P. R. Broussard

Highly spin-polarized chromium dioxide thin films prepared by chemical vapor deposition from chromyl chloride

Highly spin-polarized chromium dioxide (CrO2) thin films were deposited on (100) TiO2 substrates by chemical vapor deposition using chromyl chloride as a precursor. The spin polarization, as measured by the point contact Andreev reflection technique, was 81±3%. X-ray diffraction θ/2θ scans indicated the films grew completely (100) oriented, in registry with the (100) oriented TiO2 substrate. X-ray diffraction φ scans on the CrO2 (110) reflection indicated the expected twofold symmetry, with no evidence of misaligned material. The resistivity at room temperature was 240 μΩu200acm and decreased to 10 μΩu200acm at 5 K, consistent with metallic behavior. The films were ferromagnetic with a Curie temperature of 395 K and a coercivity of ∼100 Oe at 298 K. The use of chromyl chloride as a precursor resulted in efficient and controlled CrO2 film growth.

Characterization of transport and magnetic properties in thin film La0.67(CaxSr1−x)0.33MnO3 mixtures

We have grown thin films of (100)-oriented La0.67(CaxSr1−x)0.33MnO3 on (100)u200aNdGaO3 substrates by off-axis sputtering. We have looked at the changes in the resistivity and magnetoresistance of the samples as the Ca/Sr ratio was varied. We find that as the calcium fraction is decreased, the lattice match to the substrate decreases, and the films become more disordered, as observed in transport measurements and the variation in Curie and peak resistance temperatures. We find a correlation between the temperature independent and T2 terms to the low temperature resistivity. The room temperature magnetoresistance exhibits a maximum as the peak temperature is increased by the substitution of Sr for Ca, and a change in the field dependence to the resistivity at room temperature is observed.

Advances in the development of the magnetoquenched superconducting valve: Integrated control lines and a Nb-based device

The magnetoquenched superconducting valve is a nonvolatile switch consisting of a ferromagnetic film that spans a superconducting strip. A device state is dictated by the magnetization orientation of the ferromagnetic element and the corresponding magnetic fringe fields at its edge which control the critical current in the underlying superconductor. A Pb-based device has been fabricated with a “write wire” inductively coupled to the ferromagnetic layer as an integrated control line. A current pulse through the control line produces a small local magnetic field that is larger than the coercivity Hc of the ferromagnet and which orients the magnetization, thus switching the device state. A comparison of device switching is made between use of a current pulse through the control line and use of an external magnetic field pulse. There is good agreement between the two methods, and device switching is demonstrated with fields as small as 20 Oe. In a further effort to develop a practical device, the quenchvalve ...

MEASUREMENT OF THE SPIN POLARIZATION OF LASRMNO

A new method for determining the transport spin polarization, point contact tunneling from a low temperature superconductor into a ferromagnet, is used to determine the spin polarization of several LaSrMnO thin films and crystals. The Andreev process and its utility in measurements of spin-polarization are described. Preliminary results for the spin polarization of LSMO are presented.

Experimental evidence of resputtering of the yttria layer in a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film

Using transmission electron microscopy and energy dispersive x‐ray spectroscopy analysis, the microstructure and composition of a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film deposited on MgO by off‐axis sputtering at 670u2009°C and 100 mTorr was investigated. The YBa2Cu3O7−x layer in contact with the substrate was found to be mainly single phase. However, the top YBa2Cu3O7−x layer was embedded with 5–10 nm crystalline Y2O3 (yttria) particles, which disturbed the local microstructure. The top YBa2Cu3O7−x layer also had barium copper oxide particles covering its surface. The partial decomposition of the top YBa2Cu3O7−x layer may have been due to resputtering of the yttria layer which locally altered the deposition conditions. In particular, the oxygen pressure in the vicinity of the growing film may have increased due to sputtering of the yttria layer by plasma gas atoms and the sputtered target atoms. As a result, deposition occurred under conditions away from the thermodynamic stability line.

Characterization of thin film composite mixtures of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// and Y/sub 2/O/sub 3/

In our studies of the behavior of materials forming thin film composites with Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO), we have characterized thin film composite mixtures of YBCO and Y/sub 2/O/sub 3/ (yttria) grown onto (100) MgO and (100) SrTiO/sub 3/ substrates by off-axis sputtering. We have examined the transport properties of the films, in particular the critical current density, J/sub c/. The temperature dependence of J/sub c/ for composites on SrTiO/sub 3/ is identical to that of our pure YBCO films, while for composites on MgO the J/sub c/ is similar to measurements on our earlier composites of YBCO and Y/sub 2/Ba/sub 1/Cu/sub 1/O/sub 5/ (211). The field dependence of J/sub c/ for the YBCO/yttria composites is different from our pure YBCO films, and from our earlier composites, but the magnitude of the flux pinning is similar.<<ETX>>

X-ray photoemission characterization of La0.67(CaxSr1−x)0.33MnO3 films

The Curie temperature and x-ray photoemission spectra of thin films of La0.67(CaxSr1−x)0.33MnO3 have been studied as a function of the Ca/Sr ratio. The films were grown by off-axis cosputtering from individual targets of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) onto (100) oriented NdGaO3 substrates. The films grow with a (100) orientation, with no other orientations observed by x-ray diffraction. For the alloy mixtures, the Curie temperature, TC, varies slowly as the Ca/Sr is decreased, remaining ≈300 K, while for the LCMO and LSMO films TC is 260 and 330 K, respectively. The Mn-O valence structure is composed of two dominant peaks, whose positions undergo a change as the Ca fraction is decreased. The core lines behave as linear combinations of lines from pure LCMO and LSMO.

Structural characterization of Yba 2 Cu 3 O 7–δ /Y 2 O 3 composite films

Using 4-circle x-ray diffraction and transmission electron microscopy we have studied the microstructure and in-plane orientation of the phases present in thin film composite mixtures of YBa(2)Cu(3)O(7) and Y(2)O(3). We see a high degree of in-plane orientation and have verified a previous prediction for the in-plane order of Y(2)BaCuO(5) on (110) MgO. Transmission electron microscopy shows the composite films to be a mixture of two phases, with YBCO grain sizes of 1 micron. We have also compared our observations of the in-plane order to the predictions of a modified near coincidence site lattice model.Using 4-circle x-ray diffraction and transmission electron microscopy, we have studied the microstructure and in-plane orientation of the phases present in thin film composite mixtures of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and Y{sub 2}O{sub 3}. We see a high degree of in-plane orientation and have verified a previous prediction for the in-plane order of Y{sub 2}BaCuO{sub 5} on (110) MgO. Transmission electron microscopy shows the composite films to be a mixture of two phases, with YBCO grain sizes of {approx}1{mu}m. We have also compared our observations of the in-plane order to the predictions of a modified near coincidence site lattice model. {copyright} {ital 1998 Materials Research Society.}

INVESTIGATION OF GRANULAR FILMS COMPOSED OF INTERDISPERSED LA1/3CA2/3MNO3 PARTICLES AND METALLIC AU PARTICLES

We have carried out a study of composite, granular films made by depositing LCMO (a colossal magnetoresistive perovskite) and pure, metallic Au onto MgO and LaAlO3 substrates. Elevated temperature caused the Au to segregate into small, micron-sized granules. The films were studied by atomic force microscopy, magnetoresistivity, superconducting quantum interference device magnetization measurements, and magnetically modulated microwave absorption.

Effect of substrate smoothness on the microstructure of YBa 2 Cu 3 O 7–x /Y 2 O 3 /YBa 2 Cu 3 O 7–x trilayers

Transmission electron microscopy was used to analyze the microstructure of YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer structures deposited by off-axis sputtering on MgO substrates with varying degrees of roughness. Substrate surface hillocks with a peak-to-valley height of about 4.5 nm were found to contribute to strain that extended through the film and disrupted the smoothness of the Y2O3 layer. In some cases, these hillocks served as nucleation sites for yttria precipitates. Such defects may seriously jeopardize the realization of weak-link Josephson junctions.Transmission electron microscopy was used to analyze the microstructure of YBa{sub 2}Cu{sub 3}O{sub 7-x}/Y{sub 2}O{sub 3}/YBa{sub 2}Cu{sub 3}O{sub 7-x} trilayer structures deposited by off-axis sputtering on MgO substrates with varying degrees of roughness. Substrate surface hillocks with a peak-to-valley height of about 4.5 nm were found to contribute to strain that extended through the film and disrupted the smoothness of the Y{sub 2}O{sub 3} layer. In some cases, these hillocks served as nucleation sites for yttria precipitates. Such defects may seriously jeopardize the realization of weak-link Josephson junctions. 17 refs., 8 figs.