Yoshinori Konishi

Orbital-State-Mediated Phase-Control of Manganites

Using the epitaxial strain, the magnetic and electronic phases can be controlled for thin films of the manganites, La1-xSrxMnO3, grown on perovskite substrates with various lattice parameters. The strain-induced orbital-ordering (disordering) via coupling

ATOMICALLY DEFINED EPITAXY AND PHYSICAL PROPERTIES OF STRAINED LA0.6SR0.4MNO3 FILMS

La0.6Sr0.4MnO3 thin films were fabricated on SrTiO3 (001) substrates using pulsed laser deposition with observing persistent intensity oscillation of reflection high-energy electron diffraction. By atomic force microscopy, the surface of resulting films was confirmed to be extremely flat, showing atomically smooth terraces and 0.4 nm high steps corresponding to a unit cell height of perovskite. The surface terminating atomic layer was unambiguously assigned to the MnO2 layer by coaxial impact collision ion scattering spectroscopy. Crystal symmetry of the films is distorted into a tetragonal one due to the strain to fulfill perfect in-plane matching with the substrate even for films as thick as 100 nm. Even for films as thin as 4 nm (10 unit cells), ferromagnetic transition takes place to induce a metallic state and large negative magnetoresistance is observed as well.

Epitaxial thin films of ordered double perovskite Sr2FeMoO6

Epitaxial thin films of Sr2FeMoO6, that are ordered double perovskite with half-metallic nature, have been successfully prepared on SrTiO3 (001) and (111) substrates by pulsed laser deposition in a narrow window of the temperature and oxygen pressure. From the surface morphology analysis for the atomic scale step-and-terrace structures, the film growth is concluded to take place with the chemical formula as the growth unit when the ordering direction is normal to the surface. The films showed metallic conduction with ferromagnetic transition temperature above 400 K and intergrain tunneling type magnetoresistance even at room temperature.

Switching behavior of epitaxial perovskite manganite thin films

We have observed electric-field-induced and photoinduced switching from insulating to conducting state in stressed epitaxial Sm0.5Sr0.5MnO3 thin films fabricated by pulsed-laser deposition. Previously known only in bulk crystals of perovskite manganese oxides, the switching behavior in thin film form is more significant for potential practical utility of the strongly correlated electron systems including optical devices.We have observed electric-field-induced and photoinduced switching from insulating to conducting state in stressed epitaxial Sm0.5Sr0.5MnO3 thin films fabricated by pulsed-laser deposition. Previously known only in bulk crystals of perovskite manganese oxides, the switching behavior in thin film form is more significant for potential practical utility of the strongly correlated electron systems including optical devices.

Fabrication and physical properties of c-axis oriented thin films of layered perovskite La2−2xSr1+2xMn2O7

We have grown c-axis oriented films of a layered perovskite La2−2xSr1+2xMn2O7 (x=0.4) by pulsed laser deposition under the limited growth condition; above 900 °C and below 100 mTorr for substrate temperature and oxygen pressure (PO2), respectively. Otherwise, epitaxial but composition-unidentified films were deposited. The films show a resistive transition around 100 K in coincidence with the magnetic transition. The value of resistivity at low temperature is larger than that of a single crystal by about two orders of magnitude, perhaps due to the canted spin ordering. The films show gigantic magnetoresistance accompanied with hysteresis.

Perfect epitaxy of perovskite manganite for oxide spin-electronics

The factors of perfection in perovskite epitaxy and heterostructures to explore a novel field of spin-electronics based on half-metallic and ferromagnetic manganite compounds are defined and demonstrated. Pulsed laser deposition technique is shown to be extremely useful for making layers with such well-defined structures as atomically smooth surface and interfaces, atomically regulated thickness, and epitaxial strain caused by the substrates, when the deposition conditions are well optimized. The importance of charge transfer and spin interaction at the interfaces are highlighted for ultra-thin films and superlattices, respectively.

Perovskite superlattices as tailored materials of correlated electrons

Abstract A systematic study is presented on the control of magnetoelectronic properties of La 1− x Sr x MnO 3 thin films and their superlattices. First, it is demonstrated that the orbital/spin ordering of the thin films can be controlled by tuning epitaxial strain from the substrate even at a fixed doping level. Then, we show two prototypical cases of the manganite-based superlattice, which show gigantic magnetoelectronic responses due to the spin-modification at the interfaces. In the case of superlattices composed of ferromagnetic metal La 1− x Sr x MnO 3 ( x =0.2, 0.3, and 0.4) and non-magnetic insulator SrTiO 3 , the remarkable variation of magnetization with doping level indicates that spin canting is caused by the charge-transfer at the interface in a doping ( x )-dependent manner. In the superlattices of La 1− x Sr x MnO 3 ( x =0.4) and antiferromagnetic La 1− x Sr x FeO 3 ( x =0.4), antiferromagnetic spin correlation in La 1− x Sr x FeO 3 is transmitted to the ferromagnetic spin ordering in the La 1− x Sr x MnO 3 layer to induce the spin canting. External magnetic field restores the ferromagnetism of La 1− x Sr x MnO 3 near the interfaces, giving rise to large magnetoresistance subsisting to the lowest temperature in these superlattice systems.

Epitaxial growth and strain of manganite thin films

Thin films of La 1-x SrxMnO 3 (x = 0.4) were fabricated using pulsed laser deposition (PLD) method on various substrates for investigation of the effect of strain induced by lattice mismatch on the electro-magnetic properties. The growth conditions were optimized on SrTiO 3 substrate which has the best lattice matching (+0.9%) among the substrates used in this study. The resistivity and magnetization of thick films (> 6 nm) on SrTiO 3 were comparable to those of bulk single crystals. With increasing the lattice mismatch by using LaAlO 3 (-2.0%), the resistivity increased to show insulating behavior caused by biaxial strain due to coherent epitaxial growth. However, further increase of mismatch for NdAlO 3 (-3.1%) and YAlO 3 (-4.0%) made it possible to relax partly the strain, resulting in highly conductive films.

Orbital-driven variation of electronic structures in tetragonal La1/2Sr1/2MnO3 as investigated by optical spectroscopy

Optical conductivity spectra (σ(ω)) were investigated for coherently strained epitaxial films of tetragonal La 1/2 Sr 1/2 MnO 3 with various c / a ratios (1.04, 1.00, and 0.98). With decreasing c / a , σ(ω) in the ground state shows a successive dramatic change from an insulating gap-like to metallic but highly incoherent spectrum, then to a pseudo-gap one, which reflects the strain-induced e g orbital ordering/disordering and the resultant different spin structures. The variation of the electronic structure induced by such a strain-orbital coupling is discussed in comparison with some theoretical models.

Experimental elucidation: Microscopic mechanism of resonant X-ray scattering in manganite films

Resonant X-ray scattering experiments have been performed on perovskite manganite La 0.5 Sr 0.5 MnO 3 thin films, which are grown on three distinct perovskite substrates with a coherent epitaxial strain and have a forced ferro -type orbital ordering of Mn 3 d orbitals. Using an interference technique, we have successfully observed the resonant X-ray scattering signal from the system having the ferro -type orbital ordering and also revealed the energy scheme of Mn 4 p bands. For the forced ferro -type orbital ordering system, the present results evidence that the resonant X-ray scattering signal originates from the band structure effect due to the Jahn–Teller distortion of a MnO 6 octahedron, and not from the Coulomb interaction between 3 d and 4 p electrons.