Single molecule magnets (SMMs), made of a high-spin magnetic core surrounded by an organic ligand shell, represent an attractive workbench for the investigation of quantum magnetism. In the last few years, SMMs have also emerged as candidates for the hardware of quantum computers. Although most recent studies have focused on clusters with a polynuclear magnetic core, mononuclear SMMs (or single-ion magnets) are simpler and better suited to quantitatively test theories for magnetic quantum tunneling and quantum coherence. In this talk, we report the results of recent studies performed on inorganic molecules LnW10 and LnW30, which consist of individual lanthanide ions encapsulated by different polyoxometalate (POM) moieties. These molecules show the slow magnetic relaxation and magnetic memory that are characteristic of the SMM behavior. At very low temperatures (typically T < 1 K), the spin-lattice relaxation becomes dominated by pure quantum tunneling events, with rates that agree quantitatively with those predicted by the Prokof'ev and Stamp model [Phys. Rev. Lett. 80, 5794 (1998)]. Experiments show also that the magnetic anisotropy, as well as the rates of magnetic relaxation and quantum tunneling, are determined by the local coordination of the lanthanide ion. The molecular structure can therefore be used as a parameter to control the underlying physics. The ability to tune the magnetic properties by chemical design provides fascinating perspectives for studies of fundamental phenomena, such as the quantum spin-lattice relaxation of paramagnets and for enhancing quantum coherence. In TbW30, we find a tunnel splitting ¿/kB = 1.5 K, thus larger than the energies of all perturbations, such as dipolar and hyperfine interactions, which induce decoherence. In this regime of strong coherence, zero-point fluctuations suppress magnetic order down to T = 0, thereby inducing a quantum paramagnetic state even under zero applied field. Because of these special characteristics, some of these single-ion magnets appear also as very promising candidates to perform as spin qubits or even as magnetic coolers for the region of very low temperatures.

Resumen del trabajo presentado a la "12th Joint MMM/Intermag Conference" celebrada en Chicago (US) del 14 al 18 de enero de 2013.

Peer Reviewed