Magnetization tunneling in high-symmetry single-molecule magnets: Limitations of the giant spin approximation

Wilson, A.; Lawrence, J. M.; Yang, En‐Che; Nakano, Motohiro; Hendrickson, D. N.; Hill, Stephen

doi:10.1103/physrevb.74.140403

Cited by 90 publications

(144 citation statements)

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“…In spite of this, and as stated above, the 'effective' giant spin phenomenology continues to work very well for Mn 12 and Fe 8 . However, one obtains a renormalized ZFS parameter set via such an approach [different from what would be expected if S were exactsee section 2.3(c)], including, e.g., unphysical 4 th and higher order terms [13][14][15]45,59]. Hence, it becomes a real challenge to correlate the results of EPR and QTM measurements to underlying details of the molecular structure, something that represents the ultimate goal of spectroscopists and synthetic chemists alike.…”

Section: H Ds E S S B O B Omentioning

confidence: 99%

“…In this way, we can for example relate D mol (or E mol ) to J and d ion (as well as e ion ). At every step, our findings are informed by real molecules that have been extensively characterized by magnetic and spectroscopic (EPR) techniques which are published elsewhere [12][13][14][15][16][17][18][19]22,25]. We focus primarily on the III 3 Mn and III 6 Mn SMMs, with occasional reference to II 4 Ni .…”

Section: H Ds E S S B O B Omentioning

confidence: 99%

“…hysteresis loop step heights) and underlying molecular structure by considering a multi-spin Hamiltonian of the form [13,45]:…”

Section: H Ds E S S B O B Omentioning

confidence: 99%

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Magnetic quantum tunneling: insights from simple molecule-based magnets

et al. 2010

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This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on three families of relatively simple, low-nuclearity transition metal clusters: spin S = 4 II 4 Ni , III 3 Mn (S = 2 and 6) and III 6 Mn (S = 4 and 12). The Mn III complexes are related by the fact that they contain triangular III 3 Mn units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn III centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low-and high-spin molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn 6 ) has barely increased in the 15 years since the first studies of Mn 12 -acetate, and why the tiny Mn 3 molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn 3 and Ni 4 ). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) of the molecules highlighted in this study proves to be of crucial importance. Not only that, these simple molecules may be considered among the best SMMs: Mn 6 possesses the record anisotropy barrier, and Mn 3 is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the molecule.

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Section: H Ds E S S B O B Omentioning

confidence: 99%

Section: H Ds E S S B O B Omentioning

confidence: 99%

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Magnetic quantum tunneling: insights from simple molecule-based magnets

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“…Nevertheless, the present measurements serve a useful purpose, hinting at the significant fourth ͑and higher-order͒ anisotropy that likely results as a consequence of S mixing brought about by low-lying excited spin states. 10 Indeed, the spectra in Fig. 3͑a͒ clearly show features ͑labeled X͒ associated with the population of low-lying S Ͻ 17 2 spin states.…”

Section: Data and Discussionmentioning

confidence: 99%

Transverse anisotropy in the mixed-valent Mn2IIMn4IIIMn3IV single-molecule magnet

Datta

Milios

Brechin

et al. 2008

Journal of Applied Physics

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High-frequency electron paramagnetic resonance measurements have been performed on a single-crystal sample of a recently discovered mixed valent Mn 2 II Mn 4 III Mn 3 IV single-molecule magnet, with a spin S =17/ 2 ground state. Frequency, temperature and field-orientation dependent studies confirm previously reported axial magnetic anisotropy parameters and also provide clear evidence for higher order ͑fourth and sixth͒ transverse terms that are responsible for the magnetic quantum tunneling observed in this system.

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“…͑1͒ may result from a finite ratio of the ͑second order͒ single ion anisotropies to exchange constant. 15 To prepare dilute solutions of Ni 4 , dried crystals were dissolved in a 1:1 mixture of toluene and dichloromethane. This glass was chosen due to solubility constraints.…”

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High frequency EPR on dilute solutions of the single molecule magnet Ni4

Loubens¹,

Kent²,

Krymov³

et al. 2008

Journal of Applied Physics

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Dilute frozen solutions of the single molecule magnet Ni 4 ͑S =4͒ have been studied using 130 GHz electron paramagnetic resonance ͑EPR͒. Despite the random orientation of the molecules, well defined EPR absorption peaks are observed due to the strong variation of the splittings between the different spin states on magnetic field. Temperature dependent studies above 4 K and comparison with simulations enable identification of the spin transitions and determination of the Hamiltonian parameters. The latter are found to be close to those of Ni 4 single crystals. No echo was detected from Ni 4 in pulsed experiments, which sets an upper bound of about 50 ns on the spin coherence time. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2834447͔Single molecules magnets ͑SMMs͒ have been suggested as candidates for qubits in quantum processors. 1 However, the coherence time ͑T 2 ͒ of high-spin molecules has not yet been determined. Quantum tunneling of magnetization ͑QTM͒ has been widely studied in SMMs. 2-4 A recent focus is on coherent QTM in which the tunneling rates may be faster than the rate of decoherence. [5][6][7] Due to inhomogeneous broadening, only a lower bound ͑Ϸ0.5 ns on most of the measured SMMs͒ of T 2 can be extracted. Furthermore, in SMM crystals the strong dipolar interactions between the molecules ͑separated by Ϸ1 nm͒ is expected to drastically reduce the coherence time. In order to determine the latter ͑e.g., through spin echo͒ and to coherently manipulate the magnetization with microwave pulses in SMMs, it thus may be necessary to work with dilute ensembles of molecules.SMMs may be diluted in solvents. The drawback of such a method is that the molecules are randomly oriented. However, magnetometry 8 and EPR experiments 9 on randomly oriented powder have been successfully performed. Only recently have dilute frozen solutions been studied by high frequency ͑HF͒ techniques. Frequency domain spectroscopy showed that the zero field splitting ͑ZFS͒ parameters of Mn 12 clusters diluted in a frozen solvent were almost the same as in the crystal form. 10 Even more interesting, standard microwave pulse experiments have determined a long intrinsic coherence time ͑few s at 2 K͒ in doped antiferromagnetic wheels ͑low spin͒. 11 In this paper we present continuous wave ͑cw͒ HF EPR experiments performed on dilute frozen solutions of the SMM Ni 4 ͑S =4͒ as a function of temperature between 6 and 30 K. Well defined transitions are observed in the 0 -7 T range at f = 130 GHz. Simulations using ZFS parameters close to the ones determined from single crystals 12 and D and g strains qualitatively reproduce the spectra measured. No spin echo from the Ni 4 clusters was observed in pulse experiments, which sets an upper limit for T 2 of about 50 ns.͓Ni͑hmp͒͑dmb͒Cl͔ 4 , henceforth referred to as Ni 4 , is a particularly clean SMM with no solvate molecules present in its crystal phase and only 1% ͑natural abundance͒ of nuclear spins on the transition metal sites. 13 This results in narrower EPR peaks than in many SMMs. 12...

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Magnetization tunneling in high-symmetry single-molecule magnets: Limitations of the giant spin approximation

Cited by 90 publications

References 20 publications

Magnetic quantum tunneling: insights from simple molecule-based magnets

Magnetic quantum tunneling: insights from simple molecule-based magnets

Transverse anisotropy in the mixed-valent Mn2IIMn4IIIMn3IV single-molecule magnet

High frequency EPR on dilute solutions of the single molecule magnet Ni4

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