Magnetizing a material without applying an external magnetic field is suggested by researchers at São Paulo State University (UNESP), Brazil, in an article published in the journal Scientific reports, where they describe the experimental approach used to achieve this goal.
The study was part of the Ph.D. research conducted by Lucas Squillante under the supervision of Mariano de Souza, Professor at the UNESP Department of Physics in Rio Claro. Contributions were also provided by Isys Mello, another doctoral student. candidate supervised by Souza and Antonio Seridonio, Professor at the UNESP Department of Physics and Chemistry at Ilha Solteira. The group was supported by FAPESP.
“In short, magnetization occurs when a salt is compressed adiabatically, without exchanging heat with the external environment,” Souza said. “Compression raises the temperature of the salt and at the same time reorganizes the rotation of the particles. As a result, the total entropy of the system remains constant and the system remains magnetized at the end of the process.”
To understand the phenomenon, it is worth recalling the basics of spin and entropy.
Spin is a quantum property that causes elementary particles (quarks, electrons, photons, etc.), composite particles (protons, neutrons, mesons, etc.) and even atoms and molecules to behave like small magnets pointing north or south – up and spin and down spin – when exposed to a magnetic field.
“Paramagnetic materials such as aluminum, which is a metal, are magnetized only when an external magnetic field is applied. Ferromagnetic materials, including iron, can show limited magnetization even in the absence of an applied magnetic field because they have magnetic domains,” Souza explained.
Entropy is basically a measure of available configurations or states in the system. The greater the number of available states, the greater the entropy. The Austrian physicist Ludwig Boltzmann (1844-1906) associated with a statistical approach entropy in a system, which is a macroscopic size, with the number of possible microscopic configurations that make up its macrostat. “In the case of a paramagnetic material, entropy embodies a distribution of probabilities that describes the number of up or down spins in the particles it contains,” Souza said.
In the recently published study, a paramagnetic salt was compressed in one direction. “Application of uniaxial stress reduces the volume of the salt. Since the process is performed without any heat exchange with the environment, compression gives an adiabatic increase in the temperature of the material. An increase in temperature means an increase in entropy. To maintain total entropy in the system constant, there must be a entropy that compensates for the rise in temperature.As a result, the spin tends to adapt, leading to magnetization of the system, says Souza.
The total entropy of the system remains constant and adiabatic compression results in magnetization. “Experimentally, adiabatic compression is achieved when the sample is compressed for a shorter time than required for thermal relaxation – the typical time it takes for the system to exchange heat with the environment,” Souza said.
The researchers also suggest that the adiabatic temperature rise could be used to investigate other interacting systems, such as Bose-Einstein condensates in magnetic insulators and dipolar spin-ice systems.
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Lucas Squillante et al, Elastocaloric effect-induced adiabatic magnetization in paramagnetic salts due to the mutual interactions, Scientific reports (2021). DOI: 10.1038 / s41598-021-88778-4
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