T. Hasegawa

Rutile-type oxide-diluted magnetic semiconductor: Mn-doped SnO2

Epitaxial films of an oxide-diluted magnetic semiconductor with rutile structure, Mn-doped SnO2, have been fabricated by pulsed-laser deposition. As the Mn content increases, systematic changes in lattice constants and in-gap absorption are observed. Magnetization measurements show almost paramagnetic behavior. The injection of n-type carrier over 1020 cm−3 is achieved by Sb doping. A Sn0.95Mn0.05O2:Sb film shows giant positive magnetoresistance as large as 60% at 5 K.

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.

Scanning tunneling microscopy/spectroscopy of dangling-bond wires fabricated on the Si(100)-2×1-H surface

Abstract We present a scanning tunneling microscopy/spectroscopy (STM/STS) study of atomic-scale dangling-bond (DB) wires on a hydrogen-terminated Si(100)-2×1-H surface. In the case of DB wires made of paired DBs, the STS shows semiconductive electronic states with a band gap of approximately 0.5 eV. The DB wires made of both single and paired DBs show a finite density of states at Fermi energy and do not show semiconductive band gaps. We have succeeded in making an atomic-scale wire that has a finite density of states at Fermi energy on a semiconductive surface. The results are in good agreement with a recent first-principles theoretical calculations.

Full compensation of oxygen vacancies in EuTiO3 (001) epitaxial thin film stabilized by a SrTiO3 surface protection layer

We fabricated highly insulating EuTiO3 (001) epitaxial thin films capped with SrTiO3 protection layers on SrTiO3 (001) substrates by combining pulsed laser deposition and post-annealing processes. The epitaxial SrTiO3 protection layer played a significant role in compensation of oxygen vacancies in the EuTiO3 thin films by preventing excess oxidation of the films and by “locking” the EuTiO3 perovskite structure in an epitaxial manner from the top during the air-annealing process. The obtained EuTiO3 thin films demonstrated an antiferromagnetic transition at 5.4 K, quantum paraelectricity down to ∼25 K, and a magnetoelectric coupling comparable to that of bulk EuTiO3.

High-throughput characterization of composition-spread manganese oxide films with a scanning SQUID microscope

We have performed high-throughput characterization of composition-spread La 1-x Ca x MnO 3 (LCMO) and Nd 1-x Sr x MnO 3 (NSMO) films, fabricated by the precursor technique, with a scanning SQUID microscope (SSM). In both films, SSM successfully observed spatial variation of magnetic field, corresponding to magnetic phase transitions with respect to chemical composition. The obtained magnetic phase diagrams basically reproduced those reported in bulk materials. However, several distinctive differences have also been noted. For instance, the region identified as a charge ordered insulator in LCMO revealed intense field, suggesting the occurrence of phase separation into ferromagnetic and non-magnetic states. These results confirm that SSM possesses sufficient analytical performance for high-throughput characterization of combinatorial magnetic libraries in composition spread form.

Influence of CDW stacking disorder on metal-insulator transition in 1T-TaS2

Abstract We have systematically studied the metal–insulator (MI) transition in 1T-Ta1−xS2 single crystals grown by the iodine vapor transport method under various sulfur pressures. From the chemical analyses and X-ray diffraction measurements, it is concluded that the obtained crystals contain Ta vacancies up to x∼5%. The MI transition temperature TMI is found to decrease with decreasing x. However, this can hardly be interpreted by the carrier doping effect on the Mott transition, in which TMI should be very sensitive to band filling, suggesting that the formation of Ta disorders in the nearly stoichiometric compounds may play an important role. STM images of a nearly stoichiometric crystal which shows no MI transition reveal characteristic mesh-like microstructures and the MI transition induced by the electric field of the STM tip. This also indicates that CDW stacking faults along the c-axis, possibly induced by Ta disorders, is important in determining the transition temperature.

A composition-spread approach to investigate band-filling dependence on magnetic and electronic phases for Perovskite manganite

Abstract As-grown epitaxial composition-spread thin films of a colossal magnetoresistive compound, La 1− x Sr x MnO 3 (0≤ x ≤1), are fabricated by a combinatorial laser molecular beam epitaxy (laser MBE). High throughput characterization for studying x dependencies of structural, magnetic and electronic properties is demonstrated by the combination of a concurrent X-ray diffractometer, a scanning superconducting quantum interference device (SQUID) microscope and an infrared microscope spectrometer, respectively.

Anomalous domain structure in 1T-TaS2-xSex observed using scanning tunneling microscopy

On using scanning tunneling microscopy we observed charge-density-wave (CDW) structures in layered compounds 1T-TaS2−xSex (x=0, 0.3, 0.5) at room temperature, and found a new domain structure for the x=0.3 compound. The new domain structure has a mesh-like pattern, in which the domain boundaries are imaged with brighter contrast only at specific sample voltages, implying that the domain structure has an electronic origin. Moreover, the CDW arrangement is irregularly distorted, while its amplitude is spatially unchanged. These features are in clear contrast to those of the hexagonal domains in the x=0 compound, suggesting that the structural transformation with respect to x is not a well-defined phase transition with abrupt disappearance of the hexagonal domains, but a continuous restructuring of CDW. We argue for the mesh-like structure to be a kind of moire pattern, originating from the local variation of CDW stacking along the c-axis.

Enhanced coercivity of half-metallic La0.7Sr0.3MnO3 by Ru substitution under in-plane uniaxial strain

In this paper, La0.7Sr0.3Mn1-yRuyO3 (y = 0, 0.05, and 0.1) (001) epitaxial thin films were grown on NdGaO3 (110) substrates with in-plane uniaxial strain, and the coercivity (Hc) and in-plane magnetic anisotropy were evaluated. The coercivity of the epitaxial film grown on the NdGaO3 substrate, in which the easy and hard axes correspond to the NdGaO3 [11¯0] and NdGaO3 [001] directions, respectively, was significantly increased in comparison with a corresponding film grown on the SrTiO3 (001) substrate. With increasing Ru content, the Hc along NdGaO3 [11¯0] was almost twice as large as that along NdGaO3 [001], due to enhanced antiferromagnetic coupling by Ru substitution under the uniaxial strain.

Microtip-assisted metal–insulator transition in a layered chalcogenide

The layered compound 1T–TaS1.7Se0.3 forms a nanoscale domain structure, separated by mesh-like domain walls, above its bulk metal–insulator transition temperature TMI of ∼180 K. Scanning tunneling microscopy and spectroscopy of the compound demonstrated that each metallic domain can be converted to insulating one by successive scans of the probe tip just above TMI. This tip-assisted phenomenon is consistently explained by assuming that the domain structure arises from irregular distortion of charge density waves, and that the stacking pattern of charge density waves plays an essential role in the metal–insulator transition.