Resonant Tunneling in Truly Axial SymmetryMn12Single-Molecule Magnets: Sharp Crossover between Thermally Assisted and Pure Quantum Tunneling

Abstract

Magnetization measurements of a truly axial symmetry Mn12-tBuAc molecular nanomagnet with a spin ground state of S = 10 show resonance tunneling. This compound has the same magnetic anisotropy as Mn12-Ac but the molecules are better isolated and the crystals have less disorder and a higher symmetry. Hysteresis loop measurements at several temperatures reveal a well-resolved step fine-structure which is due to level crossings of excited states. All step positions can be modeled by a simple spin Hamiltonian. The crossover between thermally assisted and pure quantum tunneling can be investigated with unprecedented detail.PACS numbers: 75.50. Xx, 75.60.Jk, 75.75.+a, 75.45.+j Mn 12 -Ac for short, was the first single-molecule magnet (SMM), exhibiting slow magnetization relaxation of its S = 10 spin ground state which is split by axial zero-field splitting [1]. It was also the first system to show thermally assisted tunneling of magnetization [2,3,4]. During the last several years, many more SMMs have been discovered and they are now among the most promising candidates for observing the limits between classical and quantum physics since they have a well defined structure, spin ground state and magnetic anisotropy [5,6,7,9].Nevertheless, Mn 12 -Ac is still the most widely studied SMM [10,11,12,13,14,15,16,17,18,19,20,21,22]. While a rough understanding of the quantum phenomena in Mn 12 -Ac was clear from the early studies, a detailed understanding has not yet emerged. For example, current theoretical models assume the presence of quadratic and quartic transverse crystal-field interactions in the spin Hamiltonian, where the former has been ascribed to solvent disorder [21]. However, these interactions, which contain only even powers of the raising and lowering operators, do not provide an explanation for the observation of odd tunneling steps in the hysteresis loops. It has been proposed that easy-axis tilting might give the missing odd transverse interactions [18]. Although such solvent disorder induced tilts exist, the tilt values are still unclear [24]. Hyperfine, dipolar, and Dzyaloshinsky-Moriya interactions were also proposed to be responsible for odd transverse terms [10,13].Other theoretical and experimental studies concern the crossover between thermally assisted and pure quantum tunneling [11,12,13,14,15,22,23]. The crossover can occur in a narrow temperature interval with the field at which the system crosses the anisotropy barrier shifting abruptly with temperature, or the crossover can occur in a broad interval of temperature with this field changing smoothly [15,22,23]. The first studies were published on Mn 12 -Ac [11,12,13] We show that this compound has the same magnetic anisotropy as Mn 12 Ac but the molecules are better isolated and the crystals contain less disorder and a higher symmetry. Hysteresis loop measurements at several temperatures reveal a fine structure of steps which is due to the dominating energy level crossings. All step positions can be modeled by a simple spin Hamiltonian. The cr...

Investigation of the stability of Mn12 single molecule magnets

“…In principle, Mn 12 SMMs could be used as basic units for data storage or quantum computing [5]. The magnetic properties of bulk Mn 12 SMMs have been extensively studied during the past decade [6,7]. However, experiments on the magnetic properties of individual Mn 12 molecules require the ability to deposit and address intact molecules on a surface.…”

Dynamics of periodic anticrossings: Decoherence, pointer states, and hysteresis curves

“…These special molecules contain transition metal atoms with strongly exchange-coupled spins, which causes them to behave as a single, large spin. Experiments on the magnetization dynamics of these molecular crystals have shown the presence of a series of steps in the magnetization curve at sufficiently low temperatures [48,49,50,51]. This behavior is a consequence of quantum mechanical tunneling of spin states through the anisotropy energy barrier and occurs when the external field brings two levels at different sides of the barrier into resonance via Zeeman interaction.…”

Ultra-low temperature structure determination of a Mn12 single-molecule magnet and the interplay between lattice solvent and structural disorder

“…It has been suggested that this permits a transverse anisotropy and thus, provides an explanation for the odd-to-odd QTM steps in the hysteresis loops. 18 However, more recently published measurements on a Mn 12 analogue (Mn 12-tBuAc) 19 with full crystallographic axial symmetry show that quantum tunnelling occurs at odd-to-odd steps, even though this should be forbidden. Similarly there is no easy explanation for the odd-to-even and even-to-odd transitions observed for 1.…”