No abstract

Topological Hall effect (THE), appearing as bumps and/or dips in the Hall resistance curves, is considered as a hallmark of the skyrmion spin texture originated from the inversion symmetry breaking and spin–orbit interaction. Recently, Néel‐type skyrmion is proposed based on the observed THE in 5d transition metal oxides heterostructures such as SrRuO3/SrIrO3 bilayers, where the interfacial Dzyaloshinskii–Moriya interaction (DMI), due to the strong spin–orbit coupling (SOC) in SrIrO3 and the broken inversion symmetry at the interface, is believed to play a significant role. Here the emergence of THE in SrRuO3 single layers with thickness ranging from 3 to 6 nm is experimentally demonstrated. It is found that the oxygen octahedron rotation in SrRuO3 also has a significant effect on the observed THE. Furthermore, the THE may be continuously tuned by an applied electrical field. It is proposed that the large SOC of Ru ions together with the broken inversion symmetry, mainly from the interface, produce the DMI that is responsible for the observed THE. The emergence of the gate‐tunable DMI in SrRuO3 single layer may stimulate further investigations of new spin–orbit physics in strong SOC oxides.

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The correlated charge, spin, orbital, and lattice degrees of freedom in transition metal oxides make them an attractive platform to explore rich physics and potential applications. [ 1,2 ] Oxide interfaces offer even more exciting opportunities due to the emerging effects such as broken inversion symmetry, interfacial exchange interaction, and spatial confi nement. [ 3,4 ] Heterostructures composed by non-magnetic band insulators LaAlO 3 (LAO) and SrTiO 3 (STO) have attracted much attention recently, and the unexpected conduction is often linked to the electronic reconstruction at the polar (LAO)-nonpolar (STO) heterointerface. [ 5,6 ] One defi ning feature of the interface conduction is the thickness dependence, i.e., it is insulating when the LAO thickness is below four unit cells (u.c.), whereas the conduction emerges above the critical thickness. [ 7 ] Furthermore, the carriers in the two-dimensional electron gas (2DEG) are confi ned within several nm near the interface as a result of broken symmetry, and the profi le of distribution has a sensitive dependence on the carrier density. [8][9][10][11][12][13] Based on the 2DEG at the LAO/ STO interface, nanoscale control of the insulator-to-metal transition using local probes has also been demonstrated. [ 14,15 ] The recent revelation of magnetism at the LAO/STO interface added another facet to the exciting physics. [16][17][18][19][20] Two notable sources were proposed for leading to the weak magnetism discovered in the LAO/STO heterostructures: One is the intrinsic electron reconstruction [ 16,21 ] and the other is the extrinsic oxygen vacancies. [ 22 ] In addition, the inhomogeneous distribution of magnetic dipoles as demonstrated in a recent scanning superconducting quantum interference device (SQUID) microscopy study clearly underscores the critical role of disorder in forging the magnetic landscape. [ 20 ] In spite of the efforts, [ 16,[18][19][20]23 ] the nature of magnetism discovered in samples prepared in a wide range of conditions remains as an issue of debate.To shed light on the physics of LAO/STO interface and to modulate its properties, electric fi eld effect was proven to be a powerful tool. [ 7,[24][25][26][27] In particular, an insulator-to-metal transition has been achieved by using electric fi eld gating at room temperature. [ 7 ] In these previous efforts, the STO substrates were used as the dielectric insulator in the back-gate confi guration, and high voltages of tens to hundreds of volts were required. Recently, ferroelectric Pb(Zr 0.2 Ti 0.8 )O 3 was used to bias the LAO/STO interface in a top-gate confi guration, and nonvolatile modulation of the 2DEG was demonstrated. [ 28 ] As a breakthrough in such electric fi eld effect studies, electric double-layer transistors (EDLTs) with the top-gate confi guration were employed to trigger the transformation of ground states in some key materials. [29][30][31] However, so far there has been no report on applying this powerful technique to exploring the transport properties of oxide interfaces.In...

Flexoelectricity induced by the strain gradient is attracting much attention due to its potential applications in electronic devices. Here, by combining a tunable flexoelectric effect and the ferroelectric photovoltaic effect, we demonstrate the continuous tunability of photoconductance in BiFeO3 films. The BiFeO3 film epitaxially grown on SrTiO3 is transferred to a flexible substrate by dissolving a sacrificing layer. The tunable flexoelectricity is achieved by bending the flexible substrate which induces a nonuniform lattice distortion in BiFeO3 and thus influences the inversion asymmetry of the film. Multilevel conductance is thus realized through the coupling between flexoelectric and ferroelectric photovoltaic effect in freestanding BiFeO3. The strain gradient induced multilevel photoconductance shows very good reproducibility by bending the flexible BiFeO3 device. This control strategy offers an alternative degree of freedom to tailor the physical properties of flexible devices and thus provides a compelling toolbox for flexible materials in a wide range of applications.