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Uncovering the Mystery of Heisler Compounds: How Does Their Composition Affect Magnetism?
Heisler compounds are a class of magnetic intermetallic compounds known for their face-centered cubic crystal structure and specific chemical composition, usually XYZ (half Heisler) or X2YZ (full Heisler) ). This term originated from the German mining engineer and chemist Friedrich Heisler. As early as 1903, he conducted in-depth research on the beryllium-zinc alloy Cu2MnAl.
These compounds have received widespread attention for their potential applications in spintronics, which include the magnetoresistance effect, variations of the Hall effect, and phenomena such as ferromagnetism, antiferromagnetism, and ferromagnetism, which make them Research hotspots in materials science.
The magnetism of Heisler compounds comes from the double exchange mechanism between adjacent magnetic ions. Manganese is the first magnetic ion found in the Heisler compound Cu2MnAl.
Diverse chemical composition
Traditionally, Heisler compounds are thought to have the composition XYZ and X2YZ. However, research since 2015 has uncovered many different Heisler compounds that were often not expected, such as XY0.8Z and X1.5YZ, among others. Most of these "non-stoichiometric" Heisler compounds are semiconductors, and their stability and electrical properties are greatly affected by temperature.
These Heisler compounds have low thermal conductivity, making them suitable for thermoelectric material applications. The unique structure allows the transition metal X to serve as both an electron donor and an acceptor during the synthesis of X1.5YZ, further enhancing its stability and functionality.
Half Heisler thermoelectric materials
Half-Heisler compounds are highly tunable, making them ideal for use as thermoelectric materials. Recent research shows that up to 481 semi-Heisler compounds can be stably predicted based on high-throughput calculations combined with machine learning techniques. The composition of these compounds is flexible, different combinations of metal elements form different physical properties, and they have low toxicity and good mechanical properties compared to other thermoelectric materials in medium and high temperature applications.
However, the inherently high thermal conductivity of semi-Heisler thermoelectric materials makes them generally less efficient than other classes of thermoelectric materials, which is one of the main challenges in research.
Changes in magnetic characteristics
The magnetic properties of the early all-Heisler compound Cu2MnAl are closely related to its heat treatment and composition. The saturation magnetic induction intensity of this compound at room temperature is approximately 8,000 Gauss, which is higher than the value of approximately 6100 Gauss for the nickel element, but not as good as 21500 for iron. Gaussian. The study identified that its ferromagnetic phase at room temperature is a completely ordered structure, and its structure will undergo different phase changes in different temperature ranges.
Mechanical properties and thermoelectric applications
The mechanical properties of Heisler compounds are essential considerations in designing thermoelectric applications. Although there are relatively few experimental studies on its mechanical properties in the literature, considerable attention has been paid to its material's resistance to cracking when subjected to intense and repeated thermal cycling and vibration. The elastic modulus of Heisler alloys shows different trends with changes in doping components, and studies have shown that the elastic modulus of semi-Heisler alloys ranges from 83 to 207 GPa.
Amid varying mechanical strength and brittleness requirements, there is a delicate balance between crack resistance and strength of Heisler alloys.
Semi-metallic ferromagnetic Heisler compounds
Half-metallic ferromagnets exhibit metallic behavior in one spin channel and insulating behavior in the other spin channel. Among them, NiMnSb is the first semi-metallic ferromagnetic Heisler compound to be explored. Its semi-metallic property makes complete polarization of conducting electrons possible, which makes it very potential in spintronics applications.
Conclusion
As the research on Heisler compounds continues to deepen, their complex composition and unique physical properties make these materials play an important role in modern technology. How will these compounds further change our understanding and application of magnetic materials in the future?
