Surface effects and dipolar interaction in NP

Surface effects on the magnetic behavior of nanoparticle assemblies.
G. Margaris, K. Trohidou, H. Kachkachi
Phys. Rev. B 85, 024419 (2012)

Magnetization as function of the applied field for a random anisotropy interacting assembly (g = 0.1) and for different
values of the surface anisotropy coefficients. (a) The value of the quadratic term (σ = 1) is smaller than the absolute value of the quartic term (w = ±2,±5,±8) together with the curve with only the core (quadratic) anisotropy term (w = 0). (b) The quadratic term (σ = 1) is equal or bigger than the absolute value of the quartic term (w = ±0.5,±1).