Preparation method of room-temperature stable gamma-MnS with exchange bias effect

Abstract

The invention discloses a preparation method of gamma-MnS with an exchange bias effect and stable at room temperature. According to the method, a normal-pressure chemical vapor deposition process is adopted, in the cooling stage after material growth is completed, sulfur vacancies are introduced into crystals by accurately controlling sulfur partial pressure and regulating and controlling the sulfur atmosphere where the material is located, and gamma-MnS which has the exchange bias effect and is stable at the room temperature is prepared. The EB effect is achieved through the sulfur vacancy induced intrinsic magnetic heterostructure in the single-phase gamma-MnS, dependence on an artificial heterogeneous interface is avoided, and potential application prospects are achieved in the field of development of next-generation high-performance, high-stability and low-power-consumption spin electronic devices and the like.

Description

Technical Field

[0001] This invention belongs to the field of preparation technology of γ-phase wurtzite structured manganese sulfide, and relates to a method for preparing room temperature stable γ-MnS with exchange bias effect. Background Technology

[0002] Manganese sulfide (MnS), as an important class of transition metal chalcogenides, possesses a variety of crystal structures (such as α-phase rock salt structure, β-phase zincblende structure, and γ-phase wurtzite structure). This structural diversity endows it with rich physicochemical properties, showing potential application value in catalysis, photoelectric conversion, and spintronics. Under thermodynamic equilibrium conditions, the stable α-phase is the main form of MnS, while the γ-phase with a wurtzite structure is a metastable phase. Due to its high energy state, it is usually difficult to obtain γ-MnS directly and in high purity through conventional equilibrium synthesis methods.

[0003] In recent years, researchers have made progress in the preparation of metastable phases such as γ-MnS through non-equilibrium synthesis strategies. Representative methods include: mineral-assisted solvothermal synthesis of metastable microcrystals [J.Am.Ceram.Soc.89(9):2926-2928(2006)]; preparation of colloids based on the pyrolysis of Mn(S2CNEt2)2 molecular precursor in hexadecylamine [J.Am.Chem.Soc.124(4):615-619(2002)]; biomolecule-guided synthesis of γ-MnS under hydrothermal conditions [Mater.Chem.Phys.115(2-3):502-506(2009)]; and synthesis of γ-MnS under hydrothermal conditions using an ethylene glycol / water mixed solvent system [Cryst.Growth]. [Des.24(16):6625-6631(2024)]; and the preparation of γ-MnS / rGO nano-hybrid films by sol-gel spin-coating assisted layer-by-layer self-assembly [Surf.Interfaces 68(1):106705(2025)]. However, these methods generally suffer from problems such as complex process control, difficulty in obtaining precursors, non-uniform product morphology / size, and difficulty in simultaneously achieving room temperature stability and controllable magnetism of γ-MnS. How to precisely control the formation conditions, stability, and phase transition path and kinetics of metastable phases under external stimuli (such as temperature and pressure) remains a key scientific and technological challenge. This challenge is directly related to the controllability of metastable phase material properties and its practical application prospects. Summary of the Invention

[0004] This invention provides a method for preparing room-temperature stable γ-MnS with an exchange bias effect. The method employs atmospheric pressure chemical vapor deposition (CVD). During the cooling stage after material growth, by precisely controlling the sulfur partial pressure, the sulfur atmosphere surrounding the material is modulated, introducing sulfur vacancies into the crystal. This overcomes the inherent thermodynamic metastability of γ-MnS, achieving its stable preparation. Furthermore, the sulfur vacancies induce the formation of localized ferromagnetic nanodomains. These nanodomains couple with the surrounding antiferromagnetic matrix through the interface, endowing γ-MnS with an exchange bias effect.

[0005] The technical solution of the present invention is as follows:

[0006] A method for preparing room-temperature stable γ-MnS with exchange bias effect includes the following steps:

[0007] (1) Precursor loading: A dual-temperature zone tube furnace is used to mix manganese chloride (MnCl2) powder and sodium chloride (NaCl) powder evenly and place them in a quartz boat in the high-temperature zone, while sulfur (S) powder is placed separately in a quartz boat in the low-temperature zone.

[0008] (2) Substrate placement: The freshly cleaved fluorophlogopite (Mica) substrate is horizontally suspended directly above the quartz boat containing the MnCl2 / NaCl mixture;

[0009] (3) Growth process: First, argon (Ar) is introduced into the reaction system to remove air, and then the argon / hydrogen (Ar / H2) mixed gas is switched as the carrier gas. The temperature of the high temperature zone is raised to 620℃~700℃, and the temperature of the low temperature zone is maintained at 200±10℃. The reaction is kept at this temperature for 5min~10min.

[0010] (4) Post-processing: After the reaction is completed, immediately stop the flow of mixed gas and turn off the heating system to allow the reaction system to cool down to room temperature quickly and naturally, so as to obtain room temperature stable γ-MnS nanosheets with exchange bias effect.

[0011] Furthermore, in step (1), the mass ratio of MnCl2 powder to NaCl powder is 10:1.

[0012] Furthermore, in step (3), the volume percentage of H2 in the Ar / H2 mixed gas is 10% to 20%.

[0013] Furthermore, in step (3), the Ar flow rate in the Ar / H2 mixed gas is 70 sccm and the H2 flow rate is 15 sccm.

[0014] Furthermore, in step (3), the temperature of the high-temperature zone is raised to 650°C, the temperature of the low-temperature zone is maintained at 200°C, and the reaction is kept at this temperature for 8 minutes.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] (1) This invention solves the room temperature stability problem of γ-MnS by controllably introducing sulfur vacancies. Sulfur vacancies induce local lattice distortion in the γ-MnS lattice, effectively reducing the driving force for the transition to the stable α phase (i.e., the nucleation driving force of the α phase). First-principles calculations confirm that when the sulfur vacancy concentration is increased to a certain threshold, the free energy of the γ phase is lower than that of the α phase, thereby achieving thermodynamic locking of this metastable phase. The sulfur vacancy engineering (intrinsic defect engineering) developed in this invention successfully overcomes the technical barrier that makes it difficult for γ-MnS to exist stably at room temperature, and pioneers a new approach to achieve material phase stability by controlling intrinsic defects (rather than traditional ion doping), effectively avoiding the adverse effects that external dopants may have on the intrinsic electrical properties of the material.

[0017] (2) This invention also solves the problem of magnetic modulation of γ-MnS by controllably introducing sulfur vacancies. The introduction of sulfur vacancies induces the formation of localized FM nanodomains in the γ-MnS host AFM matrix. These localized ferromagnetic regions form a ferromagnetic / antiferromagnetic (FM / AFM) interface with the surrounding antiferromagnetic matrix. The FM / AFM interface leads to a significant exchange bias effect, which is typically characterized by a horizontal shift of the hysteresis loop (MH curve) along the magnetic field direction. This invention achieves the exchange bias effect in a single-phase MnS material system for the first time, providing an important material platform for the development of novel spintronic devices based on MnS (such as spin valves, magnetic tunnel junctions, etc.). Attached Figure Description

[0018] Figure 1 This is a comparison diagram of the crystal structures and formation energies of α-MnS and γ-MnS.

[0019] Figure 2 (a) is a schematic diagram of the apparatus for preparing MnS by dual-temperature zone chemical vapor deposition, and (b) is a schematic diagram of the synthesis of α-MnS and γ-MnS in sulfur-rich and sulfur-poor environments, respectively.

[0020] Figure 3 (a) Optical micrograph of γ-MnS synthesized in Example 1 and (b) Atomic force microscopy test results.

[0021] Figure 4 The X-ray photoelectron spectroscopy (XPS) results are shown for α-MnS synthesized in Comparative Example 1 and γ-MnS synthesized in Example 1.

[0022] Figure 5 (a) is a high-resolution transmission electron microscope (HRTEM) image of α-MnS and (b) the corresponding selected area electron diffraction (SAED) pattern, and (c) is an HRTEM image of γ-MnS and (d) the corresponding SAED pattern.

[0023] Figure 6 (a) Hysteresis loop of γ-MnS synthesized in Example 1 at low temperatures of 50K to 400K, (b) Thermomagnetic curve of magnetic field strength of 300Oe and (c) Thermomagnetic curve of magnetic field strength of 3000Oe, and (d) Hysteresis loop of α-MnS synthesized in Comparative Example 1 at low temperatures of 50K to 400K, (e) Thermomagnetic curve of magnetic field strength of 300Oe and (f) Thermomagnetic curve of magnetic field strength of 3000Oe.

[0024] Figure 7 (a) Hysteresis loops of γ-MnS synthesized in Example 1 at room temperature along the in-plane and out-of-plane directions, and (b) Hysteresis loops of γ-MnS synthesized in Example 1 at 50K, 100K, and 200K.

[0025] Figure 8 (a) Hysteresis loop of γ-MnS synthesized in Example 1 after field cooling at 50K and (b) Hysteresis loop of γ-MnS synthesized in Example 1 after field cooling at 100K.

[0026] Figure 9 (a) is a structural diagram of a Hall device based on γ-MnS and (b) is the negative magnetoresistance effect of γ-MnS synthesized in Example 1 at 160K to 300K.

[0027] Figure 10 (a,b) are the output characteristic curves (I) of the γ-MnS-based field-effect transistor (FET). ds -V ds (c) Transfer characteristic curves of γ-MnS-based FETs at different temperatures (I ds -V bg (d) different source-drain voltages (V) at room temperature (300K). ds The transfer characteristic curve of the γ-MnS-based FET (I) ds -V bg ).

[0028] Figure 11 (a) The change of γ-MnS resistance with temperature from 150K to 300K and (b) the lnR vs. 1000 / T curve.

[0029] Figure 12 (a) TEM image and EDS elemental distribution map of γ-MnS, (b) band structure of intrinsic γ-MnS calculated by first principles, (c) band structure of Na-substituted Mn-doped γ-MnS calculated by first principles, and (d) XPS test results of γ-MnS.

[0030] Figure 13(a) Electron paramagnetic resonance (EPR) spectra of α-MnS and γ-MnS, (b) Formation energy of α-MnS and γ-MnS as calculated by first-principles calculations as a function of sulfur vacancy concentration, (c) EPR spectra of γ-MnS at different temperatures from 100K to 300K, and (d) EPR characteristic peak integral intensity of γ-MnS as a function of temperature. Detailed Implementation

[0031] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0032] Example 1

[0033] The specific steps for preparing room-temperature stable γ-MnS nanosheets with exchange bias effect are as follows:

[0034] (1) Precursor loading: Mix 0.5g MnCl2 powder (99% purity) with 0.05g NaCl powder and place it in the first quartz boat; place 1.0g sulfur powder (99.9% purity) in the second quartz boat.

[0035] (2) Substrate placement: The freshly cleaved fluorophlogopite substrate (10mm×10mm) is horizontally suspended 1mm above the first quartz boat.

[0036] (3) Reactor setup: The first quartz boat is placed in the center of the high temperature zone of the dual-temperature zone tube furnace, and the target temperature is set to 650℃. The second quartz boat is placed in the center of the low temperature zone, and the target temperature is set to 200℃.

[0037] (4) Atmosphere control: Introduce 700 sccm of high-purity argon gas for 3 minutes to remove oxygen from the reaction tube, and then switch to Ar / H2 mixed carrier gas with Ar flow rate of 70 sccm and H2 flow rate of 15 sccm, maintaining normal pressure.

[0038] (5) Growth process: After heating to the target temperature, keep warm for 8 minutes.

[0039] (6) Termination treatment: After the reaction is completed, immediately stop the flow of mixed gas and turn off the heating system to allow the reaction system to cool naturally to room temperature, thus obtaining room temperature stable γ-MnS nanosheets with exchange bias effect.

[0040] Comparative Example 1

[0041] The specific steps for preparing α-MnS nanosheets are as follows:

[0042] (1) Precursor loading: Same as in Example 1.

[0043] (2) Base placement: Same as in Example 1.

[0044] (3) Reactor setup: Same as in Example 1.

[0045] (4) Atmosphere control: Same as in Example 1.

[0046] (5) Growth process: Same as in Example 1.

[0047] (6) Sulfur enrichment annealing treatment: After growth, stop the H2 flow, maintain only the Ar carrier gas flow of 70 sccm and keep the low temperature zone at 200℃, continue to heat the sulfur source, and cool the high temperature zone to 300℃ at a rate of 10℃ / min, and keep it at this temperature for 60min.

[0048] (7) Termination process: After the isothermal maintenance is completed, stop heating and continue to introduce Ar gas until the system cools down to room temperature.

[0049] Figure 1 This diagram compares the crystal structures and formation energies of α-MnS and γ-MnS. The formation energy is a negative value; the larger the absolute value, the more stable and easier the structure is to synthesize. Under thermodynamic equilibrium conditions, the stable α phase is the dominant form of MnS, while the γ phase, with its wurtzite structure, is a metastable phase.

[0050] Figure 2 A schematic diagram of the apparatus for preparing MnS by dual-temperature zone chemical vapor deposition (a), and a schematic diagram of the synthesis of α-MnS and γ-MnS under sulfur-rich and sulfur-poor environments, respectively (b).

[0051] like Figure 3 As shown, Example 1 synthesized γ-MnS nanosheets with a thickness of approximately 4.54 nm on a mica substrate. The nanosheets were triangular in shape and had protruding particles on their surface, which were presumably formed due to oxidation.

[0052] Figure 4 XPS test results were used to compare the α-MnS synthesized in Example 1 and the γ-MnS synthesized in Example 1. XPS spectra confirmed that both were composed of MnS. The XPS spectrum of γ-MnS showed that the S2p peak of γ-MnS shifted towards lower binding energies compared to α-MnS, indicating that γ-MnS has a higher sulfur vacancy concentration than α-MnS; the Mn 2p spectrum showed the presence of Mn on the surface. 3+ Oxidation signal. Simultaneously... Figure 13 (a) The EPR spectra of α-MnS and γ-MnS also show that the γ-MnS signal is stronger, indicating that γ-MnS has a higher sulfur vacancy concentration than α-MnS.

[0053] like Figure 5 As shown, HRTEM and SAED characterization confirmed that the product synthesized in Example 1 was a single wurtzite phase (γ-MnS), while the product synthesized in Comparative Example 1 was a single rock salt phase structure (α-MnS).

[0054] Example 2

[0055] This embodiment explores the relationship between sulfur vacancy concentration gradient regulation and phase structure evolution. The specific experimental steps are as follows:

[0056] (1) Precursor loading: Same as in Example 1.

[0057] (2) Base placement: Same as in Example 1.

[0058] (3) Reactor setup: Same as in Example 1.

[0059] (4) Atmosphere control: Same as in Example 1.

[0060] (5) Growth process: Same as in Example 1.

[0061] (6) Annealing treatment: After growth, stop the H2 flow and maintain only the Ar gas flow at 70 sccm. Keep the low temperature zone at 200°C and continue heating the sulfur source. The high temperature zone is cooled down to 300°C at a rate of 8°C / min. After reaching 300°C, keep the temperature constant at this temperature for t minutes, where t = 0, 30, 60, and 90.

[0062] (7) Termination process: After the constant temperature period ends, stop heating and continue to introduce Ar gas until the system cools to room temperature.

[0063] The phase structures of MnS obtained by different isothermal annealing times are shown in Table 1. The results indicate that the isothermal treatment time is a key parameter for controlling the sulfur vacancy concentration and the final MnS phase structure. Continuous sulfur source supply and isothermal annealing in the post-growth stage are key conditions for the formation of the α phase. The post-growth isothermal annealing treatment replenishes sulfur to repair vacancies, promoting the phase transformation of the stable γ-phase MnS with high sulfur vacancy concentration to the stable α-phase MnS.

[0064] Table 1. Phase structures of MnS obtained with different isothermal treatment times.

[0065]

[0066] Example 3

[0067] Device fabrication and performance testing: The γ-MnS nanosheets synthesized in Example 1 were transferred onto a SiO2 / Si substrate. Metal electrodes were fabricated using ultraviolet lithography (or electron beam exposure) and electron beam evaporation (or thermal evaporation, magnetron sputtering) processes to form Hall bars or field-effect transistor (FET) structures. Electrical transport tests and magnetic characterization were then performed.

[0068] Magnetic characterization showed that the γ-MnS synthesized in Example 1 exhibited antiferromagnetic properties at low temperatures. Figure 6 It exhibits significant magnetic anisotropy, with the easy magnetization direction being in-plane (ac), and displays obvious magnetic anisotropy. Figure 7 a). Figure 7 (b) The hysteresis loops of γ-MnS at 50K, 100K, and 200K show a horizontal offset. Figure 8 The results show that the hysteresis loop of γ-MnS after field cooling at 50K and 100K is deflected along the direction of the magnetic field, indicating that γ-MnS has a significant exchange bias effect under low-temperature field cooling conditions. Figure 9 The γ-MnS synthesized in Example 1 exhibits a negative magnetoresistance effect. Electrical characterization shows that it possesses semiconductor behavior with p-type conductivity. Figure 10 The resistance decreases as the temperature increases. Figure 11 (a)), its lnR vs. 1000 / T curve, fitted by the Arrhenius model, shows the electrical activation energy E in the high-temperature range. a ≈0.334eV ( Figure 11 (b)). Figure 12 (a) TEM image and EDS elemental distribution map of γ-MnS, showing that Mn, S and Na elements are uniformly distributed. (b, c) First-principles calculation of the band structure of intrinsic γ-MnS and Na-doped γ-MnS, showing that Na doping introduces acceptor level above the Fermi level to form a p-type semiconductor. The XPS test results of γ-MnS show Na element signal, indicating that p-type conductivity mainly comes from the substitutional doping of Mn by the precursor Na. Figure 13 (b) shows the relationship between the formation energy of α-MnS and γ-MnS and the sulfur vacancy concentration calculated by first-principles calculations. It indicates that when the concentration threshold is reached, the formation energy of γ-MnS is lower and can break through the thermodynamic metastable state limit. The increased sulfur vacancy concentration significantly reduces the formation energy of γ-MnS, which is the key to its room temperature stability. Figure 13 (c, d) further shows that the integral intensity of the characteristic peak of γ-MnS decreases nearly linearly with increasing temperature, which is speculated to be due to the depolarization of ferromagnetic nanodomains induced by sulfur vacancies and the intensification of spin-spin relaxation.

[0069] In summary, this invention reveals for the first time that the introduction of sulfur vacancies into the γ-MnS system disrupts the local charge-neutral balance, alters the coordination environment and electronic state of Mn ions, and thus fundamentally affects the magnetic interactions between Mn ions. Traditional theory and experiments show that intrinsic γ-MnS typically exhibits an antiferromagnetic (AFM) ground state. However, theoretical calculations and some experimental evidence from this invention suggest that sulfur vacancies may induce ferromagnetic (FM) coupling between adjacent Mn ions under specific conditions by enhancing the covalent nature of Mn-S bonds, altering the local spin density distribution, or promoting specific exchange pathways (such as double exchange mechanisms). When the sulfur vacancy concentration reaches a critical threshold, a magnetic domain structure can be formed within the γ-MnS material, where an antiferromagnetic matrix and ferromagnetic regions coexist and are strongly coupled at the nanoscale. The formation of this intrinsic magnetic heterostructure causes single-phase γ-MnS to exhibit a significant exchange bias (EB) effect under specific conditions (such as after field cooling treatment)—that is, the phenomenon of the hysteresis loop shifting along the magnetic field direction. This discovery is groundbreaking: it reveals the possibility of achieving a significant exchange bias effect within single-phase materials (rather than at traditional ferromagnetic / antiferromagnetic heterojunction interfaces). Traditional EB materials rely on ferromagnetic / antiferromagnetic heterojunction interfaces, and their performance is extremely sensitive to interface quality, lattice matching, and coupling strength, leading to complex fabrication processes and often unstable performance. In contrast, utilizing the intrinsic magnetic heterostructure induced by sulfur vacancies in single-phase γ-MnS to achieve the EB effect theoretically avoids dependence on artificial heterojunction interfaces. This provides a highly attractive single-phase material platform for developing next-generation high-performance, high-stability, low-power spintronic devices (such as high-density magnetic random access memories, high-sensitivity magnetic sensors, and spin logic devices). Its core advantages include significantly simplified device structure design, reduced fabrication process complexity, improved consistency and reliability, and excellent miniaturization and high integration potential.

Claims

1. Room temperature stable with exchange bias effect γ The method for preparing -MnS is characterized by, Includes the following steps: (1) Precursor loading: A dual-temperature zone tube furnace is used to mix MnCl2 powder and NaCl powder evenly and place them in a quartz boat in the high-temperature zone, while S powder is placed separately in a quartz boat in the low-temperature zone. (2) Substrate placement: The freshly cleaved fluorophlogopite substrate is horizontally suspended directly above the quartz boat containing the MnCl2 / NaCl mixture; (3) Growth process: First, Ar is introduced into the reaction system to remove air, and then Ar / H2 mixed gas is switched as carrier gas. The temperature of the high temperature zone is raised to 620℃~700℃, and the temperature of the low temperature zone is maintained at 200±10℃. The reaction is kept at this temperature for 5 min~10 min. (4) Post-treatment: After the reaction is completed, immediately stop the flow of mixed gas and turn off the heating system to allow the reaction system to cool rapidly and naturally to room temperature, thereby obtaining a room temperature stable system with exchange bias effect. γ -MnS nanosheets.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of MnCl2 powder to NaCl powder is 10:

1.

3. The preparation method according to claim 1, characterized in that, In step (3), the volume percentage of H2 in the Ar / H2 mixed gas is 10%~20%.

4. The preparation method according to claim 1, characterized in that, In step (3), the Ar flow rate in the Ar / H2 mixed gas is 70 sccm and the H2 flow rate is 15 sccm.

5. The preparation method according to claim 1, characterized in that, In step (3), the temperature of the high-temperature zone is raised to 650°C, the temperature of the low-temperature zone is maintained at 200°C, and the reaction is kept at this temperature for 8 minutes.

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