Evidence for ferromagnetic ordering of He-3 films on graphite

摘要

Recent experiments at USC have shown that two-dimensional films of He-3 on graphite order ferromagnetically at finite temperatures (T > 0) for densities above 20 atomsm(2). These results, obtained by measuring NMR on these films in the zero field limit, appear to contradict several studies which concluded that there is no spontaneous order by measuring magnetic susceptibility through conventional NMR in finite magnetic fields, and heat capacity in zero field. The low field limit is important in understanding these two-dimensional magnetic systems. At higher temperatures and/ or fields, these films have been described in terms of the multiple spin exchange model. However, two-dimensional systems are extremely sensitive to anisotropies such as nuclear dipole interactions, even when they are several orders of magnitude less than the dominant exchange mechanism. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 18]