Highly diluted rare-earth (RE) compounds with a large uniaxial anisotropy provide new systems to study mesoscopic quantum magnetism.
Indeed, a Ho3+ doped LiYF4 single crystal was shown to be a model system to study an ensemble of weakly-coupled atomic magnets, the axial crystal-field anisotropy giving rise to hysteresis at low temperature. Besides, magnetization quantum jumps were observed at slow field sweep rates for resonant values of a longitudinal applied field, along the easy-axis, showing the effect of low-frequency quantum fluctuations. Quantum tunnelling of the magnetization (QTM) is associated with the coherent rotation of both electronic momentum and nuclear spin of a rare-earth ion, and occurs at avoided level crossings. These tunnel splittings allow for the decay out of a field-induced metastable state and the resonance conditions can be understood on the basis of electron-nuclear states of a single Ho3+ ion. Recently, ac-susceptibility anomalies also gave a direct evidence of a many-body quantum dynamics due to cross-spin relaxations (CSR), thus corroborating the observation of additional magnetization steps at fast field sweep rates. In particular, the first demonstration of co-tunnelling processes allowed us to underline the crucial role of anisotropic interactions in the raise of quantum fluctuations at larger scales.
Moreover, dissipation and decoherence phenomena, which are of much current interest in mesoscopic physics, can now be thoroughly investigated in these very versatile compounds, by varying the crystal fields (anisotropy) or fluctuating superhyperfine fields (homogeneous broadening), as well as the average dipolar coupling strength. Therefore, the very promising RE-based diluted magnets, with high crystalline quality, appear to be much more suitable than single molecule magnets, such as Mn12-acetate, to study the effect of environmental degrees of freedom, which strongly affect the tunneling dynamics. Such mesoscopic quantum systems could also be used as quantum bits. In this context, the role of multi-spin quantum fluctuations induced by dipolar interactions is a key ingredient to be investigated. CSR should give an improved knowledge of the nature of quantum fluctuations in complex systems, as in the quantum spin glass or disordered ferromagnet states already evidenced in the high concentration regime.
Diluted rare-earth compounds as well as RE-based molecular crystals will be synthesized and characterized (XRD, EPR and optical spectroscopy). Their quantum properties will be studied in detail using new developed techniques (micro-SQUID and micro-Hall magnetometers, time-resolved submillimeter spectroscopy), as well as a unique laser polarimetry experiment or more conventional ac-susceptibility. NMR measurements will give a direct access to the nuclear spin bath dynamics. In addition, a theoretical support to the experimental results will be based on both calculated thermal transition rates and tunneling splittings.
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