Quantum Tunneling of Magnetization and Related Phenomena in Molecular Materials

Abstract

Molecules comprising a large number of coupled paramagnetic centers are attracting much interest because they may show properties which are intermediate between those of simple paramagnets and classical bulk magnets and provide unambiguous evidence of quantum size effects in magnets. To date, two cluster families, usually referred to as Mn12 and Fe8, have been used to test theories. However, it is reasonable to predict that other classes of molecules will be discovered which have similar or superior properties. To do this it is necessary that synthetic chemists have a good understanding of the correlation between the structure and properties of the molecules, for this it is necessary that concepts such as quantum tunneling, quantum coherence, quantum oscillations are understood. The goal of this article is to review the fundamental concepts needed to understand quantum size effects in molecular magnets and to critically report what has been done in the field to date.

Effects of quantum mechanics on the deflagration threshold in the molecular magnetMn12acetate

“…The magnetic properties of the Mn 12 -Ac single crystals were found to be similar to those previously published. 4,6,[14][15][16] We used a relatively large crystal of Mn 12 -Ac that was about 2 mm long and 0.5 mm wide. Smaller crystals did not show magnetic avalanches on sweeping the magnetic field due to rapid escape of heat through the surface ͑i.e., the magnetization relaxes slowly with increasing magnetic field before the avalanche condition is achieved͒.…”

From single-molecule magnetism to long-range ferromagnetism inHpyr[Fe17O16(OH)Vecchini1, Ryan2, Cranswick3 et al. 2008Phys. Rev. B151100View full textAdd to dashboardCite

“…[1][2][3] A uniaxial molecular anisotropy due to the crystal field generated by the surrounding organic ligands may also be present and lead to a spin ground state that is parallel to the anisotropy axis. Molecular magnets are in many ways superior to similar sized magnetic particles as, by their nature, they exhibit ideal monodispersity in size, volume, shape, and charge.…”

Nonlinear response of single-molecule magnets: Field-tuned quantum-to-classical crossovers

“…2͑b͔͒ the second and third harmonic components measured at /2 = 2 kHz. ͑The high noise-to-signal ratio prevented us from obtaining reliable 3 data for H z Ͼ 1 kOe.͒ The equilibrium nonlinear susceptibilities, also shown, were obtained by differentiating 1 eq , measured at the lowest frequency /2 = 1 Hz.…”

“…Actually, many experiments have shown that tunneling takes place at those fields where states of opposite orientation are degenerate, H n Ӎ nH 1 ͑n =0,1,2,... with H 1 =2g B D Ӎ 4200 Oe in Mn 12 ͒, whereas it is suppressed for intermediate fields. [6][7][8] Such a resonant tunneling enables the spins to approach faster their equilibrium state, giving rise to steps in the hysteresis loops around H z = H n , 8,9 and to maxima in the linear dynamical susceptibility 1 . In our previous work, 10,11 we found that resonant tunneling at H z = 0 induces an extra contribution to the nonlinear response 3 , making it larger ͑in magnitude͒ than the equilibrium one and having peaks reversed with respect to the classical predictions.…”