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Peculiarities of the Process of Magnetization of Nanoparticles at Temperatures Below the Blocking Temperature

Author: Simon Chkhaidze
Co-authors: A. Ugulava
Keywords: ferromagnetism, nanoparticles

The energy of magnetic anisotropy of nanoparticles forms two potential wells, located symmetrically. In the absence of a magnetic field, the particles are uniformly distributed in the wells and oriented in opposite directions. In the case of powder compounds above the blocking temperature, above-barrier transitions occur between the particles of different wells and a superparamagnetic state can occur, and below this temperature the particles are "blocked" in the wells, while maintaining an uniform distribution of the particles in them. In a magnetic field, the contours of potential wells lose their symmetry. Both the location of the barrier and the bottom of the wells are shifted relative to each other. When the barrier is shifted to the right, in the left well (in which the particles normally oriented along the field in the absence of a field), a certain region appears for particles oriented opposite the field, which is filled with particles from the same well. As a result, the inclusion of the magnetic field leads to a decrease, and not to an increase, as usual, of the magnetization from the left well. The right well (in which nanoparticles oriented against the field in the absence of a field) narrows and, therefore, the particles from the peripheral regions move to the center of the right well. This process leads to a decrease in the Curie coefficient compared to its usual value, obtained by Langevin theory approximately 0.66 times, and when passing through the blocking temperature from bottom to top to the superparamagnetic state, its approximate increase is threefold.

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