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Capping layers and their roles in polar catastrophe scenario of LaAlO3/SrTiO3 (001) systems
Cho Jinhyung,Jeen Hyoungjeen 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.81 No.10
Under the assumption that epitaxially strained growth is taking place under strained growth conditions, the metallic and insulating properties of LaAlO3 (LAO) flms grown on TiO2-terminated SrTiO3 (STO) (001) substrates are theoretically investigated in this study. We studied the electronic properties LaAlO3/SrTiO3 flms grown on the periodic superstructure of √2 × √2 in-plane squares of STO substrate to accommodate the structural strain of heterogeneous flm by allowing tilting of octahedra, as well as distortion. This enables us to study electronic and structural properties by allowing structural relaxation in a more realistic manner. This study examined spin-polarized Ti density functional calculations which promote metal–insulator transition toward thinner LAO/STO than in cases without spin polarization. Using the density of states at the Fermi level obtained from fully relaxed structures, the study shows that a capping layer of LaAlO3, Al2O3, and La2O3 on top of a stoichiometric AlO2 layer promotes toward-metallic transition, causing thin LAO flms to undergo metallic transitions sooner than stoichiometric LAO flms to do so.
Mickel, Patrick R.,Jeen, Hyoungjeen,Kumar, Pradeep,Biswas, Amlan,Hebard, Arthur F. American Physical Society 2016 Physical Review B Vol.93 No.13
<P>We report a giant linear magnetoelectric coupling in strained BiMnO3 thin films in which the disorder associated with an islanded morphology gives rise to extrinsic relaxor ferroelectricity that is not present in bulk centrosymmetric ferromagnetic crystalline BiMnO3. Strain associated with the disorder is treated as a local variable, which couples to the two ferroic order parameters, magnetization (M) over right arrow and polarization (P) over right arrow. A straightforward 'gas under a piston' thermodynamic treatment explains the observed correlated temperature dependencies of the product of susceptibilities and the magnetoelectric coefficient together with the enhancement of the coupling by the proximity of the ferroic transition temperatures close to the relaxor freezing temperature. Our interpretation is based on a trilinear coupling term in the free energy of the form (L) over right arrow . ((P) over right arrow x (M) over right arrow), where (L) over right arrow is a hidden antiferromagnetic order parameter, previously postulated by theory for BiMnO3. This phenomenological invariant not only preserves inversion and time-reversal symmetry of the strain-induced interactions but also explains the pronounced linear magnetoelectric coupling without using the more conventional higher order biquadratic interaction proportional to ((P) over right arrow . (M) over right arrow)(2).</P>
Strain-Induced Spin States in Atomically Ordered Cobaltites
Choi, Woo Seok,Kwon, Ji-Hwan,Jeen, Hyoungjeen,Hamann-Borrero, Jorge E.,Radi, Abdullah,Macke, Sebastian,Sutarto, Ronny,He, Feizhou,Sawatzky, George A.,Hinkov, Vladimir,Kim, Miyoung,Lee, Ho Nyung American Chemical Society 2012 NANO LETTERS Vol.12 No.9
<P>Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO<SUB>3</SUB>, a zero spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO<SUB>3</SUB> epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co<SUP>3+</SUP>, thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO<SUB>3</SUB> films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2012/nalefd.2012.12.issue-9/nl302562f/production/images/medium/nl-2012-02562f_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl302562f'>ACS Electronic Supporting Info</A></P>