Applying local pressure to image and tune SrTiO3 domain wall polarity
We measure the voltage of a LaAlO3/ SrTiO3 (LAO/STO) sample as a function of local pressure. We found that the pressure we apply has very strong response on the STO domain walls. Using simultaneous temperature dependent scanning SQUID measurements and computer simulations we realized that this response has to do with STO domain wall polarity that is pressure tunable. We also understand now that the origin for the current stripes we previously observed is this domain wall polarity.
Anisotropic transport at the LaAlO3/SrTiO3 interface explained by microscopic imaging of channel-flow over SrTiO3 domains
The current density at the LAO/STO interface is modulated over STO domain walls. Here we show that these modulations are significant to the global electrical properties of the interface. The figure shows two measurement configurations on a square pattern. If the sample was homogenous there would not be any difference between the two measurements. But in this case we see stripy modulations. If the current is injected along (Vertical) the stripes the resistance of the interface can be up to 4 times smaller than the other direction perpendicular to the stripes (Horizontal).
Applied Material & Interfaces (2016)
Journal of Superconductivity and Magnetism (2015)
Locally enhanced conductivity in LAO/STO due to STO domains
The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO3 and TiO2-terminated SrTiO3 (STO). To better understand the interface conductivity, we used scanning SQUID microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flow was modulated over narrow paths oriented along the crystallographic axes. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure.
The structural domain walls of STO move under the appliance of electric fields. The images below show the same location of the sample under 4 different back gate voltages. It is apparent that domain wall on the right becomes longer and then shorter again when the voltage is set back down to zero.
Critical thickness for magnetism in LAO/STO
LaAlO3 and SrTiO3 are two non-magnetic insulating materials. After growing at least 4 unit cells of LAO on a STO substrate, the polar/nonpolar interface exhibits a number of interesting properties including a high mobility 2D conductivity, superconductivity below 100 mK, magnetism and an electric field-controlled metal-insulator and superconductor-insulator transition.
We found that there is a critical thickness to the magnetism in this material, which means that the electronic reconstruction is essential for the magnetism. At the same time we found that disorder plays an important role in inducing the magnetism.
Nature Communications (2012)
With: Kathryn Moler, Harold Hwang @Stanford
Tuning the magnetism in LAO/STO
We are trying to learn more about the microscopic nature of this magnetism by tuning it in different ways. For example we found that local strain manipulates the ferromagnetic patches in LAO/STO. The images below show repeated scans of a small group of ferromagnetic patches that change as a result of local strain we apply with the tip of the SQUID’s chip.
Scanning Probe Manipulation of Magnetism at the LaAlO3/SrTiO3 Heterointerface
Nano Letters (2012)
With: Kathryn Moler, Harold Hwang @Stanford