Researchers propose a method to magnetize a material without applying an external magnetic field

The magnetization of a material without applying an external magnetic field is proposed by researchers at the State University of São Paulo (UNESP), Brazil, in an article published in the journal Scientific reports, where they detail the experimental approach used to achieve this goal.

The study was part of the doctorate. research carried out by Lucas Squillante under the supervision of Mariano de Souza, professor in the Department of Physics of UNESP in Rio Claro. Contributions were also made by Isys Mello, another PhD. candidate supervised by Souza, and Antonio Seridonio, professor at the Department of Physics and Chemistry of UNESP in Ilha Solteira. The group was supported by FAPESP.

“Very briefly, magnetization occurs when a salt is compressed adiabatically, without heat exchange with the external environment,” Souza said. “Compression raises the temperature of the salt and at the same time rearranges the spins of its particles. As a result, the total entropy of the system remains constant and the system remains magnetized at the end of the process.”

To help understand the phenomenon, it is worth remembering the basics of spin and entropy.

Spin is a quantum property that makes elementary particles (quarks, electrons, photons, etc.), compound particles (protons, neutrons, mesons, etc.) and even atoms and molecules behave like tiny magnets, pointing north or south – up and spin down – when subjected to a magnetic field.

“Paramagnetic materials like aluminum, which is a metal, are magnetized only when an external magnetic field is applied. Ferromagnetic materials, including iron, can display finite magnetization even in the absence of a field. magnetic applied because they have magnetic domains, ”Souza explained. .

Entropy is essentially a measure of the configurations or accessible states of the system. The greater the number of accessible states, the greater the entropy. The Austrian physicist Ludwig Boltzmann (1844-1906), using a statistical approach, associated the entropy of a system, which is a macroscopic quantity, with the number of possible microscopic configurations which constitute its macro-state. “In the case of a paramagnetic material, entropy embodies a probability distribution that describes the number of ascending or descending spins in the particles it contains,” Souza said.

In the recently published study, a paramagnetic salt was compressed in only one direction. “The application of uniaxial stress reduces the volume of the salt. Because the process is conducted without any heat exchange with the environment, the compression produces an adiabatic rise in the temperature of the material. A rise in temperature means a rise entropy. To keep the total entropy constant in the system, there must be a local reduction component of the entropy which compensates for the increase in temperature. As a result, the spins tend to align , leading to the magnetization of the system, ”Souza said.

The total entropy of the system remains constant and adiabatic compression causes magnetization. “As an experiment, adiabatic compression is achieved when the sample is compressed for less time than is necessary for thermal relaxation, the typical time taken by the system to exchange heat with the environment,” said Souza.

The researchers also propose that the adiabatic increase in temperature could be used to study other interacting systems, such as Bose-Einstein condensates in magnetic insulators and dipolar spin-ice systems.