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.
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.
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