Anisotropy barrier reduction in fast-relaxing Mn_{12} single-molecule magnets (original) (raw)
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Solid State Communications, 2006
We show that X-band electron paramagnetic resonance (EPR) measurements using a dual-mode resonance cavity can directly probe the levels near the top of the magnetization reversal barrier in the single-molecule magnet (SMM) Mn 12 -acetate. The observed transitions are much sharper than those reported in high-field EPR studies. The observed temperature dependence of the line positions points to the presence of a spindiffusional mode. The correlation time for such fluctuations is of the order of 6!10 K8 s at 10 K, and follows an Arrhenius activation energy of 35-40 K. These results open a new avenue for understanding the mechanism of tunneling and spin-lattice relaxations in these SMMs. q
Magnetization tunneling in Mn12 and Mn4 single-molecule magnets
Journal of Applied Physics
The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecule magnets ͑SMMs͒. SMMs are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of ͓Mn 12 O 12 (O 2 CC 6 H 4 -p-Me) 16 (H 2 O) 4 ͔•solvate. The two Mn 12 complexes are isomers, differing both in the positioning of the H 2 O and carboxylate ligands and also in the orientations of the Jahn-Teller elongation at the Mn III ions. The magnetization vs magnetic field hysteresis loops are quite different for the two isomeric Mn 12 complexes. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate vs temperature responses of two mixed-valence Mn 4 complexes. In both cases the Arrhenius plot of the logarithm of the magnetization relaxation rate vs the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region.
Particle-size dependence of magnetization relaxation in Mn12 crystals
Physical Review B, 2009
We show that reducing the crystal size of ͓Mn 12 O 12 ͑O 2 C 6 H 5 ͒ 16 ͑H 2 O͒ 4 ͔ single-molecule magnets from 11.5 to 0.4 m strongly affects the molecular magnetic anisotropy and magnetic-relaxation rates. The effective activation energy for the spin reversal of the standard clusters decreases by 13% with decreasing size, whereas it remains approximately constant for the "fast relaxing" species. The pre-exponential factor 0 increases with decreasing crystal size for both. The observed decrease in the effective energy barrier for the slow relaxing species seems to be associated with the existence of a distribution of second-order transverse anisotropy terms, centered on E = 0, which broadens as the crystal size decreases. By contrast, the expected changes in the axial anisotropy parameter D with decreasing crystal size are too small to account for the change in U. The different effects that the reduction in crystal size has on the fast and slow relaxing species are discussed.
Origin of Second-Order Transverse Magnetic Anisotropy in Mn12-Acetate
Physical Review Letters, 2002
The symmetry breaking effects for quantum tunneling of the magnetization in Mn12-acetate, a molecular nanomagnet, represent an open problem. We present structural evidence that the disorder of the acetic acid of crystallization induces sizable distortion of the Mn(III) sites, giving rise to six different isomers. Four isomers have symmetry lower than tetragonal and a nonzero second-order transverse magnetic anisotropy, which has been evaluated using a ligand field approach. The result of the calculation leads to an improved simulation of electron paramagnetic resonance spectra and justifies the tunnel splitting distribution derived from the field sweep rate dependence of the hysteresis loops.
Magnetization tunneling in Mn[sub 12] and Mn[sub 4] single-molecule magnets
Journal of Applied Physics, 2002
The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecule magnets ͑SMMs͒. SMMs are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of ͓Mn 12 O 12 (O 2 CC 6 H 4-p-Me) 16 (H 2 O) 4 ͔•solvate. The two Mn 12 complexes are isomers, differing both in the positioning of the H 2 O and carboxylate ligands and also in the orientations of the Jahn-Teller elongation at the Mn III ions. The magnetization vs magnetic field hysteresis loops are quite different for the two isomeric Mn 12 complexes. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate vs temperature responses of two mixed-valence Mn 4 complexes. In both cases the Arrhenius plot of the logarithm of the magnetization relaxation rate vs the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region.
Physical Review Letters, 2006
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.
A spectroscopic comparison between several high-symmetry S=10 Mn12 single-molecule magnets
Journal of Applied Physics, 2005
We report angle-dependent high-field electron-paramagnetic-resonance data collected for single-crystal samples of Mn12–Ac. The spectra reveal fine structures associated with various Mn12 species corresponding to different disordered local environments. Each of the fine structures exhibits a distinct dependence on the field orientation, thereby highlighting the discrete nature of the disorder. We compare these data with the spectra obtained for two recently discovered analogs of Mn12–Ac, differing only in their ligand and solvent molecules. None of the fine structures seen for Mn12–Ac are found for the recently discovered Mn12 complexes, thus confirming that the solvent significantly influences the magnetization dynamics in Mn12–Ac.
Spin relaxation in Mn-12-acetate
1999
We present a comprehensive theory of the magnetization relaxation in a Mn12acetate crystal based on thermally assisted spin tunneling induced by quartic anisotropy and weak transverse magnetic fields. The overall relaxation rate as function of the magnetic field is calculated and shown to agree well with data including all resonance peaks. The Lorentzian shape of the resonances is also in good agreement with recent data. A generalized master equation including resonances is derived and solved exactly. It is shown that many transition paths with comparable weight exist that contribute to the relaxation process. Previously unknown spin-phonon coupling constants are calculated explicitly.
Inorganic Chemistry, 2012
The synthesis and properties are reported of a rare example of a Mn 12 single-molecule magnet (SMM) in truly axial symmetry (tetragonal, I4̅). [Mn 12 O 12 (O 2 CCH 2 Bu t) 16 (MeOH) 4 ]•MeOH (3•MeOH) was synthesized by carboxylate substitution on [Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 ]•2MeCO 2 H•4H 2 O (1). The complex was found to possess an S = 10 ground state, as is typical for the Mn 12 family, and displayed both frequency-dependent out-of-phase AC susceptibility signals and hysteresis loops in single-crystal magnetization vs DC field sweeps. The loops also exhibited quantum tunneling of magnetization steps at periodic field values. Single-crystal, high-frequency electron paramagnetic resonance spectra on 3•MeOH using frequencies up to 360 GHz revealed perceptibly sharper signals than for 1. Moreover, careful studies as a function of the magnetic field orientation did not reveal any satellite peaks, as observed for 1, suggesting that the crystals of 3 are homogeneous and do not contain multiple Mn 12 environments. In the single-crystal 55 Mn NMR spectrum in zero applied field, three wellresolved peaks were observed, which yielded hyperfine and quadrupole splitting at three distinct sites. However, observation of a slight asymmetry in the Mn 4+ peak was detectable, suggesting a possible decrease in the local symmetry of the Mn 4+ site. Spin− lattice (T 1) relaxation studies were performed on single crystals of 3•MeOH down to 400 mK in an effort to approach the quantum tunneling regime, and fitting of the data using multiple functions was employed. The present work and other recent studies continue to emphasize that the new generation of truly high-symmetry Mn 12 complexes are better models for thorough investigation of the physical properties of SMMs than their predecessors such as 1.
Mn12-acetate: a prototypical single molecule magnet
Solid State Communications, 2003
Single molecule magnets display fascinating quantum mechanical behavior, and may have important technological applications for information storage and quantum computation. A brief review is given of the physical properties of Mn 12acetate, one of the two prototypical molecular nanomagnets that have been most intensively investigated. Descriptions and discussions are given of the Mn 12 magnetic cluster and the fundamental process of quantum tunneling of a nanoscopic spin magnetization; the distinction between thermally-assisted tunneling and pure quantum tunneling, and a study of the crossover between the two regimes; and a review of earlier investigations that suggest that the tunneling in this system is due to locally varying second-order crystal anisotropy which gives rise to a distribution of tunnel splittings. In the second part of the paper, we report results obtained by a new experimental method that confirm our earlier conclusion that the tunnel splittings in Mn 12 are distributed rather than single-valued, as had been generally assumed. q