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The term 'molecular magnet' generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S (ref. 1). Such complex multi-spin systems provide attractive targets for the study of quantum effects at the mesoscopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field [2][3][4] . There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits-'qubits', the basic building blocks of a quantum computer-based on molecular magnets [5][6][7][8] . But strong decoherence 9 must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations (quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave 10 ) of a molecular magnet in a hybrid system, in which discrete and wellseparated magnetic V IV 15 clusters are embedded in a self-organized non-magnetic environment. Each cluster contains 15 antiferromagnetically coupled S 5 1/2 spins, leading to an S 5 1/2 collective ground state [11][12][13] . When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of longlived quantum coherence. The present observation of quantum oscillations suggests that low-dimension self-organized qubit networks having coherence times of the order of 100 ms (at liquid helium temperatures) are a realistic prospect.In the context of quantum computing, it was recently discussed how the decoherence of molecular magnet spin quantum bits could be suppressed, with reference to the discrete low spin clusters V 15 and Cr 7 Ni (ref. 7; see also refs 8 and 14). In both systems, their low spin states cause weak environmental coupling 7 , making them candidates for the realization of a long-lived quantum memory. Measurement of the spin relaxation time t 2 in Cr 7 Ni was subsequently reported and found to be interestingly large 15,16 ; however, the important Rabi quantum oscillations were not observed, probably because electronic and nuclear degrees of freedom were too strongly linked to each other. As these oscillations have until now only been observed in non-molecular spin systems (see, for example, refs 17-20), it has remained an open question whether quantum oscillations could in principle be realized in molecular magnets 7,8 . This question is now answered by our observation of quantum oscillations of the Rabi type in V 15 . The main reason for this success lies in the fact that the important pairwise decoherence mechanism 7,8 associated with dipolar interactions could be strongly reduced.Before discussing the observed quantum oscillations, we first briefly describe the magnetic/electronic structure of the V IV 15 species as determined experimentally. Following the synthesis of the ...

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Time resolved magnetization measurements have been performed on a spin 1/2 molecular complex, so-called V15. Despite the absence of a barrier, magnetic hysteresis is observed over a time scale of several seconds. A detailed analysis in terms of a dissipative two-level model is given, in which fluctuations and splittings are of the same energy. Spin-phonon coupling leads to long relaxation times and to a particular "butterfly" hysteresis loop.

We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped Zn 1Ϫx Mn x Te epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance, and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida ͑RKKY͒ model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.

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