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Physical Review B (Condensed Matter and Materials Physics) -- March 1, 1999 -- Volume 59, Issue 10, pp. 6956-6965

Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

R. Prozorov

Loomis Laboratory of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801

Y. Yeshurun

Institute of Superconductivity, Department of Physics, Bar-Ilan University, 52900 Ramat-Gan, Israel

T. Prozorov and A. Gedanken

Department of Chemistry, Bar-Ilan University, 52900 Ramat-Gan, Israel

(Received 21 July 1998)

Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three Fe₂O₃ samples composed of regular amorphous, acicular amorphous, and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time-dependent barrier yields a natural explanation for the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define an energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data. ©1999 The American Physical Society

URL: http://link.aps.org/abstract/PRB/v59/p6956
PACS: 75.50.Tt, 75.60.Lr, 75.30.Gw        Additional Information

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