VT-STM STUDIES OF SUPERPARAMAGNETISM AND THE VERWEY TRANSITION IN MAGNETITE NANOCRYSTALS

Magnetic and electronic properties of nanomaterials and their temperature dependence have been the subjects of many scientific works in the past two decades due their disctinction from their analogues in bulk materials. Variable temperature scanning tunneling microscopy (VT-STM) and spectroscopy techniques are able to probe the density of states of nanostructures and observe its change as a function temperature. VT-STM was applied in the two projects presented in this work.

In the first project, the magnetization dynamics of individual magnetite nanocrystals (NCs) was probed by magne­toresistive VT-STM, in which a magnetoresistive junction is formed between the substrate and the magnetic NC under study. When the temperature was tuned close to the magnetization blocking of a superparamagnetic NC, the slow magnetization switching of the particle caused fluctuations in the tunnel current passing through the particle, which appeared as telegraph noise in current vs. time measurements. Analysis of the current fluctuations yielded estimates for local magnetic fields sensed by the NC, its magnetic anisotropy energy, and a low limit for its spin-polarization degree.

In the second project, the temperatureevolution of the density of states in magnetite NCs around the Verwey transition temperature was probed using VT-STM. The transition was observed as a significant change in the electronic structure around the Fermi level. Since the NCs studied are only few unit cells in size, it is quite a surprise that such a sharp transition was observed. The transition temperature was invariably observed around 101K for different NCs, as opposed to 123K typically found in stoichiometric bulk crystals. This suggests that the lowering of the transition temperature is an intrinsic finite size effect.