J.P. Sénateur

Magnetic anisotropy of ferromagnetic La0.7Sr0.3MnO3 epitaxial thin films: Dependence on temperature and film thickness

The magnetic anisotropy of (001) oriented La0.7Sr0.3MnO3 films of thickness t=7–156 nm, deposited on LaAlO3 substrates, was measured by torque magnetometry in the temperature range T=10–300 K. For t⩾50 nm and H rotating out of plane the anisotropy Ku agrees well with shape anisotropy. For thinner films, Ku is reduced and its sign is reversed for t=7 nm and T<70 K; this can be explained by a perpendicular anisotropy Kue due to lattice strain. The crystal anisotropy constant K1 was determined from the biaxial in-plane anisotropy. At T=100 K K1 differed by no more than 50% from the mean value −8 kJ/m3 in the thickness region investigated. K1 was much less dependent on the thickness t and strain relaxation in the films than Kue.

Metallurgical reinvestigation of rare earth silicides

Abstract We have reinvestigated the relation between the composition of rare earth silicides, the nature of the rare earth and the occurrence of the different structures. These relations have to be known to understand the interaction of rare earth thin films with silicon. In addition to the well known structures, several possibilities of vacancy ordering leading to superstructures have been obtained. Single crystals of some of these materials have been grown, and their magnetic and transport properties measured.

Magnetoresistant La1−xSrxMnO3 films by pulsed injection metal organic chemical vapor deposition: effect of deposition conditions, substrate material and film thickness

Abstract High quality epitaxial La 1− x Sr x MnO 3 films were deposited by pulsed injection metal organic chemical vapor deposition, using as precursor materials metal-2,2,6,6-tetramethyl-3,5-heptandionates dissolved in monoglyme. The influence of various deposition conditions, substrate material and film thickness on film properties was investigated. The best films were deposited on LaAlO 3 substrates at 825 °C: films exhibited a sharp semiconductor–metal transition and high magnetoresistance (∼40%) at a close-to-room temperature and in a rather low field of 1.5 T. In-situ or ex-situ high-temperature annealing in oxygen increased the temperature of semiconductor–metal transition, but decreased the magnetoresistance and the temperature coefficient of resistance. Biaxial strain imposed by the lattices’ mismatch was clearly observed in thinner La 1− x Sr x MnO 3 films on perovskite substrates. Tensile strain was present in the films on SrTiO 3 and compressive strain in the films on LaAlO 3 (and less clearly on NdGaO 3 ). Both tensile and compressive strains decreased the temperature of electric transition and the values of magnetoresistance. The strain was completely relaxed in the films more than ∼100 nm thick.

Analysis of the electrical resistivity of Ti, Mo, Ta, and W monocrystalline disilicides

The electrical resistivity of monocrystalline TiSi2, TaSi2, MoSi2, and WSi2 has been measured from 4.2 to 1100 K. These disilicides are metallic, yet there is a remarkable difference in the temperature dependence of their intrinsic resistivities. TiSi2 and TaSi2 are found to exhibit a T5 dependence in the temperature range of 13<T<30 K and 15<T<28 K, respectively, while MoSi2 and WSi2 show a T3.8 dependence from 15 to 40 K. For TiSi2, along the three crystallographic directions 〈100〉, 〈010〉, and 〈001〉, the phonon contribution to the resistivity was found to be linear in temperature above 300 K. The same behavior was observed for TaSi2 along the 〈0001〉 axis, while a negative deviation from the linearity followed by a quasisaturation was observed with the current, parallel to the 〈1010〉 axis. The resistivity data of WSi2 and MoSi2 with the current parallel to 〈001〉 and 〈110〉 crystallographic directions showed a positive deviation from linearity. The data are fitted to several theoretical expressions at low...

RAMAN STUDY OF BAFE12O19 THIN FILMS

We report on Raman spectra of BaFe12O19 thin films. These thin films have been deposited by injection chemical vapor deposition on three different substrates: Al2O3 (001), Gd3Ga5O12 (111), and Si (100). The observed Raman-active vibrations of the films are compared with recently published Raman spectra from bulk compounds. Surprisingly, we observed nearly the same spectra for all the films, although x-ray diffraction indicates polycrystalline (Si), textured (Gd3Ga5O12), and epitaxial (Al2O3) structure. We interpret these results by supposing the coexistence of well oriented regions and randomly oriented microcrystallites, which are not detectable by x-ray diffraction. Furthermore, by Raman spectroscopy we identified an additional phase for the films deposited on Al2O3 which has not been observed by x-ray diffraction either.

Synthesis of Pd-doped SnO2 films on silicon and interaction with ethanol and CO

Abstract Pure and Pd-doped polycrystalline SnO2 thin films have been synthesized by a modified chemical vapour deposition process using the pyrolysis of an aerosol generated by ultrahigh frequency spraying of a volatile precursor solution. The depositions have been carried out on oxidized 〈100〉 silicon. Growth rate, microstructure and composition of the films are well-controlled by the deposition temperature (460–560 °C) and the moisture of the carrier gas. Palladium particles (metal or oxide) are dispersed on SnO2 grains. At low temperature the moisture increases the growth rate of tin oxide without increasing the palladium one. The influence of adsorption phenomena on the surface composition has been studied by annealing under ethanol and CO, and afterwards by analysing Sn, O and C contents by Auger spectroscopy. The heat treatments induce an increase of O and C concentration. The introduction of palladium in the films largely increases the gaseous adsorption, showing a maximum at low temperature. These results are correlated with electrical conductance measurements, showing the influence of CO adsorption on electrical properties.

Some transport properties of single crystals of group Va transition metal disilicides

Abstract Although transition metal disilicides are used in the form of thin films, single crystals are needed to study their intrinsic properties. Single crystals of Group Va transition metal disilicides, namely VSi 2 , NbSi 2 and TaSi 2 , have been successfully grown by Czochralski pulling from a levitated melt. The good quality of our crystals is shown by the high residual resistance ratios (VSi 2 : RRR 20; NbSi 2 : RRR 220; TaSi 2 : RRR 400). The three compounds crystallize in the same hexagonal structure (C40, space group P6 2 22). Crystallographic and temperature dependences of the resistivity and of the magnetoresistance ( H ⊥ ⩽ 7.4 T) have been measured from 4.2 K up to room temperature. All the three materials show a metallic behaviour and some anisotropy, which is of the order of 2 for the three compounds. The magnetoresistance measurements show that VSi 2 and TaSi 2 are compensated metals with relatively small charge carrier concentrations (VSi 2 : n ≈ 0.15 electrons/formula unit; TaSi 2 : n ≈ 0.20–0.25 electrons/formula unit).

Resistivity and magnetoresistance of high-purity monocrystalline MoSi2

The authors report resistivity and magnetoresistance measurements (4.2<T<300K; H<or=7.6 T) of extremely pure monocrystalline MoSi2 (RRR approximately=1800). This metallic compound behaves like a compensated metal (n(h+)=n(e-)) with a very large magnetoresistivity at 4.2K which varies like H2 up to 7 T. The anisotropy of the electronic structure accounts well for the anisotropy of the results. Deviations from Matthiessens rule are also reported.

Pulsed liquid-injection metalorganic chemical vapor deposition of (La0.7Sr0.3MnO3/SrTiO3)15 superlattices

(La0.7Sr0.3MnO3/SrTiO3)15 superlattices have been grown by pulsed liquid-injection metalorganic chemical vapor deposition on monocrystalline substrates such as LaAlO3, SrTiO3, and MgO. The pulsed-injection technique allows one to control precisely the amount of precursors delivered to the deposition chamber and thus the thickness of each individual layer. The period of the superlattices depends indeed linearly on the number of injected droplets. In our deposition conditions, the average growth rates are ∼0.130 nm/injection for La0.7Sr0.3MnO3 and 0.042 nm/injection for SrTiO3, with no significant difference as regard to the substrate used. The strain’s state of the superlattices depends on the relative thicknesses of the La0.7Sr0.3MnO3 and SrTiO3 layers and also on the substrate used. Finally, the deposition of superlattices with ultrathin interlayers of few unit cells has been demonstrated.

Effects of the microstructure on the magnetotransport properties of polycrystalline manganite films grown by metalorganic chemical vapor deposition

The effects of the microstructure on the magnetoresistance of polycrystalline manganite (La1−xSrxMnO3) thin films have been investigated. The films have been grown by metalorganic chemical vapor deposition on silicon (001) substrates covered with a native silica layer. Different microstructures were obtained by varying the substrate temperature in the range (550–750 °C). The films exhibit a ferromagnetic/paramagnetic transition around 325 K. The grain size is in the range 20–100 nm and, for deposition temperature lower than 650 °C, weak links appear in the films as the grains are mostly noncohesive. The low-field contribution to the magnetoresistance is independent of the grain size in the studied range. A factor of ∼1.8 is gained in this contribution as the substrate temperature is lowered below 650 °C, which can be related to the occurrence of noncohesive grains in the films. The largest low-field magnetoresistance obtained is about 20% at 22 K (under 0.2 T). The high-field contribution to the total mag...