configured to have the thermal stability Δ of 15 or less (e.g., in a range of 0 to 15). As described with reference to the formula 1 and the formula 2, the energy barrier EBof each of the first to third free layers131,133and135may be adjusted in such a way that the thermal stability Δ of each of the first to third free layers131,133and135is 15 or less (e.g., in a range of 0 to 15). The thermal stability Δ of each of the first to third free layers131,133and135may be greater than 0 and less than 15. For example, the thermal stability Δ of each of the first to third free layers131,133and135may be equal to or greater than 1 and equal to or less than 11.

The free magnetic layer130may be configured to allow a net magnetization Mst of the free magnetic layer130to be reduced. For example, the free magnetic layer130may be configured in such a way that the net magnetization Mst of the free magnetic layer130is 0 (zero). For example, when the free magnetic layer130includes the first free layer131, the second free layer133and the third free layer135(e.g., n=3 in the formula 4), the net magnetization Mst of the free magnetic layer130may satisfy a condition of the following formula 6.

Here, ‘Ms1’ and ‘T1’ are a saturation magnetization and a thickness of the first free layer131, respectively, ‘Ms2’ and ‘T2’ are a saturation magnetization and a thickness of the second free layer133, respectively, and ‘Ms3’ and ‘T3’ are a saturation magnetization and a thickness of the third free layer135, respectively. The thickness T1 of the first free layer131, the thickness T2 of the second free layer133and the thickness T3 of the third free layer135may be measured in the direction perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The saturation magnetization Ms1 and the thickness T1 of the first free layer131, the saturation magnetization Ms2 and the thickness T2 of the second free layer133and the saturation magnetization Ms3 and the thickness T3 of the third free layer135may be adjusted in such a way that the net magnetization Mst of the free magnetic layer130is 0.

Except for the differences described above, other features and components of the magnetic tunnel junction device510according to the present embodiment may be substantially the same as corresponding features and components of the magnetic tunnel junction device500described with reference toFIG.1.

FIG.3is a cross-sectional view illustrating a magnetic tunnel junction device according to some embodiments of inventive concepts. Hereinafter, differences between the present embodiment and the embodiments ofFIGS.1and2will be mainly described for the purpose of ease and convenience in explanation.

Referring toFIG.3, a magnetic tunnel junction device520may include a pinned magnetic layer120, a free magnetic layer130, and a tunnel barrier layer TBR between the pinned and free magnetic layers120and130. The magnetic tunnel junction device520may further include a lower electrode BE connected to a source line SL, and an upper electrode TE connected to a bit line BL. In some embodiments, the pinned magnetic layer120may be disposed between the lower electrode BE and the tunnel barrier layer TBR, and the free magnetic layer130may be disposed between the upper electrode TE and the tunnel barrier layer TBR. The magnetic tunnel junction device520may further include a seed layer110between the lower electrode BE and the pinned magnetic layer120, and a capping layer140between the upper electrode TE and the free magnetic layer130.

The free magnetic layer130may include a first free layer131, a second free layer133, a third free layer135and a fourth free layer137which are sequentially stacked on the tunnel barrier layer TBR; a first spacer layer132between the first free layer131and the second free layer133; a second spacer layer134between the second free layer133and the third free layer135; and a third spacer layer136between the third free layer135and the fourth free layer137. The first free layer131may be disposed between the tunnel barrier layer TBR and the capping layer140, and the second free layer133may be disposed between the first free layer131and the capping layer140. The third free layer135may be disposed between the second free layer133and the capping layer140, and the fourth free layer137may be disposed between the third free layer135and the capping layer140. The first spacer layer132may be disposed between the first free layer131and the second free layer133, and the first free layer131and the second free layer133may be antiferromagnetically coupled to each other by the first spacer layer132. The second spacer layer134may be disposed between the second free layer133and the third free layer135, and the second free layer133and the third free layer135may be antiferromagnetically coupled to each other by the second spacer layer134. The third spacer layer136may be disposed between the third free layer135and the fourth free layer137, and the third free layer135and the fourth free layer137may be antiferromagnetically coupled to each other by the third spacer layer136.

The first free layer131, the first spacer layer132and the second free layer133may be substantially the same as the first free layer131, the spacer layer132and the second free layer133described with reference toFIG.1, respectively. The third free layer135and the second spacer layer134may be substantially the same as the third free layer135and the second spacer layer134described with reference toFIG.2.

The fourth free layer137may have a changeable magnetization direction137M, and the magnetization direction137M of the fourth free layer137may be substantially perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The fourth free layer137may be antiferromagnetically coupled to the third free layer135by the third spacer layer136, and thus the magnetization direction137M of the fourth free layer137may be antiparallel to the magnetization direction135M of the third free layer135. The fourth free layer137may include a magnetic element and may include at least one of, for example, iron (Fe), cobalt (Co), or nickel (Ni). The fourth free layer137may include at least one of the perpendicular magnetic material (e.g., CoFeTb, CoFeGd, CoFeDy), the perpendicular magnetic material having the L10structure, the CoPt alloy having the HCP lattice structure, or the perpendicular magnetic structure. The fourth free layer137may include a magnetic material (e.g., CoFeB) which has a perpendicular magnetic property by magnetic anisotropy induced by a junction of the free magnetic layer130(e.g., the first free layer131) and the tunnel barrier layer TBR. In certain embodiments, the fourth free layer137may include a Co-based Heusler alloy.

The third spacer layer136may include a non-magnetic metal. For example, the third spacer layer136may include ruthenium (Ru), iridium (Ir), tungsten (W), tantalum (Ta), or any alloy thereof.

The free magnetic layer130may be configured to have a thermal stability Δ of 15 or less (e.g., in a range of 0 to 15). For example, each of the first to fourth free layers131,133,135and137may be configured to have the thermal stability Δ of 15 or less (e.g., in a range of 0 to 15). As described with reference to the formula 1 and the formula 2, the energy barrier EBof each of the first to fourth free layers131,133,135and137may be adjusted in such a way that the thermal stability Δ of each of the first to fourth free layers131,133,135and137is 15 or less (e.g., in a range of 0 to 15). The thermal stability Δ of each of the first to fourth free layers131,133,135and137may be greater than 0 and less than 15. For example, the thermal stability Δ of each of the first to fourth free layers131,133,135and137may be equal to or greater than 1 and equal to or less than 11.

The free magnetic layer130may be configured to allow a net magnetization Mst of the free magnetic layer130to be reduced. For example, the free magnetic layer130may be configured in such a way that the net magnetization Mst of the free magnetic layer130is 0 (zero). For example, when the free magnetic layer130includes the first free layer131, the second free layer133, the third free layer135and the fourth free layer137(e.g., n=4 in the formula 4), the net magnetization Mst of the free magnetic layer130may satisfy a condition of the following formula 7.

Here, ‘Ms1’ and ‘T1’ are a saturation magnetization and a thickness of the first free layer131, respectively, ‘Ms2’ and ‘T2’ are a saturation magnetization and a thickness of the second free layer133, respectively, ‘Ms3’ and ‘T3’ are a saturation magnetization and a thickness of the third free layer135, respectively, and ‘Ms4’ and ‘T4’ are a saturation magnetization and a thickness of the fourth free layer137, respectively. The thickness T1 of the first free layer131the thickness T2 of the second free layer133, the thickness T3 of the third free layer135and the thickness T4 of the fourth free layer137may be measured in the direction perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131). The saturation magnetization Ms1 and the thickness T1 of the first free layer131, the saturation magnetization Ms2 and the thickness T2 of the second free layer133, the saturation magnetization Ms3 and the thickness T3 of the third free layer135, and the saturation magnetization Ms4 and the thickness T4 of the fourth free layer137may be adjusted in such a way that the net magnetization Mst of the free magnetic layer130is 0.

Except for the differences described above, other features and components of the magnetic tunnel junction device520according to the present embodiment may be substantially the same as corresponding features and components of the magnetic tunnel junction devices500and510described with reference toFIGS.1and2.

The free magnetic layer130may have a synthetic antiferromagnetic (SAF) structure including two free layers (e.g., the first and second free layers131and133) likeFIG.1, a synthetic antiferromagnetic structure including three free layers (e.g., the first to third free layers131,133and135) likeFIG.2, or a synthetic antiferromagnetic structure including four free layers (e.g., the first to fourth free layers131,133,135and137) likeFIG.3. However, embodiments of inventive concepts are not limited thereto.

In certain embodiments, the free magnetic layer130may have a synthetic antiferromagnetic structure including a plurality of free layers (e.g., five or more free layers) and a plurality of spacer layers disposed between the plurality of free layers, respectively. The plurality of free layers may be antiferromagnetically coupled to each other by the plurality of spacer layers. Each of the plurality of free layers may have a magnetization direction substantially perpendicular to the interface between the free magnetic layer130and the tunnel barrier layer TBR. In this case, the free magnetic layer130may be configured to have a thermal stability Δ of 15 or less (e.g., in a range of 0 to 15). For example, each of the plurality of free layers may be configured to have the thermal stability Δ of 15 or less (e.g., in a range of 0 to 15). As described with reference to the formula 1 and the formula 2, the energy barrier EBof each of the plurality of free layers may be adjusted in such a way that the thermal stability Δ of each of the plurality of free layers is 15 or less (e.g., in a range of 0 to 15). In addition, the free magnetic layer130may be configured to allow a net magnetization Mst of the free magnetic layer130to be reduced. For example, the free magnetic layer130may be configured in such a way that the net magnetization Mst of the free magnetic layer130is 0 (zero). As described with reference to the formula 4, a saturation magnetization Ms, and a thickness Tiof each of the plurality of free layers may be adjusted in such a way that the net magnetization Mst of the free magnetic layer130is 0.

FIG.4is a cross-sectional view illustrating a magnetic tunnel junction device according to some embodiments of inventive concepts. Hereinafter, differences between the present embodiment and the embodiments ofFIG.1will be mainly described for the purpose of ease and convenience in explanation.

Referring toFIG.4, a magnetic tunnel junction device530may include a pinned magnetic layer120, a free magnetic layer130, and a tunnel barrier layer TBR between the pinned and free magnetic layers120and130. The magnetic tunnel junction device530may further include a lower electrode BE connected to a source line SL, and an upper electrode TE connected to a bit line BL. In some embodiments, the free magnetic layer130may be disposed between the lower electrode BE and the tunnel barrier layer TBR, and the pinned magnetic layer120may be disposed between the upper electrode TE and the tunnel barrier layer TBR. The magnetic tunnel junction device530may further include a capping layer140between the lower electrode BE and the free magnetic layer130.

The pinned magnetic layer120may include a first pinned layer122, a second pinned layer126, and an exchange coupling layer124between the first and second pinned layers122and126. The first pinned layer122may be disposed between the upper electrode TE and the tunnel barrier layer TBR, and the second pinned layer126may be disposed between the first pinned layer122and the tunnel barrier layer TBR. The exchange coupling layer124may be disposed between the first pinned layer122and the second pinned layer126, and the first pinned layer122and the second pinned layer126may be antiferromagnetically coupled to each other by the exchange coupling layer124. The first pinned layer122, the second pinned layer126and the exchange coupling layer124may be substantially the same as the first pinned layer122, the second pinned layer126and the exchange coupling layer124described with reference toFIG.1, except the arrangement thereof.

The free magnetic layer130may include a first free layer131, a second free layer133, and a spacer layer132between the first and second free layers131and133. The first free layer131may be disposed between the tunnel barrier layer TBR and the lower electrode BE, and the second free layer133may be disposed between the first free layer131and the lower electrode BE. The spacer layer132may be disposed between the first free layer131and the second free layer133, and the first free layer131and the second free layer133may be antiferromagnetically coupled to each other by the spacer layer132. The capping layer140may be disposed between the second free layer133and the lower electrode BE. The first free layer131, the second free layer133and the spacer layer132may be substantially the same as the first free layer131, the second free layer133and the spacer layer132described with reference toFIG.1, except the arrangement thereof.

Except for the relative arrangement of the pinned magnetic layer120, the free magnetic layer130and the capping layer140, other features of the magnetic tunnel junction device530according to the present embodiment may be substantially the same as corresponding features of the magnetic tunnel junction device500described with reference toFIG.1.

FIG.5is a graph showing a relaxation time of a magnetic tunnel junction device according to a structure of a free magnetic layer.

Referring toFIG.5, a free magnetic layer according to an experimental example may have a synthetic antiferromagnetic (SAF) structure including free layers having perpendicular magnetizations. Hereinafter, as described with reference toFIG.1, it may be understood that when a component is referred to as having the perpendicular magnetization, it may have a magnetization direction substantially perpendicular to the interface between the free magnetic layer130and the tunnel barrier layer TBR. The free magnetic layer according to the experimental example may be substantially the same as the free magnetic layer130ofFIG.1. For example, the free magnetic layer according to the experimental example may include the first free layer131and the second free layer133which are antiferromagnetically coupled to each other, and the magnetization directions131M and133M of the first free layer131and the second free layer133may be substantially perpendicular to the interface between the tunnel barrier layer TBR and the free magnetic layer130(e.g., the first free layer131).

A free magnetic layer according to a comparative example 1 may include free layers having perpendicular magnetizations but may have a structure in which the free layers are ferromagnetically coupled to each other. A free magnetic layer according to a comparative example 2 may have a synthetic antiferromagnetic structure including free layers having horizontal magnetizations. Hereinafter, unlike the descriptions ofFIG.1, it may be understood that when a component is referred to as having the horizontal magnetization, it may have a magnetization direction parallel to the interface between the free magnetic layer130and the tunnel barrier layer TBR. A free magnetic layer according to a comparative example 3 may include free layers having horizontal magnetizations but may have a structure in which the free layers are ferromagnetically coupled to each other. Each of the free magnetic layers according to the experimental example and the comparative examples 1 to 3 may be configured to have a thermal stability of 15 or less (e.g., in a range of 0 to 15).

According toFIG.5, it may be recognized that a relaxation time of a magnetic tunnel junction device is lowest in the free magnetic layer (the experimental example) of the synthetic antiferromagnetic structure including the free layers having the perpendicular magnetizations. In addition, it may be recognized that the relaxation time of the magnetic tunnel junction device is 100 ns or less when the free magnetic layer according to the experimental example has a thermal stability smaller than 10.

FIG.6is a graph showing a relaxation time of a magnetic tunnel junction device according to a net magnetization of a free magnetic layer.

Referring toFIG.6, when the free layers FL1 and FL2 (e.g., the first and second free layers131and133ofFIG.1) constituting the free magnetic layer according to the experimental example ofFIG.5have saturation magnetizations Ms of the same magnitude, a net magnetization of the free magnetic layer may be changed by changing a ratio of thicknesses of the free layers FL1 and FL2. In this case, as the ratio of the thicknesses of the free layers FL1 and FL2 decreases, the net magnetization of the free magnetic layer may decrease. According toFIG.6, it may be recognized that the relaxation time of the magnetic tunnel junction device decreases as the net magnetization of the free magnetic layer decreases.

FIG.7is a graph showing a relaxation time of a magnetic tunnel junction device according to a net magnetization of a free magnetic layer.

Referring toFIG.7, when the free layers constituting the free magnetic layer according to the experimental example ofFIG.5have the saturation magnetizations Ms of the same magnitude and the same thickness T, the net magnetization of the free magnetic layer may be changed by changing the number of the free layers. In this case, when the number of the free layers in the free magnetic layer is an even number, the net magnetization of the free magnetic layer may decrease. According toFIG.7, it may be recognized that the relaxation time of the magnetic tunnel junction device decreases as the net magnetization of the free magnetic layer decreases.

FIG.8is a block diagram illustrating a stochastic computing system according to some embodiments of inventive concepts.

Referring toFIG.8, a stochastic computing system1000may include a random bit-stream generator1, a write circuit2configured to provide a current for generating a bit in the random bit-stream generator1, a read circuit structure3including a read circuit configured to read a bit generated by the random bit-stream generator1and a sense amplifier for amplifying the generated bit, and a feedback loop4for transmitting bit data read by the read circuit structure3back to the write circuit2. The random bit-stream generator1may include at least one of the magnetic tunnel junction devices500,510,520and530according to the embodiments of inventive concepts, described with reference toFIGS.1to4. In other words, the magnetic tunnel junction device500/510/520/530according to the embodiments of inventive concepts may be used as the random bit-stream generator1of the stochastic computing system1000.

According to embodiments of inventive concepts, the free magnetic layer130of the magnetic tunnel junction device500/510/520/530may have the synthetic antiferromagnetic (SAF) structure in which the plurality of free layers having the perpendicular magnetizations are antiferromagnetically coupled to each other. The energy barrier EBof each of the plurality of free layers or the energy barrier EBof the free magnetic layer130may be adjusted in such a way that the thermal stability Δ of the free magnetic layer130is 15 or less (e.g., in a range of 0 to 15). In addition, the saturation magnetization Ms and the thickness T of each of the plurality of free layers may be adjusted in such a way that the net magnetization Mst of the free magnetic layer130is 0. Thus, the relaxation time of the magnetic tunnel junction device500/510/520/530may be reduced, and as a result, the operating speed of the stochastic computing system including the magnetic tunnel junction device500/510/520/530may be increased.