Incommensurate transverse anisotropy induced by disorder and spin-orbit-vibron coupling in Mn12 acetate

Abstract

It has been shown within density-functional theory that in Mn12-acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field.PACS numbers: 75.50. Xx, 71.15.Mb, 75.30.Gw, 75.30.Et The observation of resonant tunneling of magnetization in the single molecule magnet Mn 12 -acetate 1 (hereafter Mn 12 ) with a ground-state spin of S = 10 has led to many experimental and theoretical investigations. 2,3,4,5,6 A simple anisotropy Hamiltonian for S = 10 provides an excellent approximation to the physics occurring at low temperatures. Deriving the Hamiltonian from density-functional (DF) calculations is challenging but possible. 4,5,6 For a particular molecular geometry, a magnetic anisotropy tensor may be calculated considering spin-orbit coupling within a DF framework. In the principal-axes coordinates, the lowest-order spin Hamiltonian can be simplified to the following form:where D and E are the uniaxial and second-order transverse anisotropy parameters and S z is the easy-axis component of the spin operator S. The S 4 symmetry of the ideal Mn 12 molecule causes the value of E to vanish and the lowest-order transverse terms to be fourth order. Only transverse anisotropy terms are responsible for the resonant tunneling between energy levels that are almost degenerate. Magnetic tunneling measurements, however, showed that some of the resonant tunneling occurred at a level lower than fourth order. For the Mn 12 , the calculated D value is -0.556 K, 4 which agrees well with experiment. The calculated (measured) value of D, however, does not account for all the measured anisotropy barrier of 65 K. 3,8 One needs a longitudinal anisotropy of order higher than second-order. In this spirit a fourth-order spin-orbit-vibron (SOV) interaction was proposed by Pederson et al. 11 Ideally, this SOV interaction can only contribute to modifying the second-order barrier and to activating two fourth-order transverse terms in the spin Hamiltonian. At this level a strong SOV interaction can complicate the tunneling experiments in several different ways. The simplest complication is that there are two different longitudinal 4th-order terms which scale as S 2 S 2 z and S 4 z respectively. If the S 4 z term is dominant, then at any resonant field only a single pair of states are involved with tunneling. The fourth-order transverse terms result in departures of the period of the tunnel-splitting oscillation from ∆H x = 2 2E(|D| + E)/gµ B that has been derived by Garg. 12 Such departures were first identified by Wernsdorfer et al. 13 and later modified to include the 4th-order transverse...

Particle-size dependence of magnetization relaxation inMn12crystals

“…However, the Mn 12 complex used in our experiments does not have any interstitial solvent molecules in the structure. 24 Therefore, we should look for other sources of disorder, such as long-range strains, caused by, e.g., lattice dislocations or other defects, appearing at the early stages of the crystallization process. Their density, as suggested by our x-ray diffraction data, is probably higher for crystals with smaller sizes.…”

Magnetic Signatures of Hydroxyl‐ and Water‐Terminated Neutral and Tetra‐Anionic Mn12‐Acetate

“…Computational methodologies are capable of predicting magnetic spectroscopies using wavefunction‐based methods and density‐functional methods as a function of charge, geometrical structure, and chemical structure . When such calculations are compared with measurements on systems undergoing chemical or structural change, they offer the possibility for learning about mechanistic details associated with such changes.…”

“…When such calculations are compared with measurements on systems undergoing chemical or structural change, they offer the possibility for learning about mechanistic details associated with such changes. Past experiments and calculations provide many examples of how magnetic structure is impacted by charge, chemical, and solvent rearrangements . However, much of the past research has concentrated on how such changes would impact the performance of a magnetic molecule for miniaturized device or information applications.…”

Misalignment of transverse anisotropies as a mechanism for suppression of Kondo effect in large spin single molecule magnets

“…17 demonstrated a misaligned angle of as large as 30°between the hard axes, respectively, associated with the twofold and fourfold anisotropies. In the same system, Park et al 18 worked out a misaligned angle of 24°from the densityfunctional theory. This is acceptable because two types of anisotropies have different physics origins.…”

“…This spin Hamiltonian can be applied to explain many of experimental features 17 and to predict that the oscillation of the tunnel splitting will be entirely quenched. 18 However, so far there are yet no literatures devoted to the Kondo effect subject to the misalignment. This context gives us a great motivation to repredict the Kondo effect in the SMMs system beyond the previous considerations 10-14 based only on the commensurate axes.…”