Quantum tunneling and relaxation inMn12-acetate studied by magnetic spectroscopy

Abstract

The spin relaxation in the molecular magnet Mn12ac is investigated by a novel type of highfrequency magnetic spectroscopy. By reversing the external magnetic field the intensities of the | ± 10 → | ± 9 transitions within the ground multiplet redistribute; below T ≈ 2 K resonant quantum tunneling is directly observed as magnetic hole burning in the absorption spectra of a polycrystal. We quantitatively describe our observations by taking into account inhomogeneous line broadening, the time dependence of the level population during the relaxation process, and the resonance behavior of the relaxation rate due to quantum tunneling near the avoided level crossing.PACS numbers: 75.50.Xx, 78.20.Ls, 75.45.+j, In the last years molecular magnets comprising large clusters of coupled magnetic ions have attracted considerable interest as mesoscopic systems which exhibit new properties such as molecular magnetic bistability, macroscopic quantum tunneling of magnetization, quantum phase interference, etc. [1][2][3][4]. Among them clusters with a high-spin ground state and large anisotropy, such as Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]2CH 3 COOH·4H 2 O (abbreviated as Mn 12 ac) are of a paramount interest.Mn 12 ac cluster can be considered as nanoparticle with an effective magnetic moment of 20µ B (µ B is the Bohr magneton), corresponding to a collective spin S = 10 of the exchange coupled twelve Mn ions within the molecule. Due to the strong anisotropy, the magnetic moment freezes along one of the two easy directions at low temperatures. This is governed by a crystal field (CF) which splits the ground S = 10 multiplet and provides a quasidoublet structure | ± m of the lowest energy levels. The energy barrier ∆E ≈ 60 K between the two lowest lying states | ± 10 determines the thermally activated relaxation of the magnetization in Mn 12 ac [3,4]. Below a blocking temperature of about 3 K resonant quantum tunneling was discovered in the form of peaks in the relaxation rate and steps in the hysteresis loops at regular intervals of magnetic field corresponding to a coincidence of the CF energy levels [1,2]. Since the rate of the tunneling for a ground state is small in Mn 12 ac the tunneling occurs via appropriate thermally excited CF states for temperatures above approximately 2 K resulting in a shortcut at the top of the barrier which accelerates the relaxation; at even lower temperatures pure quantum tunneling is dominant [3,4].Various spectroscopic investigations have been applied to Mn 12 ac, including electron-spin resonance (ESR) [5,6] and inelastic neutron scattering [7]. In particular, a new type of high-frequency ESR method based on a quasioptical spectroscopy [8] made it possible to directly observe the CF transitions in an equilibrium state and to study in details their characteristics, including the line-shapes [9]. In this Letter we report on the high-frequency magnetic spectroscopy of non-equilibrium phenomena in Mn 12 ac which allows to directly and independently determine the full set of magnetic characteristics.Mn ...

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

“…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.…”

“…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.…”

Quantum dynamics of crystals of molecular nanomagnets inside a resonant cavity

“…This picture, first established by magnetic measurements, 2 has been confirmed by the careful EPR study of spin levels in Mn 12 -acetate. [9][10][11][12][13][14][15] The EPR experiments have demonstrated noticeable resonant absorption of electromagnetic radiation by molecular magnets. Most recently, it has been suggested that a crystal of magnetic molecules can also be a powerful source of coherent microwave radiation.…”

Low-temperature microwave emission from molecular clusters

“…Very recently a new field combining molecular magnets, magnetization studies, cavities and electromagnetic radiation has attracted the attention of different groups. The most recent works on this particular topic deal with the absorption and emission of microwaves [4,5,6,7, 8,9,10,11,12,13,14,15]. In this letter we report low temperature experimental studies of the detection of microwave emission of Mn 12 single crystals inside a cylindrical waveguide.…”