C. Kwon

Direct evidence for a half-metallic ferromagnet

Half-metallic materials are characterized by the coexistence of metallic behaviour for one electron spin and insulating behaviour for the other. Thus, the electronic density of states is completely spin polarized at the Fermi level, and the conductivity is dominated by these metallic single-spin charge carriers. This exotic physical property could have a significant effect on technological applications related to magnetism and spin electronics. Some ferromagnetic systems, such as Heusler compounds and chromium dioxide, have been predicted theoretically to be half-metallic. However, a half-metallic system has not been demonstrated directly and the predictions are still in doubt,. Here we report spin-resolved photoemission measurements of a ferromagnetic manganese perovskite, La0.7Sr0.3MnO3, which directly manifest the half-metallic nature well below the Curie temperature. For the majority spin, the photoemission spectrum clearly shows a metallic Fermi cut-off, whereas for the minority spin, it shows an insulating gap with disappearance of spectral weight at ∼0.6 eV binding energy.

Stress-induced effects in epitaxial (La0.7Sr0.3)MnO3 films

Abstract The transport and magnetic properties of epitaxial (La0.7Sr0.3)MnO3 (LSMO) films deposited on LaAlO3 (Sample L, under a compressive stress) and SrTiO3 (Sample S, under a tensile stress) are compared. The magnetic force microscope image of Sample L shows a ‘maze-like’ pattern indicating a sizable out-of-plane magnetization, while the magnetic image of Sample S shows a ‘feather-like’ pattern indicative of in-plane magnetization. The hysteresis loop and ferromagnetic resonance measurements indicate that the lattice mismatch between the film and the substrate is the origin of the differences in the magnetic anisotropy of the two films. Sample L has a smaller ‘peak’ resistivity and a larger saturation magnetization at room temperature than Sample S. Our results can be understood by the internal ‘pressure’ consequent upon lattice mismatch and agree qualitatively with the recent hydrostatic pressure studies on the doped manganites.

Microstructure and dielectric properties of Ba1−xSrxTiO3 films grown on LaAlO3 substrates

We report a systematic study of the microstructure and dielectric properties of barium strontium titanate, Ba1−xSrxTiO3, films grown by laser ablation on LaAlO3 substrates, where x=0.1–0.9 at an interval of 0.1. X-ray diffraction analysis shows that when x 0.4, compared with the peak temperatures of the bulk Ba1−xSrxTiO3. At room temperature, the dielectric constant and tunability are relatively high when x⩽0.4 but start to decrease rapidly as x increases.

Microstructure of epitaxial La0.7Ca0.3MnO3 thin films grown on LaAlO3 and SrTiO3

Epitaxial La0.7Ca0.3MnO3 (LCMO) thin films of a thickness ∼170 nm were grown on (001) LaAlO3 (LAO) and (001) SrTiO3 (STO) substrates by pulsed laser deposition. Transmission electron microscopy and associated techniques have been applied to investigate the microstructures introduced by lattice mismatch that are responsible for the observed differences in properties between these two films. Numerous secondary phase rods were observed in both films. For the LCMO/LAO film, Ca-deficient secondary-phase rods originated in the film after a thickness of about 25 nm and were found to be responsible for relieving in-plane compressive stress during the island growth. In the case of STO substrate, however, almost all of secondary-phase rods initiated at the film–substrate interface. The lattice mismatch between LCMO and STO is relaxed into regions of good coherent fit separated by such secondary phases, possibly resulting from interfacial reaction. The two types of substrates lead to the formation of two different c...

Large magnetoresistance in La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3 ramp-edge junctions

We report on the fabrication of ferromagnet–insulator–ferromagnet junction devices using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. The maximum junction magnetoresistance (JMR) as large as 23% is observed below 300 Oe at low temperatures (T<100 K). Our ramp-edge junctions exhibit JMR of 6% at 200 K with a field less than 100 Oe. The device performance at room temperature is believed to be limited by both the nearly equivalent coercive fields in the electrodes and the magnetization process, rather than by the insulating barrier.

Spin–lattice interaction in colossal magnetoresistance manganites

The metal–insulator transition and underlying spin dynamics in La0.7D0.3MnO3 (D=Ca, Sr) are investigated using optical pump–probe spectroscopy at 1.5 eV. Our measurements, which span the ferromagnetic–paramagnetic transition temperature, reveal that the dynamics of the optically induced spectral weight transfer follow the temperature dependence of the magnetic specific heat. This dependence reflects the intrinsic interdependence between the optical conductivity and magnetism in the manganites allowing for the determination of the spin-lattice coupling magnitude.

Microwave surface resistance of HgBa2CaCu2O6+δ thin films

Microwave surface resistance (Rs) has been measured on c-axis-oriented superconducting HgBa2CaCu2O6+δ (Hg-1212) films. A cavity perturbation method was employed using a high-Q Nb cavity cooled at 4.2 K. For the best film, an Rs as low as ∼0.3 mΩ was observed at 10 GHz up to ∼120 K on Hg-1212 films that have smooth surface morphology and high critical current density near 2 MA/cm2 at 100 K and self-field. This result suggests that Hg-1212 films are very promising for microwave applications above 100 K.

Observation of spin-dependent transport and large magnetoresistance in La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3 ramp-edge junctions

We have fabricated ferromagnet-insulator-ferromagnet junctions using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. Pulsed laser deposition was used to deposit the multilayer thin films and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance is dependent on the relative orientation of the magnetization in the electrodes. A junction magnetoresistance (JMR) as large as 30% is observed at low temperatures and low fields. In addition, we have found that JMR is reduced at high temperatures (T>100 K) and decreases monotonically with increasing field at high fields (0.5 T

H-dependent magnetic domain structures in La0.67Sr0.33MnO3 thin films

Abstract Maze- and bubble-like magnetic domain structures have been observed under ambient conditions in La 0.67 Sr 0.33 MnO 3 (LSMO) films by magnetic force microscopy (MFM) for films grown on a compressive (LaAlO 3 ) lattice-mismatched substrate. The substrate-induced stress in this soft magnetic material appears to be a necessary but not sufficient condition for the appearance of these structures, i.e. no magnetic structure has been seen for other films grown under similar conditions on the same substrate with similar distortions to the unit cell. In this study, in order to understand the stability of these structures, we have examined two films grown at 750°C and three films grown at 800°C by pulsed-laser deposition by scanning tunneling and magnetic force microscopy to obtain microstructure and domain structures as a function of in-plane magnetic field strength. The latter films consisted of pitted coalesced layers with some surface 3D growth. Although the films were nominally grown under the same conditions, there were subtle differences in the domain structure, wall spacing, the latter due in part to differences in film thickness, and response to an applied in-plane magnetic field. As the field was increased, the maze-like structures became stripe domains with reduced out-of-plane magnetization and decreased wall spacing, consistent with parallel rather than antiparallel alignment of in-plane spin polarization. After removal of the field, the stripe domains remained but the wall spacing and z polarization component returned to the original value. For one 800°C film the field required for changes in and disappearance of the domains paralleled the slope in the field-dependent magnetization. In contrast, the magnetic structure for one film with a thin insulating cap layer was reversible, suggesting that the walls were pinned by the cap.

Laser induced dynamic spectral weight transfer in La0.7Ca0.3MnO3

Abstract We report on optical pump-probe measurements at 1.5 eV and 3 eV on the colossal magnetoresistance material La 0.7 Ca 0.3 MnO 3 . The material response is determined in the femtosecond to the nanosecond temporal regimes. By analyzing the absorption and refractive dynamics within the Kramers–Kronig transformations we demonstrate dynamic spectral weight transfer from the intraband Drude component to the interband transitions. This spectral transfer and its strongly temperature dependent dynamics are discussed in the framework of photoinduced demagnetization due to the exchange of energy and thermal equilibration among the electronic, lattice and spin systems. This effect is observed in other manganites with different transition temperatures. The measured dynamics are found to be insensitive to the excitation photon energy.