Method for measuring magneton dispersion of anti-ferromagnetic thin film material based on terahertz spectrum

Abstract

The invention provides a method for measuring magneton dispersion of an antiferromagnetic thin film material based on a terahertz spectrum. According to the method, a ferromagnetic metal layer sample, an antiferromagnetic layer sample, a heavy metal layer sample and a three-layer film structure sample are prepared, laser pulses are used for inducing the samples to emit terahertz spectra, an electro-optical sampling technology is combined for measurement, and contribution of magneton transmission to the terahertz spectra is separated through data processing. The invention aims to solve the problem that the magneton dispersion property of an antiferromagnetic thin film material cannot be measured in real time at low cost in the prior art. Compared with a traditional method, the method has the advantages of being wide in frequency spectrum range, high in wave number resolution ratio, free of contact and low in cost, and is suitable for different types of antiferromagnetic materials. The method provides a theoretical basis and an experimental basis for researching the spin dynamic characteristics of an antiferromagnetic material and developing a high-frequency spintronics device. The method effectively fills the technical blank in the field of magneton measurement of the antiferromagnetic material.

Description

Technical Field

[0001] The present invention relates to the technical fields of magnetic materials and terahertz technology, magnetic materials and terahertz spectroscopy technology, and particularly relates to a method for measuring magnon dispersion of an antiferromagnetic thin film material based on terahertz spectroscopy. Background Art

[0002] Antiferromagnetic materials do not exhibit a net magnetic moment and have dynamic characteristics in the terahertz frequency band, showing great application prospects in the aspects of high-density integration, strong anti-interference ability, and high-speed read-write information storage and processing technologies. Antiferromagnetic magnons are the quantized form of spin waves in antiferromagnetic materials, and their dispersion relation describes the relationship between magnon frequency and wave vector. As a fundamental physical relationship, the dispersion relation of antiferromagnetic magnons is the basis for deeply understanding complex physical phenomena such as spin dynamics, heat conduction, magneto-acoustic polarons, and quantum phase transitions in antiferromagnetic materials. Therefore, accurately measuring the magnon dispersion relation in antiferromagnetic materials, especially thin film materials, is crucial for the research and development of antiferromagnetic spintronic devices.

[0003] Currently, the traditional methods for measuring magnon dispersion commonly used include neutron scattering, ferromagnetic resonance (FMR), and antiferromagnetic resonance (AFMR) measurement methods. The limitations of these traditional methods include: although they can detect magnon dispersion, the time resolution is usually limited to the microsecond level (limited by the neutron source pulse frequency), making it difficult to capture the picosecond-level ultrafast magnon dynamics in antiferromagnetic materials. The time resolution is limited by the laser pulse width (usually in the femtosecond to picosecond level), and the sensitivity to low-frequency magnons is relatively low. Moreover, the traditional measurement methods require large-sized samples (millimeter-level thickness), and the strong neutron beam may damage the thin film materials. Even these measurement methods have extremely high requirements for the surface cleanliness of the samples and require a vacuum environment. Neutron scattering relies on large-scale synchrotron radiation facilities or reactors, with scarce equipment and high operating costs. The low-temperature strong magnetic field environment (such as superconducting magnets) is often used in traditional measurements to stabilize the magnetic ordered state, increasing the experimental complexity. And real-time low-cost measurement, non-contact measurement, wide applicability, etc. are the goals that researchers in the optical and magnetic fields have been looking forward to. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for measuring magnon dispersion of an antiferromagnetic material based on terahertz spectroscopy to solve the problem in the prior art that the magnon dispersion properties in antiferromagnetic thin film materials cannot be measured in real time at low cost.

[0005] The technical solution adopted by the present invention is as follows: A method for measuring the magnon dispersion of an antiferromagnetic thin film material based on terahertz spectroscopy, comprising the following steps: (1) placing a three-layer thin film sample successively composed of a ferromagnetic metal thin film, an antiferromagnetic thin film, and a heavy metal thin film in an experimental device; (2) applying a weak magnetic field to saturate the magnetization of the ferromagnetic metal thin film; (3) using a terahertz pulsed laser to pump and excite the three-layer thin film sample to induce the three-layer thin film sample to emit terahertz spectroscopy; (4) measuring the terahertz spectroscopy after magnons in the antiferromagnetic thin film are transmitted to the heavy metal thin film by the electro-optic sampling method; (5) flipping the magnetic field direction and repeating the measurement of the terahertz spectroscopy to ensure data accuracy; (6) performing Fourier analysis on the terahertz spectroscopy to obtain the phase data of the frequency spectrum; (7) extracting the relationship between the wave number and frequency in the phase data to obtain the dispersion relationship of magnons in the antiferromagnetic thin film material.

[0006] Further, in the step (4), the terahertz spectroscopy emitted by the three-layer thin film sample is subtracted from the terahertz spectroscopy emitted by the magnetic dipoles in the ferromagnetic metal thin film to obtain the terahertz spectroscopy after magnons in the antiferromagnetic thin film are transmitted to the heavy metal thin film.

[0007] Beneficial effects: The present invention has the following significant advantages:

[0008] Wide spectral range and high wave number resolution: Since the terahertz spectrum can cover a wide spectral range, the present invention can directly detect the dispersion relationship of magnons in antiferromagnetic materials in a wide spectral range, and the dispersion relationship data is in a quasi-continuous form with high wave number resolution.

[0009] Real-time low-cost measurement: The present invention is based on desktop terahertz spectroscopy technology for measurement, with low equipment price and maintenance cost, and short measurement time, which is conducive to the extensive research on the magnon transport properties of antiferromagnets in laboratories.

[0010] Non-contact measurement: The present invention is a non-contact measurement method and will not cause physical damage to the sample.

[0011] Wide applicability: The present invention is not limited by the type of antiferromagnetic material and the quality of the thin film single crystal, and has broad application prospects.

[0012] The measurement method of the present invention can not only accurately reveal the magnon dispersion relationship in antiferromagnetic materials, but also provide a theoretical basis and experimental evidence for the development of high-frequency spintronic devices based on antiferromagnetic materials. Through the method of the present invention, researchers can deeply explore the spin dynamics characteristics of antiferromagnetic materials and provide important support for the design and application of new spintronic devices. Description of the drawings

[0013] Figure 1 It is a schematic diagram of the structure of a nickel-iron alloy / heavy metal thin film / nickel oxide multi-layer thin film structure constructed for the present invention.

[0014] Figure 2 This is a schematic diagram of the experimental apparatus for measuring the magnon dispersion relation of antiferromagnetic materials in the present invention.

[0015] Figure 3 This is the terahertz emission spectrum of the two-layer and three-layer film structures measured in the present invention.

[0016] Figure 4 This is the curve graph of the magnon dispersion relation of the antiferromagnetic material obtained after data processing in the present invention. Detailed implementation manners

[0017] The present invention will be described in more detail below with reference to the accompanying drawings and specific embodiments. Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in this specification in the description of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used in this specification includes any and all combinations of one or more of the related listed items.

[0018] Sample preparation.

[0019] Figure 1 The schematic diagram of the NiFe alloy / heavy metal thin film / NiO multilayer thin film structure of the present invention is shown. An antiferromagnetic thin film (NiO, nickel oxide) is grown between the ferromagnetic metal thin film (Py, NiFe alloy) and the heavy metal thin film (Pt, platinum) to form a three-layer film structure of ferromagnetic metal / antiferromagnetic / heavy metal. Among them, the thickness of the ferromagnetic metal thin film is 2 nm, the thickness of the heavy metal thin film is 2 nm, and the thickness of the antiferromagnetic thin film is 5 nm. At the same time, a control sample is prepared with a two-layer film structure of ferromagnetic metal / heavy metal, and the thicknesses of both the ferromagnetic metal thin film and the heavy metal thin film are 2 nm, which is used as a reference for no magnon transmission.

[0020] Terahertz spectroscopy measurement

[0021] Figure 2 This is a schematic diagram of the experimental apparatus for measuring the magnon dispersion relation of antiferromagnetic materials in the present invention. The prepared three-layer thin film sample is placed in the experimental apparatus, and a weak magnetic field is applied to saturate the magnetization of the ferromagnetic metal thin film. The sample is pumped and excited by a terahertz pulsed laser to induce the three-layer thin film sample to emit a terahertz spectrum. The electro-optic sampling method is used to measure the terahertz spectrum of the sample, and the spectral signal is recorded. By reversing the magnetic field direction, the above measurement process is repeated to ensure the accuracy of the data. The control sample (two-layer thin film structure of ferromagnetic metal / heavy metal) is also measured for terahertz spectrum under the same conditions for reference.

[0022] Data processing and analysis

[0023] Figure 3 The terahertz emission spectra of the two-layer film and three-layer film structures measured by the present invention are shown in Figure 1. In the three-layer film structure, the terahertz spectrum signal includes two parts: ① the terahertz spectrum emitted by the magnetic dipole in the ferromagnetic metal film; ② the terahertz spectrum emitted after the magnon is transmitted through the antiferromagnetic film to the heavy metal film. The transmission of the magnon causes a time delay between the two spectral contributions.

[0024] In the two-layer film structure, since there is no magnon transmission process, the two spectra completely overlap in time. By taking the terahertz spectrum of the two-layer film structure as a reference, the terahertz spectrum signal emitted by the magnetic dipole in the ferromagnetic metal film is subtracted from the terahertz spectrum of the three-layer film structure to obtain the terahertz spectrum in the antiferromagnetic material. The extracted terahertz spectrum in the antiferromagnetic material is subjected to Fourier analysis to obtain the phase of the spectrum. Through the phase data, the relationship between the wave number and the frequency is extracted, thereby obtaining the dispersion relation of the magnon in the antiferromagnetic film material.

[0025] Verification and optimization

[0026] Figure 4 The magnon dispersion relation curve of the antiferromagnetic material obtained after data processing in the present invention is compared with the existing theoretical model to verify the accuracy of the experimental results. According to the experimental conditions (such as sample thickness, external magnetic field strength, etc.), the measurement parameters are further optimized to improve the measurement accuracy and repeatability of the results.

[0027] Applications and Advantages

[0028] Wide spectrum range and high resolution: The terahertz band covers a wide spectrum range, which enables the present invention to detect the dispersion relation of the antiferromagnetic material magnetons, and the data is in a quasi-continuous form with high wave number resolution. Non-contact measurement: The method of the present invention is a non-contact measurement, which will not cause physical damage to the sample and is applicable to a variety of antiferromagnetic materials. Wide applicability: The method of the present invention is applicable to various antiferromagnetic materials, is not limited by the material type and crystal structure, and has a wide range of application prospects, especially suitable for the research of antiferromagnetic materials in the field of spin electronics.

[0029] In summary, antiferromagnetic thin film materials play an important role in the development of high-density integration, strong anti-interference ability, and high-speed read-write information storage and processing technologies due to their lack of net magnetic moment and dynamic characteristics in the terahertz frequency band. Antiferromagnetic magnons are the quantized manifestation of spin waves in magnetic materials, and their dispersion relation reflects the relationship between magnon frequency and wave vector, which is crucial for understanding the magnetic behavior of materials. Currently, the measurement methods for antiferromagnetic magnon dispersion characteristics mainly include techniques such as neutron diffraction and X-ray magnetic circular dichroism, but these methods rely on large-scale scientific facilities, are costly, and cannot achieve real-time measurement. Terahertz spectroscopy, as a non-contact and highly sensitive measurement method, can cover the intrinsic frequency of antiferromagnetic magnons in the frequency range. Therefore, developing a method for measuring antiferromagnetic magnon dispersion based on terahertz spectroscopy is of great significance in both scientific research and practical applications.

Claims

1. A measurement method for magnon dispersion of antiferromagnetic thin film materials based on terahertz spectroscopy, characterized in that, It includes the following steps: (1) Place a three-layer thin film sample composed of a ferromagnetic metal thin film, an antiferromagnetic thin film, and a heavy metal thin film in an experimental device in sequence; (2) Apply a weak magnetic field to saturate the magnetization of the ferromagnetic metal thin film; (3) Use a terahertz pulsed laser to pump and excite the three-layer thin film sample to induce the three-layer thin film sample to emit a terahertz spectrum; (4) Measure the terahertz spectrum after magnons in the antiferromagnetic thin film are transmitted to the heavy metal thin film by electro-optic sampling; (5) Reverse the magnetic field direction and repeat the measurement of the terahertz spectrum to ensure data accuracy; (6) Perform Fourier analysis on the terahertz spectrum to obtain the phase data of the frequency spectrum; (7) Extract the relationship between the wave number and frequency in the phase data to obtain the dispersion relationship of magnons in the antiferromagnetic thin film material.

2. The measurement method of magnon dispersion of an antiferromagnetic thin film material based on terahertz spectroscopy according to claim 1, wherein In step (4), subtract the terahertz spectrum emitted by magnetic dipoles in the ferromagnetic metal thin film from the terahertz spectrum emitted by the three-layer thin film sample to obtain the terahertz spectrum after magnons in the antiferromagnetic thin film are transmitted to the heavy metal thin film.

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