Magnetic memory element, magnetic memory, and control method for controlling magnetic memory

Abstract

This magnetic memory element comprises: a ferromagnetic layer having reversible magnetization; and a ferroelectric layer laminated with the ferromagnetic layer and having a polarization direction component in a lamination direction, wherein the ease of magnetization reversal of the ferromagnetic layer differs between a state in which the polarization direction of the ferroelectric layer is in a first polarization direction and a state in which the polarization direction is in a second polarization direction different from the first polarization direction.

Description

Magnetic memory element, magnetic memory, and control method for controlling magnetic memory

[0001] The present invention relates to a magnetic memory element, a magnetic memory, and a control method for controlling a magnetic memory.

[0002] Magnetoresistive solid-state magnetic memory (MRAM: Magnetic Random Access Memory) is known as a next-generation recording element. In MRAM, a stacked structure including magnetic elements is arranged in a matrix, and information recorded in each element is read using the magnetoresistive effect of the magnetic elements. Information is written by controlling the magnetization direction of the magnetic elements. Since MRAM replaces the magnetization state with a recorded state, it is nonvolatile in principle. Therefore, it is expected to be a nonvolatile memory that can satisfy all requirements, such as low power consumption, high speed, large capacity, write endurance, and compatibility with semiconductors. In particular, nonvolatility using magnetism can theoretically reduce the standby power required for information retention to zero, and is therefore considered important as an innovative technology that realizes green technology. The memory element of MRAM includes a magnetic tunnel junction (MTJ: Magnetic Tunneling Junction). An MTJ has a structure in which an insulating layer is disposed between two ferromagnetic layers, and the tunnel current flowing between the two ferromagnetic layers can be changed by controlling the magnetization direction of the ferromagnetic layers. Controlling the magnetization of the ferromagnetic layers has the problem of high power consumption. As a method for reducing the power consumption required for magnetization reversal, a method utilizing the VCMA (Voltage Controlled Magnetic Anisotropy) effect has been investigated (e.g., JP 2013-045840 A). The VCMA effect is a method for controlling the energy required to reverse magnetization by applying an electric field to a dielectric layer formed on a magnetic layer.

[0003] However, in conventional technologies utilizing the VCMA effect, the energy required for magnetization reversal could only be controlled when an electric field was applied to the dielectric layer, and there was a problem in that the magnetization would return to its original state if no electric field was applied.

[0004] The present disclosure has been made in consideration of these circumstances, and aims to provide a magnetic memory element, a magnetic memory, and a control method for controlling a magnetic memory that can control the energy required for magnetization reversal without continuously applying an electric field.

[0005] A magnetic memory element according to a first aspect of the present disclosure is a magnetic memory element including a ferromagnetic layer whose magnetization can be reversed, and a ferroelectric layer stacked with the ferromagnetic layer and having a polarization direction component in the stacking direction, wherein the ease of magnetization reversal of the ferromagnetic layer differs between a state in which the polarization direction of the ferroelectric layer is in a first polarization direction and a state in which the polarization direction is in a second polarization direction different from the first polarization direction.

[0006] A magnetic memory element according to a second aspect of the present disclosure is the magnetic memory element according to the first aspect, wherein the ferroelectric layer has a crystal structure other than perovskite made of oxide or nitride.

[0007] A magnetic memory element according to a third aspect of the present disclosure is the magnetic memory element according to the second aspect, wherein the crystal structure is a wurtzite structure or a fluorite structure.

[0008] A magnetic memory element according to a fourth aspect of the present disclosure is the magnetic memory element according to any one of the first to third aspects, wherein the ferroelectric layer stores electrons at a rate of 10 １３ / cm ２ It consists of a ferroelectric substance that accumulates or depletes.

[0009] A magnetic memory element according to a fifth aspect of the present disclosure is the magnetic memory element according to any one of the first to fourth aspects, wherein the ferroelectric layer has a thickness of 50 nm or less.

[0010] A magnetic memory according to a sixth aspect of the present disclosure includes a ferromagnetic layer whose magnetization is reversible, a ferroelectric layer stacked with the ferromagnetic layer and having a polarization direction component in the stacking direction, a first circuit that controls the ease of magnetization reversal of the ferromagnetic layer by controlling the polarization direction of the ferroelectric layer with an electric field applied to the ferroelectric layer, and a second circuit that controls the magnetization direction of the ferromagnetic layer with a magnetization reversal torque applied to the ferromagnetic layer.

[0011] A control method according to a seventh aspect of the present disclosure is a control method for controlling a magnetic memory including a ferromagnetic layer whose magnetization can be reversed and a ferroelectric layer stacked with the ferromagnetic layer and having a polarization direction component in the stacking direction, and includes a first step of controlling the ease of magnetization reversal of the ferromagnetic layer by controlling the polarization direction of the ferroelectric layer with an electric field applied to the ferroelectric layer, and a second step of controlling the magnetization direction of the ferromagnetic layer with a magnetization reversal torque applied to the ferromagnetic layer.

[0012] According to the present invention, the energy required for magnetization reversal can be controlled without continuously applying an electric field.

[0013] 1 shows a cross-sectional view of a stack according to the present embodiment. 2 shows the relationship between the polarization direction of the ferroelectric layer 120 and the ease of magnetization reversal of the ferromagnetic layer 110. 3 shows an example of a magnetic memory 10 according to the present embodiment. 4 shows the stack structure used in Experiment 1. 5 shows the results of XRD of AlScN. 6 shows the stack structure used in Experiment 2. 7 shows the CV characteristics of AlScN. 7 shows the stack structure used in Experiment 3. 8 shows the results of magnetic anisotropy measurement. 9 shows the stack structure used in Experiment 4. 10 shows the CV characteristics of AlScN. 11 shows the results of Kerr measurement of CoFeB. 12 shows the results of confirming the repeatability of magnetic reversal ease.

[0014] An example of an embodiment of the present disclosure will be described below with reference to the drawings. In each drawing, the same or equivalent components and parts are designated by the same reference numerals. Furthermore, the dimensional proportions in the drawings are exaggerated for the sake of explanation and may differ from the actual proportions.

[0015] FIG. 1 shows a cross-sectional view of a stack according to this embodiment. The stack has a stacked structure in which a ferromagnetic layer 110 and a ferroelectric layer 120 are stacked. Note that this figure shows, as an example, a case in which the ferroelectric layer 120 is stacked directly on the ferromagnetic layer 110. However, other layers, such as a cap layer (described later), may be interposed between the ferromagnetic layer 110 and the ferroelectric layer 120. That is, the ferroelectric layer 120 may be stacked close to the ferromagnetic layer 110 within a predetermined range.

[0016] <Ferromagnetic Layer 110> The magnetization of the ferromagnetic layer 110 is reversible. The ferromagnetic layer 110 includes a ferromagnetic material. Examples of ferromagnetic materials include ferromagnetic metals such as iron (Fe), cobalt (Co), FeCo alloys, and CoFeB alloys, as well as ferromagnetic half-metals. Ferromagnitude can be confirmed by the M-H curve (magnetization-external magnetic field curve) obtained by VSM (Vibrating Sample Magnetometer) measurement or the like showing a loop with hysteresis.

[0017] (Film Formation Method) The ferromagnetic layer 110 is formed by, for example, a sputtering method or an atomic layer deposition method. Preferably, a sputtering method may be used, and more preferably, an atomic layer deposition method may be used. The manufacturing method of the ferromagnetic layer 110 according to this embodiment may include a sputtering step.

[0018] <Ferroelectric Layer 120> The ferroelectric layer 120 is stacked with the ferromagnetic layer 110. The ferroelectric layer 120 includes a ferroelectric material. The ferroelectric material is preferably a ferroelectric material having a crystal structure other than a perovskite structure made of oxide or nitride. In particular, the ferroelectric material is preferably a ferroelectric material having a wurtzite structure or a ferroelectric material having a fluorite structure. Ferroelectrics have a size effect in which ferroelectricity is lost when made thin, and the thickness at which ferroelectricity is lost varies greatly depending on the material. Ferroelectric materials with a wurtzite structure or a fluorite structure are known to exhibit ferroelectricity even when their thickness is 50 nm or less.

[0019] Furthermore, as a characteristic of ferroelectrics, there are two stable states (polarized states) in the crystal, and they have the property of being able to maintain the polarized state even when disconnected from the power source. Ferroelectric materials with a wurtzite structure are preferred because they have a large amount of charge (residual polarization value) that remains when the power source is disconnected. The amount of charge is 10 electrons. １３ / cm ２ As will be described later, the ferroelectric layer 120 can be in either a negative charge state or a positive charge state depending on the polarization direction. １３ / cm２ The above expressions indicate that "accumulation" or "depletion" is preferable. In other words, "depletion" can be defined as meaning that negatively charged electrons are lost from the surface, resulting in a relatively large amount of positive charges being accumulated on the surface.

[0020] (Composition) Ferroelectrics having a wurtzite structure are represented by the general formula Z(1-x)M(x)W, where Z is at least one element selected from the group consisting of Al, In, and Ga, M is at least one element selected from the group consisting of Sc, B, C, Si, Gd, Cr, Zn, Y, La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Ti, Hf, and Zr, and W is at least one element selected from the group consisting of N, O, S, Se, F, Cl, Br, and I. As an example, the wurtzite structure may be mainly composed of AlN.

[0021] (Film Formation Method) The ferroelectric layer 120 is formed by, for example, a sputtering method or an atomic layer deposition method. Preferably, a sputtering method may be used, and more preferably, an atomic layer deposition method may be used. The method for manufacturing the ferroelectric layer 120 according to this embodiment may include a sputtering step. For example, in the case of an apparatus with a 3-inch target, the sputtering step may be performed by setting a DC voltage of 200 V or more, a DC current of 1 A or more, a gas pressure of 1 Pa or less, and a substrate temperature of room temperature to 400° C. or less.

[0022] The ferroelectric layer 120 has a polarization direction component in the stacking direction. In particular, the direction of ferroelectric polarization reversal of the ferroelectric layer 120 is preferably oriented, for example, within 10 degrees from an axis perpendicular to the film surface. In other words, the polarization direction of the ferroelectric layer 120 is preferably approximately perpendicular to the film surface. The thickness of the ferroelectric layer 120 is preferably 5 nm or more and 50 nm or less.

[0023] (Crystallineness) The ferroelectric layer 120 is, for example, a polycrystalline film. Being polycrystalline allows for easy quality control and stable, inexpensive production. The crystal grain size of the polycrystalline film is preferably 5 nm or more and 200 nm or less, and more preferably 5 nm or more and 50 nm or less. The crystal grain size is, for example, an average crystal grain size measured by X-ray diffraction or cross-sectional transmission electron microscope observation. For example, in X-ray diffraction, the crystal grain size can be estimated from the crystallite size using Scherrer's formula, which uses the half-width of the diffraction signal and the X-ray wavelength, and in cross-sectional transmission electron microscope observation, it can be evaluated by obtaining a dark-field image. The crystal grains are, for example, columnar.

[0024] (Ferroelectricity) A P-E hysteresis evaluation can be used to evaluate the ferroelectric properties of the ferroelectric layer 120. A P-E curve can be obtained by sandwiching a ferroelectric between electrodes and applying a voltage. The remanent polarization value can be estimated from the P-E curve. A ferroelectric property evaluation system (Toyo Corporation) can be used as an evaluation device. The upper electrode is preferably formed by a lift-off method using lithography. The electrode size is preferably 100 μm x 100 μm or less.

[0025] The inventors have found that simply stacking such a ferroelectric layer 120 on the ferromagnetic layer 110 changes the ease of magnetization reversal of the ferromagnetic layer 110. Here, the ease of magnetization reversal literally refers to the degree of ease with which magnetization can be reversed, meaning that the easier it is to reverse the magnetization, the smaller the energy required for magnetization reversal, and the more difficult it is to reverse the magnetization, the greater the energy required for magnetization reversal.

[0026] A conventional technique is known in which a dielectric layer is stacked on a magnetic layer and an electric field is applied to the dielectric layer to control the ease of magnetization reversal of the magnetic layer. In this conventional technique, applying an electric field to the dielectric layer causes charges to accumulate between the dielectric layer and the magnetic layer. The electric field created by the accumulated charges then changes the electronic state (particularly the amount of electrons) of the magnetic layer, changing the orbital magnetic moment (also known as magnetic anisotropy). This is thought to enable the ease of magnetization reversal of the magnetic layer to be controlled.

[0027] This effect is known as the VCMA effect. An MTJ element utilizing the VCMA effect can be used in an MRAM (Magnetoresistive Random Access Memory).

[0028] However, in the conventional technology, charge accumulates on the surface of the dielectric layer only when an electric field is applied, and the magnetization reversibility can be controlled only when an electric field is applied, so in the conventional technology, the magnetization reversibility returns to its original state when an electric field is not applied to the dielectric layer.

[0029] In contrast, the present disclosure utilizes the non-volatility of charge storage by replacing the dielectric layer in the conventional technology with a ferroelectric layer 120. The ferroelectric layer 120 is characterized by spontaneous polarization, and can maintain a state in which charges are stored on the surface of the ferroelectric layer 120 or a state in which the charges are depleted even without applying an electric field.

[0030] The inventors further found that the ease of magnetization reversal of the ferromagnetic layer 110 differs between states where the polarization direction of the ferroelectric layer 120 is different. Figure 2 shows the relationship between the polarization direction of the ferroelectric layer 120 and the ease of magnetization reversal of the ferromagnetic layer 110. As shown in this figure, the ease of magnetization reversal of the ferromagnetic layer 110 differs between a state where the polarization direction of the ferroelectric layer 120 is in a first polarization direction and a state where the polarization direction is in a second polarization direction different from the first polarization direction.

[0031] In this figure, the first polarization direction and the second polarization direction are shown as being perpendicular to the film surface as an example, but the first polarization direction and the second polarization direction do not necessarily have to be perpendicular to the film surface. Also, in this figure, the case where positive charges accumulate (electrons are depleted) on the surface of the ferroelectric layer 120 facing the ferromagnetic layer 110 is shown as the first polarization direction and the case where electrons accumulate is shown as the second polarization direction as an example, but the case where electrons accumulate may be shown as the first polarization direction and the case where positive charges accumulate may be shown as the second polarization direction.

[0032] For example, in a first state in which the polarization direction of the ferroelectric layer 120 is a first polarization direction, the magnetization of the ferromagnetic layer 110 may be relatively easy to reverse. In a second state in which the polarization direction of the ferroelectric layer 120 is a second polarization direction, the magnetization of the ferromagnetic layer 110 may be relatively difficult to reverse.

[0033] Here, the terms "relatively easy" and "relatively difficult" may be defined using the other of the first state and the second state as a comparison standard. Therefore, in the first state, it may be interpreted as meaning that magnetization reversal is easier than in the second state, i.e., the energy required for magnetization reversal is smaller. Similarly, in the second state, it may be interpreted as meaning that magnetization reversal is more difficult than in the first state, i.e., the energy required for magnetization reversal is larger.

[0034] Alternatively or additionally, the terms "relatively easy" and "relatively difficult" may be defined with respect to an initial state in which the ferromagnetic layer 110 is not stacked with the ferroelectric layer 120. Therefore, in the first state, the ease of magnetization reversal may be interpreted as meaning that it is easier to reverse the magnetization than in the initial state, i.e., the energy required for magnetization reversal is smaller. Similarly, in the second state, the ease of magnetization reversal may be interpreted as meaning that it is more difficult to reverse the magnetization than in the initial state, i.e., the energy required for magnetization reversal is larger.

[0035] The inventors have considered that by applying this new principle to a magnetic memory, when writing information, the polarization direction of the ferroelectric layer 120 can be controlled in a direction that makes it easy to reverse the magnetization of the magnetization free layer, thereby reversing the magnetization of the magnetization free layer, and after the magnetization reversal, the polarization direction of the ferroelectric layer 120 can be controlled in a direction that makes it difficult to reverse the magnetization of the magnetization free layer, thereby increasing the magnetization stability of the magnetization free layer. Such a magnetic memory and a method for controlling the magnetic memory will now be described in detail.

[0036] 3 shows an example of a magnetic memory 10 according to this embodiment. The magnetic memory 10 includes a magnetic memory element 100 and a control circuit 200 connected to the magnetic memory element 100. For convenience, the diagram shows an example in which the control circuit 200 is connected to the bottom and top layers of the magnetic memory element 100. However, the control circuit 200 may be connected to each layer required for control in the magnetic memory element 100.

[0037] The magnetic memory element 100 is a magnetic structure including a stack of a ferromagnetic layer 110 and a ferroelectric layer 120. In this case, the ferromagnetic layer 110 may be applied to a first ferromagnetic layer 110 that functions as a magnetization free layer (also referred to as a memory layer). That is, the magnetic memory element 100 includes a first ferromagnetic layer 110 whose magnetization is reversible, and a ferroelectric layer 120 that is stacked with the first ferromagnetic layer 110 and has a polarization direction component in the stacking direction. Furthermore, the ease of magnetization reversal of the first ferromagnetic layer 110 differs between a state in which the polarization direction of the ferroelectric layer 120 is in a first polarization direction and a state in which the polarization direction is in a second polarization direction different from the first polarization direction.

[0038] In addition to the first ferromagnetic layer 110 and the ferroelectric layer 120, the magnetic memory element 100 may further include a substrate 130, a second ferromagnetic layer 140, a tunnel insulating layer 150, a cap layer 160, and a conductive layer 170.

[0039] The substrate 130 is the base of the magnetic memory element 100, which is a magnetic structure. An electrode (not shown) that functions as a lower electrode may be provided on the rear surface of the substrate 130.

[0040] The second ferromagnetic layer 140 is stacked on the substrate 130. The magnetization direction of the second ferromagnetic layer 140 is fixed in a predetermined direction. That is, the second ferromagnetic layer 140 functions as a magnetization fixed layer (also referred to as a reference layer). The second ferromagnetic layer 140 may contain the same ferromagnetic material as the first ferromagnetic layer 110, or may contain a different ferromagnetic material.

[0041] The tunnel insulating layer 150 is stacked on the second ferromagnetic layer 140. The tunnel insulating layer 150 magnetically separates the first ferromagnetic layer 110 and the second ferromagnetic layer 140. The first ferromagnetic layer 110, the tunnel insulating layer 150, and the second ferromagnetic layer 140 form an MTJ.

[0042] The cap layer 160 is stacked on the first ferromagnetic layer 110. The cap layer 160 may be provided to prevent a reaction between the first ferromagnetic layer 110 and the ferroelectric layer 120. The cap layer 160 may also be connected to the control circuit 200 and function as an electrode. A preferable material for the cap layer 160 is, for example, tantalum (Ta).

[0043] The conductive layer 170 is stacked on the ferroelectric layer 120. The conductive layer 170 is connected to the control circuit 200 and functions as an electrode. In this case, the cap layer 160 and the conductive layer 170 may function as a pair of electrodes for applying an electric field to the ferroelectric layer 120.

[0044] In the above description, the magnetic memory element 100 includes the cap layer 160 as an example, but the magnetic memory element 100 does not necessarily include the cap layer 160. In this case, if the first ferromagnetic layer 110 is conductive, the first ferromagnetic layer 110 and the conductive layer 170 may function as a pair of electrodes for applying an electric field to the ferroelectric layer 120.

[0045] The control circuit 200 is connected to the magnetic memory element 100, which is such a magnetic structure. The control circuit 200 includes a first circuit 210 and a second circuit 220.

[0046] The first circuit 210 controls the ease of magnetization reversal of the first ferromagnetic layer 110 by controlling the polarization direction of the ferroelectric layer 120 by an electric field applied to the ferroelectric layer 120. For example, the first circuit 210 may be a voltage application circuit that applies a voltage using the cap layer 160 and the conductive layer 170 as electrodes. In this case, the first circuit 210 may reverse the polarization direction of the ferroelectric layer 120 between a first polarization direction and a second polarization direction by reversing the positive and negative polarities of the voltages applied to the cap layer 160 and the conductive layer 170.

[0047] More specifically, the first circuit 210 may reverse the polarization direction depending on whether the magnetization direction is reversed or not. For example, when the magnetization direction is reversed, the first circuit 210 may control the polarization direction of the ferroelectric layer 120 by using an electric field so that positive charges accumulate on the surface of the ferroelectric layer 120 facing the first ferromagnetic layer 110, i.e., electrons are depleted. When the magnetization direction is not reversed, the first circuit 210 may control the polarization direction of the ferroelectric layer 120 by using an electric field so that electrons accumulate on the surface of the ferroelectric layer 120 facing the first ferromagnetic layer 110.

[0048] The second circuit 220 controls the magnetization direction of the first ferromagnetic layer 110 by a magnetization reversal torque applied to the first ferromagnetic layer 110. Here, the term “magnetization reversal torque” may be interpreted as referring to a physical quantity in general that exerts energy to rotate (including reversal) the direction of magnetization of the first ferromagnetic layer 110.

[0049] For example, when a spin current flows in a region where a localized magnetic moment exists, a spin transfer torque is generated on the magnetic moment due to the interaction between the spin current and the magnetic moment. The spin transfer torque excites precession of the magnetic moment, reversing the magnetization. The magnetization reversal torque may be, for example, such a spin transfer torque. In this case, the second circuit 220 may be a spin injection circuit that injects spins into the first ferromagnetic layer 110.

[0050] Alternatively or additionally, when a current is passed through the conductor, conduction electrons are injected into the first ferromagnetic layer 110, and the conduction electron spins act on the magnetization through exchange interaction, generating a spin torque, which causes the internal magnetization of the ferromagnetic layer 110 to revolve in the direction of the magnetic field, generating a rotational force. The magnetization reversal torque may be, for example, such a rotational force. In this case, the second circuit 220 may be a magnetic field generating circuit that generates an external magnetic field to be applied to the first ferromagnetic layer 110.

[0051] Here, the first circuit 210 only controls the magnetization reversibility of the first ferromagnetic layer 110, but does not also control the magnetization direction, and the second circuit 220 controls the magnetization direction of the first ferromagnetic layer 110. In other words, the control circuit 200 has separate circuits for controlling the magnetization reversibility of the first ferromagnetic layer 110 and for controlling the magnetization direction.

[0052] Therefore, the magnetization direction of the first ferromagnetic layer 110 is not limited to depend on the polarization direction of the ferroelectric layer 120. Therefore, the magnetization direction of the first ferromagnetic layer 110 can be either an in-plane direction or a perpendicular direction.

[0053] In this way, a magnetization reversibility control circuit for the magnetic structure may be provided, which is connected to a magnetic structure including a stack of a ferromagnetic layer 110 whose magnetization can be reversed and a ferroelectric layer 120 stacked with the ferromagnetic layer 110, and controls the magnetization reversibility of the ferromagnetic layer 110.

[0054] In addition, a control circuit for the magnetic structure may be provided, which is connected to a magnetic structure including a ferromagnetic layer 110 whose magnetization can be reversed and a stack of a ferroelectric layer 120 stacked with the ferromagnetic layer 110, and has a magnetization reversibility control circuit that controls the magnetization reversibility of the ferromagnetic layer 110, and a magnetization reversal torque circuit that applies a magnetization reversal torque to the ferromagnetic layer 110.

[0055] Furthermore, a control method may also be provided for controlling a magnetic memory 10 including a ferromagnetic layer 110 whose magnetization is reversible and a ferroelectric layer 120 that is stacked with the ferromagnetic layer 110 and has a polarization direction component in the stacking direction. The control method may include a first step of controlling the ease of magnetization reversal of the ferromagnetic layer 110 by controlling the polarization direction of the ferroelectric layer 120 with an electric field applied to the ferroelectric layer 120, and a second step of controlling the magnetization direction of the ferromagnetic layer 110 with a magnetization reversal torque applied to the ferromagnetic layer 110.

[0056] There may also be provided a method for setting a state of a magnetic structure including a stack of a ferromagnetic layer 110 whose magnetization can be reversed and a ferroelectric layer 120 stacked on the ferromagnetic layer 110. The method for setting a state may include a step of making the ease of magnetization reversal of the ferromagnetic layer 110 relatively easy by setting the polarization direction of the ferroelectric layer 120 to a first polarization direction, and a step of making the ease of magnetization reversal of the ferromagnetic layer 110 relatively difficult by setting the polarization direction of the ferroelectric layer 120 to a second polarization direction different from the first polarization direction.

[0057] Also provided may be a method for writing information to a magnetic memory element 100 including a stack of a ferromagnetic layer 110 whose magnetization can be reversed and a ferroelectric layer 120 stacked on the ferromagnetic layer 110. The information writing method may include a step of setting the polarization state of the ferroelectric layer 120 to an information writing mode to reverse the magnetization of the ferromagnetic layer 110. The information writing mode may be a mode in which the magnetization of the ferromagnetic layer 110 is relatively easy to reverse.

[0058] Also provided may be a method for controlling a magnetic memory device including a magnetic memory element 100 including a stack of a ferromagnetic layer 110 whose magnetization can be reversed and a ferroelectric layer 120 stacked on the ferromagnetic layer 110, and a magnetization reversal circuit connected to the magnetic memory element 100 for reversing the magnetization of the ferromagnetic layer 110. The method for controlling a magnetic memory device may include a step of setting the polarization state of the ferroelectric layer 120 to an information write mode to reverse the magnetization of the ferromagnetic layer 110.

[0059] The present inventors conducted various experiments to verify the discovered principle.

[0060] (Experiment 1) <Evaluation of AlScN Crystallinity> FIG. 4 shows the stacked structure used in Experiment 1. To obtain the stacked structure shown in this figure, an N-type Si wafer with a resistance value of 0.02 Ω·cm or less was prepared. The oxide film on the surface was removed by washing with hydrofluoric acid. A 5 nm TiN film was then formed on the clean Si surface, and a 30 nm AlScN film was then formed on top of that as the ferroelectric layer 120. Each thin film was formed as follows:

[0061] (TiN film formation) A film was formed by RF sputtering using a Ti target. Sputtering was performed by discharging at 300 W. The film formation conditions were that the substrate temperature was room temperature, the film formation pressure was adjusted to 0.2 Pa, and Ar and N ２ The film was formed by introducing the gases at a flow rate ratio of 2:3.

[0062] (AlScN film formation) A film was formed by DC sputtering using an AlSc (composition ratio 53:47) target. Sputtering was performed by discharging at 300 W. The film formation conditions were a substrate temperature of room temperature, a film formation pressure of 0.7 Pa, and Ar and N. ２ were introduced at a flow rate ratio of 1:2.

[0063] X-ray diffraction (XRD) of the AlScN film thus formed was measured under the following conditions: Instrument used: SmartLab manufactured by Rigaku Corporation Method: 2θ-θ reflection method X-ray used: Cu-Kα ray

[0064] Figure 5 shows the results of XRD of AlScN. In this figure, the horizontal axis represents the diffraction angle, and the vertical axis represents the intensity. AlScN has a wurtzite crystal structure and is known to have a polarization axis in the (002) orientation (c-axis) direction. Therefore, compared to the (100) orientation, it has higher charge storage properties as a ferroelectric.

[0065] In addition, in this figure, the upper waveform shows the diffraction results when the film was formed at a gas pressure of 0.7 Pa, and the lower waveform shows the diffraction results when the film was formed at a gas pressure of 1.2 Pa. This figure confirms that the film formed at a gas pressure of 0.7 Pa has more (002) components and a better orientation than the film formed at a gas pressure of 1.2 Pa. In other words, the orientation of the film can be controlled by the gas pressure in the film formation process, and a film with a (002) orientation can be obtained when the film is formed at 0.7 Pa. For example, in this way, a ferroelectric layer 120 suitable for controlling the magnetic memory 10 can be obtained.

[0066] (Experiment 2) <Evaluation of ferroelectricity of AlScN> Fig. 6 shows the laminated structure used in Experiment 2. To obtain the laminated structure shown in this figure, a laminated structure similar to the laminated structure used in Experiment 1 was prepared. In this case, in Experiments 2-1, 2-2, and 2-3, AlScN was used. ０．７ Sc０．３ Layered structures were prepared in which AlScN, consisting of N, had film thicknesses of 10 nm, 20 nm, and 30 nm. A 50 μm × 50 μm Ta film was then deposited to a thickness of 3 nm on the AlScN using the lift-off method. Furthermore, 50 nm of Al was evaporated onto the backside of the Si wafer. The film deposition conditions for the thin films other than Ta and Al were the same as in Experiment 1. Ta and Al were deposited as follows.

[0067] (Ta Film Formation) The film was formed by RF sputtering using a Ta target. Sputtering was performed by discharging at 150 W. The film was formed under the conditions of a substrate temperature of room temperature, a film formation pressure of 0.5 Pa, and Ar gas.

[0068] (Al Film Formation) A film was formed using a metal Al wire as a raw material by vacuum evaporation with resistance heating.

[0069] Using the laminated structure thus obtained, a voltage was applied between the Al on the back surface and the Ta on the front surface using a prober to evaluate the voltage-capacitance characteristics (CV characteristics) of the AlScN. The evaluation was performed at room temperature.

[0070] FIG. 7 shows the CV characteristics of AlScN. In this figure, the horizontal axis represents voltage and the vertical axis represents capacitance. As shown in this figure, the CV curve has hysteresis, and inflection points can be seen on both the positive and negative voltage sides. This indicates that AlScN is ferroelectric and undergoes polarization reversal. In Experiment 2-1, where the AlScN film thickness was 10 nm, polarization reversal was observed at approximately ±2 V. In Experiment 2-2, where the AlScN film thickness was 20 nm, polarization reversal was observed at approximately ±5 V. In Experiment 2-3, where the AlScN film thickness was 30 nm, polarization reversal was observed at approximately ±10 V.

[0071] (Experiment 3) <Magnetic Anisotropy Measurement> Figure 8 shows the stacked structure used in Experiment 3. To obtain the stacked structure shown in this figure, an N-type Si wafer with a resistance of 0.02 Ω cm or less was prepared. The oxide film on the surface was removed by washing with hydrofluoric acid. A 5 nm TiN film was then formed on the clean Si surface.

[0072] In Experiment 3-1, a 2 nm thick Fe film was formed on TiN as the ferromagnetic layer 110, and a 3 nm thick Ta film was formed on Fe. That is, in Experiment 3-1, the ferroelectric layer 120 was not formed. In Experiment 3-2, a 50 nm thick Al0.7Sc0.3N film was formed as the ferroelectric layer 120 between Ta and Fe in the stacked structure of Experiment 3-1. In Experiment 3-3, a 50 nm thick Al0.7Sc0.3N film was formed as the ferroelectric layer 120 between Fe and TiN in the stacked structure of Experiment 3-1. In this case, each thin film was formed as follows.

[0073] (TiN film formation) A film was formed by RF sputtering using a Ti target. Sputtering was performed by discharging at 300 W. The film formation conditions were that the substrate temperature was room temperature, the film formation pressure was adjusted to 0.2 Pa, and Ar and N ２ The film was formed by introducing the gases at a flow rate ratio of 2:3.

[0074] (Fe Film Formation) The film was formed by RF sputtering using an Fe target. Sputtering was performed by discharging at 150 W. The film was formed under the conditions of a substrate temperature of room temperature, a film formation pressure of 0.5 Pa, and Ar gas.

[0075] (Ta Film Formation) The film was formed by RF sputtering using a Ta target. Sputtering was performed by discharging at 150 W. The film was formed under the conditions of a substrate temperature of room temperature, a film formation pressure of 0.5 Pa, and Ar gas.

[0076] (AlScN Film Formation) The film was formed by DC sputtering using an AlSc (composition ratio 53:47) target. The sputtering was performed by discharging at 300 W. The film formation conditions were a substrate temperature of room temperature, a film formation pressure of 0.7 Pa, and Ar and N. ２ were introduced at a flow rate ratio of 1:2.

[0077] In Experiment 3-2, positive charges were accumulated on the surface of the AlScN facing the Fe. On the other hand, in Experiment 3-3, negative charges were accumulated on the surface of the AlScN facing the Fe. Therefore, Experiment 3-3 can be considered to correspond to the case where the polarization direction of the AlScN, which is the ferroelectric layer 120 in the stacked structure in Experiment 3-2, is reversed.

[0078] Using such a laminated structure, the magnetic properties (MH properties) of the laminated structure in the in-plane direction were evaluated using a magnetization property evaluation device (SQUID). The evaluation was performed at room temperature.

[0079] FIG. 9 shows the results of magnetic anisotropy measurements. In this figure, the horizontal axis represents the magnetic field, and the vertical axis represents the magnetization. As shown in this figure, the ease of magnetization reversal differs in Experiments 3-1, 3-2, and 3-3. Specifically, the coercive force Hc of Fe was Hc = 3 Oe, Hc = 2 Oe, and Hc = 70 Oe in Experiments 3-1, 3-2, and 3-3, respectively. Since Experiment 3-2 obtained data showing Hc = 2 Oe (< 3 Oe < 70 Oe), it became clear that Experiment 3-2 was relatively easy to achieve magnetization reversal compared to Experiments 3-1 and 3-3. Similarly, since Experiment 3-3 obtained data showing Hc = 70 Oe (> 3 Oe > 2 Oe), it became clear that Experiment 3-3 was relatively difficult to achieve magnetization reversal compared to Experiments 3-1 and 3-2.

[0080] (Experiment 4) (1) Fabrication of a Layered Structure [Example 1 (Example)] In Example 1, a MgO layer, a CoFeB layer, a Ta layer, and a Si layer were formed on the surface of a Si substrate having a thermal oxide film on the surface. ２ O ５ layer, SiO ２ A layer, an AlScN layer, a TiN layer, and an Al layer were deposited in this order to fabricate a laminated structure. A cross-sectional schematic diagram of the fabricated laminated structure is shown in FIG.

[0081] (Si substrate) The substrate has a thermal oxide film (SiO ２ A silicon substrate (20 mm square) was prepared, which had a thermal oxide film (thermal oxide film) with a thickness of 200 nm.

[0082] (MgO layer) An MgO layer was formed as a crystallization-promoting layer on the surface layer (thermal oxide film) of a Si substrate. The film was formed by RF sputtering using an MgO target. Ar gas was introduced into the film formation chamber and the film was formed under a pressure of 0.5 Pa. The discharge power was 300 W and the substrate temperature was room temperature. The film thickness of the formed MgO layer was 2 nm.

[0083] (CoFeB Layer) Next, a CoFeB layer was deposited on the MgO layer as a ferromagnetic layer. The deposition was performed by RF sputtering using a CoFeB target. The target used contained Co, Fe, and B in an atomic ratio of Co:Fe:B = 2:6:2. Ar gas was introduced into the deposition chamber, and deposition was performed under a pressure of 0.15 Pa. The discharge power was 150 W, and the substrate temperature was room temperature. The deposited CoFeB layer had a thickness of 2.0 nm. The CoFeB layer is conductive. Therefore, it functions not only as a ferromagnetic layer, but also as a lower electrode for the ferroelectric layer (AlScN layer).

[0084] (Ta ２ O ５ Next, Ta is deposited on the CoFeB layer. ２ O ５ The layer was deposited as the lower electrode of the ferroelectric layer. At this time, a Ta layer was deposited first. The deposition was performed by RF sputtering using a Ta target. During deposition, Ar gas was introduced into the deposition chamber, and deposition was performed under the condition of a pressure of 0.5 Pa. The discharge power was 150 W, and the substrate temperature was room temperature. The thickness of the deposited Ta layer was 1 nm.

[0085] The substrate after Ta film formation was removed from the sputtering apparatus and subjected to heat treatment in a heating furnace at 360°C for 1 hour in high vacuum. This fixed the axis of easy magnetization of the CoFeB layer in the perpendicular direction. Furthermore, the substrate after heat treatment was immersed in hydrogen peroxide water to oxidize the Ta layer, thereby oxidizing the Ta. ２ O ５ layer was obtained.

[0086] (SiO ２ layer) Next, Ta ２ O ５ SiO layer ２ The film was formed using a capacitively coupled high-frequency plasma CVD apparatus (manufactured by SAMCO). During film formation, the substrate temperature was set to 200° C., and TEOS and O ２ were introduced at a flow rate ratio of 2.5 / 300 sccm so that the chamber pressure became 30 Pa, and then an RF power of 100 W was applied. ２ The thickness of the layer was 100 nm. ２A 50 μm square opening was made in the layer to expose a part of the TaO layer by wet etching.

[0087] (AlScN layer) Next, SiO ２ The substrate with the openings was placed in a sputtering device, and an AlScN layer was deposited as a ferroelectric layer. The deposition was performed by DC sputtering using an AlSc target. The target contained Al and Sc in an atomic ratio of Al:Sc = 53:47. The flow rate of Ar:N ２ Ar and N so that the ratio is 1:2 ２ The mixture was introduced into a film-forming chamber, and film formation was carried out under the conditions of a pressure of 0.7 Pa. The discharge power was 300 W, and the substrate temperature was room temperature. ２ layer, and SiO ２ Ta exposed in the opening of the layer ２ O ５ An AlScN layer was formed on the AlScN layer, with a thickness of 50 nm.

[0088] (TiN Layer) Next, a TiN layer was formed as a transparent electrode on the AlScN layer by RF sputtering using a Ti target. ２ Ar and N so that the ratio is 2:3 ２ The TiN layer was deposited at a pressure of 0.2 Pa under a discharge power of 300 W and a substrate temperature of room temperature. The deposited TiN layer had a thickness of 3 nm. The unnecessary TiN layer was then removed by dry etching.

[0089] (Al Layer) Next, an Al layer was formed as an auxiliary electrode layer. The Al layer was formed so as to partially overlap the TiN layer. The Al layer was formed using a metal Al wire as a raw material by a vacuum deposition method using resistance heating. The thickness of the formed Al layer was 100 nm.

[0090] (2) Evaluation of the laminated structure and the results thereof The laminated structures thus produced were evaluated for various properties as follows.

[0091] <Evaluation of Ferroelectric Properties of Ferroelectric Layer> For the stacked structure of Example 1, the Al layer was used as the upper electrode, and the TaO layer and CoFeB layer were used as the lower electrode. A voltage was applied between these electrodes to evaluate the CV characteristics of the AlScN layer (ferroelectric layer). The obtained CV curve is shown in Figure 11. In this figure, the horizontal axis represents the magnitude of the voltage (V), and the vertical axis represents the capacitance (capacity; C) value.

[0092] As shown in Figure 11, the CV curve exhibits hysteresis. Inflection points are observed at both the positive (+) voltage side and the negative (-) voltage side (+18 V and -21 V). This indicates that the AlScN layer has ferroelectric properties and undergoes polarization reversal at +18 V and -21 V.

[0093] <Evaluation of Magnetic Properties of Ferromagnetic Layer> The magnetic properties of the ferromagnetic layer in the presence of the ferroelectric layer were examined.

[0094] The upper electrode (TiN layer) and the lower electrode (Ta) are used to set the polarization direction of the ferroelectric layer. ２ O ５ A voltage was applied between the ferroelectric layer and the CoFeB layer. The applied voltage was set to ±25 V, which was higher than the polarization inversion voltage of the ferroelectric layer.

[0095] The state in which a negative voltage (-25 V) is applied to the upper electrode (TiN layer) and the state in which the voltage is returned to 0 from this state are called "-poling." In this case, the polarization axis of the ferroelectric layer faces upward in the direction perpendicular to the surface. In addition, the state in which a positive voltage (+25 V) is applied to the upper electrode (TiN layer) and the state in which the voltage is returned to 0 from this state are called "+poling." In this case, the polarization axis of the ferroelectric layer faces downward in the direction perpendicular to the surface.

[0096] Next, the in-plane magnetic properties of the ferromagnetic layer (CoFeB layer) were measured by Kerr measurement while controlling the polarization state of the ferroelectric layer (AlScN layer). The Kerr measurement was performed using a magneto-optical effect measurement device (manufactured by Neoark Corporation) in a magnetic field range of ±10 Oe.

[0097] The results of Kerr measurement of the ferromagnetic layer (CoFeB layer) are shown in Figure 12. Hc, which is an index showing the ease of magnetic reversal of the ferromagnetic layer, was 4.55 Oe in the case of +poling and 5.71 Oe in the case of -poling.

[0098] The results of repeating +poling and -poling to confirm the repeatability of magnetic reversal ease are shown in Figure 13. Repeatability was confirmed.

[0099] This indicates that by controlling the polarization direction of the ferroelectric layer, the ease of magnetic reversal (magnetization characteristics) of the ferromagnetic layer can be repeatedly controlled with good reproducibility.

[0100] As described above, the magnetic memory element 100 according to this embodiment includes a ferromagnetic layer 110 whose magnetization can be reversed, and a ferroelectric layer 120 stacked with the ferromagnetic layer 110 and having a polarization direction component in the stacking direction, and the ease of magnetization reversal of the ferromagnetic layer 110 differs between a state in which the polarization direction of the ferroelectric layer 120 is in a first polarization direction and a state in which it is in a second polarization direction different from the first polarization direction.

[0101] In this way, in the magnetic memory element 100 according to this embodiment, by taking advantage of the characteristics of the spontaneously polarized ferroelectric layer 120 and utilizing the non-volatility of charge storage, it is possible to control the energy required for magnetization reversal of the ferromagnetic layer 110 without continuously applying an electric field to the ferroelectric layer 120. As a result, the magnetic memory element 100 according to this embodiment can realize magnetization reversal with low power consumption.

[0102] Furthermore, the magnetic memory 10 according to this embodiment includes a ferromagnetic layer 110 whose magnetization is reversible, a ferroelectric layer 120 stacked with the ferromagnetic layer 110 and having a polarization direction component in the stacking direction, a first circuit 210 that controls the ease of magnetization reversal of the ferromagnetic layer 110 by controlling the polarization direction of the ferroelectric layer 120 by an electric field applied to the ferroelectric layer 120, and a second circuit 220 that controls the magnetization direction of the ferromagnetic layer 110 by a magnetization reversal torque applied to the ferromagnetic layer 110.

[0103] Furthermore, the control method according to this embodiment is a control method for controlling a magnetic memory 10 including a ferromagnetic layer 110 whose magnetization is reversible and a ferroelectric layer 120 stacked with the ferromagnetic layer 110 and having a polarization direction component in the stacking direction, and includes a first step of controlling the ease of magnetization reversal of the ferromagnetic layer 110 by controlling the polarization direction of the ferroelectric layer 120 with an electric field applied to the ferroelectric layer 120, and a second step of controlling the magnetization direction of the ferromagnetic layer 110 with a magnetization reversal torque applied to the ferromagnetic layer 110.

[0104] In this way, the magnetic memory 10 and the control method for controlling the magnetic memory 10 according to this embodiment separately control the magnetization reversibility of the ferromagnetic layer 110 and the magnetization direction of the ferromagnetic layer 110. As a result, the magnetic memory 10 and the control method for controlling the magnetic memory 10 according to this embodiment can increase the degree of freedom in controlling each of them.

[0105] It should be noted that the present disclosure is not limited to the above, and it goes without saying that various modifications can be made without departing from the spirit of the present disclosure.

[0106] The disclosure of Japanese Patent Application No. 2024-131094, filed on August 7, 2024, is incorporated herein by reference in its entirety. In addition, all documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A magnetic memory element including a ferromagnetic layer whose magnetization can be reversed and a ferroelectric layer stacked with the ferromagnetic layer and having a polarization direction component in the stacking direction, wherein the ease of magnetization reversal of the ferromagnetic layer differs between a state in which the polarization direction of the ferroelectric layer is in a first polarization direction and a state in which the polarization direction is in a second polarization direction different from the first polarization direction.

2. The magnetic memory element according to claim 1, wherein the ferroelectric layer has a crystal structure other than perovskite made of oxide or nitride.

3. The magnetic memory element according to claim 2, wherein the crystal structure is a wurtzite structure or a fluorite structure.

4. The ferroelectric layer has a capacity of 10 electrons. １３ / cm ２ 2. The magnetic memory element according to claim 1, comprising a ferroelectric material that accumulates or depletes.

5. A magnetic memory element according to any one of claims 1 to 4, wherein the ferroelectric layer has a thickness of 50 nm or less.

6. A magnetic memory comprising: a ferromagnetic layer whose magnetization is reversible; a ferroelectric layer stacked with the ferromagnetic layer and having a polarization direction component in the stacking direction; a first circuit that controls the ease of magnetization reversal of the ferromagnetic layer by controlling the polarization direction of the ferroelectric layer with an electric field applied to the ferroelectric layer; and a second circuit that controls the magnetization direction of the ferromagnetic layer with a magnetization reversal torque applied to the ferromagnetic layer.

7. A control method for controlling a magnetic memory including a ferromagnetic layer whose magnetization can be reversed and a ferroelectric layer stacked with the ferromagnetic layer and having a polarization direction component in the stacking direction, the control method comprising: a first step of controlling the ease of magnetization reversal of the ferromagnetic layer by controlling the polarization direction of the ferroelectric layer with an electric field applied to the ferroelectric layer; and a second step of controlling the magnetization direction of the ferromagnetic layer with a magnetization reversal torque applied to the ferromagnetic layer.

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