טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentYaari Maayan
SubjectTemperature Change during Fe8 Molecular Magnet Tunneling
and its Implications to the Landau Zener Theory
DepartmentDepartment of Physics
Supervisor Professor Amit Keren
Full Thesis textFull thesis text - English Version


Abstract

Magnetism is one of the oldest scientific disciplines, but also one at the forefront of emerging nanotechnology era. Nanomagnetism is dealing with magnetic properties of objects that have at least one dimension in the nanoscopic range. Single Molecule Magnets (SMMs) are nano-magnets with all dimensions on the nano-meter scale. These molecules containing several transition-metal ions embedded in organic matrix. They exhibit superparamagnetic-like properties at high temperatures and quantum phenomena at low temperatures.

In the last few decades, Quantum Tunneling of Magnetization (QTM) in SMM has become a focus of interest in physics and chemistry since it’s a macroscopic phenomena which is controlled by quantum laws. SMMs are very good candidates to observe this kind of phenomena, since some of them have large spin ground state and the strong magnetic anisotropy barrier that allows for QTM. QTM is a process where the magnetization versus a linear time dependent magnetic field, exhibits a staircase behavior, where the magnetization jumps at equally spaced resonant magnetic fields. This tunneling of magnetization process is usually analyzed by the Landau-Zener (LZ) theory.

In some SMMs this magnetization reversal can ignite another unique behavior which called 'Magnetic Avalanche' or ‘Magnetic Deflagration’. These avalanches were first reported by Paulsen and Park in 1995 for the Mn12ac SMM crystals. Magnetic Deflagration is a process where an abrupt reversal of the magnetization occurs at a resonant magnetic field and during this process there is a spin reversal front that propagates in a subsonic constant velocity along the crystal. Previous publications by Tom Leviant and Amit Keren show that deflagration velocity in the Fe8 SMM is of the order of ~1 m/s and the thermal diffusivity is ~2•106 m2/s. Those two parameters allow one to estimate the temperature change of the Fe8 SMM during the magnetic deflagration process according to deflagration theory.

Based on this estimation we constructed an experiment to measure the crystals temperature during different magnetization reversal processes and in particular during magnetic deflagration. We found that direct measurements of the sample temperature are well below the energy barrier height of the Fe8 SMM Hamiltonian and smaller than the temperature estimation in the previous work. Based on those results we are able to suggest an adjustment for the LZ formula in order to analyze magnetization reversal processes in SMMs.