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:
- T.Leviant, A.Keren, E.Zeldov, and Y.Myasoedov, Quantum ignition of deflagration in the Fe8 molecular magnet, Phys. Rev. B., 90, (2014) 134405.
Abstract:
We report spatially resolved, time-dependent, magnetization reversal measurements of an Fe8 single molecular magnet using a microscopic Hall bar array. We found that a deflagration process, where molecules reverse their spin direction along a moving front, can be ignited quantum mechanically (T → 0) at a resonance field, with no phonon pulse. The avalanche front velocity is of the order of 1 m/s and is sensitive to field gradients and sweep rates. We also measured the thermal diffusivity κ in Fe8. This allows us to estimate the “flame” temperature.
- T.Leviant, S.Hanany, Y.Myasoedov, and A.Keren, Testing the radiation emanating from the molecular nanomagnet Fe8 during magnetization reversals, Phys. Rev. B., 90, (2014) 054420.
Abstract:
Photons emitted by transition between the discrete levels of single molecular magnets might obey the elementary condition for Dicke’s super-radiance. We investigate this possibility in the Fe8 molecule where magnetization jumps are known to occur at discrete magnetic-field values. We found energy bursts each time the molecule undergoes a magnetization jump, confirming their quantum nature. A series of tests indicated that photons carry out the energy and that indeed these photons obey the elementary conditions for super-radiance.
- O.Shafir, A.Keren, S.Maegawa, M.Ueda, and E.Shimshoni, Magnetic quantum tunneling in Fe8 with excited nuclei, Phys. Rev. B., 82, (2010) 014419.
Abstract:
We investigate the effect of dynamic nuclear-spin fluctuation on quantum tunneling of the magnetization (QTM) in the molecular magnet Fe8 by increasing the nuclei temperature using radio frequency (rf) pulses before the tunneling measurements. Independently we show that the nuclear-spin-spin relaxation time T2 has strong temperature dependence. Hence, in principle, the rf pulses should modify the nuclear-spin dynamic. Due to very long spin-lattice relaxation time, the rf pulses do not change the electrons spin temperature. Nevertheless, we found no effect of the nuclear-spin temperature on the tunneling probability. This suggests that in our experimental conditions only the hyperfine-field strength is relevant for QTM. We demonstrate theoretically how this can occur.
- O.Shafir and A.Keren, Electromagnetic radiation emanating from the molecular nanomagnet Fe8, Phys. Rev. B., 79, (2009) 180404(R).
Abstract:
Photons emitted by transition between the discrete levels of single molecular magnets might obey the elementary condition for Dicke’s super-radiance. We investigate this possibility in the Fe8 molecule where magnetization jumps are known to occur at discrete magnetic-field values. We found energy bursts each time the molecule undergoes a magnetization jump, confirming their quantum nature. A series of tests indicated that photons carry out the energy and that indeed these photons obey the elementary conditions for super-radiance.
- A.Keren, O.Shafir, E.Shimshoni, V.Marvaud, A.Bachschmidt and J.Long, Experimental Estimates of Dephasing Time in Molecular Magnets, Phys. Rev. Lett., 98, (2007) 257204.
Abstract:
Muon spin relaxation measurements in isotropic molecular magnets (MM) with a spin value S ranging from 7/2 to 27/2 are used to determine the magnitude and origin of dephasing time τϕ of molecular magnets. It is found that τϕ ~ 10 nsec with no S or ligand dependence. This indicates a nuclear origin for the stochastic field. Since τϕ is a property of the environment, we argue that it is a number common to similar types of MM. Therefore, τϕ is shorter than the Zener and tunneling times of anisotropic MM such as Fe8 or Mn4 for standard laboratory sweep rates. Our findings call for a stochastic Landau-Zener theory in this particular case.
- O.Shafir, A.Keren, S.Maegawa, M.Ueda, A.Amato, C.Baines, Demonstrating multibit magnetic memory in the Fe8 high-spin molecule by muon spin rotation, Phys. Rev. B, 72, (2005) 092410.
Abstract:
We develop a method to detect the quantum nature of high-spin molecules using muon spin rotation and a three-step field cycle ending always with the same field. We use this method to demonstrate that the Fe8 molecule can remember six (possibly eight) different histories (bits). A wide range of fields can be used to write a particular bit, and the information is stored in discrete states. Therefore, Fe8 can be used as a model compound for multibit magnetic memory. Our experiment also paves the way for magnetic quantum tunneling detection in films.
- 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.
- 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, (2002) 132403.
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.
- 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.