Magnetic percolation transition in diluted hexaferrites

The hexagonal ferrites BaFe₁₂O₁₉, SrFe₁₂O₁₉ and PbFe₁₂O₁₉ are ferrimagnets with Neel temperatures of approximately 720K that also show interesting ferroelectric properties. By randomly substituting Ga ions for the Fe ions, the Neel temperature can be suppressed to zero. Recent experiments [1] have found the phase boundary to vary as T_N ∼ (1 - x/x_c)^2/3 over a wide x range, with x_c very close to the percolation threshold of the iron spins. The shape of the phase boundary in the neighborhood of x_c does not agree with current theoretical predictions for a classical percolation transition. We investigate the shape of the low-temperature phase boundary close to the percolation threshold using large-scale Monte Carlo simulations of a three-dimensional site-diluted Heisenberg model.

[1] S.E. Rowley at al., Phys. Rev. B 96, 020407(R) (2017)