Research Highlights

Spontaneously Polarizable Heterostructures - A New Class of Materials for Computer Memory

Recently, the research group of Prof. Christos Panagopoulos demonstrated a new fascinating example of the emergence of unprecedented physical properties in correlated electron oxides nanostructures, a study published in Nature Communications. In particular, they discovered an entirely new class of artificially grown heterostructures which exhibit emergent ferroelectric behaviour tunable by an applied magnetic field, an important step towards overcoming the scarcity of naturally occurring magnetoelectric materials.

Ferroelectricity is a property of a material which possesses a spontaneous electrical polarization reversible by the application of an external electric field. Heterostructured material systems devoid of ferroic components are presumed not to display ordering associated with ferroelectricity. In heterostructures composed of transition metal oxides, however, the disruption introduced by an interface can affect the balance of the competing interactions among electronic spins, charges and orbitals. This has led to the emergence of properties absent in the original building blocks of a heterostructure, including metallicity, magnetism and superconductivity.

Dr K. Rogdakis, the leading author of the paper, used a combination of complementary experimental probes and discovered ferroelectricity in artificial tri-layer superlattices consisting solely of non-ferroelectric layers. More precisely, they prepared thin film heterostructures (depicted in the schematic on the left) based on alternative stacks consisting of three different materials (namely, LaMnO3, SrMnO3, NdMnO3) deposited on a substrate (SrTiO3) with atom by atom precision. Interestingly, the superlattices exhibited novel properties compared to the constituent insulating materials. Ferroelectricity was observed below 40K with strong tunability by superlattice periodicity (depicted in the schematic on the upper right). Furthermore, magnetoelectric coupling resulted in 150% magnetic modulation of the polarization (depicted in the schematic on the bottom right). The magnetic control of the electrical polarization is especially interesting because of potential applications in ultra-high-density data storage based on small magnetic bits, or even to explore multiple state memory elements, where data are stored both in the electric and the magnetic polarizations.

Furthermore they performed first-principles density functional calculations which indicated that broken space inversion symmetry and mixed valency, because of cationic asymmetry and interfacial polar discontinuity, respectively, give rise to the observed behavior. Notably, the understanding of the microscopic mechanism of the reported behavior enables the design of many more materials with even better functionality.

Engineering systems with tunable ferroelectric and magnetoelectric coupling in composite devices comprised of materials abundant in synthetic chemistry would widen immensely the versatility of tunable electric polarization functionality. By selecting appropriate yet abundant starting materials one may now design a plethora of low dimensional heterostructures with the desired relative arrangement of the oxide layers, of potential utility to miniature valves with electric and magnetic field tunable functions, promising to set new standards in future electronics.

Reference: "Tunable ferroelectricity in artificial tri-layer superlattices comprised of non-ferroic components", by K. Rogdakis, J.W. Seo, Z. Viskadourakis, Y. Wang, L.F.N. Ah Qune, E. Choi, J. D. Burton, E. Y. Tsymbal, J. Lee, and C. Panagopoulos, Nature Communications 3:1064, Doi:10.1038/ncomms2061 (2012).
 

Figure: [Left] Schematic drawing of the superlattice [(NdMnO3)5 / (SrMnO3)5 / (LaMnO3)5]8. The octahedral structures and spheres represent BO6 and A-site atoms in the ABO3 perovskite structure, respectively. The arrays of the arrows represent corresponding antiferromagnetic types.
[Upper right] Temperature (T) dependence of polarization (P) measured using the pyroelectric technique for a typical electric field (Ea) of +100 V cm-1 (black curve) and -100 V cm-1 (red curve) applied perpendicular to the plane of the superlattice (SL). The T-dependence of P with the magnetic field H=6 T applied parallel to the plane of the SL in magnetic field cooling (MFC) conditions is also depicted (green curve).
[Bottom right] Normalized relative change of P (Eα= +100 V cm-1) in MFC at H=6 T for SLs having different layer periodicity.

 

October 2012

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