THE SURVIVAL OF ERNST ISING AND THE STRUGGLE TO SOLVE HIS MODEL
Reinhard Folk (Personal webpage )
Institute for Theoretical Physics, University Linz, AustriaThe live of Ernst Ising and the steps to solving the model named after him are reported in parallel . Wilhelm Lenz suggested his student Ernst Ising to explain the existence of ferromagnetism on the basis of his publication in 1920. The result, published in 1925 was disappointing, especially for Lenz as reflected in his approval of the thesis. It was unknown that the model combines extraordinary simplicity with considerable complexity in the final output including concepts of great generality.
Wolfgang Pauli who was at the same time assistant of Lenz in Hamburg published in the same year his ‘nichtklassische Zweideutigkeit ...’, later identified as the spin of the electron, and the exclusion principle. He was the first - at the Solvay Conference in 1930 - to present the Hamiltonian of the Ising model, as he called it, in the form we know it today and reignited belief that a ferromagnetic phase transition might be possible in this model. Meanwhile Ising had left university research and due to the political situation in 1938 had to leave Germany and fled to Luxemburg. This went in hand with damaging the network of researchers dealing with the problem of ferromagnetism and more generally with phase transitions and statistical physics. Such a geneological network has been identified by Elliott Montroll as the Vienna School of Statistical Thought  connecting several generations of scientists.
In 1944 Lars Onsager presented a solution of the two-dimensional case, which led to a first step to prove the importance of the model for understanding critical phenomena and the liberation of Luxemburg by the American troops rescued finally Ising’s family. In 1952 Chen-Ning Yang solved the problem of Ising’s thesis in two dimensions; one year later Ising became US citizen. The following development showed, that the model turned out to be a highway to modern physics concepts applicable also in other fields, although the final exact solution in three dimensions has not yet been reached.
 T. Ising, R. Folk, R. Kenna, B. Berche, Yu. Holovatch, The fate of Ernst Ising and the fate of his model (and reviews refered therein), Journal of Physical Studies 21(3), 3002 (2017) . arXiv:physics.hist-ph/1706.01764.
 Elliot W. Montroll, On the Vienna School of statistical thought, AIP Conference Proceedings 109, 1 (1984).