First Observation of Ferroelectricity in ∼1 nm Ultrathin Semiconducting BaTiO3 Films.

The requirements of multifunctionality in thin-film systems have led to the discovery of unique physical properties and degrees of freedom, which exist only in film forms. With progress in growth techniques, one can decrease the film thickness to the scale of a few nanometers (∼nm), where its unique physical properties are still pronounced. Among advanced ultrathin film systems, ferroelectrics have generated tremendous interest. As a prototype ferroelectric, the electrical properties of BaTiO3 (BTO) films have been extensively studied, and it has been theoretically predicted that ferroelectricity sustains down to ∼nm thick films. However, efforts toward determining the minimum thickness for ferroelectric films have been hindered by practical issues surrounding large leakage currents. In this study, we used ∼nm thick BTO films, exhibiting semiconducting characteristics, grown on a LaAlO3/SrTiO3 (LAO/STO) heterostructure. In particular, we utilized two-dimensional electron gas at the LAO/STO heterointerface as the bottom electrode in these capacitor junctions. We demonstrate that the BTO film exhibits ferroelectricity at room temperature, even when it is only ∼2 unit-cells thick, and the total thickness of the capacitor junction can be reduced to less than ∼4 nm. Observation of ferroelectricity in ultrathin semiconducting films and the resulting shrunken capacitor thickness will expand the applicability of ferroelectrics in the next generation of functional devices.

Polarity-tunable magnetic tunnel junctions based on ferromagnetism at oxide heterointerfaces.

Complex oxide systems have attracted considerable attention because of their fascinating properties, including the magnetic ordering at the conducting interface between two band insulators, such as LaAlO3 and SrTiO3. However, the manipulation of the spin degree of freedom at the LaAlO3/SrTiO3 heterointerface has remained elusive. Here, we have fabricated hybrid magnetic tunnel junctions consisting of Co and LaAlO3/SrTiO3 ferromagnets with the insertion of a Ti layer in between, which clearly exhibit magnetic switching and the tunnelling magnetoresistance effect below 10 K. The magnitude and sign of the tunnelling magnetoresistance are strongly dependent on the direction of the rotational magnetic field parallel to the LaAlO3/SrTiO3 plane, which is attributed to a strong Rashba-type spin-orbit coupling in the LaAlO3/SrTiO3 heterostructure. Our study provides a further support for the existence of the macroscopic ferromagnetism at LaAlO3/SrTiO3 heterointerfaces and opens a novel route to realize interfacial spintronics devices.

Magnetotransport properties and kondo effect observed in a ferromagnetic single-crystalline Fe(1-x)Co(x)Si nanowire.

We report unconventional magnetotransport properties of an individual Fe(1-x)Co(x)Si nanowire. We have studied the dependence of the resistivity on the angle between the directions of the magnetization and electrical current below the Curie temperature (T(C)). The observed anisotropic magnetoresistance (MR) ratio is negative, thereby indicating that the conduction electrons in a minority spin band of the Fe(1-x)Co(x)Si nanowire dominantly contribute to the transport. Unlike typical ferromagnets, positive MR is observed in the overall temperature range. MR curves are linear below T(C) and show a quadratic form above T(C), which can be explained by the change of density of states that arises as the band structures of the Fe(1-x)Co(x)Si nanowire shift under a magnetic field. The temperature dependence of the resistivity curve is sufficiently explained by the Kondo effect. The Kondo temperature of the Fe(1-x)Co(x)Si nanowire is lower than that of the bulk state due to suppression of the Kondo effect. The high single crystallinity of Fe(1-x)Co(x)Si nanowires allowed us to observe and interpret quite subtle variations in the prominent intrinsic transport properties.