Role of the poly-dispersity and the dipolar interaction in magnetic nanoparticle systems: Monte Carlo study

In this paper, roles of the poly-dispersity and the dipolar interaction in frozen ferrofluid are studied by Monte Carlo method. A sample containing the uniaxial anisotropy and the random orientation is used to investigate. The temperature dependence of the coercivity is calculated to consider the magnetic phase transition under the influence of the dipolar interaction. We show that in the poly-dispersity and interacting sample, the temperature dependence of coercive field does not follow the classical expression, H C / H A = 0.48[1 − ( T/ T B ) ½]. We find that the transition temperature, which separates the anti-ferromagnetic and ferromagnetic states of strongly interacting sample, is not unique and it strongly depends on the variation of concentration. We also discuss about the concentration dependence of the coercivity at the different size distributions. At the finite temperature, the curve expresses a cusp which is due to the competition between the blocking and super-paramagnetic state at the low concentration. Therefore, we can see that the poly-dispersity also contributes to the complexity of magnetic phase of frozen ferrofluids. ► The temperature dependence of coercive field does not follow the classical expression. ► The transition temperature, is not unique and it strongly depends on the variation of concentration. ► The competition between the blocking and super-paramagnetic state makes the coercivity increase at the low concentration. ► The strong poly-dispersity retains the anti-ferromagnetic role of the dipolar interaction.