Recently, Professor Yang Mengmeng of Anhui University, researcher Li Qian of University of Science and Technology of China, and collaborators have developed an antiferromagnetic-ferromagnetic phase transformation imaging method, which expands the imaging methods of antiferromagnetic domains and provides important insights into the magnetic anisotropy of two-dimensional magnets.
In the future, this method can be applied to other antiferromagnetic materials, so as to provide a new idea for direct imaging of antiferromagnetic domains.
Chromium-sulfur bromine is the "material protagonist" in this study. If the phase transition temperature can be increased to room temperature by some methods, combined with the semiconductor properties of the material, it can be used in spintronics memory devices.
Figure |From left to right: Pei Fangfang, Liu Daxiang, Researcher Li Qian, Yu Jingjing, Professor Yang Mengmeng, Yuan Yanan (**Data map).
Let's start with the French physicist Louis Nair
Louis Néel, the famous French physicist, was awarded the Nobel Prize in Physics in 1970. He has made important achievements in the study of antiferromagnetism and is known as the "father of antiferromagnetic materials". However, it has said that "antiferromagnetism is interesting but useless", because originally antiferromagnetic materials were only used as an exchange-biased pinning layer for magnetic memory devices, which means that it has been in a supporting role until now.
In recent years, it has been discovered that antiferromagnetism has the characteristics of zero net magnetic moment and ultra-fast resonance frequency. Compared with ferromagnetic materials, when also exposed to external magnetic fields, due to the above-mentioned intrinsic magnetic properties, antiferromagnetism has better robustness and faster response speed.
Based on this, antiferromagnetic spintronics based on antiferromagnetic materials has been developed. However, it is not easy to flip the direction of the spin of the antiferromagnetic.
For traditional antiferromagnetic materials such as nickel oxide, ferrous fluoride and other materials, their spin flip fields are on the order of a few Teslas to dozens of Teslas.
Therefore, the question of concern is: how to change the spin direction of antiferromagnetism under a small magnetic field;and how to achieve antiferromagnetic-ferromagnetic conversion in the same material.
Layered 2D materials have been extensively studied since 2007, when it was discovered that there is also magnetic long-range ordering in 2D materials. Among them, chromium-sulfur bromide, a two-dimensional antiferromagnetic material, has always been a research hotspot.
As a type A-type antiferromagnetic semiconductor, chromium-sulfur bromide has good air stability and its antiferromagnetic phase transition temperature is relatively high.
Another peculiarity of the material is that it has the shape of a long strip, and the in-plane crystals A and B correspond to the long and short sides of the strip. The magnetization axis is in the crystal axis B-axis, and the transition field is also relatively small.
Combined with magneto-optical Kerr microscopy and density functional theory calculations, the team of Yang Mengmeng and Li Qian studied the antiferromagnetic-ferromagnetic transition of chromium-sulfur bromide in this work, and found that chromium-sulfur bromide has anomalotropy related to different magnetic phases.
Under different magnetic sequence conditions, this unique magneto-optical property will bring different anisotropic optical reflectivity, so that the antiferromagnetic-ferromagnetic transition under the magnetic field can be directly imaged.
With the help of magneto-optical Kerr microscopy, the research team not only imaged the nucleation and diffusion of magnetic domains, but also identified the intermediate spin flip states during the antiferromagnetic-ferromagnetic transition.
This unique magneto-optical property, as well as the phase transition dynamics of antiferromagnetic-ferromagnetic transition in chromium-sulfur bromide, mean that two-dimensional magnetic materials have a wide range of applications in the field of spintronics.
In fact, this project was not their initial research goal, but a study derived from experimental new phenomena.
Let's start with the anomalous magneto-optical effect
According to reports, when the research group first got the chromium-sulfur bromide sample, it originally wanted to observe its hysteresis loops and magnetic domains through a magneto-optical Kerr microscope.
In the process, they discovered some anomalous magneto-optical effects, and began to further study and theoretically explain the magnetic anomalous effects.
On this basis, they used this anomalous magneto-optical effect to observe the dynamic process of antiferromagnetic-ferromagnetic transition in chromium-sulfur bromide crystals for the first time, and found that there is a spin flip intermediate state in the transition process.
First of all,They measured anomalous magneto-optical signals at low temperatures using the magneto-optical kerr effect (moke).
In the study, the research group used a vibrating sample magnetometer (VSM) and a magneto-optical Kerr effect to study the macroscopic magnetization of chromium-sulfur bromide as a function of the magnetic field.
The m-h curve from VSM measurements clearly shows the anisotropic magnetocrystalline properties of chromium-sulfur bromide, where the in-plane b-axis is the easy axis, the in-plane a-axis is the sub-easy axis, and the out-of-plane c-axis is the sub-hard axis.
Below the Néel temperature of chromium-sulfur bromide (TN 132K), VSM measurements show the antisymmetric shape of the hysteresis loop. For example, under the opposite saturation magnetic field, the opposite saturation magnetization occurs.
Kerr signals from the same chromium-sulfur bromide crystals show an almost symmetrical loop shape. For example, under the opposite saturation magnetic field, the same saturation Kerr value occurs.
These results confirm that the Kerr signal of chromium-sulfur bromide is predominantly antiferromagnetic or ferromagnetic between adjacent layers below the tn temperature, rather than a net magnetization sign.
In other words, those magnetization symbols along one axis contribute much less to the Kerr signal than the magnetically ordered states corresponding to the antiferromagnetic-ferromagnetic transitions of chromium-sulfur bromide.
Figure |Crystal structure and magneto-optical properties of chromium-sulfur bromide
And then,The team studied the magneto-optical anisotropy reflectance in the bulk chromium-sulfur bromide plane by changing the polarization direction of the linearly polarized light.
To further improve the polarization state and magnetic order of linearly polarized light, as well as the interaction between crystal axes, they designed a set of vertically placed chromium-sulfur bromide samples to exclude the background of intensity variations related to the polarization of incident light.
The results showed that the anisotropic reflectance of the incident light at different polarization angles in different magnetic phases of the chromium-sulfur bromide sample showed obvious Cos2 dependence, which confirmed the uniaxial anisotropic optical properties of chromium-sulfur bromide.
To determine the polarization-dependent reflectance caused by the antiferromagnetic-ferromagnetic phase transition in chromium-sulfur bromide, they measured the reflectance at different polarization angles when swept along the b-axis.
The measurements show that at =0°, the reflectance has little to do with the magnetic field;With the increase of , the reflectivity exhibits an obvious magnetic field dependence, which is also almost the same as the hysteresis loop of the anomalous magneto-optical Kerr effect measured earlier.
Figure |Anisotropic reflectance of antiferromagnetic, ferromagnetic, and paramagnetic states (advanced functional materials).
Subsequently, they used density functional theory to calculate the magneto-optical properties of chromium-sulfur bromide. Specifically, the research group invited Professor Gao Yang from the University of Science and Technology of China to help with theoretical calculations, so as to calculate the band structure in the ferromagnetic state and the antiferromagnetic state, and found that this is consistent with the previous research results.
They then numerically confirmed ab=0 and found that the symmetry analysis was in good agreement with each other. They then calculated the real and imaginary parts of aa and bb as a function of photon energy in both ferromagnetic and antiferromagnetic states.
Through this, they found that the reflectivity in the ferromagnetic state is always greater than the reflectance in the antiferromagnetic state. Specifically, the reflectivity of ferromagnetic and antiferromagnetic states decreases as the polarization angle of the incident light increases.
Among them, the difference is smaller at =0° and the largest at =90°, which is also in good agreement with the experimental data.
Figure |Density functional theory calculation of chromium-sulfur bromide photoresponse (**advanced functional materials).
Finally, they used a Kerr microscope to observe the antiferromagnetic-ferromagnetic domain transition of bulk chromium-sulfur bromide. Based on its unique magneto-optical properties, the team imaged chromium-sulfur bromide to further reveal the details of the antiferromagnetic-ferromagnetic phase transition.
It also captures the whole process of magnetic domain images from antiferromagnetic states to ferromagnetic states, which shows a very rich magnetic domain information. At the same time, they noticed the presence of steps on the sample, which represented different antiferromagnetic-ferromagnetic phase transition critical fields for different thicknesses.
As the magnetic field increases, ferromagnetic domains nucleate at the edge of the sample or near defects, then expand through domain wall motion, and eventually cover the entire sample area.
The team also noted an interesting phenomenon that the domain wall motion always seems to be accompanied by an additional bright white contrast before the contrast transition from black to gray.
This additional bright contrast suggests that there may be a spin-flop transition in the spins of the chromium-sulfur bromide interlayer antiferromagnetic states during the antiferromagnetic-ferromagnetic transitions (a spin-flop transition in an antiferromagnetic state, in which the spins of the two sublattices are suddenly rotated and aligned almost perpendicular to the direction of the magnetic field when the magnetic field increases and exceeds a critical value, thus being in an inclined spin flip state).
Figure |Direct imaging of the change of chromium-sulfur bromide in magnetic domains during antiferromagnetic-ferromagnetic phase transition (advanced functional materials).
At this point, the research has basically come to an end. Recently, the related ** was published in Advanced Functional Materials[ 1]。
Yu Jingjing, a master's student at Anhui University, Liu Daxiang and Ding Zhenyu, a master's student at the University of Science and Technology of China, are the co-authors, and Professor Yang Mengmeng, researcher Li Qian from the University of Science and Technology of China, and Professor Qiu Ziqiang from the University of California, Berkeley serve as co-corresponding authors.
Figure |Related** (advanced functional materials).
The reviewers gave a positive evaluation of this work. The first reviewer commented that the technical research group used this technical group to observe the nucleation and evolution of magnetic domains in the process of magnetic phase transition.
A second reviewer said: "The authors have conducted a comprehensive study of the ferromagnetic, antiferromagnetic, and paramagnetic states of chromium-sulfur bromide in two-dimensional magnets using magneto-Kerr microscopy. Their results show that the antiferromagnetic-ferromagnetic transition phase transition has spatial resolution and identifies spin flip states. ”
Based on this achievement, the research group intends to study the thin-layer sample, including testing its magneto-optical properties and controlling its magnetic domain flip. It is also possible to prepare some devices based on chromium-sulfur bromide materials from the perspective of application.
References: 1yu, j., liu, d., ding, z., yuan, y., zhou, j., pei, f., li, q.. direct imaging of antiferromagnet‐ferromagnet phase transition in van der waals antiferromagnet crsbr.advanced functional materials, 2307259（2023）.