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.

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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 Imaging of Phase Separation in Cobaltites

Perovskite oxides (ABO3) exhibit a wide array of tunable functional properties which can be manipulated by tailoring ionic distribution and stoichiometry. Specifically, pervoskite oxide heterostructures provide access to emergent functional and structural phases which are not present in the bulk constituent materials. However, the role played by nanoscale morphology, i.e., phase separation, and defects in these oxide thin films and heterostructures remains largely unknown. High-resolution (scanning) transmission electron microscopy (STEM/TEM) provides a direct method to access atomic lengthscales; however, it is restricted to extremely thin samples (<10 nm) and the sample preparation, as well as the imaging process itself, can modify the nanoscale morphology and interfacial properties. We utilize x-ray nanodiffraction to directly image the nanoscale morphology of cobaltite thin films as they are progressively transformed from the equilibrium perovskite phase to the metastable brownmillerite (BM) phase with increasing Gd thickness. Our studies show the coexistence of perovskite and BM phases with a critical oxygen vacancy concentration threshold which leads to the formation of extended BM filaments. (I. Rippy et al PRM 3, 082001 (2019))

X-ray Detection of Spin Currents

Spintronics i.e spin based solid state devices, is an active field of research where spin of an electron is utilized for information processing. Hence, detailed understanding of spin currents from ferromagnet to non-magnets is an important step in development of spin based devices. However, directly observing these spin currents is extremely challenging due to magnetic moment injected into non-magnet being really small, less than 1/10000 of a regular ferromagnet. We have imaged spin currents in a non-magnet Cu layer from a ferromagnet Co layer in a nanoscale device for the first time using x-ray microscopy. X-rays pulses from a synchrotron source allow us to focus on the magnetic properties of the non-magnet Cu only. We found two components to this transient magnetic moment detected in the Cu layer. The first component is the Co/Cu interface contribution, where the spin current applies a torque and aligns the interfacial spins in the direction of the current. The second component is the induced magnetic moment in the bulk Cu due to spin current. (R. Kukreja et al. PRL 115, 096601 (2015))


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))

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