Helium‐three magnetic behavior becomes more complicated

Evidence is mounting that the correct model of solid helium‐three cannot be the simple one that long seemed adequate. The ${\mathrm{He}}^3$ nucleus, consisting of two protons and one neutron, is a fermion of spin 1/2. Because of the large probability of individual ${\mathrm{He}}^3$ atoms exchanging places, the solid acts as a kind of atomic magnet. Until only a few years ago theorists were able to treat solid ${\mathrm{He}}^3$ in terms of an extremely simple magnetic model—a Heisenberg Hamiltonian that included only interactions between nearest‐neighbor atoms, neglecting higher‐order interactions and coupling with phonons—and no experimental data contradicted them. In 1971, however, some studies of magnetic pressure versus temperature at high magnetic fields could not be fitted into the theory, and efforts were made to revise the theory by adding higher‐order interactions. And now, two groups of experiments, one at Cornell University (William Halperin, Charles Archie, Finn Rasmussen, Robert Buhrman and Robert Richardson) and the other at the University of California, La Jolla (Jeffrey Dundon and John Goodkind) indicate that the magnetic behavior of solid ${\mathrm{He}}^3$ is much more complicated than the ${\mathrm{He}}^3$ theorists had presumed. The Cornell adiabatic‐solidification studies suggest that the magnetic ordering transition in the solid occurs at about half the predicted temperature and that it is a much sharper transition than expected. The La Jolla group sees unexpected structure in the variation of heat capacity with temperature and also infers that the transition temperature $T_{\mathrm{s}}$ is below the predicted value.

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