Theory of magnetic structure determination for small ferromagnets

An easy plane ferromagnetic nanoparticle with a magnetic field along the easy axis in the easy plane, is considered. When the applied field is reversed the system finds itself in a metastable ground state of a double well potential. Tunneling is between this state and the various states on the other, stable, side of the double well. Unlike the much studied case of Mn₁₂, the states into which the system tunnels have energies determined by harmonic oscillations about equilibrium. Specific expressions are obtained for the tunneling matrix elements examined between the various levels and for relaxation toward the ground state. The formalism involves auxiliary particle techniques and the results are contrasted with the existing theory based upon the functional integral approach.