text: The thesis of Hendrika Johanna van Leeuwen rediscovered a proof that this cannot exist under classical physics.
full_question: The thesis of Hendrika Johanna van Leeuwen rediscovered a proof that this cannot exist under classical physics. It occurs at positive values of the integral function J sub A B, and is correlated with the strength of A, the exchange stiffness constant, in a Hamiltonian model developed by Heisenberg. A lattice of +1 and -1 spins models this phenomenon in a paradigm whose two-dimensional version was solved by Lars Onsager, the Ising Model. Alnico alloys display high levels of this property, as they are composed of aluminum, nickel, cobalt, or iron. For 10 points, name this property which disappears at temperatures above the Curie Point in permanent magnets.
first_sentence: The thesis of Hendrika Johanna van Leeuwen rediscovered a proof that this cannot exist under classical physics.