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Register a new userMonte Carlo simulations of ${rm Ni Fe_2O_4}$ Nanoparticles
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We use Monte Carlo simulations to study ${rm Ni Fe_2O_4}$ nanoparticles. Finite size and surface effects differentiate them from their bulk counterparts. A continuous version of the Wang-Landau algorithm is used to calculate the joint density of states $g(M_z, E)$ efficiently. From $g(M_z, E)$, we obtain the Bragg-Williams free energy of the particle, and other physical quantities. The hysteresis is observed when the nanoparticles have both surface disorder and surface anisotropy. We found that the finite coercivity is the result of interplay between surface disorder and surface anisotropy. If the surface disorder is absent or the surface anisotropy is relatively weak, the nanoparticles often exhibit superparamagnetism.
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We study the phase diagram of a quasi-two dimensional magnetic system ${rm Rb_2MnF_4}$ with Monte Carlo simulations of a classical Heisenberg spin Hamiltonian which includes the dipolar interactions between ${rm Mn}^{2+}$ spins. Our simulations reveal an Ising-like antiferromagnetic phase at low magnetic fields and an XY phase at high magnetic fields. The boundary between Ising and XY phases is analyzed with a recently proposed finite size scaling technique and found to be consistent with a bicritical point at T=0. We discuss the computational techniques used to handle the weak dipolar interaction and the difference between our phase diagram and the experimental results.
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