A magnetoelectric heterojunction film and a preparation method thereof

Abstract

The application discloses a magnetoelectric heterojunction film and a preparation method thereof. The magnetoelectric heterojunction film comprises a strontium titanate base layer, a strontium ruthenate electrode layer, a bismuth ferrite ferroelectric material layer and a SmCo-based ferromagnetic material layer which are stacked in sequence. The magnetoelectric heterojunction film has the multi-ferroelectric layer room-temperature magnetoelectric coupling effect and the excellent magnetic performance of the ferromagnetic layer, and the magnetoelectric coupling effect of the heterojunction film can be enhanced through the strong exchange bias effect at the interface.

Description

Technical Field

[0001] This invention relates to the field of magnetoelectric composite thin film technology, and particularly to a magnetoelectric heterojunction thin film and its preparation method. Background Technology

[0002] Magnetoelectric composite materials have broad application prospects in important fields such as magnetoelectric sensors, memory, and microwave communication due to their magnetoelectric coupling effect. Generally, ferromagnetic materials are often metals, while ferroelectric materials are insulators. Due to the structural repulsion between ferromagnetism and ferroelectricity, very few materials in nature possess both ferromagnetism and ferroelectricity, and the corresponding magnetoelectric coupling effect is extremely weak. Furthermore, most materials only exhibit magnetoelectric coupling at extremely low temperatures, which greatly limits the application of single-phase multiferroic materials. Bismuth ferrite (BFO) thin films are currently the only known single-phase material exhibiting magnetoelectric coupling at room temperature; its ferroelectricity originates from Bi. 3+ The hybridization of lone pairs of electrons with other orbitals leads to the asymmetric distortion of the electron cloud center, and the helical antiferromagnetism originates from Fe. 3+ The Fe atoms are arranged in an orderly manner along the

[111] direction of the spatial spiral structure of BFO. The magnetic moments of adjacent Fe atoms have a net magnetic moment in the (111) plane, exhibiting antiferromagnetism. The antiferromagnetic easy face is perpendicular to the ferroelectric polarization direction of BFO. When BFO undergoes polarization reversal under the action of an electric field, its antiferromagnetic easy face also reverses, thereby realizing the coupling of ferroelectricity and antiferromagnetism of BFO. However, this coupling effect is very weak due to the limitation of the intrinsic magnetic properties of BFO, which seriously hinders the application of BFO in the field of magnetoelectric coupling materials.

[0003] Therefore, enhancing the magnetoelectric coupling effect by compositing BFO thin films with strongly magnetic ferromagnetic thin film materials into heterostructures, and leveraging the exchange bias effect of ferromagnetism and antiferromagnetism at the interface, is currently a research hotspot. Rare-earth magnetic materials possess a unique 4f electron layer structure, and materials formed by combining them with transition metals exhibit excellent ferromagnetic properties. Combining these two materials with typical ferroelectric / ferromagnetic properties will provide strong support for expanding magnetoelectric composite materials from theoretical research to practical applications. However, due to the significant crystal structure difference between BFO thin films and rare-earth magnetic thin film materials, the formation of heterostructures by combining BFO thin films and rare-earth magnetic thin film materials often results in severe lattice mismatch problems, ultimately leading to poor interface and magnetic coupling effects.

[0004] Therefore, it is urgent to develop a magnetoelectric heterojunction thin film that can be prepared by simple experiments and has good interfacial coupling. Summary of the Invention

[0005] The present invention aims to at least solve one of the aforementioned technical problems existing in the prior art. Therefore, the object of the present invention is to provide a magnetoelectric heterojunction thin film that simultaneously possesses the room-temperature magnetoelectric coupling effect of a multiferroic layer and the excellent magnetic properties of a ferromagnetic layer. Through a strong exchange bias effect at the interface, the magnetoelectric coupling effect of the heterojunction thin film can be enhanced.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a magnetoelectric heterojunction thin film comprising a strontium titanate substrate layer, a strontium ruthenium electrode layer, a bismuth ferroelectric material layer, and an SmCo-based ferromagnetic material layer stacked sequentially.

[0008] In some embodiments of the present invention, the thickness of the strontium titanate substrate layer is 0.1 to 1 mm, and the crystal structure of the strontium titanate includes single crystal.

[0009] In some embodiments of the present invention, the thickness of the strontium titanate substrate layer is 0.5 mm.

[0010] In some embodiments of the present invention, the thickness of the strontium ruthenate electrode layer is 40–50 nm.

[0011] In some embodiments of the present invention, the thickness of the strontium ruthenate electrode layer is 40 nm.

[0012] In some embodiments of the present invention, the thickness of the bismuth ferroelectric material layer is 100-200 nm, the bismuth ferroelectric material layer includes a rhombic phase structure, and the bismuth ferroelectric layer has ferroelectricity and G-type antiferromagnetism at room temperature.

[0013] In some embodiments of the present invention, the thickness of the bismuth ferroelectric material layer is 100 nm.

[0014] In some embodiments of the present invention, the thickness of the SmCo-based ferromagnetic material layer is 2 to 20 nm, and the crystal structure of the SmCo-based ferromagnetic material includes amorphous and single crystal.

[0015] In some embodiments of the present invention, the SmCo-based ferromagnetic material layer is distributed in a layered manner or in a lattice manner.

[0016] In some embodiments of the present invention, the magnetoelectric heterojunction thin film further includes a protective layer disposed on a ferromagnetic material layer.

[0017] In some embodiments of the present invention, the protective layer includes at least one of a metal Ta layer and a Pt layer.

[0018] In some embodiments of the present invention, the thickness of the protective layer is 2 to 20 nm.

[0019] In a second aspect, the present invention provides a method for preparing the aforementioned magnetoelectric heterojunction thin film, comprising the following steps:

[0020] S1. The strontium ruthenate electrode layer is formed on the strontium titanate substrate by a first deposition using a pulsed laser, and the bismuth ferroelectric material layer is formed on the strontium ruthenate electrode layer by a second deposition.

[0021] S2. By magnetron sputtering, the SmCo-based ferromagnetic material layer is formed on the bismuth ferroelectric material layer by a first sputtering, followed by cold treatment to obtain a magnetoelectric heterojunction thin film.

[0022] In some embodiments of the present invention, in S1, the gas pressure of the first deposition is 8-12 Pa, the temperature of the first deposition is 700-750°C, the gas pressure of the second deposition is 8-12 Pa, and the temperature of the second deposition is 670-730°C.

[0023] In some embodiments of the present invention, in S1, the gas pressure of the first deposition is 10 Pa, the temperature of the first deposition is 710 °C, the gas pressure of the second deposition is 10 Pa, and the temperature of the second deposition is 690 °C.

[0024] In some embodiments of the present invention, in S1, the energy of the first deposited laser cavity is 50-70 mJ, 6-10 Hz, medium distance, 1800-2200 Pulse.

[0025] In some embodiments of the present invention, in S1, the energy of the first deposited laser cavity is 60mJ, 8Hz, mid-range, 2000Pulse.

[0026] In some embodiments of the present invention, in S1, the laser cavity energy of the second deposition is 50-70 mJ, 6-10 Hz, medium distance, and 6000-7000 pulses.

[0027] In some embodiments of the present invention, in S1, the laser cavity energy of the second deposition is 60mJ and the mid-range of 8Hz is 6500Pulse.

[0028] In some embodiments of the present invention, in S2, the gas pressure of the first sputtering is 0.6 to 1.0 Pa, and the power of the first sputtering is 8 to 12 W.

[0029] In some embodiments of the present invention, in S2, the deposition gas pressure of the first sputtering is 0.8 Pa, and the power of the first sputtering is 10 W.

[0030] In some embodiments of the present invention, in S2, the vacuum degree of the field cooling treatment is 0.008 to 0.012 mTorr, the applied magnetic field is 1800 to 2200 Oe, and the temperature is 630 to 670 K.

[0031] In some embodiments of the present invention, in S2, the vacuum degree of the field cooling treatment is 0.01 mTorr, the applied magnetic field is 2000 Oe, and the temperature is 650 K.

[0032] In some embodiments of the present invention, the method for preparing the magnetoelectric heterojunction thin film further includes: forming a protective layer on the ferromagnetic material layer by a second sputtering before field cooling.

[0033] In some embodiments of the present invention, the gas pressure of the second sputtering is 1.0 to 1.4 Pa, and the power of the second sputtering is 70 to 90 W.

[0034] In some embodiments of the present invention, the gas pressure of the second sputtering is 1.2 Pa and the power of the second sputtering is 80 W.

[0035] In a third aspect, the invention proposes the application of the magnetoelectric heterojunction thin film in magnetoelectric sensors, memory, and microwave communication devices.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) The magnetoelectric heterojunction thin film in this invention has very strong magnetism and a large exchange bias field, which can achieve a strong magnetoelectric coupling effect. The preparation method in this invention is simple and easy to control, and the composition is easy to adjust. By adopting different thin film deposition methods according to the characteristics of different materials, the sample yield is improved, the interface coupling effect of the heterojunction thin film is enhanced, and a magnetoelectric heterojunction thin film with good interface coupling effect is formed.

[0038] (2) The magnetoelectric heterojunction thin film in this invention comprises a multiferroic oxide thin film (BFO ferroelectric material layer) and a rare-earth magnetic thin film (SmCo-based ferromagnetic material layer). It is fabricated using a two-step method to form an integral structure with multiple materials. First, a strontium ruthenium oxide electrode layer and a multiferroic oxide layer (BFO ferroelectric material layer) are epitaxially grown sequentially on an oxide (strontium titanate substrate) using pulsed laser deposition. Second, a rare-earth magnetic thin film and a protective layer are deposited on the multiferroic oxide layer using magnetron sputtering. The fabricated heterojunction thin film achieves interfacial ferromagnetic / antiferromagnetic coupling between the rare-earth magnetic layer and the multiferroic oxide layer at room temperature through field cooling, overcoming the weakness of the single multiferroic oxide layer's inherently weak magnetoelectric coupling effect. This stronger exchange bias effect enhances the magnetoelectric coupling effect. The fabrication method involves simple equipment, is easy to control, and has high reliability. It can be used in fields such as magnetoelectric sensors, memory, low-frequency magnetoelectric antennas, and microwave devices.

[0039] (3) In this invention, the magnetoelectric heterojunction thin film includes a multiferroic oxide thin film (BFO ferroelectric material layer) and a rare earth magnetic thin film (SmCo-based ferromagnetic material layer). The multiferroic oxide thin film and the rare earth magnetic thin film are epitaxially grown on an oxide substrate using different physical vapor deposition methods to form a multilayer heterojunction thin film structure. After field cooling treatment, the ferromagnetic / antiferromagnetic interface coupling of the heterojunction thin film is obtained. The coercive bias field of the magnetoelectric heterojunction thin film is 210Oe. The antiferromagnetism and exchange bias effect of the multiferroic oxide thin film are controlled by an external electric field.

[0040] (4) The magnetoelectric heterojunction thin film of the present invention has very strong magnetism and obvious exchange bias field, which can realize a strong magnetoelectric coupling effect.

[0041] (5) The thickness of the SmCo-based ferromagnetic material layer and the protective layer in the magnetoelectric heterojunction thin film of the present invention is very small, and the grain size that the SmCo-based ferromagnetic material layer and the Ta layer can form is in the nanometer scale, which has little impact on the ferroelectric properties of BFO.

[0042] (6) The magnetoelectric coupling mechanism of the magnetoelectric heterojunction thin film of the present invention belongs to the exchange bias effect mechanism, and the thickness of the film layer is more conducive to the miniaturization of the device. The STO / SRO / BFO has a weak antiferromagnetic / ferroelectric coupling effect. After being composited with the strongly magnetic rare earth SmCo through the interface, the antiferromagnetic moment of BFO and the strong ferromagnetic moment of SmCo have a strong exchange bias effect. The ferroelectric material layer BFO is polarized under the action of the electric field, and its antiferromagnetism is induced to change, which destroys the original antiferromagnetic / ferromagnetic exchange bias effect. This solves the problem of serious lattice mismatch in the formation of heterostructure by BFO film and rare earth magnetic film material. As a result, the overall magnetic properties of the magnetoelectric heterojunction thin film change significantly. Furthermore, under the action of the SRO electrode layer, BFO can undergo polarization reversal at a very small voltage (10V), and can be used as a functional device with small voltage regulation of magnetic properties.

[0043] (7) When there is no electrode layer between the STO substrate layer and the BFO ferroelectric material layer, since STO itself is a good dielectric material, when an external electric field is applied to the heterojunction, the STO substrate will induce an electric field and weaken the electric field, resulting in a limited effective electric field applied to the BFO layer. Therefore, a very high electric field is required to regulate the ferroelectric polarization of the BFO layer. This invention sets an electrode layer between the STO substrate layer and the BFO ferroelectric material layer, using STO as the electrode layer when an electric field is applied to the heterojunction, thus shielding the strong dielectric properties of STO, achieving low-voltage regulation of the polarization of the BFO layer, and reducing the dielectric loss caused by the STO substrate, thereby achieving energy saving. Attached Figure Description

[0044] Figure 1This is a schematic diagram of the magnetoelectric heterojunction thin film structure of the present invention.

[0045] Figure 2 This is a flowchart illustrating the preparation process of the magnetoelectric heterojunction thin film of the present invention.

[0046] Figure 3 This is a diagram of the exchange bias effect of the magnetoelectric heterojunction thin film at room temperature in Embodiment 1 of the present invention.

[0047] Figure 4 This is a diagram of the exchange bias effect of the magnetoelectric heterojunction thin film at room temperature in Embodiment 2 of the present invention.

[0048] Figure 5 This is a diagram of the exchange bias effect of the magnetoelectric heterojunction thin film at room temperature in Embodiment 3 of the present invention. Detailed Implementation

[0049] The present invention will be further described in detail below through specific embodiments. Unless otherwise specified, the raw materials, reagents, or apparatus used in the embodiments and comparative examples are all available from conventional commercial sources or can be obtained by existing technical methods. Unless otherwise specified, the test or experimental methods are conventional methods in the art.

[0050] BFO stands for bismuth ferrite.

[0051] STO is single-crystal strontium titanate;

[0052] SRO stands for Strontium ruthenium oxide.

[0053] Example 1

[0054] This embodiment provides a magnetoelectric heterojunction thin film, the specific process of which is as follows:

[0055] S1. Using (001) oriented single-crystal STO as the substrate, a 50 nm electrode layer SRO is grown using pulsed laser deposition technology according to the following process parameters: chamber pressure 10 Pa, deposition temperature 710 °C, and pulsed laser parameters (cavity energy 60 mJ, 8 Hz, 2000 Pulse). A 100 nm multi-iron layer BFO is grown on the above SRO film at a chamber pressure of 10 Pa, deposition temperature of 690 °C, and pulsed laser parameters (cavity energy 60 mJ, 8 Hz, 6500 Pulse). Then, a 20 nm rare earth layer SmCo (sputtering pressure 0.8 Pa, DC sputtering power 10 W) and a 10 nm protective layer Ta (sputtering pressure 1.2 Pa, DC sputtering power 80 W) are sequentially grown on the STO / SRO / BFO multi-iron film to form a dense multilayer heterojunction film.

[0056] S2. Under a high vacuum of 0.01 mTorr, an external magnetic field of 2000 Oe is applied parallel to the plane of the multilayer heterojunction film. The film is cooled from 650 K to room temperature. The hysteresis loops are measured along two opposite directions on the film plane to obtain a magnetoelectric heterojunction film with a sample surface size of 2.5*2.5 (mm). The sample is heated to 650 K with zero magnetic field and held for 1 min. Then, it is cooled to 300 K with a magnetic field of 2000 Oe, parallel to the maximum surface of the film. The applied magnetic field range is (-6000 Oe, 6000 Oe), and the field change rate is 30 Oe / s. The hysteresis loops are measured along the direction of the above-mentioned field cooling magnetic field and in the reverse method.

[0057] Depend on Figure 3 It can be seen that the heterojunction has a large coercive field (about 500 Oe) and a significant exchange bias field (about 210 Oe).

[0058] Example 2

[0059] This embodiment provides a magnetoelectric heterojunction thin film, the specific process of which is as follows:

[0060] S1. Using (001) oriented single-crystal STO as the substrate, a 50 nm electrode layer SRO is grown using pulsed laser deposition technology according to the following process parameters: chamber pressure 10 Pa, deposition temperature 730 °C, and pulsed laser parameters (cavity energy 60 mJ, 8 Hz, 2000 Pulse at mid-range). A 100 nm multi-iron layer BFO is grown on the above SRO film under the following conditions: chamber pressure 10 Pa, deposition temperature 700 °C, and pulsed laser parameters (cavity energy 90 mJ, 8 Hz, 6500 Pulse at large-range). Then, a 2 nm rare earth layer SmCo is grown sequentially on the STO / SRO / BFO multi-iron film using magnetron sputtering (with an external 800 Oe magnetic field parallel to the film surface for assisted deposition, sputtering pressure 0.8 Pa, and DC sputtering power 10 W) and a 2 nm protective layer Ta (sputtering pressure 1.2 Pa, DC sputtering power 80 W) to form a dense multilayer heterojunction film.

[0061] S2. Under a high vacuum of 0.01 mTorr, an external magnetic field of magnitude 2000 Oe is applied in a direction parallel to the plane of the heterojunction film. The film is cooled from 650 K to room temperature. The hysteresis loops measured in two opposite directions along the film plane are used to obtain a magnetoelectric heterojunction film with a sample surface size of 2.5*2.5 (mm).

[0062] Performance testing: The magnetic field is parallel to the maximum surface of the thin film, with the applied magnetic field range (-6000 Oe, 6000 Oe) and the field change rate of 30 Oe / s; the sample is heated to 650 K with zero magnetic field, held for 1 min, and then cooled to 300 K with a magnetic field of 2000 Oe. The above test is repeated.

[0063] Depend on Figure 4 It can be seen that the heterojunction does not have an exchange bias phenomenon at room temperature, but has a large coercive field (1230 Oe) under the action of substrate stress; after field cooling treatment, the coercive field is enhanced (1630 Oe), and at the same time, it has a certain exchange bias field (105 Oe).

[0064] Example 3

[0065] This embodiment provides a magnetoelectric heterojunction thin film, the specific process of which is as follows:

[0066] S1. Using (001) oriented single-crystal STO as the substrate, a 50 nm electrode layer SRO is grown using pulsed laser deposition technology according to the following process parameters: chamber pressure 10 Pa, deposition temperature 730 °C, and pulsed laser parameters (cavity energy 60 mJ, 8 Hz, 2000 Pulse at mid-range). A 100 nm multi-iron layer BFO is grown on the above SRO film at a chamber pressure of 10 Pa, deposition temperature 700 °C, and pulsed laser parameters (cavity energy 90 mJ, 8 Hz, 6500 Pulse at large range). Then, a 2 nm rare earth layer SmCo is grown sequentially on the prepared STO / SRO / BFO multi-iron film using magnetron sputtering (with an external 2000 Oe magnetic field perpendicular to the film surface for assisted deposition, sputtering pressure of 0.8 Pa, and DC sputtering power of 10 W) and a 2 nm protective layer Ta (sputtering pressure of 1.2 Pa, DC sputtering power of 80 W) to form a dense multilayer heterojunction film.

[0067] S2. Under a high vacuum of 0.01 mTorr, an external magnetic field of magnitude 2000 Oe is applied in a direction parallel to the plane of the heterojunction film. The film is cooled from 650 K to room temperature. The hysteresis loops measured in two opposite directions along the film plane are used to obtain a magnetoelectric heterojunction film with a sample surface size of 2.5*2.5 (mm).

[0068] Performance testing: The magnetic field is parallel to the maximum surface of the thin film, with the applied magnetic field range (-6000 Oe, 6000 Oe) and the field change rate of 30 Oe / s; the sample is heated to 650 K with zero magnetic field, held for 1 min, and then cooled to 300 K with a magnetic field of 2000 Oe. The above test is repeated.

[0069] Depend on Figure 5 It can be seen that heterojunctions do not have exchange bias at room temperature, but have a large coercive field (1850 Oe) under the influence of substrate stress and external field; after field cooling treatment, the coercive field (2250 Oe) is enhanced while having a certain exchange bias field (50 Oe).

[0070] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A magnetoelectric heterojunction thin film, characterized in that, It comprises a strontium titanate substrate layer, a strontium ruthenate electrode layer, a bismuth ferroelectric material layer, and an SmCo-based ferromagnetic material layer stacked sequentially; the thickness of the SmCo-based ferromagnetic material layer is 2–20 nm; the crystal structure of the SmCo-based ferromagnetic material includes amorphous, single crystal, and polycrystalline; the SmCo-based ferromagnetic material layer is distributed in a layered or lattice-like manner.

2. The magnetoelectric heterojunction thin film according to claim 1, characterized in that, The thickness of the strontium titanate substrate is 0.1~1mm, and the crystal structure of the strontium titanate includes single crystal.

3. The magnetoelectric heterojunction thin film according to claim 1, characterized in that, The thickness of the strontium ruthenate electrode layer is 40–50 nm.

4. The magnetoelectric heterojunction thin film according to claim 1, characterized in that, The thickness of the bismuth ferroelectric material layer is 100-200 nm, the bismuth ferroelectric material layer includes a rhombic phase structure, and the bismuth ferroelectric layer has ferroelectricity and G-type antiferromagnetism at room temperature.

5. The method for preparing the magnetoelectric heterojunction thin film according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1. The strontium ruthenate electrode layer is formed on the strontium titanate substrate by a first deposition using a pulsed laser, and the bismuth ferroelectric material layer is formed on the strontium ruthenate electrode layer by a second deposition. S2. By magnetron sputtering, the SmCo-based ferromagnetic material layer is formed on the bismuth ferroelectric material layer by a first sputtering, followed by cold treatment to obtain a magnetoelectric heterojunction thin film.

6. The method for preparing the magnetoelectric heterojunction thin film according to claim 5, characterized in that, In S1, the gas pressure of the first deposition is 8~12 Pa, the temperature of the first deposition is 700~750℃, the gas pressure of the second deposition is 8~12 Pa, and the temperature of the second deposition is 670~730℃.

7. The method for preparing the magnetoelectric heterojunction thin film according to claim 6, characterized in that, In S2, the gas pressure of the first sputtering is 0.6~1.0 Pa, and the power of the first sputtering is 5~15 W.

8. The application of the magnetoelectric heterojunction thin film according to any one of claims 1 to 4 in magnetoelectric sensors, memory, and microwave communication equipment.

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