Speaker
Dr.
Fernando LUIS
(Instituto de Ciencia de Materiales de Aragón, CSIC)
Description
The erasure of a bit of information encoded in any physical system is an irreversible operation bound to dissipate an amount of energy Q = kBT ln 2 [1]. As a result, work W ≥ Q has to be applied to the physical system to restore the erased information content [2-4]. This limit, called Landauer limit, sets a minimal energy dissipation inherent to any classical computation. In the pursuit of the fastest and most efficient means of computation, the ultimate challenge is to produce a memory device executing an operation as close to this limit in the shortest time possible.
Here, we use a single crystal of Fe8 molecular nanomagnets as a spin-memory device. Each molecular cluster carries a net spin S = 10 and possesses a strong uniaxial magnetic anisotropy. The ground state corresponds to maximum spin projections Sz = +10 and -10 along the anisotropy axis, common to all molecules, which define the ‘0’ and ‘1’ bit states (Fig. 1(a)). These states are separated by an energy barrier U/kB ≈ 24 K, which hinders the spin flip and gives rise to magnetic hysteresis, thus magnetic memory, below approximately 1.2 K [5,6].
In our experiments, the Landauer cycle is performed, at T = 1 K, via the application of a sequence of magnetic fields aligned along different orientations with respect to the magnetic anisotropy axis (Fig. 1(b)). The erasure is induced by a transverse magnetic field Hy, which reduces the height of the magnetic energy barrier and promotes tunneling between quasi-degenerate spin projections, thus exploiting a form of quantum annealing [7,8]. The bits are then recorded by applying a magnetic field Hz along the anisotropy axis, strong enough to polarize their magnetic moments. The cycle is then completed by reducing first Hy and then Hz back to zero.
The magnetic susceptibility along z and y has been measured (Fig. 1(c)) and then integrated to obtain the net magnetic work required to perform the erasure and recording cycle. It agrees, within experimental uncertainties, with the Landauer limit. The ac susceptibility provides also information about the magnetization dynamics and, in particular, enables us to estimate the time needed to record each bit. This time turns out to be shorter than 0.1 micro-seconds thanks to the very fast quantum spin dynamics induced by the transverse magnetic field. The performance of our device in terms of energy-time cost is then orders of magnitude better than that of any existing memory devices to date.
Acknowledgments
The research reported here was supported by the Spanish MINECO (grant MAT2015-68204-R), the Dutch Organization for Fundamental research (NWO/FOM), the Gobierno de Aragón (grant E98-MOLCHIP) and the European Union (advanced ERC grant Mols@Mols and COST 15128 Molecular Spintronics project).
References
[1] R. Landauer, IBM Journal of Research and Development 5 (1961) 183.
[2] C. H. Bennett, International Journal of Theoretical Physics 21 (1982) 905.
[3] C. H. Bennett, IBM Journal of Research and Development 32 (1988) 16.
[4] H. Leff and A. Rex, Maxwell's demon: Information, entropy, computing, Hilger and Princeton Univ. Press, Europe/USA (1990).
[5] W. Wernsdorfer, R. Sessoli, A. Caneschi, D. Gatteschi and A. Cornia, EPL 50 (2000) 552.
[6] E. Burzurí, F. Luis, O. Montero, B. Barbara, R. Ballou, and S. Maegawa, Phys. Rev. Lett. 111 (2013) 057201.
[7] J. Brooke, D. Bitko, T. F. Rosenbaum and G. Aeppli, Science 284 (1999) 779.
[8] E. Burzurí, F. Luis, B. Barbara, R. Ballou, E. Ressouche, O. Montero, J. Campo, and S. Maegawa Phys. Rev. Lett. 107 (2011) 097203.
Primary author
Dr.
Fernando LUIS
(Instituto de Ciencia de Materiales de Aragón, CSIC)
