2023-11-22 10:09:18
Ordinary magnets attached to refrigerators exhibit ferromagnetism, but recently, scientists discovered a new form of ferromagnetism in a man-made material whose magnetic moment arrangement cannot be explained by traditional exchange interactions. Substances have various magnetisms, such as paramagnetism, diamagnetism, ferromagnetism, antiferromagnetism, ferrimagnetism, superparamagnetism, etc. The most common form of magnetism is ferromagnetism, which is the magnet we stick on the refrigerator and whiteboard every day. The principle is that ferromagnetism occurs when all electron spins in the material point in the same direction, which can be said to be a perfect combination that satisfies a variety of physical effects. Other magnetisms, such as paramagnetism, are weaker forms that occur when electron spins point in random directions. But in a new study, a team at the Swiss Federal Institute of Technology in Zurich (German: ETH Zürich) has discovered a strange new form of ferromagnetism. The researchers originally wanted to explore what makes moiré materials so magnetic: experimental materials made of two-dimensional stacks of molybdenum diselenide and tungsten disulfide, which have a lattice structure that can contain electrons. In order to find out what type of magnetism the molar material has, the research team first applied a current and steadily increased the voltage to “inject” electrons into the material. Then, they irradiated the material with laser and measured the reflection intensity of light of different polarizations to measure the magnetism and uncover the electron spin. Are they pointing in the same direction (ferromagnetic) or in random directions (paramagnetic). ▲ In the molar material created by the team, if there is only one electron per lattice site (left), the electron spins are disordered; if there are multiple electrons per lattice site (right) and doublons particles are formed (red) , it will transform into ferromagnetism. (Source: ETH Zurich) Initially the material exhibited paramagnetic properties with random spin directions, but as the team added more electrons to the lattice and the material exceeded a critical electron density, it unexpectedly transitioned to ferromagnetism; Even more interesting is that the transition occurs exactly when each lattice site is filled with one electron, ruling out the exchange interaction mechanism commonly used to drive ferromagnetism. Ataç Imamoğlu, the lead author of the study, said: “This is surprising evidence of a new type of magnetism that cannot be explained by exchange interactions.” The team proposed a completely different mechanism: when multiple electrons enter the lattice vacancies, they will pair into doublons particles, and finally penetrate The quantum tunnel fills the entire lattice. When they do this, the electrons adjust their spins to minimize their kinetic energy, creating ferromagnetism. This “moving magnetism” mechanism has been predicted by theory for decades but has never been observed in solid materials before. The researchers will study the phenomenon more closely to see whether the mechanism can occur once more at higher temperatures. After all, this molar material was cooled to near absolute zero before the experiments were conducted. The new paper was published in the journal Nature. (First image source: Flickr/Andi CC BY 2.0) New scientific and technological knowledge, updated from time to time
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