Quantum Tunneling of the Magnetization

High spin molecules (HSM) are molecules consisting of ions coupled by ferromagnetic or antiferromagnetic interaction; these molecules crystallize in a lattice where neighboring molecules are very well separated, yielding at low temperatures (temperatures lower than the magnetic interaction between ions) molecules that behave like noninteracting giant spins. These molecules have a magneto-crystalline anisotropy, and the energy of a spin up and spin down states are equal. When the temperature is high enough the spin of the molecules can flip from spin up state to spin down sate thermally (over the anisotropy barrier), but when the temperature is much lower than the anisotropy barrier, the only possible relaxation mechanism is through tunneling. This behavior enables us to examine quantum effects on macroscopic properties, and is referred to as quantum tunneling of the magnetization (QTM).

More information on the subject can be found in Zaher's PhD Proposal.

We have published the following papers on the subject:

Z. Salman, A Keren, S. Megeawa, M. Ueda, O. Shafir, B. Barbara, C. Baines, Probing magnetic quantum tunneling in Fe₈ with muons, Physica B 326, (2003) 480-483.

Abstract:

We present a muSR study of the Fe₈ molecules from which an evidence of quantum tunneling of the magnetization (QTM) can be seen, in the form of drastic changes in the local field experience by the muon..

The paper in PDF format.

Z. Salman, A Keren, P. Mendels, V. Marvaud, A. Scuiller, M. Verdaguer, J. S. Lord, and C. Baines, Dynamics at T-->0 in the half-integer isotropic high spin molecules, Phys. Rev. B 65, 132403 (2002).

Abstract:

We investigate the dynamical spin-spin auto-correlation function of the isotropic high spin molecules CrCu₆(S=9/2), CrNi₆ (S=15/2) and CrMn₆ (S=27/2), using magnetization, muSR and NMR measurements. We find that the field autocorrelation time tau of the molecule's spin at zero and low fields is nearly temperature independent as T --> 50 mK, indicating that it is induced by a quantum process. The high temperatures tau is very different between the molecules. Surprisingly, it is identical (~10 nsec) at base temperature.

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Z. Salman, A. Keren, P. Mendels, A. Scuiller and M. Verdaguer, Quantum Fluctuations of the Magnetization in High Spin Molecules - a muSR Study , Physica B 289-290 (2000) 106-109.

Abstract:
Using zero field (ZF) and longitudinal field (LF) muSR we study the magnetic properties of high spin molecules (HSM) with spin S=15/2 and S=27/2. The LF-muSR at very low temperatures suggests that in both our samples dynamical field fluctuations are responsible for the muon relaxation. The relaxation rate lambda increases as the temperature decreases and then saturates below T<Tc indicating that the dynamics is of quantum nature. The fluctuation rate at T --> 0 of the different samples is compared.

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