A single-phase Fe3O4 agglomerate with room temperature exchange bias effect and its preparation method

Abstract

The application belongs to the technical field of magnetic nanometer material, and particularly relates to a single-phase Fe3O4 agglomerate with room-temperature exchange bias effect and a preparation method. The single-phase Fe3O4 agglomerate is a micron-level agglomerate formed by agglomeration of pure-phase Fe3O4 nanocrystalline grains. The Fe3O4 nanocrystalline grains contain reverse domain boundaries. The reverse domain boundaries produce pinning effect on the Fe3O4 matrix at room temperature, and form a magnetic hetero-interface with the surrounding ferromagnetic Fe3O4 matrix, so that the single-phase Fe3O4 agglomerate exhibits exchange bias effect at room temperature. The application introduces high-density reverse domain boundaries into the Fe3O4 nanocrystalline grains, so that the Fe3O4 nanocrystalline grains are functionally equivalent to a ferromagnetic / antiferromagnetic hetero-interface. Through the pinning effect of the reverse domain boundaries on the ferromagnetic matrix, the application breaks the dependence of traditional exchange bias effect on the hetero-interface, and provides a single-phase material with stable exchange bias performance at room temperature.

Description

Technical Field

[0001] This invention belongs to the field of magnetic nanomaterials technology, specifically relating to a single-phase Fe3O4 aggregate with room temperature exchange bias effect and its preparation method. Background Technology

[0002] The exchange bias effect refers to the physical phenomenon where the hysteresis loop of a magnetic material shifts along the magnetic field axis after being cooled by a magnetic field. This effect is the physical basis of spintronic devices such as spin valves and magnetic random access memories. Traditionally, the exchange bias effect is thought to originate from the exchange coupling at the interface of ferromagnetic / antiferromagnetic heterojunctions, and is typically only significantly observable at low temperatures (below the Nell temperature of antiferromagnetic materials).

[0003] Iron(III) oxide (Fe3O4), a typical ferrimagnetic material, possesses a high Curie temperature, high spin polarizability, and good biocompatibility, making it a promising candidate for applications in magnetic recording, biosensing, and catalysis. However, due to the lack of a natural ferromagnetic / antiferromagnetic heterostructure within single-phase Fe3O4 materials, they typically fail to exhibit a significant exchange bias effect at room temperature, severely limiting their practical application in room-temperature spintronic devices. Summary of the Invention

[0004] The purpose of this invention is to provide a single-phase Fe3O4 agglomerate with room temperature exchange bias effect and its preparation method. High-density anti-phase domain boundaries are introduced inside Fe3O4 nanocrystals, making them functionally equivalent to a ferromagnetic / antiferromagnetic heterostructure. Through the pinning effect of the anti-phase domain boundaries on the ferromagnetic matrix, a stable exchange bias effect at room temperature is achieved in the single-phase material system.

[0005] To achieve the above objectives, the specific technical solution provided by the present invention is as follows: The first objective of this invention is to provide a single-phase Fe3O4 agglomerate with a room-temperature exchange bias effect. The single-phase Fe3O4 agglomerate is a micron-sized agglomerate formed by the aggregation of pure-phase Fe3O4 nanocrystals. The Fe3O4 nanocrystals contain anti-phase domain boundaries, which pin the Fe3O4 matrix at room temperature and form a magnetic heterogeneous interface with the surrounding ferromagnetic Fe3O4 matrix, thereby enabling the single-phase Fe3O4 agglomerate to exhibit an exchange bias effect at room temperature.

[0006] Furthermore, the Fe3O4 nanocrystals have an inverse spinel cubic structure with a space group of Fd-3m; the cation sublattices on both sides of the inverse domain boundary have a translation vector R=a / 4

[110] .

[0007] Furthermore, the absolute value of the exchange bias field of single-phase Fe3O4 aggregates at 300K is 150Oe to 250Oe.

[0008] Furthermore, the coercivity of single-phase Fe3O4 aggregates at 300K is 350Oe to 450Oe.

[0009] A second objective of this invention is to provide a method for preparing the above-mentioned single-phase Fe3O4 aggregates with room temperature exchange bias effect, comprising the following steps: S1. Using soluble ferric iron and soluble fluorine as raw materials, they are mixed in a solvent at room temperature to generate an iron-containing precursor precipitate, which is then separated and dried to obtain the precursor.

[0010] S2. The precursor powder is calcined in an oxygen-containing atmosphere to obtain single-phase Fe3O4 aggregates with room temperature exchange bias effect.

[0011] Furthermore, the molar ratio of the soluble trivalent iron source to the soluble fluorine source is 1:2 to 4.

[0012] Furthermore, the calcination treatment is carried out at a temperature of 350℃~450℃ for 1h~3h, and the oxygen-containing atmosphere is air.

[0013] Furthermore, the soluble trivalent iron source is ferric chloride hexahydrate, and the soluble fluorine source is ammonium bifluoride.

[0014] Furthermore, the specific method of mixing in the solvent at room temperature is as follows: the soluble fluorine source solution is added dropwise to the iron source solution at a rate of 0.5 drops / s to 2 drops / s.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: The single-phase Fe3O4 aggregates provided by this invention introduce high-density anti-phase domain boundaries within Fe3O4 nanocrystals. These anti-phase domain boundaries act as intrinsic pinning sources, pinning the Fe3O4 matrix at room temperature. Due to cation misalignment and altered superexchange pathways at their interfaces, they exhibit antiferromagnetic coupling properties, forming a magnetic heterojunction with the surrounding ferromagnetic Fe3O4 matrix. This results in the single-phase Fe3O4 aggregates exhibiting an exchange bias effect at room temperature. As a structurally stable intrinsic crystal defect, the strong exchange coupling and structural disorder of the anti-phase domain boundaries at the atomic scale ensure that their pinning effect remains stable at room temperature (300K) and even higher temperatures, exhibiting a stable exchange bias field at room temperature, reaching a value of -225.3 Oe. This provides a new material system and design concept for the application of single-phase magnetic materials in room-temperature spintronic devices.

[0016] This invention employs a co-precipitation-calcination method, precisely controlling the precipitation and calcination processes of the precursor. During the growth and aggregation of Fe3O4 nanocrystals, due to the multiple possible growth modes of spinel structures nucleating on rock salt structures or face-centered cubic oxygen sublattices, atomic stacking mismatch occurs when subdomains with different orientations merge, thus forming antiphase domain boundaries at the interface. This allows for the controllable introduction of structural defects (antiphase domain boundaries) into Fe3O4 nanocrystals. The process is simple, highly reproducible, and eliminates the need for complex heterostructure construction processes, making it functionally equivalent to a ferromagnetic / antiferromagnetic heterostructure interface. Through the pinning effect of the antiphase domain boundaries on the ferromagnetic matrix, a stable exchange bias effect at room temperature is achieved in a single-phase material system. Attached Figure Description

[0017] Figure 1 This is a process flow diagram of the preparation of single-phase Fe3O4 agglomerates in Example 1 of the present invention.

[0018] Figure 2 The X-ray diffraction patterns are of the Fe3O4 powders prepared in Examples 1 to 3 of this invention.

[0019] Figure 3 This is a microstructure diagram of the Fe3O4 nanoparticles prepared in Example 1 of the present invention. Figure 3 In the diagram, (a) is a transmission electron microscope image, and the inset is a histogram of particle size distribution; (b) is a selected area electron diffraction pattern of Fe3O4 nanoparticles; (c) is a high-resolution transmission electron microscope image of a single Fe3O4 nanoparticle; (d) is the fast Fourier transform image corresponding to (c); (e) is the image after Fourier filtering of (c) using {311} reflection; (f) is the image after Fourier filtering of (c) using {220} reflection; (g) is a magnified view of the lattice fringes of the red square region in (f), where the red circles indicate the antiphase domain boundaries; (h1) is the phase diagram calculated for region (g); and (h2) is the deformation diagram calculated for region (g).

[0020] Figure 4 This is a microstructure diagram of the Fe3O4 aggregates prepared in Example 1 of the present invention. Figure 4 In the image, (a) is a scanning electron microscope image of Fe3O4 aggregates, (b) is an energy dispersive X-ray spectrum of region (a), (c) is a histogram of the size distribution of Fe3O4 aggregate particles, and (d1) to (d3) are the energy dispersive X-ray spectrum distributions of Fe and O elements in Fe3O4 aggregates, respectively.

[0021] Figure 5 The hysteresis loops of Fe3O4 aggregates prepared in Examples 1 to 3 of this invention were measured at room temperature. Detailed Implementation

[0022] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention.

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0024] In recent years, although some studies have discovered anomalous exchange bias phenomena in certain defective single-phase magnetic nanomaterials, the underlying physical mechanisms remain unclear, especially the stable exchange bias effect at room temperature, which has been rarely reported. Therefore, those skilled in the art are dedicated to developing a single-phase Fe3O4 material capable of achieving a significant exchange bias effect at room temperature and its controllable preparation method. By controlling the microscopic defect structure within the material, intrinsic magnetic pinning centers can be constructed, thereby overcoming the limitations of traditional heterojunction interfaces and improving the room-temperature magnetic properties of single-phase magnetic materials.

[0025] Based on this, the present invention provides a single-phase Fe3O4 agglomerate with room temperature exchange bias effect. The single-phase Fe3O4 agglomerate is a micron-sized agglomerate formed by the aggregation of pure phase Fe3O4 nanocrystals. The Fe3O4 nanocrystals contain antiphase domain boundaries, which act as intrinsic pinning sources. The antiphase domain boundaries pin the Fe3O4 matrix at room temperature. Due to the cation misalignment and the change in superexchange path at the interface, it has antiferromagnetic coupling characteristics, forming a magnetic heterogeneous interface with the surrounding ferromagnetic Fe3O4 matrix, so that the single-phase Fe3O4 agglomerate exhibits an exchange bias effect at room temperature.

[0026] The inventors discovered that during the growth and aggregation of Fe3O4 nanocrystals, due to the multiple possible growth pathways of spinel structures nucleating on rock salt structures or face-centered cubic oxygen sublattices, atomic stacking mismatch occurs during the merging of subcrystalline domains with different orientations, resulting in the formation of antiphase domain boundaries at the interface. These antiphase domain boundaries disrupt the Fe... 2+ and Fe 3+ The ordered arrangement of ions in tetrahedral and octahedral sublattices interrupts or alters the superexchange interaction pathways, resulting in strong antiferromagnetic coupling at the interfaces. From the perspective of microscopic crystal defect manipulation, the physical essence of antiphase domain boundary pinning magnetic moment reversal is revealed, providing theoretical and experimental basis for controlling the macroscopic magnetism of nanomaterials through defect engineering.

[0027] In this invention, the anti-ferromagnetic domain boundary with antiferromagnetic coupling characteristics is functionally equivalent to a traditional ferromagnetic / antiferromagnetic heterointerface. The Fe3O4 matrix within the crystal domains retains its ferromagnetism, while the anti-ferromagnetic domain boundary region acts as a highly anisotropic antiferromagnetic pinning layer. During magnetization reversal, the anti-ferromagnetic domain boundary exerts a pinning torque on the magnetic moment of the ferromagnetic matrix through interfacial exchange coupling, hindering its reversal with the external field. This macroscopically causes the hysteresis loop to shift along the magnetic field axis, i.e., an exchange bias effect.

[0028] In this invention, the average particle size of Fe3O4 nanocrystals can be in the range of about 20 nm to about 30 nm, and the average particle size of micron-sized aggregates can be in the range of about 1 μm to about 5 μm. It is understood that the above particle size ranges are merely exemplary observations of the product morphology obtained under certain conditions by the method of this invention, and are not intended to limit the scope of protection of this invention. The actual particle size may vary due to adjustments in the preparation parameters.

[0029] In some embodiments, the Fe3O4 nanocrystals have an inverse spinel cubic structure with space group Fd-3m; the cation sublattices on both sides of the inverse domain boundaries exhibit a translation vector R=a / 4

[110] . This displacement disrupts the ordered arrangement of charges.

[0030] In this invention, geometric phase analysis using high-resolution transmission electron microscopy revealed significant lattice phase jumps (approximately π) and linear strain fields within the nanoparticles, providing direct evidence for the existence of antiphase domain boundaries.

[0031] In some embodiments, the absolute value of the exchange bias field HEB of the single-phase Fe3O4 aggregate at 300K is 150Oe to 250Oe. The coercivity HC of the single-phase Fe3O4 aggregate at 300K is 350Oe to 450Oe.

[0032] In this invention, antiphase domain boundaries, as structurally stable intrinsic crystal defects, exhibit strong exchange coupling and structural disorder at the atomic scale, enabling their pinning effect to remain stable at room temperature (300K) and even higher temperatures. This overcomes the limitation that traditional exchange bias effects can only be observed at low temperatures. Fe3O4 aggregates exhibit a stable exchange bias field at room temperature, with a value reaching -225.3Oe, providing a new material system and design concept for the application of single-phase magnetic materials in room-temperature spintronic devices.

[0033] This invention also provides a method for preparing the above-mentioned single-phase Fe3O4 aggregates with room temperature exchange bias effect, comprising the following steps: S1. Using soluble ferric iron and soluble fluorine as raw materials, they are mixed in a solvent at room temperature to generate an iron-containing precursor precipitate, which is then separated and dried to obtain the precursor.

[0034] It is understood that the soluble ferric iron source and the soluble fluorine source are mixed in solution form. Specifically, the soluble ferric iron source is dissolved in ethanol, and the soluble fluorine source is dissolved in water. As a preferred embodiment of the invention, the soluble ferric iron source is ferric chloride hexahydrate with a purity of not less than 97%, and the soluble fluorine source is ammonium bifluoride (NH4HF2) with a purity of not less than 98%. The soluble fluorine source solution is added dropwise to the iron source solution. The molar ratio of the soluble ferric iron source to the soluble fluorine source is 1:2 to 4, preferably 1:3. The dropping rate is 0.5 drops / s to 2 drops / s. After the addition is complete, stirring continues for 0.5 min to 2 min, resulting in a precipitation reaction to form a white precipitate of (NH4)3FeF6.

[0035] In this invention, fluorine is used as a sacrificial structure-directing agent, and the final product is pure-phase Fe3O4. The purpose of introducing NH4HF2 is to act as a precipitant in the liquid-phase precipitation reaction, forming the precursor (NH4)3FeF6. The reaction formula is as follows: Fe 3+ +3NH4 + +6F - →(NH4)3FeF6. The fluorine in the final product completely decomposes and volatilizes during subsequent high-temperature calcination. Using ethanol as the main solvent helps control the precipitation rate and product morphology. A dropping rate of 0.5 drops / s to 2 drops / s is used to control the amount of fluorine in the reaction system. - The local concentration of ions is controlled to avoid explosive nucleation caused by excessive concentration, thus promoting the formation of (NH4)3FeF6 precipitates with more uniform size and better crystallinity.

[0036] In some embodiments, separation and drying specifically include: separating the precipitate from the liquid using a centrifuge at a speed of 3500 rpm to 4500 rpm, repeatedly washing the precipitate to remove soluble impurities, and then drying it in a vacuum to obtain a white (NH4)3FeF6 precursor powder.

[0037] S2. The precursor powder is calcined in an oxygen-containing atmosphere at a temperature of 350℃~450℃ for 1h~3h to obtain single-phase Fe3O4 agglomerates with room temperature exchange bias effect.

[0038] It is understood that the oxygen-containing atmosphere used in this invention is air, and the precursor is calcined at 350℃~450℃ for 1h~3h. Under high-temperature calcination conditions, the fluorine element in the precursor is completely converted into gaseous hydrogen fluoride (HF) and discharged with the tail gas. The reaction formula is as follows: (NH4)3FeF6→FeF3(s)+3NH3(g)+3HF(g) (decomposition). Subsequently, FeF3 is further oxidized in the oxygen-containing atmosphere, eventually generating a fluorine-free single-phase Fe3O4 agglomerate. The decomposition, reconstruction and volume change of the precursor during the calcination process are used to deliberately introduce structural defects (APB).

[0039] This invention utilizes a co-precipitation-calcination method to precisely control the precipitation process and calcination process of the precursor, thereby controllably introducing structural defects (anti-phase domain boundaries) into Fe3O4 nanocrystals. The process is simple, highly reproducible, and does not require a complex heterostructure building process. It is functionally equivalent to a ferromagnetic / antiferromagnetic heterointerface. Through the pinning effect of the anti-phase domain boundaries on the ferromagnetic matrix, a stable exchange bias effect at room temperature is achieved in a single-phase material system.

[0040] The following specific examples will provide further explanation.

[0041] Example 1 A method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect, such as Figure 1 As shown, it includes the following steps: S1. Weigh 2.70 g (10 mmol) of ferric chloride (III) hexahydrate (97% purity) and 1.71 g (30 mmol) of ammonium bifluoride (98% purity). Dissolve ferric chloride (III) hexahydrate in 100 mL of ethanol at room temperature with thorough stirring to obtain an orange-red transparent solution. Dissolve ammonium bifluoride in 5 mL of purified water at room temperature with thorough stirring to obtain a colorless transparent solution. While stirring continuously, add the ammonium bifluoride solution dropwise to the ferric chloride (III) hexahydrate solution at a rate of 1 drop / s. After the addition is complete, continue stirring for 1 min, observing the formation of a yellow-green supernatant and a white precipitate. Separate the precipitate from the liquid using a centrifuge at 4000 rpm. Wash the precipitate repeatedly and then dry it in a vacuum at 60 °C for 12 h to obtain a white (NH4)3FeF6 powder.

[0042] S2. Place (NH4)3FeF6 powder in a muffle furnace and calcine at 400℃ for 2 hours in air atmosphere to obtain black Fe3O4 powder, which is a single-phase Fe3O4 agglomerate with room temperature exchange bias effect.

[0043] Example 2 A method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect, such as Figure 1As shown, it includes the following steps: S1. Weigh 2.70 g (10 mmol) of ferric chloride (III) hexahydrate (99% purity) and 1.71 g (30 mmol) of ammonium bifluoride (99% purity). Dissolve ferric chloride (III) hexahydrate in 100 mL of ethanol at room temperature with thorough stirring to obtain an orange-red transparent solution. Dissolve ammonium bifluoride in 5 mL of purified water at room temperature with thorough stirring to obtain a colorless transparent solution. While stirring continuously, add the ammonium bifluoride solution dropwise to the ferric chloride (III) hexahydrate solution at a rate of 0.5 drops / s. After the addition is complete, continue stirring for 2 min, and observe the formation of a yellow-green supernatant and a white precipitate. Separate the precipitate from the liquid using a centrifuge at 3500 rpm. Wash the precipitate repeatedly and then dry it in a vacuum at 60 °C for 12 h to obtain a white (NH4)3FeF6 powder.

[0044] S2. Place (NH4)3FeF6 powder in a muffle furnace and calcine at 350°C for 3 hours in air atmosphere to obtain black Fe3O4 powder, which is a single-phase Fe3O4 agglomerate with room temperature exchange bias effect.

[0045] Example 3 A method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect, such as Figure 1 As shown, it includes the following steps: S1. Weigh 2.70 g (10 mmol) of ferric chloride (III) hexahydrate (99.9% purity) and 1.43 g (25 mmol) of ammonium bifluoride (99% purity). Dissolve ferric chloride (III) hexahydrate in 100 mL of ethanol at room temperature with thorough stirring to obtain an orange-red transparent solution. Dissolve ammonium bifluoride in 5 mL of purified water at room temperature with thorough stirring to obtain a colorless transparent solution. While stirring continuously, add the ammonium bifluoride solution dropwise to the ferric chloride (III) hexahydrate solution at a rate of 2 drops / s. After the addition is complete, continue stirring for 0.5 min, and observe the formation of a yellow-green supernatant and a white precipitate. Separate the precipitate from the liquid using a centrifuge at 4500 rpm. Wash the precipitate repeatedly and then dry it in a vacuum at 60 °C for 12 h to obtain a white (NH4)3FeF6 powder.

[0046] S2. Place (NH4)3FeF6 powder in a muffle furnace and calcine at 450℃ for 1 hour in air atmosphere to obtain black Fe3O4 powder, which is a single-phase Fe3O4 agglomerate with room temperature exchange bias effect.

[0047] The structure and properties of the Fe3O4 powders prepared in Examples 1 to 3 were tested, and the results are shown below.

[0048] Phase analysis was performed using X-ray diffraction on a Bruker D8 Advance diffractometer with a Cu Kα radiation source (λ = 1.5406 Å). Surface elemental chemical states were analyzed using X-ray photoelectron spectroscopy on a ThermoFisher ESCALAB Xi+ instrument. Microstructure and atomic arrangement were observed using transmission electron microscopy and high-resolution transmission electron microscopy on a JEOL JEM-2100F instrument. Surface morphology and elemental distribution were obtained using scanning electron microscopy combined with energy dispersive spectroscopy on a Gemini SEM 500 instrument. Magnetic parameters were determined using a superconducting quantum interference device (SQUID) on a Quantum Design, Inc. SQUID system, all at room temperature (300 K).

[0049] Figure 2 The images show X-ray diffraction patterns of the Fe3O4 powders prepared in Examples 1 to 3 of this invention. Figure 2 As shown, all diffraction peaks of the Fe3O4 powders prepared in Examples 1 to 3 correspond to the cubic inverse spinel structure Fe3O4 (PDF#99-0073), and there are no other impurity phase characteristic peaks, which confirms the phase purity of the samples.

[0050] Figure 3 This is a microstructure diagram of the Fe3O4 nanoparticles prepared in Example 1 of the present invention. Figure 3 In the diagram, (a) is a transmission electron microscope image, with an inset showing the particle size distribution histogram; (b) is a selected area electron diffraction pattern of Fe3O4 nanoparticles; (c) is a high-resolution transmission electron microscope image of a single Fe3O4 nanoparticle; (d) is the fast Fourier transform image corresponding to (c); (e) is the image after Fourier filtering of (c) using {311} reflection; (f) is the image after Fourier filtering of (c) using {220} reflection; (g) is a magnified view of the lattice fringes in the red square region of (f), where the red circles indicate antiphase domain boundaries; (h1) is the phase diagram calculated for region (g); and (h2) is the deformation diagram calculated for region (g). Figure 3 As shown in (a), the TEM image reveals that the nanoparticles exhibit a polyhedral profile, with a statistically determined average particle size of 25.1 nm. HRTEM image ( Figure 3 (c) to (g)) combined with geometric phase analysis ( Figure 3 The middle (h1) to (h2) clearly reveal the existence of antiphase domain boundaries inside the nanoparticles, which are manifested as the bending and misalignment of lattice fringes and the π-phase jump.

[0051] Figure 4 This is a microstructure diagram of the Fe3O4 aggregates prepared in Example 1 of the present invention. Figure 4In the image, (a) is a scanning electron microscope image of the Fe3O4 aggregates, (b) is the energy-dispersive X-ray spectrum of region (a), (c) is the size distribution histogram of the Fe3O4 aggregate particles, and (d1) to (d3) are the energy-dispersive X-ray spectrum distributions of Fe and O elements in the Fe3O4 aggregates, respectively. Figure 4 As shown in (a) and (c), SEM images reveal that nanoparticles aggregate to form micron-sized secondary structures. The average particle size of the aggregates was statistically determined to be 2.88 μm. EDS analysis ( Figure 4 (b) and (d1) to (d3) confirm that Fe and O elements are evenly distributed in the aggregates, with an atomic percentage close to 3:4.

[0052] Figure 5 The magnetic hysteresis loops of Fe3O4 aggregates prepared in Examples 1 to 3 of this invention were measured at room temperature. Figure 5 As shown, the Fe3O4 aggregates prepared in Example 1 exhibit typical ferromagnetic / ferrimagnetic behavior at room temperature (300 K) and a significant exchange bias effect was observed, with an exchange bias field (HEB) of -218.9 Oe and a coercivity (HC) of 386.5 Oe. The absolute value of the exchange bias field (HEB) of the obtained nanoparticles (218.9 Oe) falls within the range of 150 Oe to 250 Oe, and the coercivity (HC) falls within the range of 350 Oe to 450 Oe. The Fe3O4 aggregates prepared in Example 2 also exhibited a significant exchange bias effect at room temperature (300 K), with an exchange bias field (HEB) of -179.1 Oe and a coercivity (HC) of 377.7 Oe. This demonstrates that single-phase Fe3O4 materials with room temperature exchange bias effects can still be successfully prepared at lower calcination temperatures (350 °C) and slower dropping rates (0.5 drops / s). The absolute value of the exchange bias field HEB of the obtained nanoparticles (179.1 Oe) falls within the range of 150 Oe to 250 Oe, and the coercivity HC falls within the range of 350 Oe to 450 Oe. The Fe3O4 aggregates prepared in Example 3 exhibit a significant exchange bias effect at room temperature (300 K), with an exchange bias field HEB of -202.4 Oe and a coercivity HC of 382.7 Oe. This demonstrates that single-phase Fe3O4 materials with room temperature exchange bias effects can still be successfully prepared even at higher calcination temperatures (450 °C), faster dropping rates (2 drops / s), and a raw material ratio deviating from the stoichiometric ratio of 1:3 (1:2.5). The absolute value of the exchange bias field HEB of the obtained nanoparticles (202.4 Oe) falls within the range of 150 Oe to 250 Oe, and the coercivity HC falls within the range of 350 Oe to 450 Oe.

[0053] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, it is intended to include any modifications and variations that fall within the scope of the claims and their equivalents.

Claims

1. A single-phase Fe3O4 aggregate with room temperature exchange bias effect, characterized in that, The single-phase Fe3O4 agglomerates are micron-sized agglomerates formed by the aggregation of pure-phase Fe3O4 nanocrystals. The Fe3O4 nanocrystals contain anti-phase domain boundaries, which pin the Fe3O4 matrix at room temperature and form a magnetic heterogeneous interface with the surrounding ferromagnetic Fe3O4 matrix, causing the single-phase Fe3O4 agglomerates to exhibit an exchange bias effect at room temperature.

2. The single-phase Fe3O4 aggregate with room temperature exchange bias effect according to claim 1, characterized in that, Fe3O4 nanocrystals have an inverse spinel cubic structure with space group Fd-3m; the cation sublattices on both sides of the inverse domain boundary have a translation vector R=a / 4[110].

3. The single-phase Fe3O4 aggregate with room temperature exchange bias effect according to claim 1, characterized in that, The absolute value of the exchange bias field of single-phase Fe3O4 aggregates at 300K is 150Oe to 250Oe.

4. The single-phase Fe3O4 aggregate with room temperature exchange bias effect according to claim 1, characterized in that, The coercivity of single-phase Fe3O4 aggregates at 300K is 350Oe to 450Oe.

5. A method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Using soluble ferric iron and soluble fluorine as raw materials, they are mixed in a solvent at room temperature to generate an iron-containing precursor precipitate, which is then separated and dried to obtain the precursor. In an oxygen-containing atmosphere, the precursor powder was calcined to obtain single-phase Fe3O4 aggregates with room temperature exchange bias effect.

6. The method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect according to claim 5, characterized in that, The molar ratio of soluble ferric iron source to soluble fluorine source is 1:2 to 4.

7. The method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect according to claim 5, characterized in that, The calcination treatment temperature is 350℃~450℃, the time is 1h~3h, and the oxygen-containing atmosphere is air.

8. The method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect according to claim 5, characterized in that, The soluble trivalent iron source is ferric chloride hexahydrate, and the soluble fluorine source is ammonium bifluoride.

9. The method for preparing single-phase Fe3O4 aggregates with room temperature exchange bias effect according to claim 5, characterized in that, The specific method for mixing in the solvent at room temperature is as follows: the soluble fluorine source solution is added dropwise to the iron source solution at a rate of 0.5 drops / s to 2 drops / s.

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