Ising model

Does a 2-D magnet have a sharp critical temperature where it loses its magnetisation?

How the lab tests it

Heat a lattice of spins (Metropolis Monte-Carlo), measure the magnetisation ⟨|M|⟩ and the susceptibility χ at each temperature, and locate Tc at the χ peak.

What it checks

Onsager's exact critical temperature Tc = 2/ln(1+√2) ≈ 2.2692