Inelastic-neutron-scattering study of excited spin multiplets and low-energy phonons in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>Fe</mml:mi><mml:mn>8</mml:mn></mml:msub></mml:mrow></mml:math>nanomagnet: Implications for relaxation

Carretta, S.; Santini, P.; Amoretti, G.; Guidi, T.; Dyson, Jack; Caciuffo, R.; Stride, John A.; Caneschi, Andréa; Copley, J. R. D.

doi:10.1103/physrevb.73.144425

Cited by 14 publications

(10 citation statements)

References 24 publications

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“…with s A i = s B j = 5/2. A good agreement with the experimental data is obtained with the following set of values for the exchange constants: J A = 26 K, J A = 36 K, J AB = 200 K, J AB = 59 K [30,33,34]. As in the case of Mn 12 , we take as a reference the linear superposition between M 1 = 10 and M 2 = −10.…”

Section: The Fe8 Moleculesupporting

confidence: 65%

Size of linear superpositions in molecular nanomagnets

Troiani

Zanardi

2013

Phys. Rev. B

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Molecular nanomagnets are zero-dimensional spin systems, that exhibit quantum mechanical behavior at low temperatures. Exploiting quantum-information theoretic measures, we quantify here the size of linear superpositions that can be generated within the ground multiplet of high-and low-spin nanomagnets. Amongst the former class of systems, we mainly focus on Mn12 and Fe8. General criteria for further increasing such sizes are also outlined, and illustrated in the case of spin rings. The actual character (micro or macroscopic) of linear superpositions in low-spin systems is inherently ill-defined. Here, this issue is addressed with specific reference to the Cr7Ni and V15 molecules, characterized by an S = 1/2 ground state. In both cases, the measures we obtain are larger than those of a single s = 1/2 spin, but not proportionate to the number and value of the constituent spins. Molecular nanomagnets represent prototypical examples of engineerable quantum systems. In fact, their physical properties can be widely tuned by chemical synthesis, and quantum coherence effects show up in their spin dynamics [1,2]. These effects include quantum tunneling of the molecule spin, resulting in a speed-up of the magnetization relaxation [3,4], and quantum phase interference [5]. Besides, microwave-induced quantum coherences [6,7] and Rabi oscillations [8,9] were recently observed in a wide class of spin clusters. These experimental achievements, along with the microscopic understanding and chemical control of the decoherence processes [10,11], will possibly enable the use of molecular nanomagnets for quantum-information processing [12,13].Amongst molecular nanomagnets with dominant exchange interaction, a prominent distinction is that between high-and low-spin systems.In the former class of spin clusters, the ground S multiplet may include classical-like states that are macroscopically different from one another, and whose linear superpositions can thus be regarded as Schrödinger cat states [14]. In the latter systems, antiferromagnetic interactions result instead in ground states with low S and highly non-classical features, such as quantum entanglement [15] or Néel-vector tunneling [16][17][18]. Hereafter, we theoretically investigate the size of linear superpositions that can be -or have already beengenerated in both these kinds of molecular nanomagnets. In the case of high-spin molecules, we quantify the actual macroscopicity of linear superpositions. In other words, we determine to which extent their sizes are proportionate to the number and value of the constituent spins, and thus approach the theoretical maxima. We initially focus on the most celebrated single-molecule magnets, [ [20]. In both these nanomagnets (hereafter referred to as Mn 12 and Fe 8 ) the ground state is characterized by a ferrimagnetic ordering, with the total spin S resulting from the inequivalence of the two sublattices [1]. The role played by such inequivalence is further discussed in reference to a prototypical class of spin rings, which includes...

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Section: The Fe8 Moleculesupporting

confidence: 65%

Size of linear superpositions in molecular nanomagnets

Troiani

Zanardi

2013

Phys. Rev. B

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“…Therefore, such an undertaking is usually only possible using neutrons. 20 However, Figs. 2 and 3 clearly show that J can be estimated on the basis of the ZFS within the lowest-lying multiplet.…”

Section: ͑2͒mentioning

confidence: 97%

“…We note that the situation in Ni 4 is not dissimilar to that of many other SMMs, including the most widely studied Mn 12 -acetate, for which similar fourth-order ZFS interactions and low-lying excited spin states are found. 19,20 Finally, we note that the most unambiguous method for estimating exchange couplings in polynuclear metal complexes involves determining the exact locations of excited spin multiplets. However, the magnetic-dipole selection rule forbids transitions between states with different multiplicity.…”

Section: ͑2͒mentioning

confidence: 99%

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

et al. 2006

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Electron paramagnetic resonance ͑EPR͒ studies of a Ni 4 single-molecule magnet ͑SMM͒ yield the zerofield-splitting ͑ZFS͒ parameters D, B 4 0 , and B 4 4 , based on the giant spin approximation ͑GSA͒ with S =4; B 4 4 is responsible for the magnetization tunneling in this SMM. Experiments on an isostructural Ni-doped Zn 4 crystal establish the Ni II ion ZFS parameters. The fourth-order ZFS parameters in the GSA arise from the interplay between the Heisenberg interaction Jŝ 1 • ŝ 2 and the second-order single-ion anisotropy, giving rise to mixing of higher-lying S 4 states into the S = 4 state. Consequently, J directly influences the ZFS in the ground state, enabling its determination by EPR.

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“…A schematic diagram of the magnetic structure of the Fe 8 molecular core [28,29] is shown in Fig. 5 ration, with a ground state S = 10 and a first excited S = 9 multiplet lying about δE 9,10 /k B = 44 K above it [28][29][30][31], follows from the fact that |J 4 | > |J 3 | (all couplings are antiferromagnetic). In addition to symmetric exchange interactions, one has to consider also single-ion magnetic anisotropies, dipolar interactions and antisymmetric Dzyaloshinskii-Moriya (DM) interactions, which mix states of different S [20].…”

Section: H Xmentioning

confidence: 99%

Quantum Interference Oscillations of the Superparamagnetic Blocking in anFe8Molecular Nanomagnet

et al. 2013

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We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe8 single molecule magnet oscillate as a function of the magnetic field Hx applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S = 10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe8 amounts to approximately 11.6 %. PACS numbers:High-spin molecular clusters [1, 2] display superparamagnetic behavior, very much as magnetic nanoparticles typically do. Below a time-(or frequency-)dependent blocking temperature T b , the linear magnetic response "freezes" [3,4] and magnetization shows hysteresis [5]. The slow magnetic relaxation of these single-molecule magnets (SMMs) arises from anisotropy energy barriers separating spin-up and spin-down states. Because of their small size, the magnetic response shows also evidences for quantum phenomena, such as resonant spin tunneling [3,[6][7][8]. In the case of molecules that, like Fe 8 (cf Fig. 1A and [4]), have a biaxial magnetic anisotropy, tunneling between any pair of quasi-degenerate spin states ±m can proceed via two equivalent trajectories, which, as illustrated in Fig. 1B, cross the hard anisotropy plane close to the medium anisotropy axis. A magnetic field along the hard axis changes the phases of these tunneling paths, leading to either constructive or destructive interferences. This phenomenon is known as Berry phase interference [9,10].Experimental evidences for the ensuing oscillation of the quantum tunnel splitting ∆ m , shown in Fig. 1C, were first observed in Fe 8 [11,12] and then in some other SMMs [13][14][15][16][17][18][19] by means of Landau-Zener magnetization relaxation experiments. Interference patterns measured on Fe 8 at very low temperatures, which correspond to tunneling via the ground state doublet m = ±10, are reproduced by the following spin Hamiltonian C/k B = −2.9 × 10 −5 K are magnetic anisotropy parameters, and g = 2. The sizeable fourth-order parameter C reflects not only the intrinsic anisotropy but, mainly, it parameterizes quantum mixing of the S = 10 with excited multiplets (S-mixing) and how it influences quantum tunneling via the ground state [20].In the present paper, we study the influence of Berry phase interference on the ac magnetic susceptibility χ and T b of Fe 8 , that is, on those quantities that characterize the standard SMM (or superparamagnetic) behavior. Close to T b , magnetic relaxation is dominated by tunneling near the top of the anisotropy energy barrier, thus also near excited multiplets with S = 10. In this way, we

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Inelastic-neutron-scattering study of excited spin multiplets and low-energy phonons in theFe8nanomagnet: Implications for relaxation

Cited by 14 publications

References 24 publications

Size of linear superpositions in molecular nanomagnets

Size of linear superpositions in molecular nanomagnets

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

Quantum Interference Oscillations of the Superparamagnetic Blocking in anFe8Molecular Nanomagnet

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