Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

Measurements of the magnetic irreversibility line and time-logarithmic deca y of the magnetization are described for three Fe2O3 samples composed of re gular amorphous, acicular amorphous, and crystalline nanoparticles. The rel axation rate is the largest and the irreversibility temperature is the lowe st for the regular amorphous nanoparticles. The crystalline material exhibi ts the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimen tal results. The main new aspect of the model is the dependence of the barr ier for magnetic relaxation on the instantaneous magnetization and therefor e 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 sc ale that controls the magnetic irreversibility. Introducing this energy sca le yields a self-consistent explanation of the experimental data. [S0163-18 29(99)03009-X].