Fast Magnetization Tunneling in Tetranickel(II) Single-Molecule Magnets

Yang, En‐Che; Wernsdorfer, Wolfgang; Zakharov, Lev N.; Karaki, Yoshitomo; Yamaguchi, Akira; Isidro, Rose Marie; Lu, Guangquan; Wilson, Sarah Kate; Rheingold, Arnold L.; Ishimoto, Hidehiko; Hendrickson, David N.

doi:10.1021/ic050093r

Cited by 183 publications

(155 citation statements)

References 52 publications

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“…13 This results in narrower EPR peaks than in many SMMs. 12 The spin Hamiltonian of Ni 4 is to first approximation,…”

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confidence: 99%

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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“…13 This results in narrower EPR peaks than in many SMMs. 12 The spin Hamiltonian of Ni 4 is to first approximation,…”

mentioning

confidence: 99%

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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“…A static magnetic field H was applied along the axis of the cavity. A single crystal of Ni 4 (synthesized using published procedures [25]) was mounted on the bottom of the cavity at a position where the rf field was perpendicular to the static field. The easy axis of the crystal was manually tilted at various angles (θ H ) relative to H. We measured the reflected power as a arXiv:1510.03109v2 [cond-mat.mes-hall] 30 Sep 2016…”

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confidence: 99%

“…In addition, there are no solvate molecules in the crystal lattice and 99% (natural abundance) of Ni nuclei have spin I = 0. This SMM has been characterized by electronspin resonance (ESR) spectroscopy [17,[19][20][21][22][23][24], magnetization measurements [17,18,25] and heat capacity measurements [23,26,27]. Ni 4 can be well described as a single "giant spin" with the Hamiltonian [24]: where g is the molecule's g tensor, D and A are axial (diagonal) anisotropy parameters that define the "easy" z axis and make the m = ±4 magnetic sublevels have the lowest energy, producing an energy barrier between those two orientations; C is a transverse (off-diagonal) anisotropy parameter that affects the strength of tunneling through the barrier; and the magnetic field B = B (sin θ cos φ, sin θ sin φ, cos θ) produces a Zeeman interaction.…”

mentioning

confidence: 99%

“…We are grateful to H. Xu for her assistance in some aspects of the numerical simulations. We thank C. Euvrard Samples of Ni 4 were produced using published procedures [25]. Single crystals of typical dimension ∼ 1 mm were produced and crystal structure verified through unit cell parameters using X-ray diffractometry.…”

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Observation of Tunneling-Assisted Highly Forbidden Single-Photon Transitions in a Ni4 Single-Molecule Magnet

et al. 2016

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Forbidden transitions between energy levels typically involve violation of selection rules imposed by symmetry and/or conservation laws. A nanomagnet tunneling between up and down states violates angular momentum conservation because of broken rotational symmetry. Here we report observations of highly forbidden transitions between spin states in a Ni4 single-molecule magnet in which a single photon can induce the spin to change by several timesh, nearly reversing the direction of the spin. These observations are understood as tunnelingassisted transitions that lift the standard ∆m = ±1 selection rule for single-photon transitions. These transitions are observed at low applied fields, where tunneling is dominated by the molecule's intrinsic anisotropy and the field acts as a perturbation. Such transitions can be exploited to create macroscopic superposition states that are not typically accessible through single-photon ∆m = ±1 transitions.There has been much recent attention to using spin systems as potential qubits [1][2][3][4]. Molecular nanomagnets are particularly attractive as spin qubits [4][5][6][7][8][9][10][11][12] because many of their properties can be chemically engineered. Single-molecule magnets (SMMs) are anisotropic molecular magnets, typically with large total spin, for which the spin is impelled to point along a preferred axis, the "easy" axis [13]. They exhibit remarkable quantum dynamics including tunneling between different orientations [14] and quantum-phase interference [15]. Here we present evidence of highly forbidden transitions in the Ni 4 SMM where the transitions are enabled by tunneling, which lifts the requirement of spin angular momentum conservation. We observe transitions in which the absorption of a single photon permits a near reversal of the molecule's macrospin, grossly violating the standard ∆m = ±1 selection rule. The quantum states that can be generated through these forbidden transitions are non-classical, having a substantial "macroscopicity" by a standard measure. Our results imply that the forbidden transitions observed in this system (and similar molecules with strong anisotropy) can be exploited to create highly nonclassical states with single-photon transitions.From a quantum coherence perspective, forbidden transitions have some distinct advantages: Since the matrix elements for these transitions are small, they tend to have long lifetimes. In addition, they can be less susceptible to magnetic-field fluctuations under certain circumstances, potentially leading to longer coherence times [3,12,16]. Forbidden transitions have been seen in SMMs with very strong tunneling produced by strongly broken symmetry [11,12,17]. In contrast, in our experiments the transitions are dominated by a modest intrinsic anisotropy with an applied field acting as a perturbation.We studied the S = 4 complex [Ni(hmp)(dmb)Cl] 4 (hereafter Ni 4 ), shown in the inset of Fig. 1. The molecule's large ligands isolate the magnetic centers within a crystal from each other [18]. In addition, there a...

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Structural Effects of Sodium Cations in Polynuclear, Multicubane‐Type Mixed Na–Ni Complexes

et al. 2010

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Dedicated to Professor Herbert W. Roesky on the occasion of his 75th birthdayThe importance of transition-metal catalysts for ethylene oligo-and polymerization continues to motivate the synthesis of a wide range of heterotopic ligands and metal complexes. [1] Studies of dinuclear Ni II complexes with N,OH chelates as precatalysts for ethylene oligomerization [2] triggered our interest in comparing complexes with (pyridine-2-yl)methanol (HL) and those containing the corresponding anionic chelate, partly because N,O À ligands may lead to systems that do not require any cocatalyst. [3] Furthermore, L À is well known to promote the formation of polynuclear complexes with exciting physical properties, [4] paralleling the rich chemistry of 2-pyridones. [5] We have now obtained unusual mixed nickel/sodium polynuclear complexes and identified remarkable structural effects of the sodium cations associated with the reagent used to deprotonate the alcohol function. In addition to their intrinsic novelty, such observations are relevant to the possible in situ formation of unexpected and possibly overlooked active species in catalytic reactions.The dinuclear complex [Ni(m-Cl)(HL)] 2 Cl 2 (1) and the mononuclear, octahedral mer-[Ni(HL) 3 ]Cl 2 complex (2) have been recently reported. [2a] The reaction of 1 with 10 equivalents of NaH in THF led mainly to a complex catalytically active for ethylene oligomerization formulated as [Ni(HL)(L)Cl], [2a] but a few green crystals were also obtained, which are now shown to correspond to a very unusual mixed Ni-Na cluster (Figures 1 and 2).In this cluster, 3, [20] two fragments are held together through Na + ···O interactions (Figure 2 left): 1) a cationic trinuclear nickel moiety (Ni2, Ni3, Ni4, Figure 1 b), in which two L À ligands N,O-chelate each metal center, the Ni atoms being doubly bridged by the oxygen atoms and further capped by a m 3 -OH group; and 2) a mononuclear anionic unit in which three N,O chelating ligands form a facial arrangement around Ni1 (Figure 1 c). The alcoholate oxygen atoms O5, O7, and O9 each bridge two Na + ions and two Ni centers in the Ni 3 substructure. This generates a pseudo-cubane arrangement for Ni2, Ni3, Ni4, the m 3 -OH group (O10-H), and a missing vertex (Figure 2). The latter is shared with another defective pseudo-cubane entity containing Ni1, the three Na + ions, and O1, O2, and O3, which each bridge two Na + ions. The formation of this unprecedented nickel/sodium polynuclear complex illustrates the remarkable effect of the Na + ions. [5f, 6] A much improved synthesis of 3 was devised by reacting HL, [NiCl 2 (DME)] (DME = 1,2-dimethoxyethane), and NaH in THF in a 3.2:1:5 ratio, respectively. The presence of the m 3 -OH group most likely originates from the presence of NaOH that originates from NaH [Eq. (1), see the Supporting Information for details]. 4 ½NiCl 2 ðDMEÞ þ 9 HL þ 9 NaH þ NaOH ! ½Ni 4 Na 3 ðLÞ 9 OHCl ð3Þ þ 7 NaCl # þ9 H 2 " þ4 DMEWhen HL was stirred in THF with [NiCl 2 (DME)] for 12 h, 1 formed first, irrespective of the L/Ni r...

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Fast Magnetization Tunneling in Tetranickel(II) Single-Molecule Magnets

Cited by 183 publications

References 52 publications

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

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

Observation of Tunneling-Assisted Highly Forbidden Single-Photon Transitions in a Ni4 Single-Molecule Magnet

Structural Effects of Sodium Cations in Polynuclear, Multicubane‐Type Mixed Na–Ni Complexes

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