Y. F. Hu

Superconducting properties of nanocrystalline MgB2 thin films made by an in situ annealing process

We have studied the structural and superconducting properties of MgB2 thin films made by pulsed-laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature (Tc0) of 34 K and a zero-field critical current density (Jc) of 1.3×106 A/cm2 were obtained. The irreversibility field was ∼8 T at low temperatures, although severe pinning instability was observed. The result is a step towards making the in situ deposition process a viable technique for MgB2 Josephson junction technologies.

Role of strain in magnetotransport properties of Pr0.67Sr0.33MnO3 thin films

We have studied the strain effects on the structural and magnetotransport properties of Pr0.67Sr0.33MnO3 (PSMO) thin films. The PSMO films were epitaxially grown on LaAlO3 (001), SrTiO3 (001), and NdGaO3 (110) substrates that induce biaxial compressive, tensile, and almost no strain in the films, respectively. The film thickness t, varied between 4–400 nm, was used as another controlling parameter of strain for each type of film. There exist two distinct thickness ranges with different thickness dependence of the magnetotransport properties. For t 20 nm, the Tp and the HFMR ratio show weak t dependence. The results show evidence for the effects of the Jahn–Teller type distortion as well as disorders on the resistive transition temperature and the HFMR.

Properties of MgB2 thin films with carbon doping

We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing precursor metalorganic bis(methylcyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the film increases, the resistivity increases, Tc decreases, and the upper critical field increases dramatically as compared to clean films. The self-field Jc in the carbon doped film is lower than that in the clean film, but Jc remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with columnar nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic-field applications.

Anomalous anisotropic magnetoresistance in Pr0.67Sr0.33MnO3 thin films

Anisotropic magnetoresistance (AMR) in strained Pr0.67Sr0.33MnO3 thin films has been studied by measuring the resistance as a function of the angle between the applied magnetic field direction and the film normal with the current always perpendicular to the magnetic field. The results show that both compressive- and tensile-strained ultrathin films (50–150 A) exhibit unusually large AMR, but with opposite signs. In contrast, the almost strain free films show much smaller AMR over all the temperature and field ranges studied. The AMR decreases rapidly as the film thickness increases due to the gradual release of strain.

Thermodynamics and thin film deposition of MgB2 superconductors

The recently discovered superconductor MgB2 with Tc at 39 K has great potential in superconducting microelectronics. Thermodynamics studies with the calculation of phase diagrams (CALPHAD) modelling technique show that due to the high volatility of Mg, MgB2 is only thermodynamically stable under fairly high Mg overpressures for likely in situ growth temperatures. This provides a helpful insight into the appropriate processing conditions for MgB2 thin films, including the identification of the pressure–temperature region for adsorption-controlled growth. The initial MgB2 thin films were made by pulsed laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature of 34 K and a zero-field critical current density of 1.3 × 106 A cm−2 were obtained. The qualities of these films are limited by the thermodynamic stability conditions, which favour deposition techniques that can maintain a high flux of Mg.

Characterization of spinel iron-oxide nanocrystals grown on Fe whiskers

Passive iron-oxide nanocrystals are grown on Fe(100) and Fe(110) facets of single-crystal Fe whiskers. Transmission electron microscopy and electron diffraction characterize the oxide spinel structure and their epitaxial growth on Fe whiskers. Iron-oxide nanocrystals grown on Fe(100) facets have sizes close to that of the single magnetic domain Fe3O4 particles, which is supported by our preliminary magnetic force microscopy measurement at room temperature.

Large low-field magnetoresistance in strained ultrathin Pr0.67Sr0.33MnO3 films

Strain effect on the low-field magnetoresistance (LFMR) in epitaxially grown Pr0.67Sr0.33MnO3 thin films has been studied. Very large LFMR and MR hysteresis have been found in compressive-strain ultrathin films grown on LaAlO3 (001) substrates when a magnetic field is applied perpendicular to the film plane. The LFMR ratio as high as 360% at H=1600 Oe and T=30 K was obtained from the MR hysteresis curve. The large LFMR depends strongly on the applied magnetic field direction as well as the film thickness. It is reduced to less than 10% when the film thickness is about 20 nm. In comparison, tensile-strain films on SrTiO3(001) show positive LFMR, and almost strain free films on NdGaO3 (110) show very small LFMR (<2%), at comparable magnetic fields and temperatures. These effects were found to be closely related to the strain-induced magnetic anisotropy.

Nano-sized Superlattice Clusters Created by Oxygen Ordering in Mechanically Alloyed Fe Alloys

Creating and maintaining precipitates coherent with the host matrix, under service conditions is one of the most effective approaches for successful development of alloys for high temperature applications; prominent examples include Ni- and Co-based superalloys and Al alloys. While ferritic alloys are among the most important structural engineering alloys in our society, no reliable coherent precipitates stable at high temperatures have been found for these alloys. Here we report discovery of a new, nano-sized superlattice (NSS) phase in ball-milled Fe alloys, which maintains coherency with the BCC matrix up to at least 913 °C. Different from other precipitates in ferritic alloys, this NSS phase is created by oxygen-ordering in the BCC Fe matrix. It is proposed that this phase has a chemistry of Fe3O and a D03 crystal structure and becomes more stable with the addition of Zr. These nano-sized coherent precipitates effectively double the strength of the BCC matrix above that provided by grain size reduction alone. This discovery provides a new opportunity for developing high-strength ferritic alloys for high temperature applications.

Deposition and Properties of Superconducting MgB2 Thin Films

The recently discovered superconductor MgB2 with Tc at 39 K has great potential in superconducting electronics. In this paper, we review the deposition techniques used for MgB2 thin films in the light of a thermodynamic study of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique. This thermodynamic study identifies a growth window in the pressure–temperature phase diagram, in which the magnesium pressure is very high for likely in situ growth temperatures. A Hybrid Physical–Chemical Vapor Deposition (HPCVD) technique that successfully achieves such a high Mg pressure is shown to produce in situ epitaxial MgB2 thin films with bulk superconducting properties.

Strain-dependent spin dynamics in Nd0.67Sr0.33MnO3 near the metal-insulator transition

We report on time-resolved pump–probe measurements of spin-dependent dynamics in strained Nd0.67Sr0.33MnO3 thin films grown on three different substrates: LaAlO3 (001), SrTiO3 (001), and NdGaO3 (110). The temperature dependence of the long-lived spin-relaxation component is represented well by a power-law decay of long-range correlations, clearly showing the transition from quasilong-range ferromagnetic order to a disordered paramagnetic phase. The “disordering” temperature, TM, where the intermediate phase of quasilong-range order appears, varies according to the creation of static-distortion waves under different strain forces.