Macroscopic Quantum Coherence in Molecular Magnets

Abstract

We study macroscopic quantum coherence in antiferromagnetic molecular magnets in the presence of magnetic fields. Such fields generate artificial tunnel barriers with externally tunable strength. We give detailed semi-classical predictions for the tunnel splitting in various regimes for low and high magnetic fields. We show that the tunneling dynamics of the Néel vector can be directly measured via the static magnetization and the specific heat. We also report on a new quantum phase arising from fluctuations. The analytic results are complemented by numerical simulations.PACS numbers: 73.40. Gk, 75.60.Jp, 75.10.Jm, 03.65.Sq, 75.30.Gw Quantum spin dynamics in mesoscopic magnets has received much attention over the recent years, both from experiment and from theory [1]. A number of nanosized particles in the superparamagnetic regime have been identified as promising candidates for the observation of macroscopic quantum phenomena (MQP) such as the tunneling of the magnetization out of a metastable potential minimum, or, more strikingly, macroscopic quantum coherence (MQC), where the magnetization (or the Néel vector) tunnels coherently between classically degenerate directions over many periods. On one hand, these phenomena are interesting from a fundamental point of view as they extend our understanding of the transition from quantum to classical behavior. On the other hand, the measurement of MQP quantities such as the tunnel splitting provides independent information about microscopic parameters such as anisotropies and exchange constants.A prominent example of such MQC behavior that has attracted wide attention (but also scrutiny) is the antiferromagnetic ferritin [2]. More recently, molecular magnets [3] such as the ferric wheel or Mn 12 have emerged as promising candidates for the experimental observation of MQP [4,5] mainly for three reasons. First, molecular magnets have well-defined structures and magnetic properties. Thus, precise values for the tunneling rates can be calculated. Second, molecular magnets can be produced as single crystals that contain a macroscopic number of identical magnetic subunits, which leads to a natural amplification of the single-unit signal. Third, the typically high symmetry of these magnets reduces the number of independent parameters.In this letter we discuss novel tunneling scenarios in antiferromagnetic (AFM) molecular magnets. A key feature of our discussion is to exploit the well-known fact [6] that an effective anisotropy can be generated in an AFM by applying a magnetic field. Thus it is possible to create tunnel barriers that are tunable by an external parameter. Evidently, such control parameters are highly desirable as they open the door to systematic tests of MQC. We concentrate on ring-like structures such as Fe 6 , Fe 10 ,, where the spins interact with their nearest neighbors via exchange coupling. In particular, we show that the tunneling rates become field dependent and thus can be measured via the static magnetization and (less surprisingly) also via the S...

Quantum Phase Interference and Néel-Vector Tunneling in Antiferromagnetic Molecular Wheels

“…1a). The combined data show that the low-temperature magnetism in Fe 18 is very accurately described by the NVT scenario, as unfolded by semiclassical theory [7]. Moreover, the torque is found to exhibit wiggles that directly reflect the oscillations in the tunnel splitting with field, or quantum phase interference indeed.…”

“…1c) and interference occurs due to the phase of the wave function. This gives rise to characteristic oscillations in the tunnel splitting as function of applied magnetic field, which would be observable in static measurements such as magnetization [7].Initial attempts to establish NVT with the ferritin protein [8] unfortunately proved controversial owing to polydisperse samples and the presence of uncompensated magnetization [9]. Recent attempts with the AFM molecular wheels CsFe 8 and Fe 10 were encouraging steps forward [10,11], but were criticized with arguments that the tunneling actions S 0 / are too small and the NVT picture only approximately valid [11].…”

Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering

“…It has been proposed [5] that the low-temperature dynamics of antiferromagnetic rings could be characterized by the coherent tunneling of the Néel vector…”

“…The so-obtained correlation functions allow a model-free picture of the quantum dynamics of the ring. For example, the way a quantum fluctuation propagates along the ring or the degree of validity of a Néel vector tunneling description [5,23] are determined.…”

“…Shells of organic ligands provide magnetic separation between adjacent magnetic cores, which behave as identical and independent zero-dimensional units [1]. The magnetic dynamics are characterized by strong quantum fluctuations and this makes MNMs of great interest in quantum magnetism as model systems to investigate a range of phenomena, such as quantum-tunnelling of the magnetization [2][3][4], Néel-vector tunnelling (NVT) [5,6], quantum information processing [7][8][9], quantum entanglement [10][11][12][13] or decoherence [14][15][16]. Besides their fundamental interest, MNMs are also the focus of intense research for the potential technological applications as classical or quantum bits [1,[7][8][9]17] and as magnetocaloric refrigerants [18].…”