טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentSalman Zaher
SubjectA mu-SR and NMR Study of Quantum Tunneling of the
Magnetization in High Spin Molecules
DepartmentDepartment of Physics
Supervisor Professor Amit Keren


Abstract

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. When these molecules have a magneto-crystalline anisotropy along the z axis, 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).


Tunneling between different spin states can only be induced by terms in the spin Hamiltonian which do not commute with the spin in the z direction Sz. This term can be dipolar interaction between neighboring molecules, hyperfine interaction between nuclear spins and the electronic spin of the molecules, high order spin terms, transverse magnetic field on the molecular spins etc...



In this work we aim at pointing out the term dominating QTM in anisotropic molecules by studying the spin dynamics in a family of simple isotropic high spin molecules. These molecules are CrCu6 with spin S=9/2, CrNi6 with spin S=15/2, and CrMn6 with spin S=27/2.


We use DC-Susceptibility measurements to measure the magnetic properties of these molecules, and we perform muon spin lattice relaxation measurements using mSR and proton spin lattice relaxation measurements using NMR. We find that the spin dynamics in these molecules is independent of the spin value, strongly indicating that the spin dynamics at low temperatures is dominated by hyperfine interactions.