J. Q. Zheng

Magnetization of compositionally modulated CuNi films

We report static magnetization measurements on the compositionally modulated ferromagnetic alloy Cu/Ni, which, contrary to earlier ferromagnetic resonance measurements, show the moment per Ni atom is reduced relative to pure Ni. The low‐temperature magnetization is found to vary linearly with modulation amplitude, but, surprisingly, the Curie temperature is found to be almost amplitude independent.

The magnetization and Curie temperature of compositionally modulated Cu/Ni films

We report detailed measurements of the magnetization of Cu/Ni composition modulated foils as a function of temperature, magnetic field, composition wavelength, and composition amplitude. We find a Curie temperature that initially increases rapidly with wavelength but quickly saturates. We also find that the Curie temperature is independent of composition amplitude which we ascribe to the existence of disk‐like Ni clusters.

Effect of oxygen partial pressure on the in situ growth of Y‐Ba‐Cu‐O thin films on SrTiO3

The evolution of YBCO film growth with thickness at various oxygen pressures was observed by in situ synchrotron x‐ray diffraction in real time. When the films were deposited at 2 A/s and 730 °C under higher oxygen partial pressures (in an Ar/O2 mixture of 90 mTorr), the nucleation was observed to have c‐axis orientation. After the films reached a critical thickness, the growth of the YBCO film changed from c axis to a axis and then propagated epitaxially. This provides evidence that a‐axis epitaxial growth nucleates on a c‐axis base. The critical thickness reflects the competition between the growth of the c and a axes, which is determined by the oxygen partial pressure in the process of thin‐film formation. The a‐axis oriented films showed a very sharp rocking curve (less than 0.1°) which indicates a very high structural quality. For very low oxygen partial pressures, the in situ growth process was very similar, but the initial nuclei involve a second phase mixed with a small amount of c‐axis ‘‘123’’ ph...

Neutron diffraction analysis of a compositionally modulated alloy of nickel-copper☆

Abstract A compositionally modulated Ni 0.4 Cu 0.6 alloy, made in a thin film of 1.8 μm thickness, was examined by polarized neutron diffraction. The results show that the material is ferromagnetic, with a large enhancement of the magnetic moment (0.3μ B /Ni) with respect to a disordered alloy. The behaviour of the magnetic moments gives strong evidence of chemical clustering.

Superconducting and transport properties of NbTi layered metals

Abstract We report data on the superconducting transition temperature and room temperature resistivity of layered NbTi for thicknesses in the range 7 – 70 A.

A miniature x‐ray compatible sputtering system for studying in situ high Tc thin film growth

We describe a miniature sputtering system incorporating an unconventional magnetron gun and a right angle sputtering geometry. The design essentially eliminates the negative ion resputtering effect encountered in the sputtering of high Tc oxide films. The geometry is compatible with the in‐plane and conventional θ–2θ diffraction geometries and a chamber with two appropriate sector x‐ray windows has been constructed and operated at a synchrotron. Detailed data on the performance of the sputtering gun are presented.

Dual electron beam evaporator for the preparation of composition‐modulated structures

We describe the construction of an UHV evaporator with two electron beam guns and a heated substrate turntable which is used for the preparation of composition‐modulated structures (CMS’s). Using a suitably controlled stepping motor, up to 20 different CMS’s can be prepared in the same run. The system has been used to deposit V/Fe CMS’s and preliminary structural measurements indicate the samples are of high quality.

Proximity effect coupled VCr superlattices

Abstract V m Cr n superlattices, where m and n denote the number of atomic planes of V and Cr in each layer, with m equal to 98, 147, 175 and 197 and n varying between 3 and 100 have been prepared. The films were grown in an ultra-high vacuum chamber (base pressure ≈1×10 −9 Torr) equipped with dual e-guns and a rotating substrate holder. Films were grown at 250°C on “c” cut sapphire substrates. The zero field transition temperature and parallel upper critical field of the films have been measured. A dimensional crossover in the parallel upper critical field is observed at n equal to about 80. The zero field transition temperature data has been modeled using the procedures and of Werthamer and Auvil and Ketterson. The magnetic scattering time, τ, in the Cr layers has been found to be dependent on the vacuum during deposition: 4.5×10 −14 sec for films deposited at ≤ 4×10 −7 Torr and 5.5×10 −14 sec for films deposited at ≤ 4×10 −8 Torr. For the V 175 Cr x series with n >- 80 the best fits occur for infinite scattering time τ.

In situ synchrotron studies of the structural properties of Y-Ba-Cu-O thin films during growth

We report the first real time, in situ synchrotron x‐ray studies of Y‐Ba‐Cu‐O thin‐film growth on (100) SrTiO3 using a miniature, faced‐magnetron sputtering system. A combination of the substrate temperature and the deposition rate determines whether the film grows along the a, c, or multiple axes.

Phase relationships in Cu‐O thin films prepared by sputtering

The conditions for deposition of Cu and Cu‐oxide thin films by reactive dc sputtering have been investigated by characterizing the crystal structure of growing films both during and immediately following deposition using an in situ x‐ray diffraction technique. The relationship between sputtering conditions and the phases deposited was established for a variety of conditions. At each temperature studied, increasing the oxygen pressure in the system resulted in a systematic change in the phases deposited. Of significant importance was the identification of temperature‐pressure regimes in which CuO was stable during deposition but reverted to Cu2O after the sputtering plasma was extinguished, suggesting a shift in the oxidizing potential in the plasma environment. These results also suggest that the in situ analysis technique may be ideally suited for the investigation of phase relationships and phase diagrams in other systems.