Origin of Second-Order Transverse Magnetic Anisotropy in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow><mml:mi mathvariant="normal">M</mml:mi><mml:mi mathvariant="normal">n</mml:mi></mml:mrow><mml:mn>12</mml:mn></mml:msub></mml:math>-Acetate

Cornia, Andrea; Sessoli, Roberta; Sorace, Lorenzo; Gatteschi, Dante; Barra, Anne‐Laure; Daiguebonne, Carole

doi:10.1103/physrevlett.89.257201

Cited by 162 publications

(232 citation statements)

References 23 publications

(30 reference statements)

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“…[20]). Furthermore, we have seen these extra peaks in all of the samples that we have studied, as have Barra et al [10] and Cornia et al [23]. There do, however, appear to be noticeable differences in the positions and intensities of anomalous EPR peaks obtained by different groups.…”

Section: Introductionsupporting

confidence: 70%

“…One particular proposal considers long-range strains caused by dislocations [19,20]. However, a more likely scenario involves acetate ligand disorder which would give rise to distinct Mn 12 Á/Ac isomers with local site symmetry lower than S 4 [7,12,17,23]. Indeed, recent experiments clearly show that a narrow distribution of tunnel splittings exists [7], a fact which is consistent with the ligand disorder scenario.…”

Section: Introductionmentioning

confidence: 83%

“…Thus, it is unlikely that one would observe distinct splittings in such a case, but rather a broadening of the resonance linewidths, with an EPR lineshape that reflects the distribution. Cornia et al [23] have also recently published data which show a splitting of a-resonances (b-resonances are also seen in their 95 GHz spectra). However, this splitting is more subtle, since it is only clearly observed using field modulation, or by taking derivatives of absorption spectra [17].…”

Section: Introductionmentioning

confidence: 85%

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On the origin of anomalous EPR peaks observed in Mn 12 –Ac

Hill¹,

Edwards²,

North³

et al. 2003

Polyhedron

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We present a detailed investigation of the temperature and frequency dependence of the anomalous electron paramagnetic resonance (EPR) transitions first observed in Mn 12 Á/Ac by Hill et al. [Phys. Rev. Lett. 80 (1998) 2453. The most dominant of these transitions manifest themselves as an extra series of EPR absorption peaks for spectra obtained with the DC field applied within the hard magnetic plane. Recent studies by Amigó et al. [Phys. Rev. B 65 (2002) 172403] have attributed these additional peaks to a strain-induced transverse quadratic anisotropy which gives rise to distinct Mn 12 Á/Ac site symmetries, each having a distinct EPR spectrum; on the basis of these measurements, it has been suggested that this transverse anisotropy is responsible for the tunneling in Mn 12 Á/Ac. Our temperature-and frequency-dependent measurements demonstrate unambiguously that these anomalous EPR absorptions vanish as the temperature tends to zero, thereby indicating that they correspond to transitions from an excited state of the molecule. We argue that this low-lying excited state corresponds to an S0/9 multiplet having very similar zero-field crystal parameters to the S0/10 state. These findings compare favorably with available neutron scattering data. #

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Section: Introductionsupporting

confidence: 70%

Section: Introductionmentioning

confidence: 83%

Section: Introductionmentioning

confidence: 85%

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On the origin of anomalous EPR peaks observed in Mn 12 –Ac

Hill¹,

Edwards²,

North³

et al. 2003

Polyhedron

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“…Magnetic tunneling measurements, however, showed that some of the resonant tunneling occurred at a level lower than fourth order. 2 To understand this anomaly in the tunneling, Cornia et al 7 proposed that inherent disorder in solvent molecules may break the S 4 symmetry and provide nonzero values of E. Recent electron paramagnetic resonance (EPR) experiments 8,9 and magnetic tunneling measurements 9,10 revealed that the model of Cornia et al 7 needed to be refined for a quantitative comparison with experiment. Our DF calculations 6 on the Mn 12 showed that the values of E for possible configurations of the disordered solvent molecules are in good quantitative agreement with experiment.…”

mentioning

confidence: 99%

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

Park

Pederson

Baruah

et al. 2005

Journal of Applied Physics

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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...

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“…13,18 The imperfections in large complex molecules such as Mn 12 could be of many types, such as dislocations 7 and the disorder caused by the various orientations of the acetate ligand and of the solvent molecules in a unit cell. 8 So far the origin of the possible defects has not yet been clarified. In the study, 13,14 we modeled that the defects could be represented by the distributions in the uniaxial anisotropy parameter D (second-order anisotropy) and the g-factor.…”

Section: Introductionmentioning

confidence: 99%

Effect of defects on the line shape of electron paramagnetic resonance signals from the single-molecule magnet Mn12: A theoretical study

Park

Dalal

Rikvold

2002

The Journal of Chemical Physics

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We herein estimate the effect of lattice defects on the line shape of electron paramagnetic resonance (EPR) signals from a single crystal of the S = 10 single-molecule magnet Mn 12 with the external magnetic field along the crystal c axis. A second-order perturbation treatment of an effective single-spin Hamiltonian indicates that a small, random, static misorientation of the magnetic symmetry axes in a crystalline lattice can lead to asymmetric EPR peaks. Full spectra are simulated by calculating probability-distribution functions for the resonant fields, employing distributions in the tilt angle of the easy axis from the c axis, in the uniaxial anisotropy parameter, and in the gfactor. We discuss conditions under which the asymmetry in the EPR spectra becomes prominent. The direction and magnitude of the asymmetry provide information on the specific energy levels involved with the EPR transition, the EPR frequency, and the distribution in the tilt angle.

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Origin of Second-Order Transverse Magnetic Anisotropy inMn12-Acetate

Cited by 162 publications

References 23 publications

On the origin of anomalous EPR peaks observed in Mn 12 –Ac

On the origin of anomalous EPR peaks observed in Mn 12 –Ac

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

Effect of defects on the line shape of electron paramagnetic resonance signals from the single-molecule magnet Mn12: A theoretical study

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