Susceptibility and percolation in two-dimensional random field Ising magnets

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© 2001 American Physical Society (APS). This is the accepted version of the following article: Seppälä, E. T. & Alava, Mikko J. 2001. Susceptibility and percolation in two-dimensional random field Ising magnets. Physical Review E. Volume 63, Issue 6. 066109/1-14. ISSN 1539-3755 (printed). DOI: 10.1103/physreve.63.066109, which has been published in final form at http://journals.aps.org/pre/abstract/10.1103/PhysRevE.63.066109.
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Journal ISSN
Volume Title
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Date
2001
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Mcode
Degree programme
Language
en
Pages
066109/1-14
Series
Physical Review E, Volume 63, Issue 6
Abstract
The ground-state structure of the two-dimensional random field Ising magnet is studied using exact numerical calculations. First we show that the ferromagnetism, which exists for small system sizes, vanishes with a large excitation at a random field strength-dependent length scale. This breakup length scale Lb scales exponentially with the squared random field, exp(A/Δ2). By adding an external field H, we then study the susceptibility in the ground state. If L>Lb, domains melt continuously and the magnetization has a smooth behavior, independent of system size, and the susceptibility decays as L−2. We define a random field strength-dependent critical external field value ±Hc(Δ) for the up and down spins to form a percolation type of spanning cluster. The percolation transition is in the standard short-range correlated percolation universality class. The mass of the spanning cluster increases with decreasing Δ and the critical external field approaches zero for vanishing random field strength, implying the critical field scaling (for Gaussian disorder) Hc∼(Δ−Δc)δ, where Δc=1.65±0.05 and δ=2.05±0.10. Below Δc the systems should percolate even when H=0. This implies that even for H=0 above Lb the domains can be fractal at low random fields, such that the largest domain spans the system at low random field strength values and its mass has the fractal dimension of standard percolation Df=91/48. The structure of the spanning clusters is studied by defining red clusters, in analogy to the “red sites” of ordinary site percolation. The sizes of red clusters define an extra length scale, independent of L.
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Keywords
Ising model, domain walls, domain structures, random magnets, percolation studies of phase transitions
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Citation
Seppälä, E. T. & Alava, Mikko J. 2001. Susceptibility and percolation in two-dimensional random field Ising magnets. Physical Review E. Volume 63, Issue 6. 066109/1-14. ISSN 1539-3755 (printed). DOI: 10.1103/physreve.63.066109.