A Caveat for Single-Molecule Magnetism: Non-linear Arrhenius Plots (original) (raw)
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Anisotropy barrier reduction in fast-relaxing Mn_{12} single-molecule magnets
Physical Review B, 2009
A novel angle-swept high-frequency electron paramagnetic resonance (HFEPR) technique is described that facilitates efficient in-situ alignment of single-crystal samples containing lowsymmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based technique involves recording HFEPR spectra at fixed frequency and field, while sweeping the applied field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the extensively studied spin S = 10 Mn 12 SMMs (e.g. Mn 12-acetate). The lowsymmetry complex also exhibits SMM behavior, but with a significantly reduced effective barrier to magnetization reversal (U eff 43 K) and, hence, faster relaxation at low temperature in comparison with the higher-symmetry species. Mn 12 complexes that crystallize in lower symmetry structures exhibit a tendency for one or more of the Jahn-Teller axes associated with the Mn III atoms to be abnormally oriented, which is believed to be the cause of the faster relaxation. An extensive multi-high-frequency angle-and field-swept electron paramagnetic resonance study of [Mn 12 O 12 (O 2 CCH 2 Bu t) 16 (H 2 O) 4 ]•-CH 2 Cl 2 •MeNO 2 is presented in order to examine the influence of the abnormally oriented Jahn-Teller axis on the effective barrier to magnetization reversal. The reduction in the axial anisotropy, D, is found to be insufficient to account for the nearly 40% reduction in U eff. However, the reduced symmetry of the Mn 12 core gives rise to a very significant 2 nd order transverse (rhombic) zero-field-splitting anisotropy, E D/6. This, in turn, causes a significant mixing of spin projection states well below the top of the classical anisotropy barrier. Thus, magnetic quantum tunneling is the dominant factor contributing to the effective barrier reduction in fast relaxing Mn 12 SMMs.
Spin-spin cross relaxation in single-molecule magnets
Physical Review Letters, 2002
The one-body tunnel picture of single-molecule magnets (SMMs) is not always sufficient to explain the measured tunnel transitions. An improvement to the picture is proposed by including also two-body tunnel transitions such as spin-spin cross relaxation (SSCR) ...
Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets
Nature, 2010
A fundamental step towards atomic-or molecular-scale spintronic devices has recently been made by demonstrating that the spin of an individual atom deposited on a surface 1 , or of a small paramagnetic molecule embedded in a nanojunction 2 , can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets 3-6 (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface 7 was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices 8,9 . Here we show that a preferential orientation of Fe 4 complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs-their stepped hysteresis loop, which results from resonant quantum tunnelling of the magnetization 5,6 -can be clearly detected using synchrotron-based spectroscopic techniques. With the aid of multiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.
Magnetization tunneling in single-molecule magnets
Polyhedron, 2001
The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecules magnets (SMM's). SMM's 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 that both differ 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 versus magnetic field hysteresis loop is quite different for the two isomeric Mn 12 complexes. One Mn 12 complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn 12 isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn 4 complexes. In all three cases, the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. The temperature-independent magnetization rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.
Crystal lattice desolvation effects on the magnetic quantum tunneling of single-molecule magnets
Physical Review B, 2009
High-frequency electron paramagnetic resonance ͑HFEPR͒ and alternating current ͑ac͒ susceptibility measurements are reported for a new high-symmetry Mn 12 complex, ͓Mn 12 O 12 ͑O 2 CCH 3 ͒ 16 ͑CH 3 OH͒ 4 ͔ •CH 3 OH. The results are compared to those of other high-symmetry spin S =10 Mn 12 single-molecule magnets ͑SMMs͒, including the original acetate, ͓Mn 12 ͑O 2 CCH 3 ͒ 16 ͑H 2 O͒ 4 ͔ •2CH 3 CO 2 H•4H 2 O, and the ͓Mn 12 O 12 ͑O 2 CCH 2 Br͒ 16 ͑H 2 O͒ 4 ͔ •4CH 2 Cl 2 and ͓Mn 12 O 12 ͑O 2 CCH 2 Bu t ͒ 16 ͑CH 3 OH͒ 4 ͔ •CH 3 OH complexes. These comparisons reveal important insights into the factors that influence the values of the effective barrier to magnetization reversal, U ef f , deduced on the basis of ac susceptibility measurements. In particular, we find that variations in U ef f can be correlated with the degree of disorder in a crystal which can be controlled by desolvating ͑drying͒ samples. This highlights the importance of careful sample handling when making measurements on SMM crystals containing volatile lattice solvents. The HFEPR data additionally provide spectroscopic evidence suggesting that the relatively weak disorder induced by desolvation influences the quantum tunneling interactions and that it is under-barrier tunneling that is responsible for a consistent reduction in U ef f that is found upon drying samples. Meanwhile, the axial anisotropy deduced from HFEPR is found to be virtually identical for all four Mn 12 complexes, with no measurable reduction upon desolvation.
Origin of the hysteresis of magnetoconductance in a supramolecular spin-valve based on a<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:mrowmml:miTbmml:msubmml:miPcmml:mn2single-molecule magnet
Physical review, 2020
We present a time-dependent microscopic model for Coulomb blockade transport through an experimentally realized supramolecular spin-valve device driven by an oscillating magnetic field, in which the 4f-electron magnetic states of an array of TbPc 2 single-molecule magnets (SMMs) were observed to modulate a sequential tunneling current through an underlying substrate nanoconstriction. Our model elucidates the dynamical mechanism at the origin of the observed hysteresis loops of the magnetoconductance, a signature of the SMMmodulated spin-valve effect, in terms of a phonon-assisted multi-spin-reversal cascade relaxation process, which mediates the switching of the device between the two conductive all-parallel spin configurations of the SMM array. Moreover, our proposed model can explain the zero-bias giant magnetoresistive transport gap measured in this device, solely within the incoherent transport regime, consistently with the experimental observations, as opposed to previous interpretations invoking Fano-resonance conductance suppression within a coherent ballistic transport regime. Finally, according to the proposed Coulomb blockade scenario, the SMM-mediated giant magnetoresistance effect is predicted to increase with the number of SMMs aligned on the nanoconstriction surface, on account of the increased number of intermediate nonconducting spin-flip states intervening in the phonon-assisted multi-spin-reversal cascade relaxation process necessary to switch between the two conducting all-parallel SMM spin configurations.
Defects, Tunneling, and EPR Spectra of Single-Molecule Magnets
MRS Proceedings, 2002
We examine theoretically electron paramagnetic resonance (EPR) lineshapes as functions of resonance frequency, energy level, and temperature for single crystals of three different kinds of single-molecule nanomagnets (SMMs): Mn 12 acetate, Fe 8 Br, and the S = 9/2 Mn 4 compound. We use a density-matrix equation and consider distributions in the uniaxial (second-order) anisotropy parameter D and the g factor, caused by possible defects in the samples. Additionally, weak intermolecular exchange and electronic dipole interactions are included in a mean-field approximation. Our calculated linewidths are in good agreement with experiments. We find that the distribution in D is common to the three examined single-molecule magnets. This could provide a basis for a proposed tunneling mechanism due to lattice defects or imperfections. We also find that weak intermolecular exchange and dipolar interactions are mainly responsible for the temperature dependence of the lineshapes for all three SMMs, and that the intermolecular exchange interaction is more significant for Mn 4 than for the other two SMMs. This finding is consistent with earlier experiments and suggests the role of spin-spin relaxation processes in the mechanism of magnetization tunneling.
Magnetic Quantum Tunneling in the Single-Molecule Magnet Mn12-Acetate
Journal of Low Temperature Physics, 2005
The symmetry of magnetic quantum tunneling (MQT) in the single molecule magnet Mn12-acetate has been determined by sensitive low-temperature magnetic measurements in the pure quantum tunneling regime and high frequency EPR spectroscopy in the presence of large transverse magnetic fields. The combined data set definitely establishes the transverse anisotropy terms responsible for the low temperature quantum dynamics. MQT is due to a disorder induced locally varying quadratic transverse anisotropy associated with rhombic distortions in the molecular environment (2 nd order in the spin-operators). This is superimposed on a 4 th order transverse magnetic anisotropy consistent with the global (average) S4 molecule site symmetry. These forms of the transverse anisotropy are incommensurate, leading to a complex interplay between local and global symmetries, the consequences of which are analyzed in detail. The resulting model explains: (1) the observation of a twofold symmetry of MQT as a function of the angle of the transverse magnetic field when a subset of molecules in a single crystal are studied; (2) the non-monotonic dependence of the tunneling probability on the magnitude of the transverse magnetic field, which is ascribed to an interference (Berry phase) effect; and (3) the angular dependence of EPR absorption peaks, including the fine structure in the peaks, among many other phenomena. This work also establishes the magnitude of the 2 nd and 4 th order transverse anisotropy terms for Mn12-acetate single crystals and the angle between the hard magnetic anisotropy axes of these terms. EPR as a function of the angle of the field with respect to the easy axis (close to the hard-medium plane) confirms that there are discrete tilts of the molecular magnetic easy axis from the global (average) easy axis of a crystal, also associated with solvent disorder. The latter observation provides a very plausible explanation for the lack of MQT selection rules, which has been a puzzle for many years.
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