Tong-yun Zhao

Laser molecular beam epitaxy and characterization of perovskite oxide thin films

More than 10 kinds of perovskite oxide thin films and their heterostructures have been successfully fabricated on SrTiO3(STO) (0 0 1) substrates by laser molecular beam epitaxy (laser MBE). Measurements of atomic force microscopy (AFM) and high-rt solution transmission electron microscopy (HRTEM) reveal that the surfaces and interfaces are Atom level smooth. The unit cell layers and the lattice structure are perfect. The enhancement of secund-harmonic generation (SHG) in BaTiO3/SrTiO3 9BTO/STO) superlattices and the thickness dependent structural characteristics of BaTiO3 (BTO) thin fillns were observed. The all-prrovskite oxide P-N junctions have been successfully fabricated and better I-V curves were observed

Superconductivity of Mg(B1-xCx)(2) ternary compounds

The structural properties and superconductivity of Mg(B1−xCx)2 compounds were investigated by means of powder x-ray diffraction (XRD) and magnetization measurements. Powder XRD Rietveld analysis indicates that the samples crystallize in a hexagonal AlB2-type structure. The lattice parameter a decreases slightly with increasing carbon content, while c remains unchanged. The addition of carbon results in a decrease of TC and an increase in the superconducting transition width, while the critical current density Jc data indicate that MgB1.8C0.2 manifests comparable superconducting properties with MgB2.

Entropy changes due to the first-order phase transition in the Gd5SixGe4−x system

Entropy changes due to magnetostructrual phase transition in Gd5SixGe4−x intermetallics have been studied based on a systematic analysis of experiment data and mean-field theory calculations. It is found that the magnetic and lattice entropy changes have the same sign. Further analysis indicates that the main entropy change (∼60%–∼80%) comes from the field-induced change of the magnetic order, while the rest arise from the entropy difference of the two crystallographic modifications joined by the structural transition, probably due to the variation of the lattice vibration mode. The present work reveals the importance of lattice entropy for a system experiencing a first-order transition.

Sb-doped SrTiO3 transparent semiconductor thin films

Optically transparent Sb-doped SrTiO3 thin films with a transmittance higher than 95% in most of the visible region have been grown on SrTiO3 (001) substrate by pulsed laser deposition. The films behave as an n-type semiconductor between 10 K and room temperature. The carrier concentration and mobility of the films at room temperature are ∼5.8×1017 cm−3 and ∼6.4 cm2/V s, respectively. X-ray photoelectron spectroscopy measurement reveals that the delocalized electrons from the Sb dopants give rise to deep impurity levels within the band gap of the parent compound, which are responsible for the electrical conduction observed. The wide band gap and low density of states in the conduction band account for transparency of the films.

The magnetic properties of Gd2Co17−xGax compounds

X-ray diffraction (XRD) and magnetization measurements were performed to investigate the effect of Ga substitution for Co on the structural and magnetic properties of Gd2Co17−xGax compounds (x=0, 1, 2, 3, 4, 5, 6, 7, and 8). Crystal-structure studies indicate that all samples are single phase with the rhombohedral Th2Zn17-type structure except for the samples with x=7 and 8, which contain a small amount of Co. The Ga substitution for Co in Gd2Co17 compounds leads to a monotonic increase in the unit cell volume and an approximately linear decrease in the saturation magnetization. The Curie temperature TC is found to decrease monotonically from 1210 K for x=0 to 30 K for x=8. The compensation points are observed for the Gd2Co17−xGax samples with x=5 and 6. The value of the compensation temperature, determined from the temperature dependence of the magnetization measured in a magnetic field of 1000 Oe, is 120 and 152 K for x=5 and 6, respectively. It is noteworthy that the substitution of Ga for Co in the Gd...

STRUCTURE AND MAGNETIC-PROPERTIES OF SM2(FE1-XALX)17NY COMPOUNDS

By means of x‐ray phase analysis, it is shown that all the interstitial Sm2(Fe1−xAlx)17Ny nitrides and the parent compounds crystallize in Th2Zn17‐type structure for x≤0.4. The lattice constants of the parent compounds increase linearly with Al concentration. The introduction of nitrogen leads to a further increase in lattice constants, but the amplitude of the increase decreases with increasing Al concentration. The introduction of nitrogen leads to an increase of Curie temperature Tc . The composition dependence of Tc of the parent compounds exhibits a maximum, whereas Tc of the nitride decreases monotonously with increasing Al concentration from 750 K for x=0 to 313 K for x=0.4. The mean magnetic moments of iron ions in both nitrides and parents decrease monotonously with increasing Al concentration.

Growth and characterization of SrMoO3 thin films

Highly conductive SrMoO3 thin films with good crystallinity and smooth surface were grown on SrTiO3 (0 0 1) substrates by pulsed laser deposition. The effects of substrate temperature and oxygen pressure on the structure, surface morphology, and electrical properties were studied. In the range of substrate temperatures from 560 degreesC to 640 degreesC and oxygen pressure from 10 (-3) to 10 (-4) Pa studied in our experiments, high-quality SrMoO3 thin films were produced. Beyond the top temperature or oxygen pressure limit, SrMoO4 appears as an impurity phase. High-resolution transmission electron microscopy (HRTEM) study shows that the SrMoO3 film has high-quality crystallinity and an epitaxial nature. The root-mean-square surface roughness of the film deposited at 2.5 x 10 (-4) Pa is 3.9 Angstrom. The films exhibit metallic conduction, which results from the delocalized electrons from Mo. X-ray photoelectron spectroscopy (XPS) measurements characterized its core level spectra and the valence band

Hard magnetic properties in isotropic nanocomposite Pr2Fe14B/α-Fe ribbons

Abstract We have studied the demagnetization behavior for melt-spun nanocomposite Pr 8 Fe 87 B 5 ribbons in this paper. We have defined H ex which reflects the magnetic hardening of α-Fe and H irr which expresses the irreversible reversal of magnetic hard phase. It is found that magnetization behavior can be well described using H ex and H irr . A magnetic single-phase behavior is observed as H irr approximately equals to H ex . Remanence enhancement is not only determined by the intergrain exchange coupling but also dominated by the anisotropy of Pr 2 Fe 14 B phase. To achieve a high energy product (BH) max , the ideal microstructure is discussed. The magnetic reversal in the ribbons is mainly controlled by inhomogeneous pinning of nucleated type.

Multiple magnetic transitions and large magnetoresistance of the Y0.5Ho0.5Mn6Sn6 compound

Magnetic transitions and magnetoresistance of the HfFe6Ge6-type Y0.5Ho0.5Mn6Sn6 compound have been investigated in the temperature range of 5–380 K. It was found that the compound displays paramagnetism, ferrimagnetism, antiferromagnetism, and reentrant ferrimagnetism with decreasing temperature. The temperature range of the antiferromagnetic state can be narrowed by increasing the magnetic field, and the metamagnetic transition from the antiferro- to ferrimagnetic state can be induced by a fairly small threshold field (<6 kOe). The antiferro-ferrimagnetic transition is accompanied by a large magnetoresistance effect of about −16% at 100 K. The magnetic transitions with temperature and magnetic field are both of the second order.

Large coercivity in nanocrystalline TbMn6Sn6 permanent magnets prepared by mechanical milling

Isotropic TbMn6Sn6 was prepared by mechanical milling and subsequent annealing. Although the crystalline grain size was a little larger than 15 nm, no remanence enhancement resulting from intergrain exchange coupling was observed. The coercivity μ0Hc = 0.96 T at 293 K was much larger than that expected from magnetocrystalline anisotropy. The smallest effective anisotropy constant is suggested to be 0.25 MJ m−3 when the coercivity mechanism is controlled by coherent rotation of magnetization in a single-domain grain. The contributions of shape anisotropy and magnetoelastic anisotropy are considered in order to explain the large coercivity in the magnets.