Our group focuses on imaging ultrafast dynamics of electronic and magnetic materials. The main focus is on understanding and manipulating the switching in magnetic materials (up to down spin domains) and phase transitions in metal-insulator materials, ferroelectric materials on femtosecond timescales and nano-meter length scales. Probing in time domain allows us to discern role played by different components (electrons, spins and lattice) in these materials following excitation by laser or electric field or current. The main characterization tools utilized in our research group vary from in-lab capabilities such as thin film deposition, Magnetic Property Measurement System (MPMS) and Versalab for characterizing electrical and magnetic properties of materials to time resolved X-ray diffraction performed at synchrotrons and x-ray lasers housed in National User Facilities. Thin film growth is done using magnetron sputtering and thermal evaporation. Photo-lithography and ebeam lithography at CNM2 at UC Davis is utilized to fabricate nanometer scale structures.
Left to Right: Ultrafast manipulation of magnetite via optical laser, spin injection from cobalt to copper layer in a spintronic device, speckle pattern obtained from orbital ordering in magnetite.
X-ray Detection of Spin Currents
Ultrafast Phase Separation in Magnetite
Correlated oxides including materials showing metal-insulator transition have recently gained interest as potential candidates for novel electronics and photonics devices. Magnetite (Fe3O4), is the first oxide where a relationship between electrical conductivity and fluctuating/localized charges was observed by Verwey in 1939. The Verwey transition is also accompanied by a structural change from monoclinic to cubic symmetry. Nevertheless, the exact mechanism of the metal-insulator transition has long remained inaccessible. We utilized time-resolved soft x-ray scattering to investigate the metal insulator transition (Verwey transition) in magnetite triggered by optical excitation. We measured the real time response of insulating magnetite to optical excitation and found it to be a two-step process. After an initial 300 fs destruction of individual trimeron (three Fe-site lattice distortions), phase separation into metallic and insulating regions occurs on a timescale of 1.5 picosecond. This work establishes the speed limit for switching in future oxide electronics. (S de Jong* and R. Kukreja* et al. Nature Materials 12, 882 (2013))