Quantum tunneling in ferromagnetic nanoparticles interacting with a spin thermostat: Effective Hamiltonian

An effective Hamiltonian is obtained which describes quantum tunneling in ferromagnetic nanoparticles in the presence of a hyperfine interaction of the electron spins in the nanoparticle with the microscopic spins in the environment (such as paramagnetic impurities or nuclear spins). A criterion of transition between ferromagnetic and antiferromagnetic tunneling regimes is found. The validity of the equations obtained is checked by the method of exact diagonalization. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 27–32 (10 January 1998)