Yasushi Ogimoto

Phase control through anisotropic strain in Nd0.5Sr0.5MnO3 thin films

Strain effect in charge- and orbital-ordered state has been investigated for Nd0.5Sr0.5MnO3 thin films deposited on (100), (110), and (111)-oriented substrates of SrTiO3. Films on (001) and (111) substrates have a monotonous temperature dependence for magnetic and transport properties showing no first-order phase transition. On the other hand, films on (110) substrate show a clear ferromagnetic-antiferromagnetic and metal-insulator transition around 170K similar to that in a bulk single crystal, which is a manifestation of the charge and orbital order. Precise control of the hole concentration was also demonstrated around half doping.

Novel Orbital Ordering Induced by Anisotropic Stress in a Manganite Thin Film

A novel structure of orbital ordering is found in a Nd0.5Sr0.5MnO3 thin film, which exhibits a clear first-order transition, by synchrotron x-ray diffraction measurements. Lattice parameters vary drastically at the metal-insulator transition at 170 K (= T(MI)), and superlattice reflections appear below 140 K (= T(CO)). The electronic structure between T(MI) and T(CO) is identified as A-type antiferromagnetic with a d(x2-y2) ferro-orbital ordering. The new type of antiferro-orbital ordering characterized by the wave vector (1/4 1/4 1/2) in cubic notation emerges below T(CO). The accommodation of the large lattice distortion at the first-order phase transition and the appearance of the novel orbital ordering are brought about by the anisotropy in the substrate, a new parameter for the phase control.

Resistance switching memory device with a nanoscale confined current path

The impact of a nanoscale confined current path is demonstrated on a resistance switching memory device. The memory element consists of a Ti layer constricted by an insulating self-assembly nanogap oxide, in which a redox reaction is anticipated for the resistance change without an aid of a forming process. The device exhibits a resistance ratio greater than 100, a data retention longer than 3.7×106s, an endurance of more than 2000cycles, and a switching operation temperature up to 125°C at an operation condition of ±2.5V with 50–100ns duration, evidencing an essential role of the nanostructure on the device performance.

Pseudomorphic strain effect on the charge-orbital ordering pattern in Pr0.5Sr0.5MnO3 epitaxial thin films

We accomplished coherent epitaxial growth of a Pr0.5Sr0.5MnO3 thin film on (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 (110) substrate, which demonstrates a robust charge-orbital ordering (COO) phase transition. In addition to the isotropic transport properties, a drastic recovery of high-resistivity state (>20Ωcm) from a field-cooled metallic state (∼0.5mΩcm) was observed at 5K with a release of a magnetic field. These results indicate a possible modification of COO pattern into CE-type COO state [d(3x2−r2∕3y2−r2)] in the film from A-type stripe COO state [d(x2−y2)] observed in bulk samples by a substrate-induced strain.

Tunneling Magnetoresistance above Room Temperature in La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3 Junctions.

We have investigated the undesired reduction of spin polarization in La1-xSrxMnO3 (LSMO)/SrTiO3 (STO)/LSMO spin tunnel junctions by employing superlattices of LSMO/STO with various x of 0.2, 0.3, and 0.4. The systematic data revealed that the instability towards the A-type (layered) antiferromagnetic phase dominates the suppressed ferromagnetism by spin canting in the vicinity of the interface as x increases. Another boundary condition of reduced Curie temperature (TC) for x<0.3 gives us a trade-off for the choice of x in spin tunnel junctions. Indeed, atomically regulated LSMO/STO/LSMO tunnel junctions with x=0.3 show tunneling magnetoresistance above room temperature up to 320 K, which agrees well with TC of the superlattices with the same x.

Interface Effect and Its Doping Dependence in La1-xSrxMnO3/SrTiO3 Superlattices

We have investigated the interface effect on magnetic and transport properties in the superlattices composed of ferromagnetic La 1- x Sr x MnO 3 (LSMO) and non magnetic SrTiO 3 (STO) ( x = 0.2, 0.3, and 0.4). The interface of LSMO with STO was proved to show spin-canting, the degree of which critically depends on the doping level x and the layer thickness ( t LSMO ) of LSMO. Large magnetoresistance subsisting at low temperature appears in the superlattices with specific sets of ( x , t LSMO ) parameters that shows a metal-insulator crossover behavior with appreciable spin-canting. Implications of the present results in the tunneling magnetoresistance for the LSMO/STO/LSMO junction are also discussed.

Strain-stabilized charge ordering and magnetorelaxor behaviors in Cr-doped Pr0.5Ca0.5MnO3 epitaxial thin films

Thin films of a magnetorelaxor compound, Pr0.5Ca0.5Mn1−yCryO3 were grown on various substrates. Strain-free and polycrystalline thin films on MgO substrates well reproduce the properties reported for the bulk samples. Pseudomorphically strained and atomically smooth epitaxial thin films grown on perovskite substrates show strain-driven modification of the properties. Being consistent with the orbital ordering accompanied with charge ordering, tensile strain anomalously stabilizes the charge-ordered (CO) state. Compressive strain also suppresses ferromagnetism but high-Cr doping (y=0.10) induces magnetorelaxor behaviors, where the volume ratio of coexisting CO and ferromagnetic phases can be tuned and memorized by the history of the magnetic-field application.

Gate Control of Electronic Phases in a Quarter-Filled Manganite

Electron correlation often produces a variety of electrically insulating states caused by self-organization of electrons, which are particularly stable at commensurate fillings. Although collapsing such ordered states by minute external stimuli has been a key strategy toward device applications, it is difficult to access their true electronic phase boundaries due to the necessity of fine-tuning of material parameters. Here, we demonstrate the ambipolar resistance switching in Pr1−xSrxMnO3 thin films (x = 0.5; an effectively 1/4-filled state) by quasi-continuous control of the doping level x and band-width W using gate-voltage and magnetic field, enabled by the extreme electric-field formed at the nanoscale interface generated in an electrolyte-gated transistor. An electroresistance peak with unprecedented steepness emerges on approaching a critical point in the x-W phase diagram. The technique opens a new route to Mott-insulator based transistors and to discovering singularities hitherto unnoticed in conventional bulk studies of strongly correlated electron systems.

Direct observation of photoinduced magnetization in a relaxor ferromagnet

Persistent ferromagnetic magnetization (M) as large as 0.6 μB per Mn site can be induced by irradiating visible laser pulses on a thin film of 1% Cr-doped Pr0.5Ca0.5MnO3. The enhancement of M upon the photoexcitation was also confirmed by the scanning superconducting quantum interference device microscope as an increase of magnetic field from ferromagnetic domains. The observed photoinduced transition from a metastable charge/orbital ordered state into a ferromagnetic one is likely to be assisted by the microscopic phase separation characteristic of such a relaxor ferromagnet as the present Cr-doped manganite.

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.