Magnetic phase transitions can be either first order or second order. The nature of the transition can be inferred from the Arrott plot based on the slope of the magnetic isotherms. If the lines are all positive slope, the phase transition is second order, whereas if there are negative slope lines, the phase transition is first order. This condition is known as the Banerjee criterion.
The Banerjee criterion is not always accurate for evaluating inhomogeneous ferromagnets, since the slopes can all be positive even when the transition is first-order.