monte carlo simulation of ising model and phase transition studies by gelman evgenii l.
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Monte Carlo Simulation of Ising Model and Phase Transition Studies By Gelman Evgenii. Introduction to Magnetism. Magnetic susceptibility χ : Types of magnetic materials: 1. D iamagneti c : χ <0 and constant (H elium );

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introduction to magnetism
Introduction to Magnetism
  • Magnetic susceptibilityχ:
  • Types of magnetic materials:
  • 1. Diamagnetic: χ<0 and constant (Helium);
  • 2. Paramagnetic: magnetic susceptibility χ>0 and χ∝1/T (Rare earth);
  • 3. Ferromagnetic: Iron. Below a critical temperature (Curie temperature), χ depends on magnetic field, and the M-H diagram shows a hysteresis loop; above this temperature, the material becomes paramagnetic;
  • 4. Anti-Ferromagnetic: Below a critical temperature, χ∝T; above this temperature, the material becomes paramagnetic. (MnO)

Hysteresis loop

ising model 2d
Ising Model(2D)
  • A lattice model proposed to interpret ferromagnetism in materials(1925).
  • Basic idea: Elementary particles have an intrinsic property called “spin”. Spins carry magnetic moments. The magnetism of a bulk material is made up of the magnetic dipole moments of the atomic spins inside the material.
  • Ising model postulates a lattice with a spin σ(or magnetic dipole moment) on each site, defining the following Hamiltonian:
  • E is total energy of the system, J is the nearest spin-spin interaction energy, H is external magnetic field. σ=+1 or -1.
ising model 2d4
Ising Model(2D)
  • Thermalproperties are defined, and computed, by the partition function, which is the normalization factor of the probability of a thermodynamic state:
  • Using Z(T), we can calculate the specific heat C, and magnetic susceptibilityχ
phase transitions
Phase transitions
  • The abrupt sudden change in physical properties of the thermodynamic system around some critical value of thermodynamic variables(such astemperature). A particular quantity is the specific heat.
  • Ehrenfest classification of Phase Transition:
  • First-order phase transitions exhibit a discontinuity in the first derivative of the chemical potential with a thermodynamic variable. Such as solid/liquid/gas transitions.
  • Second-order phase transitions(also called continuous phase transition)have a discontinuity or divergence in a second derivative of the chemical potential with thermodynamic variables.
phase transitions6
Phase transitions
  • C and χaresecond derivative of chemical potential with T and H separately.
  • Onsager (1944) obtained the exact solution for 2D Ising model without external field. The solution shows that there exists second order phase transition in C andχ, because they diverge at some critical value of temperature (Tc≈2.269 in unit of (1/Boltzmann constant)). The studies can explain the ferromagnetic to paramagnetic transition of materials.
  • Monte Carlo simulations also reveal the phase transition properties of Ising model.
monte carlo method and
Monte Carlo method and
  • Monte Carlo:A method using pseudorandom number to simulate the random thermal fluctuation from state to state of a system;
  • The probability of a particular state αfollows Boltzmann distribution:
  • In theory, sum over all possible states to calculate the statistical mean values of a physical quantity, weighing each state based on its Boltzmann factor;
  • Metropolis algorithm (importance sampling technique):

1.Flip one randomly picked spin;

2.Calculate the total energy difference between new and old spin state δE=E(new)-E(old);

3. If δE>0, the probability to accept the new state P(old->new) = exp[-δE/kT],otherwiseP(old->new) = 1.

simulation settings
Simulation settings
  • Set the spin-spin interaction energy J=1, Boltzmann constant k=1, Bohr magneton
  • The unit of Energy is J; the unit of temperature T is
slide10

Low temperature

High magnetization

High temperature

Low magnetization

slide11
Results: C versus T. Specific heat divergence is shown more clearly at Tc≈2.269 in this figure. Second order phase transition occurs.
slide13
Results: Magnetization per spin versus External field H at T= 0.2. It shows a hysteresis loop, characteristic of ferromagnetic materials.
summary of results
Summary of Results
  • Demonstrate that second order phase transition of specific heat C and magnetic susceptibilityχoccur at Tc≈2.269, as predicted by Onsager’s exact solution.
  • Demonstrate the existence of spontaneous magnetization and hysteresis loop below Tc≈2.269 (J>0). These show that the system is ferromagnetic below Tc.
  • Combing these results, the ferromagnetic to paramagnetic phase transition of 2D Ising model is demonstrated.
plans
Plans
  • Write parallel algorithm (MPI or Pmatlab)
  • Check the performance as a function of:
  • Number of processors.
  • Lattice size.
  • Load equalization.