Spin glass is a system formed by disorder-oriented spin freezing, and it is also the simplest and most typical type of glass. For more than half a century, many new theoretical concepts and models have been proposed in the study of spin glass, which have strongly promoted the development of statistical physics, and also have important guiding significance in the fields of protein folding, neural networks, and optimization algorithms. In 2021, Italian scientist Giorgio Parisi was awarded the Nobel Prize in Physics for his theoretical work on spin glass. There are many novel physical phenomena in spin glass materials, such as spin frustration and chirality-induced topological magnetic structure and topological Hall effect, so they also have potential application value in spintronic devices.
Figure 1The direct current (a-c) and alternating current (d-f) magnetization of fcnm bulk alloys varies with temperature.
Composed. Alloys formed by four or more than five kinds of metals of equal or approximately equal amounts are called high-entropy alloys, which have attracted extensive attention as a new type of disordered alloy materials in recent years. They are usually made up of:
The composition of four or five or more metal elements with near-equal atomic ratios breaks through the traditional alloy composition design concept of a single element, so it is also called a multi-principal element alloy or a chemically complex alloy. Different from previous experience, chemically complex alloys tend to form stable solid solutions rather than complex intermetallic compounds due to the influence of their high mixing entropy. And it often exhibits unique properties that are not available in any set of elements. In addition, compared with traditional alloys, high-entropy alloys jump out of the design framework of traditional alloys, and have huge compositional space and rich physical properties, so they have broad application prospects.
Recently, Dr. Yu Jihao, a student from the team of Academician Wang Weihua of the Key Laboratory of Extreme Condition Physics of the Institute of Physics of the Institute of Physics of the Chinese Academy of Sciences and Beijing National Research Center for Condensed Matter Physics, under the guidance of Prof. Bai Haiyang and Associate Prof. Sun Baoan, observed a spin glass state with a very high glass transition temperature in the Fe-Co-Ni-Mn quaternary high-entropy alloy (hereinafter referred to as FCNM alloy) system. By adjusting the composition, the spin glass transition temperature of this alloy can reach up to above room temperature, which is the highest spin glass transition temperature reported to date. This spin glass state can be stable over a wide range of components, and its phase transition temperature can be continuously adjusted over a wide range (see Figure 1).
Figure 2(a,b) DFT calculates the electronic wave-splitting density and magnetic moment distribution of different components of FCNM alloy system. (c) Magnetic phase diagram of the FCNM bulk alloy.
In order to explain the formation mechanism of the spin glass phase and the origin of the ultra-high transition temperature, they found that the magnetic moments of Fe, Co, and Ni atoms in the system tend to be parallel, while the Mn atoms tend to be counterparallel (see Fig. 2). When they form alloys, the four elements randomly occupy the face-centered cubic lattice, creating a high degree of magnetic resistance frustration that ultimately leads to the formation of the spin glass phase.
In addition, the direct interaction strength between magnetic atoms in this system is much higher than that of the rkky indirect interaction in traditional spin glasses such as dilute magnetic alloys, which makes the magnetic resistance degree of the system higher, the spin glass state more stable, and the transition temperature significantly increased. When the proportion of Mn is adjusted, the degree of frustration in the system will also change, thus changing the transition temperature of the spin glass state. The discovery of this new alloy system not only puts forward a new idea for the design of spin glass and magnetoresistive frustration materials, but also provides more possibilities for the practical application of spin glass materials due to its ultra-high transition temperature and adjustable composition. The work was published in Physical Review Materials 6, L091401 (2022) under the title "Robust Spin Glass State with Exceptional Thermal Stability in a Chemically Complex Alloy".
Figure 3(a-c) Principle and measurement method of topological Hall effect in FCNM alloy films. (d, e) Topological Hall effect intensity as a function of magnetic field and temperature.
The topological Hall effect is the electrical transport response of chiral spin structures, which can be used as a powerful tool to detect and understand such novel magnetically ordered structures. So far, the topological Hall effect has only been found and reported in some non-centrally symmetric superlattice systems, magnetic heterojunctions, and triangular lattice magnets, and their formation mechanism is that the DM interaction due to symmetry breakage destroys the collinear arrangement of spins, or the geometric frustration changes the magnitude and sign of the RKKY interaction. The existence of a strong magnetoresistive frustration effect caused by the random occupation of atoms in high-entropy alloys also leads to the destruction of long-range magnetic order and the generation of non-collinear magnetism, so it can be used as a new generation mechanism of chiral spin structures.
Further, they first prepared the FCNM alloy into thin films by physical vapor deposition, and then subjected them to systematic electrical transport and magnetic characterization. The results showed that intensities up to 1The topological Hall effect is significant at 92 cm and can exist over a wide range of temperatures and magnetic fields (see Figure 3). In order to further understand the mechanism of the topological Hall effect, they performed Lorentz electron microscopy and small-angle neutron scattering experiments, and found that the sample transforms into a single-domain structure at a very low magnetic field, but the magnetic moment within the domain is still non-collinear (see Figure 4). The DFT calculation results show that at 0 K, even considering the influence of SoC effect and allowing the magnetic moment to rotate in three-dimensional space, the magnetic moment still tends to be arranged in a collinear manner, which indicates that the chiral spin structure in FCNM alloy is a thermally activated phenomenon. The Monte Carlo simulation results not only further confirm this conclusion, but also find that the temperature dependence of the topological Hall effect is related to the perpendicular magnetic anisotropy. Combined with the above experimental, computational, and simulation results, they attribute the physical origin of the topological Hall effect to the fact that the originally isotropic spin arrangement has a preferential orientation under the action of an external magnetic field, resulting in the scalar spin chirality (defined as the angular size of the solid tensioned by the adjacent local spin vectors) no longer canceling each other out and showing a net value as a whole.
Figure 4(a-c) Lorentz electron microscopy to observe the evolution of the magnetic domain wall in the sample with the magnetic field. (d, e) Scattering intensity of small-angle neutrons at different magnetic fields and their angular integration along the direction of parallel and perpendicular magnetic fields.
Since the topological Hall effect is proportional to the spin chirality of the system, its intensity is zero in both the absence of magnetic field and the saturation magnetic field, but appears in the intermediate magnetic field. This origin is completely different from the previously reported systems with chiral spins, and opens up a new direction for the design and preparation of chiral spin systems. In addition, FCNM high-entropy alloys have a simple face-centered cubic structure, simple preparation process, and easy to control properties, which not only provides a good material system for the study of chiral spin structures, but also may be applied to spintronic devices in the future. The work was published in Advanced Materials 202308415 (2024) under the title "Observation of topological hall effect in a chemically complex alloy". Yu Jihao is the first author, Sun Baoan, associate researcher of the Institute of Physics, Bai Haiyang researcher and Professor Li Zi'an of Guangxi University are the co-corresponding authors, and collaborators include Ke Yubin, a researcher at the Dongguan Spallation Neutron Source.
This series of research has been supported by the major projects of the National Natural Science Commission of China and the Basic Science Center Project, the National Key R&D Program of the Ministry of Science and Technology, the Strategic Priority Science and Technology Project of the Chinese Academy of Sciences, and the Major Basic and Applied Foundation Projects of Guangdong Province.
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