A Cr2Se3 single crystal and its preparation method

Abstract

This invention provides a Cr2Se3 single crystal and its preparation method, relating to the field of crystal materials technology. The Cr2Se3 single crystal preparation method, from raw material proportioning and powder preparation to single crystal growth, involves coordinated steps to suppress impurity introduction and impurity phase formation, while simultaneously promoting ordered atomic arrangement and eliminating lattice vacancies and dislocations, resulting in a Cr2Se3 single crystal with good single-phase properties, high crystallinity, and extremely low defect density. The high-purity, low-defect single crystal structure ensures its semiconductor transport properties and antiferromagnetic order; it also eliminates interference from impurity scattering and lattice distortion, optimizing the magnetic field's control over the magnetic moments of Cr atoms, enabling the single crystal to exhibit a strong CMR effect approaching -100%. Furthermore, the coordinated steps ensure consistency in size, phase composition, and structural integrity of the prepared Cr2Se3 single crystal, significantly improving the success rate, process controllability, and repeatability.

Description

Technical Field

[0001] This invention relates to the field of crystal materials technology, specifically to a Cr2Se3 single crystal and its preparation method. Background Technology

[0002] The development of existing spintronic devices and high-density magnetic storage technologies has placed higher demands on the key performance characteristics of core magnetoresistive materials, such as the "magnetic field-resistance" coupling strength, response speed, and power consumption. However, traditional magnetic metals and alloys mainly rely on anisotropic magnetoresistance (AMR) or giant magnetoresistance (GMR) effects, which have significant performance bottlenecks: on the one hand, their magnetoresistance change rate is generally less than 100%, resulting in insufficient "magnetic field-resistance" coupling strength, which limits the accuracy and sensitivity of device signal modulation; on the other hand, they usually require high operating magnetic fields (on the order of Tesla), leading to high device power consumption, limited signal-to-noise ratio, and insufficient thermal stability. These performance bottlenecks are no longer sufficient to meet the development needs of next-generation high-sensitivity magnetic sensors, low-power magnetic random access memories (MRAM), and efficient quantum spin devices, thus restricting further improvements in the integration, energy efficiency, and reliability of magnetoelectronic devices.

[0003] The advent of giant negative magnetoresistance (CMR) materials has provided a completely new path to overcome the aforementioned difficulties. Since the late 20th century, perovskite-type manganese oxides (such as La...) have... 1-x Sr x Since the discovery of the CMR effect in the MnO3 series, its unique property of significantly reduced resistance under magnetic field has broken the transport limit of traditional magnetic materials, laying an important foundation for breakthroughs in cutting-edge technologies such as high-density magnetic storage and magnetic sensors. However, perovskite manganese oxides have significant drawbacks: their Curie temperatures are generally lower than room temperature, requiring a low-temperature operating environment, which significantly increases the cost of device cooling and integration difficulty; high-quality samples need to be prepared through epitaxial thin film processes, which can easily lead to lattice mismatch and phase separation problems, resulting in poor crystal quality and insufficient performance stability; and bulk single crystals are difficult to obtain, which cannot provide an ideal experimental platform for studying the intrinsic magneto-electric coupling mechanism of strongly correlated systems, nor can they be directly used to construct three-dimensional vertical magnetic storage arrays, thus limiting their practical application scenarios.

[0004] Cr₂Se₃, a novel chalcogenide predicted by theory, possesses both antiferromagnetic order and a significant CMR effect. Its magnetic transition temperature (≈38K) is suitable for low-temperature spintronics applications, and its simple crystal structure and high chemical stability make it a promising alternative to traditional CMR materials. However, the preparation of Cr₂Se₃ single crystals currently faces significant technical bottlenecks: traditional melting methods or Bridgman processes can only produce millimeter-scale polycrystalline materials, and grain boundary scattering severely masks its intrinsic transport properties; while existing chemical vapor transport processes have narrow parameter windows and lack systematic optimization in terms of transport agent selection, temperature gradient, growth cycle, and cooling procedures, resulting in small single crystal size, high defect density, weak CMR effect, and poor reproducibility, which cannot meet the material requirements for basic property research and prototype device development.

[0005] Therefore, there is an urgent need in this field to develop a chemical vapor transport method for producing high-quality, large-size Cr2Se3 single crystals with strong CMR effects that is stable, reproducible, and controllable, in order to overcome the technical bottleneck in the single crystal growth of Cr2Se3 and provide a reliable crystal material basis for the study of its intrinsic physical properties and related spintronic device applications. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a Cr2Se3 single crystal and its preparation method. The preparation method of the Cr2Se3 single crystal can stably, repeatedly, and controllably prepare high-quality, large-size Cr2Se3 single crystals with strong CMR effect.

[0007] The specific technical solution of this invention is as follows:

[0008] In a first aspect, the present invention provides a method for preparing Cr2Se3 single crystal, comprising the following steps:

[0009] S1. Weighing raw materials: Weigh chromium and selenium under inert gas protection, mix them to obtain reaction raw materials; the molar ratio of chromium to selenium is (1.9-2.1):3;

[0010] S2, sintering powder growth: Under vacuum conditions, the reaction raw materials obtained in step S1 are sintered to react the chromium and the selenium to generate Cr2Se3 sintered powder;

[0011] S3. Grinding and mixing: The Cr2Se3 powder obtained in step S2 is ground and then mixed with a volatile halogen / halide transport agent to obtain a mixed raw material; the volatile halogen / halide transport agent is used to promote the gas phase transport and single crystal growth of Cr2Se3 during crystal growth.

[0012] S4. Crystal growth: Under vacuum, the mixed raw materials obtained in step S3 are subjected to chemical vapor transport growth. The temperature of the high-temperature region of the chemical vapor transport growth is 780-830 ℃, the temperature of the low-temperature region is 690-750 ℃, and the temperature difference between the high-temperature region and the low-temperature region is 80-130 ℃. The heating rate is ≤1 ℃ / min, and the chemical vapor transport growth time is ≥8 days. After the growth is completed, the crystal is cooled to obtain Cr2Se3 single crystal.

[0013] In one possible implementation, the weighing of raw materials in step S1 is carried out in a glove box, and the inert gas is argon or nitrogen.

[0014] Furthermore, the molar ratio of chromium to selenium in step S1 is 2:3.

[0015] In one possible implementation, the purity of both the chromium and the selenium is ≥99.99%.

[0016] In one possible implementation, the heating rate of the sintering process in step S2 is ≤0.5 ℃ / min, the temperature is 480-530 ℃, and the holding time is 36-60 h.

[0017] In one possible implementation, the volatile halogen / halide transporter in step S3 is selected from iodine, bromine, iodine chloride, and iodine bromide.

[0018] In one possible implementation, the mass ratio of the Cr2Se3 powder to the volatile halogen / halide transporter in step S3 is 100:(0.4-1.5).

[0019] Furthermore, when the volatile halogen / halide transporter is iodine, the mass ratio of the Cr2Se3 powder to the iodine is 100:(0.6-1.5).

[0020] Furthermore, when the volatile halogen / halide transporter is bromine, the mass ratio of the Cr2Se3 powder to the bromine is 100:(0.4-0.8).

[0021] Furthermore, when the volatile halogen / halide transporter is iodine chloride, the mass ratio of the Cr2Se3 cooked powder to the iodine chloride is 100:(0.5-0.9).

[0022] Furthermore, when the volatile halogen / halide transporter is iodine bromide, the mass ratio of the Cr2Se3 powder to the iodine bromide is 100:(0.6-1.2).

[0023] In one possible implementation, the process of forming the vacuum environment in steps S2 and S4 is as follows: in an environment protected by inert gas, the reaction raw material / mixed raw material is placed in a container, the outlet of the container is sealed, the container is taken out from the environment protected by inert gas, the inert gas in the container is extracted, and then the reaction raw material / mixed raw material is vacuum sealed in the container using a hydrogen-oxygen generator.

[0024] In one possible implementation, the temperature of the high-temperature zone in step S4 is 800-820 ℃, the temperature of the low-temperature zone is 700-720 ℃, and the temperature difference between the high-temperature zone and the low-temperature zone is 90-100 ℃, and the chemical vapor transport growth time is 8-14 days.

[0025] Secondly, the present invention provides a Cr2Se3 single crystal, wherein the Cr2Se3 single crystal is an antiferromagnetic material with a melting temperature of 38 K, and has both semiconductor transport properties and negative magnetoresistance effect, and is prepared by the above-mentioned method for preparing Cr2Se3 single crystal.

[0026] The positive and progressive effects of this invention are as follows:

[0027] This invention provides a Cr₂Se₃ single crystal and its preparation method. The Cr₂Se₃ single crystal preparation method involves controlling the molar ratio of Cr to Se to (1.9-2.1):3, low-temperature sintering to generate Cr₂Se₃ powder, and controlling the temperature, heating rate, and single crystal growth time in the high and low temperature zones during single crystal growth. Each step progressively suppresses the introduction of impurities and the formation of impurity phases, while promoting the ordered arrangement of atoms and eliminating lattice vacancies and dislocations. This results in a Cr₂Se₃ single crystal with good single-phase properties, high crystallinity, and extremely low defect density. The high-purity, low-defect single crystal structure ensures its intrinsic performance of semiconductor transport properties and antiferromagnetic order. Simultaneously, it eliminates the interference of impurity scattering and lattice distortion, optimizes the magnetic field's control over the magnetic moments of Cr atoms, and enables the single crystal to possess a strong CMR effect close to -100%, providing a high-quality core material for spintronic devices. Furthermore, through the coordinated efforts of each step, the prepared Cr2Se3 single crystals were ensured to be consistent in size, phase composition, and structural integrity, which greatly improved the success rate of preparation and the controllability and repeatability of the process. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the crystal lattice structure of the Cr2Se3 single crystal obtained in Example 1.

[0029] Figure 2 The image shows the X-ray diffraction pattern of the Cr2Se3 single crystal prepared in Example 1.

[0030] Figure 3The magnetic moment-temperature curve is shown for the Cr2Se3 single crystal prepared in Example 1.

[0031] Figure 4 The resistivity-temperature curve of the Cr2Se3 single crystal prepared in Example 1 is shown.

[0032] Figure 5 The curve of the huge negative magnetoresistance (CMR) effect of the Cr2Se3 single crystal prepared in Example 1 at 2 K temperature.

[0033] Figure 6 This is a photograph of the Cr2Se3 single crystal prepared in Example 1. Detailed Implementation

[0034] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. It should be noted that the following embodiments are only used to illustrate the implementation methods and typical parameters of the present invention, and are not intended to limit the parameter range described in the present invention. Reasonable variations derived therefrom are still within the protection scope of the present invention.

[0035] It should be noted that the endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0036] Unless otherwise defined, all terms, symbols, and other scientific terms used herein are intended to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In some instances, terms having a conventional meaning are defined herein for clarification or ease of reference, and such definitions should not be construed as indicating a significant difference from conventional understanding in the art. The technical methods described or referenced herein are generally well understood by those skilled in the art and employed by conventional methods. Unless otherwise stated, the use of commercially available kits, reagents, and instruments shall be performed according to the manufacturer's instructions and parameters.

[0037] The specific technical solution of this invention is as follows:

[0038] In a first aspect, the present invention provides a method for preparing Cr2Se3 single crystal, comprising the following steps:

[0039] S1. Weighing raw materials: Weigh chromium and selenium under inert gas protection, mix them to obtain reaction raw materials; the molar ratio of chromium to selenium is (1.9-2.1):3;

[0040] In step S1, by controlling the molar ratio of Cr to Se to be (1.9-2.1):3, a material basis is laid for the subsequent solid-phase reaction to generate single-phase Cr2Se3 powder and prepare high-purity Cr2Se3 single crystals. The raw material mixing is completed under the protection of inert gas, which can effectively isolate oxygen and moisture, avoid oxidation or hydrolysis of raw materials, and eliminate the introduction of impurity phases such as oxides and hydroxides from the source, thus ensuring the purity, phase uniformity and performance stability of the subsequent Cr2Se3 powder and single crystals.

[0041] S2, sintering powder growth: Under vacuum conditions, the reaction raw materials obtained in step S1 are sintered to react the chromium and the selenium to generate Cr2Se3 sintered powder;

[0042] In step S2, the vacuum environment effectively isolates air and moisture, preventing the Cr and Se raw materials from reacting with oxygen during the sintering process to form impurity phases such as Cr2O3 and SeO2. It also prevents nitrogen from penetrating the raw material lattice and forming interstitial impurities, ensuring that the resulting Cr2Se3 powder is a high-purity single-phase product. The heat energy provided by the sintering process activates the diffusion of Cr and Se atoms, allowing the two elemental atoms to overcome the barriers at the particle interface and fully undergo a solid-phase reaction to generate Cr2Se3.

[0043] S3. Grinding and mixing: The Cr2Se3 powder obtained in step S2 is ground and then mixed with a volatile halogen / halide transport agent to obtain a mixed raw material; the volatile halogen / halide transport agent is used to promote the gas phase transport and single crystal growth of Cr2Se3 during crystal growth.

[0044] In step S3, the grinding operation can reduce the particle size of the sintered Cr2Se3 powder to the micrometer or even submicrometer level, significantly increasing the specific surface area of ​​the powder. This larger specific surface area allows for sufficient contact between the powder and the transport agent, ensuring a uniform consumption rate of raw materials during subsequent gas-phase transport and improving the stability of single crystal growth. Volatile halogen / halide transport agents are the core medium for realizing the gas-phase transport of Cr2Se3. These transport agents can undergo reversible chemical reactions with the Cr2Se3 powder to generate volatile Cr-halogen and Se-halogen intermediates. These gaseous compounds diffuse under the drive of a concentration gradient, providing a sufficient source of materials for single crystal growth.

[0045] S4. Crystal growth: Under vacuum, the mixed raw materials obtained in step S3 are subjected to chemical vapor transport growth. The temperature of the high-temperature region of the chemical vapor transport growth is 780-830 ℃, the temperature of the low-temperature region is 690-750 ℃, and the temperature difference between the high-temperature region and the low-temperature region is 80-130 ℃. The heating rate is ≤1 ℃ / min, and the chemical vapor transport growth time is ≥8 days. After the growth is completed, the crystal is cooled to obtain Cr2Se3 single crystal.

[0046] In step S4, the vacuum environment isolates air and moisture, preventing air components from reacting with Cr2Se3 or the transport agent at high temperatures to form oxides, nitrides, and other impurities. The high-temperature zone is the reaction and volatilization zone, with a temperature range of 780-830 ℃, which is suitable for the reaction requirements of Cr2Se3 and halogen / halide transport agents, and can efficiently generate volatile Cr-halogen and Se-halogen intermediate compounds. The low-temperature zone is the deposition and crystallization zone. In the low-temperature zone of 690-750 ℃, the intermediate compounds undergo a reversible decomposition reaction due to the decrease in temperature, precipitating high-purity Cr2Se3 and epitaxially growing it into single crystals. The transport agent returns to the high-temperature zone in gaseous form to participate in the reaction, greatly improving the utilization rate of raw materials. 80-130 A temperature gradient of ℃ can drive the directional diffusion of gaseous intermediate compounds from the high-temperature region to the low-temperature region, ensuring that Cr2Se3 preferentially grows epitaxially on the inner wall of the quartz tube or on the crystal nucleus in the low-temperature region, forming Cr2Se3 single crystals with regular size. The gas phase transport rate can be precisely controlled by adjusting the temperature difference, achieving controllable adjustment of the single crystal growth rate. The slow heating mode with a heating rate ≤1℃ / min allows the reaction between the calcined powder and the transport agent to proceed gradually, ensuring the stability of the gas phase raw material supply. The long growth cycle of ≥8 days provides sufficient time for the atoms to arrange in an orderly manner, and in conjunction with the above process conditions, it synergistically reduces defects such as lattice vacancies and dislocations, significantly improving the crystallinity of the single crystal. At the same time, it promotes the fusion growth of small grains, realizing the preparation of large-size single crystals. The high purity and low defect characteristics of Cr2Se3 single crystals are the core guarantee for its strong CMR effect: no impurity scattering, few lattice defects, and a more significant control effect of the magnetic field on the magnetic moment of Cr atoms, thereby achieving a sharp decrease in resistivity and exhibiting an excellent huge negative magnetoresistance effect.

[0047] The method for preparing Cr2Se3 single crystals provided by this invention optimizes and controls the preparation of high-quality, large-size, and strongly CMR-effect Cr2Se3 single crystals through the synergistic cooperation of the technical features of each step, from the dimensions of raw material regulation, reaction environment, and process parameters.

[0048] In one possible implementation, the weighing of raw materials in step S1 is carried out in a glove box, and the inert gas is argon or nitrogen. The glove box is a sealed environment that can isolate impurities such as oxygen and moisture in the outside air, providing a stable and controllable environment; argon or nitrogen, as inert gases, are chemically stable and will not react with Cr and Se raw materials, forming a protective atmosphere around the raw materials to ensure that the weighed and mixed reaction raw materials contain only elemental Cr and Se.

[0049] Furthermore, the molar ratio of chromium to selenium in step S1 is 2:3. The theoretical stoichiometric ratio of Cr2Se3 is Cr:Se = 2:3. Limiting the raw material molar ratio to 2:3 can promote the subsequent solid-phase reaction to proceed entirely in the direction of generating single-phase Cr2Se3, which is beneficial to improving the phase purity of the subsequent Cr2Se3 powder and single crystal.

[0050] In one possible implementation, the purity of both chromium and selenium is ≥99.99%. A purity of ≥99.99% for both Cr and Se can significantly reduce the impurity content in the raw materials, overcome the negative impact of impurities on the single crystal structure and performance, and ensure that the prepared Cr2Se3 single crystal possesses both high crystallinity, stable semiconductor transport properties, and a significant CMR effect.

[0051] In one possible implementation, the heating rate of the sintering treatment in step S2 is ≤0.5 ℃ / min, the temperature is 480-530 ℃, and the holding time is 36-60 h. The ultra-low heating rate of ≤0.5 ℃ / min matches the kinetic rate of the Cr-Se solid-state reaction, allowing the diffusion reaction of Cr and Se atoms to gradually advance from the particle surface to the interior. 480-530 ℃ is the thermodynamically optimized range for the solid-state reaction of Cr and Se to form Cr2Se3, providing sufficient kinetic energy for atomic thermal motion, breaking the lattice constraints of Cr and Se elements, and promoting the formation of Cr-Se bonds. Simultaneously, it effectively suppresses excessive volatilization of Se, ensuring the directional and efficient formation of single-phase Cr2Se. 3。 The long holding time of 36-60 h provides sufficient time for the long-range diffusion of Cr and Se atoms, promotes the growth of Cr2Se3 crystal nuclei and lattice perfection, reduces crystal defects such as vacancies and dislocations, and improves the crystallinity of the calcined powder. The calcined powder with high crystallinity has a lower surface energy and more stable reactivity with subsequent halogen / halide transport agents, providing high-quality raw materials for the growth of large-size single crystals.

[0052] In one possible implementation, the volatile halogen / halide transport agent in step S3 is selected from iodine, bromine, iodine chloride (ICl), and iodine bromide (IBr). Iodine, bromine, iodine chloride, and iodine bromide are all highly volatile halogen compounds with excellent reversibility in reacting with Cr2Se3. They can react rapidly with Cr2Se3 powder to generate volatile Cr-halogen and Se-halogen intermediates. These intermediates can diffuse directionally to the low-temperature region driven by a concentration gradient and undergo a reversible decomposition reaction as the temperature decreases, precipitating high-purity Cr2Se3 and epitaxially growing it into a single crystal. The transport agent then returns to the high-temperature region in gaseous form to participate in the reaction. Furthermore, the gas-phase transport reaction involving iodine, bromine, iodine chloride, and iodine bromide is a reversible reaction without byproducts, and no other impurity compounds are generated during the reaction.

[0053] In one possible implementation, the mass ratio of the Cr2Se3 calcined powder to the volatile halogen / halide transporter in step S3 is 100:(0.4-1.5). When the mass ratio of Cr2Se3 calcined powder to the volatile halogen / halide transporter is 100:(0.4-1.5), the concentration of the gaseous intermediate compound generated by the reaction of the transporter and the calcined powder is moderate, the deposition rate of the gaseous material in the low-temperature region is slow, and the atoms have sufficient time to arrange themselves in an orderly manner along the optimal crystal orientation, which is beneficial for preparing single-crystal Cr2Se3 with high crystallinity and low defect density.

[0054] Furthermore, when the volatile halogen / halide transporter is iodine, the mass ratio of the Cr2Se3 calcined powder to the iodine is 100:(0.6-1.5). Iodine has relatively mild volatility and a stable reaction rate with the Cr2Se3 calcined powder. The mass ratio range of 100:(0.6-1.5) provides sufficient iodine to react with the calcined powder, generating a gaseous intermediate compound of appropriate concentration. This promotes its directional diffusion under the temperature gradient, achieving efficient and stable gas-phase transport, which is beneficial for obtaining high-quality, large-size Cr2Se3 single crystals.

[0055] Furthermore, when the volatile halogen / halide transporter is bromine, the mass ratio of the Cr2Se3 powder to the bromine is 100:(0.4-0.8). Bromine is significantly more volatile than iodine, resulting in a faster reaction rate with the Cr2Se3 powder. The low ratio range of 100:(0.4-0.8) ensures that the reaction rate of bromine with the Cr2Se3 powder matches the dual-temperature zone process parameters, achieving stable and controllable gas-phase transport, which is beneficial for obtaining high-quality, large-size Cr2Se3 single crystals.

[0056] Furthermore, when the volatile halogen / halide transport agent is iodine chloride, the mass ratio of the Cr2Se3 cooked powder to the iodine chloride is 100:(0.5-0.9). Iodine chloride, as an interhalogen compound, has reactivity between chlorine and iodine, and its volatility is better than iodine but weaker than chlorine. The 100:(0.5-0.9) ratio range ensures a high degree of compatibility between the reaction rate of iodine chloride and the Cr2Se3 cooked powder and the dual-temperature zone process parameters, achieving stable and controllable directional gas-phase transport, which is beneficial for obtaining high-quality, large-size Cr2Se3 single crystals.

[0057] Furthermore, when the volatile halogen / halide transport agent is iodine bromide, the mass ratio of the Cr2Se3 calcined powder to the iodine bromide is 100:(0.6-1.2). Iodine bromide, as an interhalogen compound, has reactivity and volatility between bromine and iodine. The 100:(0.6-1.2) ratio range ensures a high degree of compatibility between the reaction rate of iodine bromide and the calcined powder and the dual-temperature zone process parameters, achieving stable and directional gas-phase transport, which is beneficial for obtaining high-quality, large-size Cr2Se3 single crystals.

[0058] In one possible implementation, the vacuum environment formation process in steps S2 and S4 is as follows: In an environment protected by an inert gas, the reactant / mixed material is placed in a container, the container's outlet is sealed, the container is removed from the inert gas-protected environment, the inert gas in the container is extracted, and then the reactant / mixed material is vacuum-sealed in the container using an oxyhydrogen generator. Metallic chromium and selenium are extremely sensitive to oxygen and moisture, especially at high temperatures, where they readily oxidize to form oxide impurities. Completing the loading and temporarily sealing the nozzle in the inert gas environment of a glove box ensures that the material remains isolated from the atmosphere throughout the entire process from weighing to transfer. Connecting the temporarily sealed container to the vacuum system completely removes the inert gas inside, establishing a high vacuum. This process utilizes both the oxygen-free operation advantage of the glove box and the high sealing strength and airtightness of the oxyhydrogen flame, achieving the goal of creating a high-vacuum reaction chamber in an atmospheric environment.

[0059] Furthermore, in step S4, the temperature of the high-temperature zone is 800-820 ℃, the temperature of the low-temperature zone is 700-720 ℃, and the temperature difference between the high-temperature zone and the low-temperature zone is 90-100 ℃. The chemical vapor transport growth time is 8-14 days. This process, through synergistic regulation from the perspectives of thermodynamics (temperature zone, temperature difference) and kinetics (growth time), precisely controls the gas-phase reaction, material diffusion, and atomic deposition processes. It suppresses impurity generation and reduces lattice defects from the source, achieving a dual improvement in the purity and lattice integrity of Cr2Se3 single crystals. This makes the semiconductor transport characteristics and antiferromagnetic order of Cr2Se3 single crystals more stable, and also makes the magnetic field's control effect on the magnetic moments of Cr atoms more significant, thereby strengthening the CMR effect of the single crystal.

[0060] Secondly, the present invention provides a Cr2Se3 single crystal, wherein the Cr2Se3 single crystal is an antiferromagnetic material with a Nell temperature of 38 K, and has both semiconductor transport properties and negative magnetoresistance effect, and is prepared by the above-mentioned Cr2Se3 single crystal preparation method.

[0061] The technical solution of the present invention will be further described below with reference to specific embodiments and comparative examples. All reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing. The instruments used in the embodiments are also commercially available.

[0062] Example 1

[0063] This embodiment provides a Cr2Se3 single crystal with a Nell temperature of 38 K. Its in-plane resistivity increases monotonically with decreasing temperature, and its resistance decreases sharply with increasing external magnetic field at low temperatures. It combines semiconductor transport properties with a significant large negative magnetoresistance effect.

[0064] It is prepared through the following steps:

[0065] S1. Weigh the raw materials:

[0066] S1.1 Pretreatment: In advance, control the oxygen content and water content of the nitrogen-protected glove box to ≤1ppm respectively to ensure that the glove box is in an anhydrous and oxygen-free environment to avoid oxidation or hydrolysis of Cr and Se raw materials; at the same time, place the raw materials, crucible, quartz tube and electronic balance into the glove box and let them stand for more than 30 minutes to make the temperature of the raw materials, crucible, quartz tube and electronic balance consistent with the ambient temperature inside the glove box to reduce weighing errors; select elemental Cr powder with a purity ≥99.99% and a particle size ≤50 μm and elemental Se powder with a purity ≥99.99% and a particle size ≤100 μm as raw materials, and remove trace dust impurities that may be attached to the surface of the raw materials through the vacuum suction filter in the glove box before use;

[0067] S1.2 Raw material mixing: Place the crucible on the electronic balance in the glove box and weigh 4.06 g of Cr powder and 9.26 g of Se powder in sequence. After weighing, use a ceramic stirring rod to gently stir and mix the Cr powder and Se powder in the crucible to form a reaction raw material. Then, slowly transfer the crucible containing the reaction raw material into a quartz tube. After plugging the outlet of the quartz tube with quartz wool, remove the quartz tube from the glove box.

[0068] S2, Powder Growth: The quartz tube taken out in step S1 is installed on a vacuum sealing device, and nitrogen gas is extracted from inside the quartz tube until the vacuum degree inside the quartz tube is ≤1×10⁻⁶. -3 Pa, the quartz tube was vacuum-sealed using a hydrogen-oxygen generator; the vacuum-sealed quartz tube was placed horizontally in the center of the homogenization zone of the muffle furnace, the muffle furnace was started, and the furnace temperature was uniformly increased from room temperature to 500 ℃ at a heating rate of 0.5 ℃ / min. The slow heating helps Se and Cr to react gradually; then it was held at 500 ℃ for 48 hours. This holding stage is the key to the full progress of the solid-phase reaction, ensuring that chromium and selenium atoms react completely through diffusion to generate a single-phase Cr2Se3 powder; after the holding period, the power to the muffle furnace was turned off, and the quartz tube was allowed to cool naturally to room temperature with the furnace.

[0069] S3. Grinding, Mixing, and Vacuum Sealing: Remove the cooled quartz tube from step S2 and place it in a nitrogen-protected glove box (oxygen content ≤1 ppm, water content ≤1 ppm); carefully open the sealing port using a quartz tube cutting tool, remove the crucible from the quartz tube, and use a dry ceramic scraper to completely transfer the Cr2Se3 powder from the crucible to an agate mortar. Then grind the Cr2Se3 powder to a particle size ≤5 μm; weigh 0.2 g of sublimation-grade iodine with a purity ≥99.9% on an electronic balance in the glove box; in the glove box, pour the ground Cr2Se3 powder into a dry quartz tube with a length of 250 mm and an inner diameter of 100 mm, then pour the weighed iodine into the quartz tube and sprinkle it evenly on the surface of the Cr2Se3 powder. Then gently stir and mix the powder evenly with a clean quartz rod to form a mixed raw material; after plugging the outlet of the quartz tube with quartz wool, remove the quartz tube from the glove box;

[0070] S4. Crystal Growth: Install the quartz tube taken out in step S3 onto a vacuum sealing device, and extract nitrogen gas from inside the quartz tube until the vacuum level inside the quartz tube is ≤1×10⁻⁶. -3 Pa, using a hydrogen-oxygen generator, vacuum seals the quartz tube; the vacuum-sealed quartz tube is then placed horizontally in the center of the homogenization zone of a dual-zone tube furnace, with one end of the quartz tube containing the mixed raw material placed in the high-temperature zone and the other end (empty end) placed in the low-temperature zone; simultaneously, the temperature control programs for the high-temperature and low-temperature zones are activated, setting the high-temperature zone to uniformly heat from room temperature to 820 °C at a rate of 0.82 °C / min, and the low-temperature zone to uniformly heat from room temperature to 720 °C at a rate of 0.72 °C / min, establishing and maintaining a temperature gap of approximately 100 °C between the two zones. A stable axial temperature gradient of ℃ serves as the driving force for chemical transport. After reaching the set temperature, the temperature is maintained for 8 days. During this period, the Cr2Se3 powder in the high-temperature zone reacts with iodine to generate gaseous iodides. These gaseous substances diffuse to the low-temperature zone under the influence of concentration gradient and convection. Since the temperature in the low-temperature zone is lower than the crystallization temperature of the material system, the gaseous substances undergo a reversible reaction at this end to precipitate solid Cr2Se3, which preferentially grows epitaxially into single crystals on the inner wall of the quartz tube or on the crystal nucleus. After the isothermal growth is completed, the heating power of the dual-temperature zone furnace is turned off, and the quartz tube is allowed to cool naturally to room temperature with the furnace. The sealed quartz tube is then removed and opened, and a Cr2Se3 single crystal with a metallic luster can be obtained at the low-temperature end.

[0071] Example 2

[0072] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0073] The amount of iodine used in step S3 is 0.08 g.

[0074] Example 3

[0075] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0076] In step S4, the insulation temperature of the high-temperature zone is 800 ℃, the insulation temperature of the low-temperature zone is 720 ℃, and the insulation time is 14 days.

[0077] Example 4

[0078] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0079] In step S4, the insulation temperature of the high-temperature zone is 830 ℃, the insulation temperature of the low-temperature zone is 750 ℃, and the insulation time is 10 days.

[0080] Example 5

[0081] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0082] In step S4, the insulation temperature of the high-temperature zone is 780 ℃, the insulation temperature of the low-temperature zone is 690 ℃, and the insulation time is 18 days.

[0083] Example 6

[0084] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0085] The heat preservation temperature used in step S2 is 530 ℃, and the time is 36 h.

[0086] Example 7

[0087] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0088] The heat preservation temperature used in step S2 is 480 ℃, and the time is 60 h.

[0089] Example 8

[0090] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0091] In step S3, bromine is used as the transport agent, and the amount of bromine used is 0.11 g.

[0092] Example 9

[0093] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0094] In step S3, bromine is used as the transport agent, and the amount of bromine used is 0.06 g.

[0095] Example 10

[0096] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0097] In step S3, iodine chloride is used as the transport agent, and the amount of iodine chloride used is 0.12 g.

[0098] Example 11

[0099] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0100] In step S3, iodine chloride is used as the transport agent, and the amount of iodine chloride used is 0.07 g.

[0101] Example 12

[0102] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0103] In step S3, iodine bromide is used as the transport agent, and the amount of iodine bromide used is 0.16 g.

[0104] Example 13

[0105] This embodiment provides a Cr2Se3 single crystal, the preparation method of which differs from that of Example 1 in that:

[0106] In step S3, iodine bromide is used as the transport agent, and the amount of iodine bromide used is 0.08 g.

[0107] Performance Testing and Results Analysis

[0108] The performance of the Cr2Se3 single crystals prepared in the examples was tested using the following methods:

[0109] Single-crystal X-ray diffraction (XRD) test: A single-crystal X-ray diffractometer was used with Cu-Kα as the radiation source (wavelength λ=0.15406 nm) and the scanning range was 2θ=5°-90°.

[0110] Magnetic susceptibility test: A superconducting quantum interference device (SQUID) magnetometer was used. The test mode was zero-field cold-field cold dual-mode test, and the applied magnetic field strength was set to 100 Oe. The test temperature range was 2 K-300 K, and the cooling rate was 2 K / min. When the temperature dropped to around 38 K, the cooling rate was adjusted to 0.5 K / min.

[0111] Resistivity test: The DC four-probe resistivity test module in the Physical Performance Testing System (PPMS) was used. The test temperature range was 2 K-300 K and the cooling rate was 2 K / min. A constant DC current of 100 μA was applied during the test. The voltage values ​​between the electrodes at different temperatures were recorded simultaneously. The in-plane resistivity ρ=V×S / (I×L) was calculated according to Ohm's law, where S is the electrode contact area and L is the electrode spacing.

[0112] CMR effect testing: A physical performance testing system (PPMS) was used, integrating variable temperature and variable magnetic field testing functions; the test temperature was 2 K; the applied magnetic field strength range was 0-5 T, and the magnetic field loading rate was 0.1 T / min; under each magnetic field strength, the magnetic field was kept stable for 5 min before resistivity testing. The sample resistivity ρ(H) under different magnetic field strengths was recorded, and the magnetoresistivity change rate MR=[ρ(H)-ρ(0)] / ρ(0)×100%, where ρ(0) is the resistivity under zero magnetic field, and ρ(H) is the resistivity under the applied magnetic field H.

[0113] Figure 1 This is a schematic diagram of the crystal structure of the Cr₂Se₃ single crystal obtained in Example 1. In the diagram, blue spheres represent Se atoms, and red spheres represent Cr atoms. They constitute the basic unit of Cr₂Se₃ in a stoichiometric ratio of 2:3. The single crystal structure of Cr₂Se₃ belongs to the rhombohedral crystal system. The a, b, and c axes are crystal axes. The a and b axes lie in the in-layer plane, and the c axis represents the interlayer stacking direction. The atoms within the layers have strong bonds, while the interlayer bonds are weak, exhibiting typical layered structure characteristics. Within the ab plane, Cr atoms and Se atoms form a hexagonal close-packed two-dimensional grid. Each Cr atom (red) is coordinated with six Se atoms (blue), forming a distorted octahedral structure (Cr-Se₆ octahedrons). These Cr-Se₆ octahedrons are connected by sharing edges / corners, forming a continuous two-dimensional layered network. The Cr-Se bonds within the layers are strong covalent bonds, which are the core support for lattice stability. Along the c-axis, multiple two-dimensional Cr-Se layers are stacked in an ABAB-type manner. The Se atoms and Cr atoms in adjacent layers are bonded by weak van der Waals forces (rather than strong covalent bonds), so the interlayer forces of Cr2Se3 are relatively weak. There is a certain misalignment between the Cr atoms and Se atoms in the interlayer. This stacking method affects the formation of its magnetic order (antiferromagnetism). The magnetic moments of Cr atoms in adjacent layers will be antiparallel, ultimately exhibiting antiferromagnetic properties.

[0114] Figure 2The X-ray diffraction pattern of the Cr2Se3 single crystal prepared in Example 1 is consistent with the typical standard data of the Cr2Se3 rhombohedral crystal system in the International Crystallography Data Center (ICDD) standard card 00-027-0159. Furthermore, the XRD pattern shows a narrower full width at half maximum (FWHM) peak, indicating superior crystallinity. The diffraction peaks in the pattern all correspond to the characteristic crystal planes of the Cr2Se3 rhombohedral crystal system. The (003) peak near 15°, the (006) peak near 30°, and the (0012) peak near 60° are all layered crystal plane diffraction along the c-axis, corresponding to its layered stacked structure, indicating high growth order along the c-axis direction. The pattern only shows the characteristic diffraction peaks of Cr2Se3, with no diffraction peaks of impurities, indicating that the grown crystal is a high-purity Cr2Se3 single crystal. The diffraction peaks are sharp and symmetrical, without obvious peak broadening or splitting, indicating that the Cr2Se3 single crystal has high crystallinity, good atomic arrangement order, low internal defect density, high lattice integrity, and large crystal domain size. Figure 2 The X-ray diffraction pattern of the Cr2Se3 single crystal shows that the method of the present invention has successfully prepared high-purity Cr2Se3 single crystals with highly preferred orientation in the c-axis direction and excellent crystal quality.

[0115] Figure 3 The magnetic moment-temperature curve of the Cr2Se3 single crystal prepared in Example 1 is shown, with an external magnetic field strength of 0.1T. When the temperature drops from 300 K to around 38 K, the magnetic moment increases rapidly with decreasing temperature; this is because Cr2Se3 reaches the Nell temperature (T0). n Below 38 K, Cr atoms exhibit antiferromagnetic order. At low temperatures, the magnetic moments of Cr atoms tend to be more ordered, resulting in a stronger response to weak magnetic fields and a significant increase in magnetic moment. Above 38 K, the magnetic moment continuously decreases with increasing temperature. This is because above the Nell temperature, thermal motion disrupts the antiferromagnetic order, causing the magnetic moments of Cr atoms to become disordered, weakening their response to magnetic fields, and gradually reducing the magnetic moment. The curve shows an inflection point near 38 K, corresponding to the antiferromagnetic-paramagnetic phase transition temperature (Nell temperature) of Cr₂Se₃, consistent with the theoretical 38 K.

[0116] Figure 4The resistivity-temperature curve of the Cr2Se3 single crystal prepared in Example 1 illustrates its semiconductor transport characteristics. When the temperature drops from 300 K to 2 K, the resistivity increases sharply, a typical characteristic of semiconductor transport: Cr2Se3 has a band gap, and at low temperatures, the thermal excitation ability of electrons is weak, resulting in a significant reduction in the number of charge carriers participating in conduction, thus significantly increasing the resistivity. Above 50 K, the resistivity decreases rapidly and then levels off. At high temperatures, thermal excitation is enhanced, more electrons gain energy to jump across the band gap and participate in conduction, increasing the charge carrier concentration and thus decreasing the resistivity. When the temperature is sufficiently high, the charge carrier concentration approaches saturation, and the change in resistivity gradually flattens out. Furthermore, the curve shows continuous changes without abrupt changes, extra impurity peaks, or abnormal fluctuations, indicating high crystallinity of the single crystal and the absence of obvious defects or impurity phases interfering with the transport process.

[0117] Figure 5 The figure shows the giant negative magnetoresistance (CMR) effect curve of the Cr2Se3 single crystal prepared in Example 1 at 2 K, demonstrating the ability of a magnetic field to control its resistivity. Near zero magnetic field, the CMR reaches an extreme value of approximately -100%, indicating extremely high resistivity at zero magnetic field. As the magnetic field approaches zero, the resistivity drops sharply, almost to zero, indicating that the Cr2Se3 single crystal has a very strong ability to control resistivity with a magnetic field at low temperatures, making it a typical giant negative magnetoresistance material. With increasing absolute value of the magnetic field, the CMR approaches zero from its extreme value, and the resistivity recovers from its extremely low value. This is because the magnetic field, by controlling the magnetic moment arrangement of Cr atoms, changes the electron transport path. When the magnetic field reaches a certain strength, the orderliness of the magnetic moment arrangement tends to stabilize, and the change in resistivity gradually decreases. The magnetic field affects electron transport by controlling magnetic order (antiferromagnetic arrangement), demonstrating the magnetoelectric coupling effect of the material. This is also the basis for the application of this Cr2Se3 single crystal in spintronic devices. The curves show a consistent trend under both positive and negative magnetic fields, indicating that the CMR effect is a function of the magnetic field strength, which is consistent with the magnetic response characteristics of antiferromagnetic materials.

[0118] Figure 6 The image shows a physical photograph of the Cr2Se3 single crystal prepared in Example 1. This Cr2Se3 single crystal exhibits a uniform black appearance and a regular polyhedral geometric shape, a typical characteristic of highly crystalline single crystals. This indicates that atoms are orderly arranged along specific crystal orientations of the rhombohedral crystal system during gas-phase transport growth. The single crystal surface is smooth, without obvious discolored spots or microcrystal agglomerations, indicating that the preparation process effectively suppressed the introduction of impurities and the formation of impurity phases. This proves that the process can stably prepare high-purity, low-defect Cr2Se3 single crystals. Each square in the image is 1 mm in size, indicating that the maximum size of the prepared Cr2Se3 single crystal is approximately 3 mm. This demonstrates that the preparation process of this invention successfully achieves the growth of large-size single crystals, meeting the basic size requirements for subsequent property testing or device fabrication.

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing Cr2Se3 single crystal, characterized in that, Includes the following steps: S1. Weighing raw materials: Weigh chromium and selenium under inert gas protection, mix them to obtain reaction raw materials; the molar ratio of chromium to selenium is (1.9-2.1):3; S2, sintering powder growth: Under vacuum conditions, the reaction raw materials obtained in step S1 are sintered to react the chromium and the selenium to generate Cr2Se3 sintered powder; S3. Grinding and mixing: The Cr2Se3 powder obtained in step S2 is ground and then mixed with a volatile halogen / halide transport agent. The volatile halogen / halide transport agent is selected from iodine, bromine, iodine chloride and iodine bromide to obtain a mixed raw material. The volatile halogen / halide transport agent is used to promote the gas phase transport and single crystal growth of Cr2Se3 during crystal growth. S4. Crystal growth: Under vacuum, the mixed raw materials obtained in step S3 are subjected to chemical vapor transport growth. The temperature of the high-temperature region of the chemical vapor transport growth is 780-830 ℃, the temperature of the low-temperature region is 690-750 ℃, and the temperature difference between the high-temperature region and the low-temperature region is 80-130 ℃. The heating rate is ≤1 ℃ / min, and the chemical vapor transport growth time is ≥8 days. After the growth is completed, the crystal is cooled to obtain Cr2Se3 single crystal.

2. The method for preparing Cr2Se3 single crystal according to claim 1, characterized in that, The weighing of raw materials in step S1 is carried out in a glove box, and the inert gas is argon or nitrogen.

3. The method for preparing Cr₂Se₃ single crystal according to claim 1, characterized in that, In step S1, the molar ratio of chromium to selenium is 2:3, and the purity of both chromium and selenium is ≥99.99%.

4. The method for preparing Cr2Se3 single crystal according to claim 1, characterized in that, The heating rate of the sintering treatment in step S2 is ≤0.5 ℃ / min, the temperature is 480-530 ℃, and the holding time is 36-60 h.

5. The method for preparing Cr2Se3 single crystal according to claim 1, characterized in that, In step S3, the mass ratio of the Cr2Se3 cooked powder to the volatile halogen / halide transporter is 100:(0.4-1.5).

6. The method for preparing Cr2Se3 single crystal according to claim 5, characterized in that, When the volatile halogen / halide transporter is iodine, the mass ratio of the Cr2Se3 powder to the iodine is 100:(0.6-1.5); Alternatively, when the volatile halogen / halide transporter is bromine, the mass ratio of the Cr2Se3 powder to the bromine is 100:(0.4-0.8); Alternatively, when the volatile halogen / halide transporter is iodine chloride, the mass ratio of the Cr2Se3 cooked powder to the iodine chloride is 100:(0.5-0.9); Alternatively, when the volatile halogen / halide transporter is iodine bromide, the mass ratio of the Cr2Se3 powder to the iodine bromide is 100:(0.6-1.2).

7. The method for preparing Cr₂Se₃ single crystal according to claim 1, characterized in that, The process of forming the vacuum environment in steps S2 and S4 is as follows: In an environment protected by inert gas, the reaction raw material / mixed raw material is placed in a container, the outlet of the container is sealed, the container is taken out from the environment protected by inert gas, the inert gas in the container is extracted, and then the reaction raw material / mixed raw material is vacuum sealed in the container using a hydrogen-oxygen generator.

8. The method for preparing Cr2Se3 single crystal according to claim 7, characterized in that, In step S4, the temperature of the high-temperature zone is 800-820 ℃, the temperature of the low-temperature zone is 700-720 ℃, and the temperature difference between the high-temperature zone and the low-temperature zone is 90-100 ℃. The chemical vapor transport growth time is 8-14 days.

9. A Cr₂Se₃ single crystal, characterized in that, The Cr2Se3 single crystal is an antiferromagnetic material with a Nell temperature of 38 K, and possesses both semiconductor transport properties and negative magnetoresistance effect. It is prepared by the method for preparing Cr2Se3 single crystal according to any one of claims 1-8.

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