Preparation method of core-shell structure soft and hard magnetic nano double-phase particles with controllable shell thickness

Abstract

The invention discloses a preparation method of core-shell structure soft and hard magnetic nano double-phase particles with controllable shell thickness. The preparation method comprises the following steps: preparing soft magnetic phase nano particles; preparing a precursor solution containing soft magnetic phase nanoparticles; and carrying out spray pyrolysis on the precursor solution to obtain the soft and hard magnetic nano double-phase particles with a one-shell multi-core structure. The one-shell multi-core soft and hard magnetic nano double-phase particles with double controllable core layers and shell layers are obtained through a spray pyrolysis method, the thickness and components of the shell layers can be accurately regulated and controlled at the nanoscale, good soft and hard magnetic matching and exchange coupling performance is achieved, a new way is provided for structure regulation and control and performance optimization of a high-performance soft and hard magnetic composite material, and the method is suitable for large-scale popularization and application. And the complex sol-gel coating or acid pickling step is not needed, the technological process is simplified, and the applicability is high.

Description

Technical Field

[0001] This invention relates to the field of magnetic composite material preparation technology, and in particular to a method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness. Background Technology

[0002] Soft and hard magnetic dual-phase composite materials, possessing both high saturation magnetization and high coercivity, have shown significant application potential in high-density magnetic recording media, storage devices, high-performance permanent magnet motors, and spintronic devices. To meet the demands of modern technological advancements for these devices, enhancing the saturation magnetization of permanent magnet materials has become essential. The saturation magnetization of single-phase magnets is often limited, making the introduction of soft magnetic materials with high saturation magnetization a viable approach. However, achieving both high saturation magnetization and high coercivity requires the clamping effect of the hard magnetic phase on the soft magnetic phase to prevent domain flipping. This places higher demands on the spatial distribution and interfacial coupling between the soft and hard magnetic phases, necessitating precise control of the core-shell structure and shell thickness to achieve effective nanoscale coupling between the soft and hard magnetic phases, thereby balancing high saturation magnetization and high coercivity.

[0003] On the other hand, in magnetic recording media, while relying solely on a highly magnetocrystalline anisotropic hard magnetic phase can ensure thermal stability, the external magnetic field required for its reversal often exceeds the maximum field strength of the write head, making information writing difficult. By introducing a soft magnetic layer into the hard magnetic layer and forming an exchange-coupled structure, the soft magnetic layer can undergo magnetic moment reversal first under an external magnetic field, and through coupling, it can drive a change in the magnetization direction of the hard magnetic layer, thereby effectively reducing the field strength required for information writing without affecting the stability provided by the hard magnetic layer. Thus, the composite of soft and hard magnetic phases can not only improve the overall saturation magnetization, but also significantly improve the write performance in magnetic storage applications while maintaining high coercivity, demonstrating the potential to obtain ultra-high magnetic energy product.

[0004] Currently, there are several methods for preparing soft and hard magnetic nanophases in the laboratory, including mechanical ball milling, co-precipitation, chemical vapor deposition, sol-gel method, and hydrothermal method. All of these methods have problems such as complex preparation steps and uneven composition, which not only prevent the soft and hard magnetic phases from achieving the ideal coupling effect, but also limit their further promotion and application.

[0005] Spray pyrolysis, as a continuous and scalable chemical preparation process, can complete the atomization, drying, and decomposition of precursor solutions in a short time, offering significant advantages such as controllable particle size, uniform composition, mild reaction conditions, and simplified process. By controlling the atomized particle size, precursor concentration, and atmosphere composition, the controllable construction of multiphase structures at the nanoscale can be achieved, providing a new approach for the preparation of composite magnetic materials. Especially in the construction of core-shell structures, the droplet evaporation rate and solute migration behavior during spray pyrolysis enable the shell layer to tightly encapsulate the core layer during growth, allowing the core-shell structure to spontaneously form in an integrated process.

[0006] However, most existing publicly available spray pyrolysis preparation processes focus on single-phase powders or simple core-shell systems, with limited research on the simultaneous controllable design of the number of core layers and shell thickness. Particularly in soft and hard magnetic two-phase systems, how to precisely control the core-shell ratio and structural distribution by adjusting the solution concentration and soft magnetic particle size remains a challenge and a gap in current technology.

[0007] In existing technologies, such as the patent CN118955113B applied for by He Xiankun et al. from the University of Electronic Science and Technology of China, a soft-hard magnetic microwave ferrite coupling material and its preparation method are disclosed. This method uses a mixture of YIG and BaM two-phase materials for sintering, and achieves coupling through Bi2O3 and SiO2 doping, thereby improving the material's coercivity and dielectric loss. However, this technical route relies on high-temperature solid-state reaction and ball milling processes. The coupling interface is limited by grain contact, resulting in uneven distribution of the soft and hard magnetic phases, making it difficult to achieve coating and uniform coupling at the nanoscale.

[0008] For example, patent CN113753958A applied for by Yang Hangfu et al. discloses a method for preparing high-performance soft and hard magnetic composite materials using high-pressure ultrasonic thermal decomposition. This method involves atomizing a metal salt solution using a high-pressure ultrasonic atomizing device and then thermally decomposing it in a tube furnace to prepare soft magnetic phase A. 1-X Ln X Fe 12 O 19 (Where A is Sr or Ba, and Ln is La or Ca) and a hard magnetic phase / CoFe2O4 are combined, and then composited under high pressure heat treatment to obtain soft and hard magnetic materials. The composite material prepared by this method has good magnetic properties, but the soft and hard magnetic particles are prepared independently and then composited, resulting in insufficient contact between the two phase interfaces and limited coupling efficiency. The resulting composite structure is mostly randomly stacked micron-sized particles, which limits its application in miniaturized magnetic devices or high-density magnetic storage.

[0009] For example, patent CN106024030A applied for by Lu Haipeng et al. from the University of Electronic Science and Technology of China discloses a soft / hard magnetic exchange coupling structure. By layering the structure into upper and lower parts and isolating them with a soft magnetic layer, the coercivity of highly magnetocrystalline anisotropic materials (such as L10-FePt) is reduced. However, this structure is a planar layered design, and the coupling between the soft and hard magnetic phases is limited to the two-dimensional interface, making it difficult to achieve three-dimensional uniform coating and particle-level structural control.

[0010] Furthermore, patent CN119750583A filed by Song Huaihe et al. of Shenzhen Jintang New Energy Technology Co., Ltd. discloses a method for preparing a silicon-carbon composite material with a one-shell multi-core structure and rich internal voids. This method prepares a one-shell multi-core structure with internal voids through spray pyrolysis and acid washing steps to alleviate the volume expansion of lithium-ion battery anode materials during charging and discharging. Although this method is somewhat inspiring in terms of structural construction, its design concept mainly serves the electrochemical system and does not address the interfacial continuity, thickness control, and exchange coupling characteristics between the soft and hard magnetic phases in the soft-hard magnetic nanobiphase material.

[0011] Patent CN110124665A, filed by Lin Qian et al. from Guizhou University, discloses a method for preparing a Pd@HCS catalyst with a multi-core yolk-shell structure. This method involves steps such as sol-gel coating, carbonization, and etching to encapsulate multiple Pd nanoparticles within a carbon shell, thereby constructing a multi-core structure, which is then applied to the catalytic synthesis of hydrogen peroxide. Their research focuses on noble metal catalytic systems, which differs significantly from key issues in soft and hard magnetic nanomaterials, such as phase-to-phase magnetic matching, interfacial exchange coupling, and magnetic property regulation. Furthermore, the synthesis method is more complex than that of spray pyrolysis. Summary of the Invention

[0012] The purpose of this invention is to provide a technical solution for preparing core-shell structured soft and hard magnetic dual-phase nanoparticles with controllable shell thickness, addressing the shortcomings of existing technologies. This preparation method is simple, obtaining one-shell multi-core soft and hard magnetic dual-phase nanoparticles with controllable core and shell layers through spray pyrolysis. It can precisely control the shell thickness and composition at the nanoscale, achieving good soft and hard magnetic matching and exchange coupling performance. This provides a new approach for the structural control and performance optimization of high-performance soft and hard magnetic composite materials, and eliminates the need for complex sol-gel coating or acid washing steps, simplifying the process and making it highly applicable.

[0013] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0014] A method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness, characterized by comprising the following steps:

[0015] S1. Preparation of soft magnetic phase nanoparticles;

[0016] S2. Prepare a precursor solution containing soft magnetic phase nanoparticles;

[0017] S3. Spray pyrolysis of the precursor solution to obtain soft and hard magnetic nanoparticles with a one-shell multi-core structure.

[0018] By adjusting the concentration of the precursor solution, atomization method, atomization frequency, carrier gas flow rate, and pyrolysis temperature, the soft magnetic phase forms multiple nuclei inside the particles, while the hard magnetic phase forms a continuous shell on the outer layer, achieving dual controllability of the core and shell thicknesses.

[0019] The preparation method is simple and obtains one-shell multi-core soft and hard magnetic nanoparticles with controllable core and shell layers through spray pyrolysis. It can precisely control the shell thickness and composition at the nanoscale to achieve good soft and hard magnetic matching and exchange coupling performance. It provides a new way for structural control and performance optimization of high-performance soft and hard magnetic composite materials. Moreover, it does not require complicated sol-gel coating or acid washing steps, and the process is simplified and has strong applicability.

[0020] Furthermore, the soft magnetic phase nanoparticles in step S1 are prepared by co-precipitation, hydrothermal method or sol-gel method.

[0021] Furthermore, the coprecipitation method specifically includes the following steps:

[0022] Fe 3+ / Fe 2+ The precursor dissolves in deionized water to form a metal salt solution with a total metal ion concentration of 0.05–1.0 mol / L;

[0023] The pH was adjusted to 9–11 with an alkalizing agent under stirring conditions, the reaction temperature was 60–95℃, and the reaction time was 0.5–4 h, so that the soft magnetic phase precursor precipitated out.

[0024] The precipitate was washed, centrifuged and dried to obtain nanoparticles.

[0025] Furthermore, the soft magnetic phase nanoparticles are ferrite-based soft magnetic materials, including Fe3O4, NiFe2O4, and Ni... X Zn 1-X One or more combinations of Fe2O4.

[0026] Furthermore, the soft magnetic phase nanoparticles have a particle size of 5–100 nm, preferably 10–30 nm, and can be further dispersed in a solvent to form a stable suspension, providing a good dispersibility basis for the subsequent formation of a core-shell structure and obtaining good magnetic properties.

[0027] Furthermore, the preparation of the precursor solution containing soft magnetic phase nanoparticles in step S2 specifically includes the following steps:

[0028] S2.1 The hard magnetic phase metal salt is dissolved in deionized water or an ethanol-water mixed solvent in stoichiometric ratio to form a mother liquor with a total metal ion concentration of 0.05–1.0 mol / L, preferably 0.03–0.2 mol / L. The metal salt can be a nitrate, acetate, or chloride. A mother liquor with a total metal ion concentration of 0.05–1.0 mol / L is beneficial for obtaining higher product preparation efficiency. In the spray pyrolysis method, if the concentration of the precursor solution used is too low, the product preparation efficiency will decrease, while if the concentration of the precursor solution is too high, it will increase the difficulty of ultrasonic atomization, thus also affecting the product preparation efficiency.

[0029] S2.2 Under stirring conditions, a complexing agent, such as citric acid, EDTA, acetylacetone, etc., is added to the mother liquor. The metal ions and complexing agent are mixed in a molar ratio of 1:(1~3). The mixture is ultrasonically stirred for 10~60min to form a homogeneous solution. This promotes the uniform complexation of metal ions in the solution and achieves control of the metal ion ratio to inhibit hydrolysis.

[0030] S2.3 Add the soft magnetic phase nanoparticles obtained in step S1 to the homogeneous solution and sonicate for 5 to 15 minutes to fully disperse the soft magnetic phase nanoparticles.

[0031] S2.4 Adjust the pH of the solution to 3-5 to obtain the precursor solution for spray pyrolysis.

[0032] The amount added is determined based on the target core-shell ratio, and is generally calculated based on the final molar ratio of the soft magnetic phase to the hard magnetic phase.

[0033] The solute concentration in the precursor solution can be adjusted according to the target shell thickness to achieve controllable shell growth during spray pyrolysis. The mass ratio of the soft magnetic core to the hard magnetic shell can also be controlled by adjusting the amount of soft magnetic nanoparticles added. For example, soft magnetic phase nanoparticles can be added to the precursor solution in different proportions, and the dispersibility can be improved by surface ligands (such as citric acid) to promote uniform coating of the subsequent shell.

[0034] Furthermore, the hard magnetic phase is SrFe 12 O 19 BaFe 12 O 19 PbFe 12 O 19 One or more combinations thereof, optionally modified by one or more of Sm, Co, Zn, and Ca to improve magnetic properties.

[0035] Furthermore, step S3, which involves spray pyrolysis of the precursor solution to obtain soft and hard magnetic nanoparticles with a one-shell, multi-core structure, specifically includes the following steps:

[0036] S3.1 A uniformly prepared precursor solution (containing a soft magnetic core dispersion and a hard magnetic metal salt solution) is injected into an atomizing device to form droplets through ultrasonic atomization or airflow atomization, wherein the ultrasonic atomization frequency is 20-40 kHz, the airflow atomization pressure is 0.2-1.2 MPa, and the average droplet diameter is 0.5-20 μm;

[0037] S3.2 The droplets are transported to a tubular furnace under the action of a carrier gas (air, nitrogen, or a nitrogen / oxygen mixture). The diameter of the tubular furnace is 50–150 mm, and the carrier gas flow rate is 3–50 L / min. Different carrier gas atmospheres can be adjusted according to the different atmospheres required for the production target product.

[0038] S3.3 Adjust the concentration of the precursor solution, the atomization flow rate and the gas flow rate to make the shell thickness adjustable in the range of 10 to 300 nm;

[0039] The powder collected in S3.4 is washed to remove salts and template agents, and then annealed at 600–900℃ for 10–60 min. This process helps improve crystallinity, stabilize the phase structure, and further regulate the degree of interfacial diffusion to optimize the bonding strength between soft and hard magnetic phases. Increasing the solute concentration of the precursor solution or decreasing the atomization flow rate can thicken the shell, while increasing the carrier gas flow rate or diluting the concentration is beneficial for forming a thinner shell. Since increasing the saturation magnetization in soft and hard magnetic nanophases while maintaining coercivity requires a certain thickness of both the shell and core layers, the shell thickness can be adjusted within the range of 10–300 nm by controlling these parameters.

[0040] Furthermore, in step S3.2, the tubular heating furnace is equipped with a multi-temperature zone structure along its length:

[0041] First temperature zone: 100~300℃, used for solvent evaporation and gelation, so that the surface of the droplet first solidifies to form a shell framework;

[0042] Second temperature zone: 350~650℃, used for organic combustion and precursor decomposition, enabling hard magnetic phase to nucleate on the surface of nuclei;

[0043] The third temperature zone, 800–1250℃, is used for shell crystallization and phase formation to obtain core-shell composite particles with a dense structure and uniform distribution.

[0044] Furthermore, the combination of soft and hard magnetic phases in step S3 includes Fe3O4 / SrFe 12 O 19 ZnFe2O4 / BaFe 12 O 19 Ni X Zn 1-X Fe2O4 / SrFe 12 O19 One of them.

[0045] The present invention, by adopting the above-described technical solution, has the following beneficial effects:

[0046] The preparation method of this invention is simple. It obtains one-shell multi-core soft and hard magnetic nanoparticles with controllable core and shell layers through spray pyrolysis. It can precisely control the shell thickness and composition at the nanoscale, and achieve good soft and hard magnetic matching and exchange coupling performance. It provides a new way for structural control and performance optimization of high-performance soft and hard magnetic composite materials. Moreover, it does not require complicated sol-gel coating or acid washing steps, and the process is simplified and has strong applicability. Attached Figure Description

[0047] The present invention will be further described below with reference to the accompanying drawings:

[0048] Figure 1 This is a flowchart of a method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to the present invention.

[0049] Figure 2 This is a schematic diagram illustrating the specific preparation steps in this invention;

[0050] Figure 3 The image shows the transmission electron microscope (TEM) and corresponding energy dispersive spectroscopy (EDS) distribution of elements (Sm, Sr, Fe, O) for the sample with a soft magnetic phase to hard magnetic phase ratio of 1:2 in this invention. Detailed Implementation

[0051] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0052] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0053] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0054] Example 1

[0055] Weigh out FeCl3·6H2O and FeCl2·4H2O, and make Fe... 2+ / Fe 3+ The metal salt solution was obtained by dissolving it in deionized water at a molar ratio of 2:1, and the total concentration of metal ions was adjusted to 0.1 mol·L⁻¹. -1 Under stirring at 600 rpm, 25 wt.% NH3·H2O was added dropwise to the solution to adjust the pH to 10. The reaction temperature was raised to 80℃ and maintained for 1 h to allow the Fe3O4 precursor to fully precipitate. The resulting precipitate was washed three times with deionized water, centrifuged at 8000 rpm for 5 min, and then vacuum dried at 60℃ for 6 h to obtain soft magnetic phase Fe3O4 nanoparticles.

[0056] Weigh out Sr(NO3)2, Fe(NO3)3·9H2O and Sm(NO3)3·6H2O, and dissolve them in deionized water according to the target composition ratio Sr:Fe:Sm=1:11.8:0.2, controlling the Fe content. 3+ The concentration is 0.2 mol·L⁻¹ -1 While maintaining stirring, citric acid was added to the mother liquor of the metal salt as a complexing agent to achieve a molar ratio of metal ions to complexing agent of 1:2. Stirring was continued for 20 minutes until the system became clear and homogeneous. The prepared Fe3O4 nanoparticles were then processed according to the formula M(SrSm). 0.2 Fe 11.8 O 19 The metal salt-complexing agent solution was slowly added in a ratio of 1:1 (Fe3O4):M, and the mixture was treated under ultrasonic conditions for 10 minutes to promote the full dispersion of the soft magnetic nuclei. The resulting homogeneous suspension is the precursor solution used for spray pyrolysis.

[0057] The prepared precursor solution was injected into an ultrasonic nebulizer, with the atomization frequency set to 30 kHz. Using air at a flow rate of 10 L / min as the carrier gas, the atomized droplets were transported to a tube furnace. The tube furnace was set with three temperature zones: zone one at 300°C; zone two at 600°C; and zone three at 1180°C. During the heating process, the precursor droplets underwent evaporation, decomposition, and sintering. Soft magnetic Fe3O4 particles were coated onto hard magnetic SrSm... 0.2 Fe 11.8 O 19 Within the shell formed by the precursor, multi-core or core-shell type soft and hard magnetic dual-phase particles are formed. The powder obtained from spraying is collected through a G5 sand core funnel. Finally, Fe3O4@SrSm is obtained. 0.2 Fe 11.8 O 19 Multi-core soft and hard magnetic nanoparticles with a single shell.

[0058] The performance parameters of composite powders obtained by different ratios of soft magnetic phase and hard magnetic phase (1:1, 1:2, 1:3) are shown in Table 1.

[0059] Soft magnetic phase: Hard magnetic phase Ms (saturation magnetization) / emu / g Mr (remanence) / emu / g Hcj (Innate Coping Power) / kOe (BH)max (maximum magnetic energy product) / MGOe 1:3 70.35 38.40 6.23 3.02 1:2 77.64 44.05 5.85 3.43 1:1 85.78 43.45 4.52 2.85

[0060] Table 1

[0061] As shown in Table 1, the ratio of soft magnetic phase to hard magnetic phase has a significant impact on the magnetic properties of the composite material, as detailed below:

[0062] (1) Ms increases with the increase of the proportion of soft magnetic phase. When the content of soft magnetic phase increases (the ratio of soft magnetic phase to hard magnetic phase changes from 1:3 to 1:1), Ms increases from 70.35 emu / g to 85.78 emu / g. This is because the intrinsic saturation magnetization of the hard magnetic phase (strontium ferrite) is lower than that of the soft magnetic phase (Fe3O4); as the proportion of soft magnetic phase increases, the overall Ms of the system shows an upward trend.

[0063] (2) The remanent magnetization (Mr) showed a trend of first increasing and then decreasing with the increase of the soft magnetic phase ratio. Mr first increased from 38.40 emu / g to 44.05 emu / g and then decreased to 47.45 emu / g. This is because when the soft magnetic phase content is low, there is a strong exchange coupling effect in the composite structure, which makes the soft magnetic phase partially "pinned" by the hard magnetic phase, thereby improving the remanent magnetization. However, when the soft magnetic phase ratio is high, it is easier to form a continuous or semi-continuous soft magnetic network structure between particles, so that the reverse magnetic domains inside the soft magnetic phase can start to rotate or flip under a lower external magnetic field. At this time, the "pinning effect" of the hard magnetic phase on the reverse magnetic domains is weakened, which makes the Mr of the system decrease to a certain extent compared with the 1:2 sample.

[0064] (3) The intrinsic coercivity (Hcj) increases significantly with the increase of the proportion of hard magnetic phase. Hcj increases from 4.52kOe to 6.23kOe. This phenomenon indicates that with the increase of the content of hard magnetic phase, the anisotropy of hard magnetic phase and the contribution of magnetocrystalline structure become dominant, which enhances the demagnetization resistance of the composite system.

[0065] (4) The maximum magnetic energy product ((BH)max) reaches its maximum value of 3.43 MGOe when the soft-hard magnetic ratio is 1:2. This is because although Hcj is high at 1:3, Ms is relatively low, which limits the magnetic energy product. Although Ms is high at 1:1, Hcj is low because the soft magnetic ratio is high and the magnetic domains are easy to flip, which cannot support a large magnetic energy product; at 1:2, the soft-hard magnetic ratio reaches a more favorable exchange coupling balance state.

[0066] Transmission electron microscopy (TEM) images and corresponding energy dispersive spectroscopy (EDS) elemental (Sm, Sr, Fe, O) distributions of a sample with a soft magnetic phase:hard magnetic phase ratio of 1:2 prepared according to the method of Example 1 are shown below. Figure 2 As shown.

[0067] TEM images reveal that the particles exhibit a near-spherical structure with multiple Fe3O4 nanonuclei inside, showing weak Sm and Sr elemental signals. The outer layer is a continuous, shell-like coating. The boundaries of the elemental distribution areas are clear, with no obvious interpenetration, indicating a well-defined interface between the soft magnetic core and the hard magnetic shell. The shell coating is complete and continuous, fully demonstrating the effective controllability of the method of this invention over the core position and shell thickness.

[0068] The above results demonstrate that the magnetic properties of the soft and hard magnetic composite materials prepared in this invention can be effectively controlled by adjusting the ratio of the soft magnetic phase to the hard magnetic phase. In particular, when the ratio of soft magnetic phase to hard magnetic phase is 1:2, the material achieves superior overall magnetic properties in terms of saturation magnetization, coercivity, and energy product, making it suitable as one of the application solutions for high-performance permanent magnet composite materials.

[0069] Comparative Example 1

[0070] Under the same process conditions as described in Example 1, single-phase hard magnetic phase SrSm was obtained by spray pyrolysis. 0.2 Fe 11.8 O 19 And the preparation of single-phase soft magnetic phase Fe3O4 by co-precipitation method.

[0071] SrSm 0.2 Fe 11.8 O 19 The powder properties of Fe3O4 are shown in Table 2.

[0072] sample Ms (saturation magnetization) / emu / g Mr (remanence) / emu / g Hcj (Innate Coping Power) / kOe (BH)max (maximum magnetic energy product) / MGOe <![CDATA[SrSm 0.2 Fe 11.8 O 19 ]]> 66.86 34.22 6.70 2.80 <![CDATA[Fe3O4]]> 91.35 2.83 0.047 /

[0073] Table 2

[0074] While hard magnetic phases possess high coercivity, their Ms (magnetic moment) is low, preventing them from simultaneously achieving high magnetization and high energy product. Soft magnetic phases, while possessing high Ms, exhibit almost no coercivity. Compared to the aforementioned single-phase materials, this invention constructs a dual-controllable core-shell multi-core soft-hard magnetic nanostructure, achieving close interfacial contact between the soft and hard magnetic phases at the nanoscale. By introducing controllable shell thickness, the soft magnetic phase can partially harden under the exchange field constraint of the hard magnetic phase, while the hard magnetic phase achieves a higher Ms due to the high magnetic moment compensation of the soft magnetic phase. This significantly enhances saturation magnetization (compared to single-phase hard magnets); maintains an appropriate coercivity level (far superior to single-phase soft magnets); and achieves a higher maximum energy product (higher than both single-phase hard and soft magnets). Simultaneously, it improves the Ms–Hc performance balance, which is difficult to achieve in traditional physical mixing methods or simple composite structures.

[0075] The above are merely specific embodiments of the present invention, but the technical features of the present invention are not limited thereto. Any simple changes, equivalent substitutions, or modifications made based on the present invention to achieve substantially the same technical effect are all covered within the protection scope of the present invention.

Claims

1. A method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness, characterized in that... Includes the following steps: S1. Preparation of soft magnetic phase nanoparticles; S2. Prepare a precursor solution containing the soft magnetic phase nanoparticles; S3. Spray pyrolysis of the precursor solution to obtain soft and hard magnetic nano-biphase particles with a one-shell multi-core structure. In this process, by adjusting the concentration, atomization method, atomization frequency, carrier gas flow rate, and pyrolysis temperature of the precursor solution, the soft magnetic phase forms multiple nuclei inside the particles, and the hard magnetic phase forms a continuous shell on the outer layer, thus achieving dual controllability of the core layer and shell layer thickness.

2. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 1, characterized in that: The soft magnetic phase nanoparticles in step S1 are prepared by coprecipitation, hydrothermal method or sol-gel method.

3. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 2, characterized in that: The coprecipitation method specifically includes the following steps: Fe 3+ / Fe 2+ The precursor dissolves in deionized water to form a metal salt solution with a total metal ion concentration of 0.05–1.0 mol / L; The pH was adjusted to 9–11 with an alkalizing agent under stirring conditions, the reaction temperature was 60–95℃, and the reaction time was 0.5–4 h, so that the soft magnetic phase precursor precipitated out. The precipitate was washed, centrifuged and dried to obtain nanoparticles.

4. A method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 2 or 3, characterized in that: The soft magnetic phase nanoparticles are ferrite-based soft magnetic materials, including Fe3O4, NiFe2O4, and Ni. X Zn 1-X One or more combinations of Fe2O4.

5. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 4, characterized in that: The soft magnetic phase nanoparticles have a particle size of 5–100 nm.

6. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 1, characterized in that: The preparation of the precursor solution containing soft magnetic phase nanoparticles in step S2 specifically includes the following steps: S2.1 Dissolve the hard magnetic phase metal salt in deionized water or ethanol-water mixed solvent according to the stoichiometric ratio to form a mother liquor with a total metal ion concentration of 0.05-1.0 mol / L; S2.2 Under stirring conditions, a complexing agent is added to the mother liquor. The metal ions and the complexing agent are mixed in a molar ratio of 1:(1-3). The mixture is ultrasonically stirred for 10-60 minutes to form a homogeneous solution. S2.3 Add the soft magnetic phase nanoparticles obtained in step S1 to the homogeneous solution and sonicate for 5 to 15 minutes to fully disperse the soft magnetic phase nanoparticles. S2.4 Adjust the pH of the solution to 3-5 to obtain the precursor solution for spray pyrolysis.

7. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 6, characterized in that: The hard magnetic phase is SrFe. 12 O 19 BaFe 12 O 19 PbFe 12 O 19 One or more combinations thereof, optionally modified by one or more doping of Sm, Co, Zn, and Ca.

8. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 1, characterized in that: Step S3, which involves spraying the precursor solution to obtain soft and hard magnetic nanoparticles with a one-shell, multi-core structure, specifically includes the following steps: S3.1 The precursor solution is formed into droplets by ultrasonic atomization or airflow atomization, wherein the ultrasonic atomization frequency is 20-40 kHz, the airflow atomization pressure is 0.2-1.2 MPa, and the average droplet diameter is 0.5-20 μm; S3.2 The droplets are transported to the tubular furnace under the action of a carrier gas, and the carrier gas flow rate is 3-50 L / min; S3.3 Adjust the concentration of the precursor solution, the atomization flow rate and the gas flow rate to make the shell thickness adjustable in the range of 10 to 300 nm; S3.4 Collect the obtained powder, wash it, and then anneal it at 600-900℃ for 10-60 minutes.

9. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 8, characterized in that: The tubular heating furnace described in step S3.2 is equipped with a multi-temperature zone structure along its length: First temperature zone: 100~300℃, used for solvent evaporation and gelation, so that the surface of the droplet first solidifies to form a shell framework; Second temperature zone: 350~650℃, used for organic combustion and precursor decomposition, enabling hard magnetic phase to nucleate on the surface of nuclei; The third temperature zone, 800–1250℃, is used for shell crystallization and phase formation to obtain core-shell composite particles.

10. The method for preparing core-shell structured soft and hard magnetic nanoparticles with controllable shell thickness according to claim 1, characterized in that: The combination of the soft magnetic phase and the hard magnetic phase in step S3 includes Fe3O4 / SrFe 12 O 19 ZnFe2O4 / BaFe 12 O 19 Ni X Zn 1-X Fe2O4 / SrFe 12 O 19 One of them.

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