S. M. Watts

Epitaxial growth of CrO2 thin films by chemical-vapor deposition from a Cr8O21 precursor

Presently, the best epitaxial thin films of CrO2 are made by chemical-vapor deposition (CVD) in a two-zone furnace with oxygen flow from a CrO3 precursor. The growth mode has previously been described as CrO3 vaporizing in the first zone, and thermally decomposing at higher temperature in the second zone onto a substrate. In the more recent works, the focus has been on the properties of the obtained layers rather than on deposition mechanisms. In the present experimental work, we attack the epitaxial growth of CrO2 by two completely different methods, namely, molecular-beam epitaxy (MBE) and CVD. We focus on the CVD process itself, and show the importance of an intermediate compound, Cr8O21, for the growth of CrO2 films. We show that it is not necessary to start the CVD from CrO3; instead, one can prepare Cr8O21 ex situ, and use it directly for the growth of high-quality CrO2 epitaxial layers, avoiding any contamination caused by the decomposition of CrO3 to Cr8O21. We discuss in parallel our failed attem...

Strain effects in thin films of CrO2 on rutile and sapphire substrates

Detailed x-ray diffraction characterizations were made of chromium dioxide films fabricated by chemical vapor deposition onto (100) and (110) oriented TiO2 and (0001) Al2O3 substrates. Pole figures were used to examine the epitaxy of these systems, and the lattice parameters were calculated using x-ray area maps. The film on (100) TiO2 exhibits the best epitaxy of the three; however, it is significantly strained relative to bulk CrO2. The film on (110) TiO2 is distorted from the ideal tetragonal structure by 0.17° in the angle between the a and b lattice directions, and also exhibits significant mosaicity. The film on sapphire contains crystallites that have grown in three in-plane orientations, but exhibit the best rocking curve widths and the least degree of strain of the films studied. Magnetization and magnetotransport measurements are shown to demonstrate effects of strain and crystalline structure on the physical properties of these films.

Characterization of films of La1−xSrxMnO3−δ grown by means of metal organic chemical vapor deposition

An effort to develop La manganites for possible sensor applications has led to the successful growth of La1−xBxMnO3−δ (B=Sr, Ca) thin films on single crystal MgO, Al2O3, and LaAlO3 substrates. Here we concentrate on Sr-doped material on LaAlO3 with Curie temperature Tc=262±2 K. Tc was determined from magnetization measurements using Arrott plots, and agrees well with the observed resistivity peak in zero applied magnetic field (H). This Tc corresponds closely to a Sr content x=0.17 reported in bulk materials. At H=60 kOe the maximum magnetoresistance Δρ/ρ≈0.95. No discernible hysteresis, as might be expected for structural phase transitions, was observed. When plotted as a function of reduced temperature T/Tc, the magnetization extrapolated to zero H is best fit by a Brillouin function with an effective spin value 〈S〉 much larger than the 〈S〉=1.9 indicated by the composition. This may be indicative of magnetic polaron formation, especially near the magnetic and transport transition temperatures. At temper...

Effect of ion-irradiation-induced disorder on the low-field magnetoresistance of La0.67A0.33MnO3 (A=Sr, Ca)

We report on a systematic study of the correlation between the disorder produced by ion irradiation and the magnetoresistance (MR) behavior of two doped manganite films: a highly textured La0.67Sr0.33MnO3 film on a (100) LaAlO3 substrate and a film of La0.67Ca0.33MnO3 on a YSZ substrate with a high degree of large-angle grain boundary disorder. The films were grown via metal organic chemical vapor deposition and exposed to irradiation of 200 keV Ne ions. The temperature and field dependence of the resistivity were measured on the same samples at different levels of damage. For the Sr-doped film, the bulk magnetic properties were observed to degrade faster with cumulative damage than transport. For both films at low temperature, a low-field hysteretic MR was either induced or enhanced by ion damage. These effects are discussed in terms of diffusive transport through “granular” magnetically disordered regions.

GRAIN BOUNDARY EFFECTS ON TRANSPORT IN METALORGANIC CHEMICAL VAPOR DEPOSITION-GROWN, CA-DOPED LANTHANUM MANGANITES

The transport properties of metalorganic chemical vapor deposited films of lanthanum manganite grown on various substrates are investigated. The more disordered films show a magnetoresistance that is both large and relatively temperature independent over a wide temperature range. At low magnetic fields, a linear field dependence is observed and is attributed to spin-polarized intergrain tunneling. In addition, at low fields a hysteretic dependence of resistivity on the magnetic field has been observed. This effect has been attributed to the scattering of spin-polarized carriers at the grain boundary.

Grain Boundary Effets On Transport In MOCVD-Grown, Ca-Doped Lanthanum Manganites

THURSDAY AFTERNOON, JANUARY 8,1998 259 BAYVIEW, 2:OO TO 5:OO Session FD CMR MATERIALS AND THEIR PROPERTIES R. B. Van Dover, Cochair Bell Labs, ID-154, 600 Mountain Road, Murray Hill, NJ 07974 A. Gupta, Cochair IBM Research Center, P.O. Box 218, Yorktown Heights, NY 10598