title: A two rotor model with spin for magnetic nanoparticles.
abstract: We argue that a kind of magnetic nanoparticle might exist characterized by the locking of the constituent spins with the density profile of the macrospin. We represent such a nanoparticle by two interacting rigid rotors, one of which has a large spin attached to the body, namely a two rotor model with spin. By this model we can describe in a unified way the cases of nanoparticles free and stuck in an elastic or a rigid matrix. We evaluate the magnetic susceptibility for the latter case and under some realistic assumptions we get results in closed form. A crossover between thermal and purely quantum hopping occurs at a temperature much higher than that at which tunneling becomes important. Agreement with some experimental data is remarkable.
authors.affiliation: Scienze e Tecnologie, Universita' di Camerino, Via Madonna delle Carceri 9, 62032 Camerino, Italy.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Hatada
authors.forename: Kuniko
authors.initials: K
authors.lastname: Hayakawa
authors.forename: Augusto
authors.initials: A
authors.lastname: Marcelli
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Palumbo