An antiferromagnetic magnetometer and a method for characterizing antiferromagnetic magnetization

Abstract

The present invention discloses an antiferromagnetic magnetometer, comprising a femtosecond laser, a terahertz time-domain spectrum detection element and an electromagnet, wherein the femtosecond laser and the terahertz time-domain spectrum detection element constitute a terahertz emission spectrum optical path, an antiferromagnetic sample is used as a terahertz source in the terahertz emission spectrum optical path, the antiferromagnetic sample is a three-layer structure consisting of a substrate, an antiferromagnetic layer and a heavy metal layer, and the electromagnet applies a magnetic field to the antiferromagnetic sample. The present invention also discloses a method for characterizing antiferromagnetic magnetization, wherein the antiferromagnetic sample is pumped by a femtosecond laser, the relationship between the amplitude of the generated terahertz emission spectrum and the external magnetic field is measured, and the magnetization intensity of the antiferromagnetic sample is calibrated. The method of characterizing antiferromagnetic magnetism by using the terahertz emission spectrum of the antiferromagnetic sample of the present invention has the advantages of good economy, easy adjustment and strong real-time performance, and realizes antiferromagnetic magnetism characterization with high sensitivity and easy integration in ultrafast optical systems.

Description

Technical Field

[0001] The invention relates to an antiferromagnetic magnetometer, in particular to a high-sensitivity antiferromagnetic magnetometer, and more particularly to a magnetometer which uses a terahertz emission spectrum to characterize the magnetic characteristics of an antiferromagnetic sample. Background Art

[0002] With the development of spintronic devices, the research on devices using antiferromagnetism as the spin current injection source has received widespread attention. Unlike the sensitivity of ferromagnetic materials to external magnetic fields, antiferromagnetism has a very small response to external magnetic fields, which makes it difficult to characterize antiferromagnetic devices. In low-frequency electrical transport measurements, antiferromagnetic devices can be measured using the spin Hall anisotropic magnetoresistance principle, which shows a response relationship that is 90° phase different from ferromagnetism. However, the low-frequency characteristics of electrical transport cannot measure the resonant characteristics of antiferromagnetism in the terahertz frequency band, which is not conducive to the development of antiferromagnetic devices into ultrafast devices. In recent years, ultrafast antiferromagnetic experimental schemes based on femtosecond lasers have gradually developed. Therefore, it is of great use value to develop an antiferromagnetic magnetic measurement scheme that is friendly to optical experimental measurement equipment.

[0003] The commonly used device for characterizing antiferromagnetism is the Superconducting quantum interference device-vibrating sample magnetometer (SQUID-VSM). The principle is based on the SQUID detection technology. By applying a DC or AC magnetic field to the magnetic material, the DC magnetization intensity or AC magnetization intensity of the sample to be tested is obtained as a function of temperature or field strength. This method requires a high SQUID preparation process, a low-temperature working environment, and strong temperature jitter control. In addition, the device is large in size and cannot be conveniently equipped with an optical window. It cannot be used in a compact ultrafast optical experimental optical path. A highly sensitive, real-time monitoring, and low-cost antiferromagnetic characterization system is a goal that researchers in the field of optics and condensed matter physics have been looking forward to. Summary of the invention

[0004] Technical problem: In view of the problems and shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide an antiferromagnetic magnetometer and a method for antiferromagnetic magnetization characterization that are highly sensitive and easy to integrate into an optical system. By pumping an antiferromagnetic sample placed in a scanning magnetic field with a femtosecond laser and utilizing the response of the terahertz emission spectrum of the antiferromagnetic sample to the antiferromagnetic magnetization intensity, highly sensitive antiferromagnetic magnetization characterization can be achieved.

[0005] Technical solution: To achieve the above-mentioned invention purpose, the first technical solution provided by the present invention is an antiferromagnetic magnetometer, including a femtosecond laser, a terahertz time-domain spectrum detection element and an electromagnet. The femtosecond laser and the terahertz time-domain spectrum detection element constitute a terahertz emission spectrum optical path. The antiferromagnetic sample serves as a terahertz source in the terahertz emission spectrum optical path. The antiferromagnetic sample is a three-layer structure consisting of a substrate, an antiferromagnetic layer and a heavy metal layer. The electromagnet applies a magnetic field to the antiferromagnetic sample. The present invention uses a femtosecond laser to excite the terahertz emission spectrum of the antiferromagnetic material, and uses the response of the terahertz signal to the external magnetic field to obtain the characterization of the magnetization characteristics of the antiferromagnetic material. The amplification effect of the heavy metal layer in the antiferromagnetic sample on the terahertz radiation signal makes it possible to detect the antiferromagnetic magnetization intensity with high sensitivity.

[0006] Furthermore, the terahertz emission spectrum optical path is a transmission optical path.

[0007] Furthermore, the terahertz time-domain spectrum detection element includes an electro-optic crystal, a quarter-wave plate, a Wollaston prism and a photodetector. The femtosecond laser passes through the electro-optic crystal and the quarter-wave plate, is split by the Wollaston prism, and is detected by the photodetector.

[0008] Furthermore, the heavy metal layer is usually made of platinum and has a thickness of no more than 5 nm.

[0009] Furthermore, the substrate of the antiferromagnetic sample is a substrate material that is transparent to laser or terahertz waves.

[0010] The second technical solution provided by the present invention is a method for characterizing antiferromagnetic magnetization, which simultaneously uses the time domain spectrum and frequency domain spectrum information of the terahertz emission spectrum to measure the amplitude change of the terahertz emission spectrum of the antiferromagnetic sample in different magnetic fields to characterize the magnetic properties of the antiferromagnetic material, including the following steps:

[0011] 1. The applied magnetic field is greater than the anisotropy field of the antiferromagnetic sample;

[0012] 2. Set the scanning step of the optical delay line in the terahertz optical path, and scan point by point to obtain the time domain spectrum information of the terahertz emission spectrum;

[0013] 3. Read the maximum amplitude of the time domain spectrum of the terahertz emission spectrum;

[0014] 4. Perform Fourier transform on the time domain spectrum of the terahertz emission spectrum to obtain the frequency domain spectrum;

[0015] 5. Read the spectrum amplitude at the characteristic frequency of the antiferromagnetic sample in the frequency domain;

[0016] 6. Set the magnetic field scanning step, scan the magnetic field point by point, and repeat steps 3 to 5 to obtain the relationship between the magnetic field and the maximum amplitude of the characteristic spectrum;

[0017] 7. Compare the signal-to-noise ratio of the magnetization curve obtained by time domain spectrum and frequency domain spectrum;

[0018] 8. Use the signal amplitude of the standard reference sample to calculate the magnetization intensity per unit volume of the antiferromagnetic sample under study.

[0019] Beneficial effects: The antiferromagnetic magnetometer proposed by the present invention does not need to be equipped with a quantum superconducting interference device, which reduces the use cost and use conditions, and can be conveniently integrated in the optical path of ultrafast optics. Compared with the previous antiferromagnetic magnetization characterization method, the method of using the terahertz emission spectrum of the laser-induced antiferromagnetic sample can measure the magnetization intensity of the antiferromagnetic with high sensitivity and provide real-time magnetization property characterization. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the structure of an antiferromagnetic magnetometer.

[0021] Figure 2 Schematic diagram of the structure of the terahertz time-domain spectrum detection element.

[0022] Figure 3 Schematic diagram of the antiferromagnetic sample structure.

[0023] Figure 4 This is the magnetization curve of the antiferromagnetic sample obtained by time domain spectroscopy.

[0024] Figure 5 This is the magnetization curve of the antiferromagnetic sample obtained by frequency domain spectroscopy. DETAILED DESCRIPTION

[0025] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of use of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0026] 1. A scheme for characterizing antiferromagnetic magnetization with high sensitivity and easy integration into optical systems

[0027] The present invention uses a femtosecond laser to excite the terahertz emission spectrum of an antiferromagnetic sample, and uses the response of the terahertz emission spectrum to an external magnetic field to obtain the characterization of the magnetization characteristics of the antiferromagnetic material. The femtosecond laser can excite the antiferromagnetic magnon mode in the terahertz frequency band, and the ultrafast magnetodynamic process can be used as a spin source to pump spin currents to the adjacent heavy metal layer. The strong spin-orbit coupling in the heavy metal layer causes the spin current to be converted into a charge flow. Compared with the direct terahertz emission spectrum of the magnetodynamic process, the transient charge flow emits terahertz waves outward with higher efficiency. The terahertz emission spectrum is based on the spin pumping process of the antiferromagnetic film layer, so it carries the magnetization intensity information in the magnetic film and can be used to characterize the magnetization characteristics of the antiferromagnetic sample.

[0028] Figure 1 The schematic diagram of the structure of the antiferromagnetic magnetometer of the present invention is shown. The femtosecond laser 1 is vertically incident on the antiferromagnetic sample 2, and the terahertz wave 3 generated by the pump continues to transmit along the transmission direction. After the necessary beam expansion and focusing, the terahertz wave 3 can be measured by electro-optical sampling using the terahertz time domain spectrum detection element 4. The advantage of this measurement scheme is that the optical path is easy to calibrate, and the core gap of the electromagnet 5 can be reduced to less than 5mm to enhance the magnetic field strength. Figure 2 The specific structure of the terahertz time-domain spectrum detection element is shown, including an electro-optic crystal 6, a quarter-wave plate 7, a Wollaston prism 8 and a photodetector 9. A portion of the light beam in the femtosecond laser 1 is split in advance and used as a detection light, passing through the electro-optic crystal 6 and the quarter-wave plate 7, and then split by the Wollaston prism 8 and detected by the photodetector 9.

[0029] According to Figure 3 The structure of the measured antiferromagnetic sample is shown, and the antiferromagnetic sample 2 includes a substrate 10, an antiferromagnetic layer 11 and a heavy metal layer 12. The material of the antiferromagnetic layer 11 can be selected from α-Fe having a weak net magnetic moment due to the spin sublattice tilt. 2 O 3. The antiferromagnetic layer 11 is prepared by a laser pulse deposition method and has a thickness of 20 nm. The nonlinear process in the antiferromagnetic layer 11 leads to the appearance of transient net magnetization, and then forms a terahertz emission spectrum based on the spin pumping process from the antiferromagnetic layer 11 to the heavy metal layer 12. The nonlinear optical process of the antiferromagnetic layer 11 itself directly contributes very little to the terahertz emission spectrum, while the spin current pumped by the ultrafast magnetodynamic process of the antiferromagnetic layer 11 is converted into a charge current in the adjacent heavy metal layer 12, thereby improving its terahertz radiation efficiency. The amplitude of the terahertz emission spectrum after the spin current is converted into a charge current is more than two orders of magnitude stronger than the amplitude of the terahertz emission spectrum of the magnetodynamic process. The material thickness of the antiferromagnetic layer 11 can be selected according to actual conditions, and nanofilms or bulk materials can be used. The thickness of the heavy metal layer 12 is about 3 nm. A thickness lower than the optimized heavy metal layer 12 will lead to a decrease in the spin-to-charge conversion efficiency, thereby leading to a decrease in the terahertz emission spectrum signal. A thickness of the heavy metal layer 12 higher than the optimal value will cause the femtosecond laser 1 or the terahertz wave 3 to be consumed by the heavy metal layer 12, which also results in a decrease in the terahertz signal. In specific implementation, multiple samples with different thicknesses of the heavy metal layer 12 can be prepared for comparison according to the specific sample preparation conditions to achieve the purpose of optimizing the thickness of the heavy metal layer 12. The antiferromagnetic layer 11 is prepared on a substrate 10 that is transparent to terahertz waves or lasers, and is usually made of MgO or Al 2 O 3 The isoparamagnetic single crystal substrate avoids signal attenuation caused by the substrate's strong absorption of the laser 1 or the terahertz wave 3, and does not bring interfering strong background signals to the magnetic measurement.

[0030] 2. Method for magnetic characterization using terahertz emission spectrum of antiferromagnetic samples

[0031] In actual measurement, the time domain spectrum of the antiferromagnetic sample 2 with a strong terahertz emission spectrum signal can be analyzed. Figure 4 For the antiferromagnetic sample α-Fe 2 O 3 The change of the time domain spectrum amplitude of the terahertz emission spectrum in the range of ±1T. As the magnetic field is scanned, the α-Fe 2 O 3 The magnetization orientation of each magnetic domain tends to be consistent, resulting in an increase in the amplitude of the terahertz signal. When the magnetic field is reversed, obvious hysteresis characteristics appear. 2 O 3 The hysteresis characteristics of the magnetic field are similar to the signal-to-noise ratio of the hysteresis loop measured by SQUID-VSM. At the same time, the magnetic anisotropy field obtained by the measurement summary is also consistent. These measurement results prove the effectiveness of this method.

[0032] Another effective way is to analyze the frequency domain spectrum information of the terahertz emission spectrum of the antiferromagnetic sample 2. After Fourier transforming the time domain terahertz waveform, rich terahertz frequency domain spectrum information can be obtained. The uniform precession of antiferromagnetism in the terahertz frequency band is the spectrum characteristic of antiferromagnetic materials. This spectrum often shows a sharp resonance peak, and the spectrum components that deviate from the resonance frequency are almost zero, so it can provide a higher signal-to-noise ratio. The amplitude of the characteristic spectrum changes during the scanning magnetic field, so it can be used to analyze the antiferromagnetic magnetization.

[0033] This analysis method is applicable to antiferromagnetic materials with two tilted spin sublattices. This antiferromagnetic material has a weak net magnetization in each spin domain. After the antiferromagnetic sample 2 is excited by a femtosecond laser 1, its antiferromagnetic resonance mode depends on the direction of the net magnetization. After applying an external magnetic field, the Née vectors of all spin domains tend to be perpendicular to the external magnetic field, resulting in a net magnetization of the entire sample. Therefore, the amplitude and polarity of the antiferromagnetic resonance mode are sensitive to the external magnetic field. Taking the antiferromagnetic material α-Fe 2 O 3 For example, a magnetic field with a magnetic field strength of 1.5 T is first applied. Since the external magnetic field is greater than the anisotropy field of the antiferromagnetic sample 2, the antiferromagnetic material α-Fe 2 O 3 The Née vector of each spin domain in the terahertz optical path tends to be arranged in a direction perpendicular to the external magnetic field. Then, the scanning step length of the optical delay line in the terahertz optical path is set to 10 μm, and the time domain spectrum information of the terahertz emission spectrum is obtained by scanning point by point. After the scanning is completed, the maximum value of the amplitude of the terahertz emission spectrum in the time domain is read. In addition, the frequency domain spectrum is obtained by Fourier transforming the time domain spectrum of the terahertz emission spectrum. At room temperature, the antiferromagnetic material α-Fe 2 O 3 The resonance frequency of the antiferromagnetic resonance mode is 0.2 THz, which is a sharp resonance peak in the Fourier spectrum and can be used as the characteristic frequency of the material. The magnetic field scanning step is set to 0.15T. When measuring, a smaller scanning step can be set near the anisotropic field value, and the magnetic field can be scanned point by point to obtain the antiferromagnetic material α-Fe under different magnetic fields. 2 O 3 The amplitude change of the characteristic spectrum. Figure 5 Demonstrated the antiferromagnetic material α-Fe 2 O 3 The variation of the antiferromagnetic resonance mode amplitude with magnetic field in the range of ±1.5T. Figure 4 The antiferromagnetic material α-Fe was measured by time domain spectroscopy of terahertz emission spectroscopy. 2 O 3 Compared with the magnetization curve of Figure 5The magnetization curve measured by the frequency domain spectrum of the terahertz emission spectrum shown in has a better signal-to-noise ratio. Finally, the signal amplitude of the standard reference sample can be used to calculate the magnetization intensity per unit volume of the antiferromagnetic sample under study.

[0034] In summary, we have proposed a method for characterizing antiferromagnetic magnetization with high sensitivity and easy integration in optical systems, including using femtosecond laser to excite antiferromagnetic samples to induce terahertz emission spectra, and using this to characterize the magnetization of antiferromagnetic materials. This method of characterizing antiferromagnetic magnetism using terahertz emission spectra of antiferromagnetic samples has the advantages of good economy, easy adjustment and strong real-time performance, and realizes the high-sensitivity and ultrafast optical system integrated antiferromagnetic magnetism characterization.

Claims

1. A method for characterizing antiferromagnetic magnetization, It is characterized in that The time domain spectrum and frequency domain spectrum information of the terahertz emission spectrum are simultaneously used to measure the amplitude change of the terahertz emission spectrum of the antiferromagnetic sample in different magnetic fields to characterize the magnetic properties of the antiferromagnetic material, including the following steps: (1) applying a magnetic field greater than the anisotropy field of the antiferromagnetic sample; (2) setting the scanning step length of the optical delay line in the terahertz optical path, and obtaining the time domain spectrum information of the terahertz emission spectrum after scanning point by point; (3) reading the maximum amplitude of the time domain spectrum of the terahertz emission spectrum; (4) performing Fourier transform on the time domain spectrum of the terahertz emission spectrum to obtain the frequency domain spectrum; (5) reading the spectrum amplitude at the characteristic frequency of the antiferromagnetic sample in the frequency domain; (6) setting the magnetic field scanning step length, scanning the magnetic field point by point, and repeating steps (3) to (5) to obtain the relationship between the magnetic field and the maximum amplitude of the characteristic spectrum; (7) comparing the signal-to-noise ratio of the magnetization curve obtained by the time domain spectrum and the frequency domain spectrum; (8) using the signal amplitude of the standard control sample to calculate the magnetization intensity per unit volume of the antiferromagnetic sample.

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