A ferromagnetic free layer, preparation method and application thereof

Abstract

The present invention relates to a ferromagnetic free layer, a preparation method and an application thereof. The thickness of each film layer in a magnetic film alloy included in the ferromagnetic free layer decreases from a first end to a second end in order to break the in-plane structural symmetry, and the film includes a heavy metal film and a ferromagnetic metal film, which are arranged in an alternating manner to break the out-of-plane crystal symmetry. When a current is applied to the magnetic film alloy, a spin-orbit torque is generated to directly drive the magnetic moment of the magnetic film alloy to undergo a deterministic magnetization reversal. By using the magnetic film alloy as a ferromagnetic free layer, there is no need to add an additional heavy metal layer in a magnetic tunnel junction, and no need for additional magnetic field assistance, so that the spin-orbit torque driving the magnetic moment reversal can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of data storage, and in particular to a ferromagnetic free layer, a preparation method and application thereof. Background Art

[0002] In magnetic memory devices, magnetic tunnel junctions based on ferromagnetic pinned layer / oxide / ferromagnetic free layer have been widely used as storage units. Computer binary "0" and "1" correspond to the two magnetization states of the ferromagnetic free layer. At present, spin transfer torque and spin-orbit torque based on spin current driven magnetic moment reversal have been widely studied. Compared with spin transfer torque, spin-orbit torque can be designed as a three-terminal storage unit to separate the storage and reading paths of information and extend the service life of magnetic memory devices. However, spin-orbit torque usually requires an additional heavy metal layer to be added to the magnetic tunnel junction to provide spin current, and in order to ensure the deterministic reversal of the magnetic moment in the ferromagnetic free layer, the spin-orbit torque also needs to apply an external magnetic field in the direction of the current, which has a complex structure. Summary of the invention

[0003] The purpose of the present invention is to provide a ferromagnetic free layer, a preparation method and its application. By changing the structure of the ferromagnetic free layer, there is no need to add an additional heavy metal layer in the magnetic tunnel junction, and no additional magnetic field assistance is required, so that the spin-orbit torque-driven magnetic moment deterministic reversal can be achieved.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] A ferromagnetic free layer, the ferromagnetic free layer comprises a magnetic thin film alloy, the magnetic thin film alloy comprises a plurality of thin films stacked, the thickness of each layer of the thin films decreases from the first end to the second end of the magnetic thin film alloy; the thin films comprise a heavy metal film and a ferromagnetic metal film, the heavy metal film and the ferromagnetic metal film are arranged alternately; the thickness of the heavy metal film increases or decreases in sequence from the top layer to the bottom layer, and the thickness of each layer of the ferromagnetic metal film is the same; or, the thickness of each layer of the heavy metal film is the same, and the thickness of the ferromagnetic metal film increases or decreases in sequence from the top layer to the bottom layer; or, the thickness of the heavy metal film increases in sequence from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film decreases in sequence from the top layer to the bottom layer; or, the thickness of the heavy metal film decreases in sequence from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film increases in sequence from the top layer to the bottom layer.

[0006] A method for preparing a ferromagnetic free layer, comprising:

[0007] A heavy metal film and a ferromagnetic metal film are cross-prepared by an inclined growth method, and the heavy metal film and the ferromagnetic metal film are stacked to obtain a ferromagnetic free layer; the thickness of the heavy metal film increases or decreases in sequence from the top layer to the bottom layer, and the thickness of each layer of the ferromagnetic metal film is the same; or, the thickness of each layer of the heavy metal film is the same, and the thickness of the ferromagnetic metal film increases or decreases in sequence from the top layer to the bottom layer; or, the thickness of the heavy metal film increases in sequence from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film decreases in sequence from the top layer to the bottom layer; or, the thickness of the heavy metal film decreases in sequence from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film increases in sequence from the top layer to the bottom layer.

[0008] A magnetic tunnel junction comprises a stacked ferromagnetic pinning layer, an oxide layer and a ferromagnetic free layer, wherein the ferromagnetic free layer adopts the above-mentioned ferromagnetic free layer.

[0009] A magnetic storage device comprises a plurality of storage units, wherein the storage units are the magnetic tunnel junctions mentioned above.

[0010] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0011] The invention is used to provide a ferromagnetic free layer, a preparation method and an application thereof. The ferromagnetic free layer comprises a magnetic thin film alloy, the magnetic thin film alloy comprises a plurality of thin films stacked, the thickness of each thin film layer decreases from the first end to the second end of the magnetic thin film alloy in sequence to break the structural symmetry within the surface of the magnetic thin film alloy, and the thin films comprise a heavy metal film and a ferromagnetic metal film, the crystal symmetry outside the surface of the magnetic thin film alloy is broken by means of a composition gradient, when a current is applied within the surface of the magnetic thin film alloy, a spin-orbit torque is generated, and the magnetic moment of the magnetic thin film alloy is directly driven to undergo deterministic magnetization reversal, and the magnetic thin film alloy is used as a ferromagnetic free layer, without adding an additional heavy metal layer in the magnetic tunnel junction, and without requiring additional magnetic field assistance, the spin-orbit torque can be used to drive the magnetic moment to undergo deterministic magnetization reversal, the structure of the magnetic tunnel junction and the magnetic storage device can be simplified, and the magnetic storage device has great practical application value in the preparation and design of high-density and low-power consumption magnetic storage devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A schematic diagram of oblique sputtering of a CoPt magnetic thin film alloy using a magnetron sputtering apparatus provided in Example 1 of the present invention;

[0013] Figure 2 A schematic diagram of the hysteresis loop of the anomalous Hall resistance (RH) and the pulse current Ix under different external magnetic fields Hx provided in Example 1 of the present invention;

[0014] Figure 3 This is an image of magnetic domain reversal driven by current under zero magnetic field provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0015] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0016] Embodiment 1:

[0017] This embodiment is used to provide a ferromagnetic free layer, such as Figure 1 As shown, the ferromagnetic free layer includes a magnetic thin film alloy, and the magnetic thin film alloy includes multiple layers of thin films stacked, and two adjacent thin films are bonded. The thickness of each thin film decreases from the first end to the second end of the magnetic thin film alloy, that is, the shape of each thin film is wedge-shaped. Figure 1 (a) is a schematic diagram of a structure in which a thin film is deposited on a substrate. Figure 1 The direction of the y-axis is the direction from the first end to the second end of the magnetic thin film alloy, and the thickness of each thin film decreases in sequence along the y-axis direction. It should be noted that the length (along the y-axis direction) and width (along the x-axis direction) of each thin film are the same, and the first end of the magnetic thin film alloy is the first end of each thin film, and the second end of the magnetic thin film alloy is the second end of each thin film.

[0018] Each layer of the film can be prepared by an inclined growth method, and then the thickness of each layer of the film decreases successively along the y-axis direction, thereby breaking the structural symmetry within the surface of the magnetic film alloy through the inclined growth method.

[0019] The film includes a heavy metal film and a ferromagnetic metal film, and the heavy metal film and the ferromagnetic metal film are arranged alternately. The materials used for the heavy metal film include Pt, Ta, Tb and W, and the materials used for the ferromagnetic metal film include Fe, Co and Ni, and the types of magnetic film alloys include but are not limited to CoPt, FePt and CoTb. In addition, the thickness of the heavy metal film increases or decreases in order from the top layer to the bottom layer, and the thickness of each layer of the ferromagnetic metal film is the same; or, the thickness of each layer of the heavy metal film is the same, and the thickness of the ferromagnetic metal film increases or decreases in order from the top layer to the bottom layer; or, the thickness of the heavy metal film increases in order from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film decreases in order from the top layer to the bottom layer; or, the thickness of the heavy metal film decreases in order from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film increases in order from the top layer to the bottom layer, and the composition gradient is used to break the crystal symmetry outside the magnetic film alloy surface.

[0020] A current is applied in the plane of the magnetic thin film alloy, and the direction of the current is perpendicular to the direction from the first end to the second end and parallel to the horizontal plane, that is, along Figure 1 The x-axis shown in the figure applies current, and a spin-orbit torque is generated by the current. Since both the in-plane symmetry and out-of-plane symmetry of the magnetic thin film alloy are broken, the spin-orbit torque can realize the deterministic magnetization reversal of the magnetic moment of the magnetic thin film alloy. When the magnetic thin film alloy is used as a ferromagnetic free layer in a magnetic tunnel junction, since the ferromagnetic free layer can realize the deterministic magnetization reversal of the magnetic moment under the drive of pure current, there is no need to add an additional heavy metal layer in the magnetic tunnel junction, and no additional external magnetic field assistance is required, which can simplify the structure of the magnetic tunnel junction and the magnetic storage device, and has great practical application value in the preparation and design of high-density, low-power magnetic storage devices.

[0021] As an optional implementation, the ferromagnetic free layer of this embodiment further includes a buffer layer and a cover protection layer, wherein the buffer layer is disposed below the magnetic thin film alloy, and the cover protection layer is disposed above the magnetic thin film alloy, so as to protect the magnetic thin film alloy by using the buffer layer and the cover protection layer. Figure 1 (b) is a schematic diagram of the structure of a ferromagnetic free layer including a buffer layer, a magnetic thin film alloy and a covering protective layer, wherein the buffer layer is a Ru layer and the covering protective layer is a MgO layer.

[0022] The following is a specific experiment to demonstrate the effect of the ferromagnetic free layer:

[0023] Figure 1 (a) is a schematic diagram of the magnetron sputtering apparatus tilting the CoPt magnetic thin film alloy. Figure 1 As shown in (a), the substrate rotation function is turned off and the substrate position can be translated along the y-axis, which causes the target atoms to be incident on the substrate obliquely, resulting in each film being a wedge-shaped film. At the same time, the Co component and the Pt component are cross-sputtered to prepare a CoPt magnetic thin film alloy.

[0024] Figure 1 (b) shows the Ru(2.0nm) / CoPt / MgO(2.0nm) ferromagnetic free layer structure prepared, where the Ru and MgO layers are used as buffer layer and cover protection layer respectively, and the structure of CoPt magnetic thin film alloy from bottom to top is Pt(0.7nm) / Co(0.3nm) / Pt(0.5nm) / Co(0.5nm) / Pt(0.3nm) / / Co(1.0nm). The metal layers Ru, Pt, and Co are sputtered by DC, with growth rates of 100 nm per second respectively. MgO is grown by radio frequency sputtering at a rate of 1000 s. The thickness of each film can be determined by setting the sputtering time. The background vacuum of the chamber before pre-sputtering is better than 10-8 Torr, and the argon pressure during sputtering is 3mTorr. In addition, Figure 1 (b) shows the thickness of the CoPt magnetic thin film alloy measured by X-ray reflectivity. The results show that the thickness gradient caused by oblique sputtering is only 0.015nm / mm.

[0025] The anomalous Hall effect is a measurement method to experimentally characterize the magnetic moment state of magnetic thin film alloys. Here, a CoPt magnetic thin film alloy Hallbar structure with a width of 5μm and a length of 80μm was prepared by photolithography and argon ion etching. The Hallbar structure and the test wiring of the anomalous Hall effect are shown in Figure 1. Figure 1 (b) As shown in the figure below, the current channel is along the x-axis, and the Hall voltage is measured along the y-axis. In the experiment, a Keithley 6221 current source is used to apply a pulse current along the positive or negative direction of the x-axis with a pulse width of 10 milliseconds.

[0026] Figure 2 The hysteresis loops of the anomalous Hall resistance (RH) and the pulse current Ix under different external magnetic fields Hx are shown, where the low resistance state corresponds to the state with the magnetic moment downward, and the high resistance state corresponds to the state with the magnetic moment upward. Figure 2 The measurement of the mid-loop is to first apply a pulse current Ix of corresponding amplitude (pulse width is 10ms) to generate spin-orbit torque to drive the magnetic moment to flip, and then use a small current of 0.1mA to measure the anomalous Hall resistance (RH), that is, the state of the magnetic moment. Figure 2 It can be seen that even under the condition of no magnetic field assistance, that is, when Hx = 0, the positive current causes the magnetic moment to flip downward, while the negative current causes the magnetic moment to flip upward. Therefore, the experimental results prove that the symmetry-broken CoPt magnetic thin film alloy prepared in this embodiment can achieve a deterministic reversal of the magnetic moment driven by pure current without an additional heavy metal layer and without the assistance of a magnetic field.

[0027] Figure 3 For the measurement of magneto-optical Kerr effect, the nucleation and expansion evolution of reverse magnetic domains in CoPt magnetic thin film alloy after stimulation by pulse current of different amplitudes are given. Figure 3 The gray area in the middle represents the magnetic moment along the -z direction, and the black area represents the magnetic moment along the +z direction. It can be seen that as the positive current amplitude increases, the area of ​​the black area in the CoPt magnetic thin film alloy decreases and the area of ​​the gray area increases, that is, the magnetic moment along the +z direction gradually flips to the -z direction under the action of the positive current. On the contrary, the negative current causes the gray area to decrease and the black area to increase, that is, the negative current causes the magnetic moment to flip to +z, and the result is consistent with Figure 2 The results of Hall resistance measurements are consistent.

[0028] This embodiment proposes a ferromagnetic free layer structure that does not rely on additional heavy metal layers and external magnetic field assistance, and drives the magnetic moment of the ferromagnetic free layer to undergo deterministic magnetization reversal by pure current to achieve spin-orbit torque. The principle based on this is to simultaneously break the symmetry of the magnetic thin film alloy in both the in-plane and out-of-plane directions, where the in-plane refers to the same horizontal plane, such as Figure 1 The xy plane shown in the figure, the in-plane direction can generally be divided into the orthogonal x-axis and y-axis. It can be a direction in the plane that is broken, such as the x-axis direction or the y-axis direction, or it can be two orthogonal directions in the plane that are broken, such as the x-axis direction and the y-axis direction are broken at the same time. The out-of-plane refers to the direction parallel to the normal of the horizontal plane, such as Figure 1 The z-axis shown, symmetry objects include but are not limited to structural symmetry, lattice symmetry, etc. This embodiment realizes deterministic regulation of the spin-orbit torque on the magnetic moment direction in the magnetic thin film alloy by combining the inclined growth mode with the composition gradient method.

[0029] Embodiment 2:

[0030] The present embodiment is used to provide a method for preparing a ferromagnetic free layer, comprising: cross-preparing a heavy metal film and a ferromagnetic metal film by an inclined growth method, and stacking the heavy metal film and the ferromagnetic metal film to obtain a ferromagnetic free layer. The thickness of the heavy metal film increases or decreases in order from the top layer to the bottom layer, and the thickness of each layer of the ferromagnetic metal film is the same; or, the thickness of each layer of the heavy metal film is the same, and the thickness of the ferromagnetic metal film increases or decreases in order from the top layer to the bottom layer; or, the thickness of the heavy metal film increases in order from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film decreases in order from the top layer to the bottom layer; or, the thickness of the heavy metal film decreases in order from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film increases in order from the top layer to the bottom layer.

[0031] As an optional implementation, the tilted growth method includes pulsed laser deposition, magnetron sputtering, electron beam evaporation and molecular beam epitaxy. It should be noted that the tilted growth method means that the target material is incident and deposited on the surface of the substrate to be plated at a small angle to the substrate surface, growing a wedge-shaped film.

[0032] The following is an introduction to the process of preparing CoPt magnetic thin film alloy using the tilted growth method of a magnetron sputtering instrument:

[0033] 1) The (001) oriented Si / SiO2 substrate is fixed on a three-dimensionally movable sample holder.

[0034] 2) Adjust the position of the sample holder so that the substrate deviates from the sputtering center area of ​​the target material to form a certain inclined incident angle. By changing the position of the substrate, thin films with different inclined angles can be prepared. At the same time, for the preparation of CoPt magnetic thin film alloy, a simpler method is to prepare it by alternately sputtering Pt and Co components, and adjust the sputtering time to make the Pt component and / or Co component have a thickness gradient. The thickness gradient refers to the gradual increase or decrease in the thickness of the two components from the bottom layer to the top layer.

[0035] Using oblique growth methods such as pulsed laser deposition, magnetron sputtering, electron beam evaporation, and molecular beam epitaxy, a magnetic thin film alloy with an out-of-plane composition gradient and perpendicular magnetic anisotropy is obliquely grown. Due to the oblique growth method, the thickness of the film is uneven, the thickness is thicker where the inclination angle is small, and the thickness is thinner where the inclination angle is large, forming a wedge-shaped structure. Therefore, the structural symmetry in the plane is broken. The alternating oblique growth and the film having a thickness gradient result in a composition gradient in the out-of-plane direction, so the out-of-plane crystal symmetry is also broken. Applying current in the plane of the magnetic thin film alloy drives the magnetic moment to flip, where the current direction is perpendicular to the direction of the film's in-plane symmetry breaking.

[0036] This embodiment provides a preparation method for current-driven single magnetic layer magnetic moment reversal without additional heavy metal layers and without external magnetic field assistance. This method has great practical application value in the preparation and design of high-density, low-power magnetic storage devices, and has the advantages of strong universality and easy preparation.

[0037] Embodiment 3:

[0038] This embodiment is used to provide a magnetic tunnel junction, including a stacked ferromagnetic pinning layer, an oxide layer and a ferromagnetic free layer. The ferromagnetic free layer adopts the ferromagnetic free layer described in Example 1.

[0039] Embodiment 4:

[0040] A magnetic storage device includes a plurality of storage units, wherein the storage units are magnetic tunnel junctions as described in the third embodiment.

[0041] The above embodiments are only used to help understand the method and core idea of ​​the present invention; at the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A ferromagnetic free layer, characterized in that: The ferromagnetic free layer includes a magnetic thin film alloy, and the magnetic thin film alloy includes a plurality of thin films stacked in layers, and the thickness of each layer of the thin film decreases from the first end to the second end of the magnetic thin film alloy; the thin film includes a heavy metal film and a ferromagnetic metal film, and the heavy metal film and the ferromagnetic metal film are staggered; the thickness of the heavy metal film increases or decreases in sequence from the top layer to the bottom layer, and the thickness of each layer of the ferromagnetic metal film is the same; or, the thickness of each layer of the heavy metal film is the same, and the thickness of the ferromagnetic metal film increases or decreases in sequence from the top layer to the bottom layer; or, the thickness of the heavy metal film increases in sequence from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film decreases in sequence from the top layer to the bottom layer; or, the thickness of the heavy metal film decreases in sequence from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film increases in sequence from the top layer to the bottom layer; A heavy metal film and a ferromagnetic metal film are cross-prepared by an inclined growth method, and the heavy metal film and the ferromagnetic metal film are stacked to obtain a ferromagnetic free layer; A current is applied in the plane of the magnetic thin film alloy, and the direction of the current is perpendicular to the direction from the first end to the second end and parallel to a horizontal plane.

2. The ferromagnetic free layer according to claim 1, characterized in that: The ferromagnetic free layer further comprises a buffer layer and a covering protection layer; the buffer layer is arranged below the magnetic thin film alloy; the covering protection layer is arranged above the magnetic thin film alloy.

3. The ferromagnetic free layer according to claim 1, characterized in that: The materials used for the heavy metal film include Pt, Ta, Tb and W.

4. The ferromagnetic free layer according to claim 1, characterized in that: The materials used for the ferromagnetic metal film include Fe, Co and Ni.

5. The ferromagnetic free layer according to claim 2, characterized in that: The buffer layer is a Ru layer; the covering protection layer is a MgO layer.

6. A method for preparing a ferromagnetic free layer, characterized in that: include: A heavy metal film and a ferromagnetic metal film are cross-prepared by an inclined growth method, and the heavy metal film and the ferromagnetic metal film are stacked to obtain a ferromagnetic free layer; the ferromagnetic free layer includes a magnetic film alloy, and the magnetic film alloy includes a plurality of stacked films, and the thickness of each layer of the film decreases from the first end to the second end of the magnetic film alloy; the film includes a heavy metal film and a ferromagnetic metal film, and the heavy metal film and the ferromagnetic metal film are staggered; the thickness of the heavy metal film increases or decreases in order from the top layer to the bottom layer, and the thickness of each layer of the ferromagnetic metal film is the same; or, the thickness of each layer of the heavy metal film is the same, and the thickness of the ferromagnetic metal film increases or decreases in order from the top layer to the bottom layer; or, the thickness of the heavy metal film increases in order from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film decreases in order from the top layer to the bottom layer; or, the thickness of the heavy metal film decreases in order from the top layer to the bottom layer, and the thickness of the ferromagnetic metal film increases in order from the top layer to the bottom layer; A current is applied in the plane of the magnetic thin film alloy, and the direction of the current is perpendicular to the direction from the first end to the second end and parallel to a horizontal plane.

7. The preparation method according to claim 6, characterized in that: The tilted growth method includes pulse laser deposition, magnetron sputtering, electron beam evaporation and molecular beam epitaxy.

8. A magnetic tunnel junction, comprising a stacked ferromagnetic pinned layer, an oxide layer and a ferromagnetic free layer, characterized in that: The ferromagnetic free layer adopts the ferromagnetic free layer as claimed in any one of claims 1-5.

9. A magnetic storage device comprising a plurality of storage units, characterized in that: The memory cell is the magnetic tunnel junction as claimed in claim 8.

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