Fig 6Ising boardHover to heat · hold and drag for BkT/Jd 0.45 · B/Jd 0.00 · m/N 0.97 · ordered
Needle head: spin up (σ = +1) when it points to the backHeater knob sets kT/Jd, coil knob sets B/JdThirty-six spins σᵢ = ±1 with E = −BΣσᵢ − JΣσᵢσⱼ. Mean-field theory replaces the neighbours by their average, an effective field B + Jd·m/N, so m/N = tanh(β(B + Jd·m/N)). With B = 0 the board orders below T_c = Jd/k (kT/Jd = 1) and is half up, half down above it. The needles show the fraction up, (1 + m/N)/2, rounded to a whole needle. With B ≠ 0 the root aligned with the field is shown.