= Arrott plot
{c}
{title2=$M^2=(\mu_0/b)(H/M)-a/b$}
An <Arrott plot> presents squared magnetisation against field divided by magnetisation. For the uniform SI Landau equation $\mu_0H=aM+bM^3$, it is a straight line
$$
M^2=\frac{\mu_0}{b}\frac HM-\frac ab.
$$
The critical mean-field isotherm has $a=0$ and passes through the origin. Horizontal intercept $a/\mu_0$ changes sign across the transition; a positive vertical intercept estimates spontaneous magnetisation squared. The original criterion concerns internal <magnetic field> and the molecular-field regime. Demagnetizing fields and non-mean-field <critical exponents> require additional care.<Criterion for Ferromagnetism from Observations of Magnetic Isotherms by Anthony Arrott>[1957, abstract: cubic magnetisation proportional to internal field at the Curie point]{ref}
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