Thickness dependence of the stability of the charge-ordered state in Pr0.5_{0.5}Ca0.5_{0.5}MnO3_{3} thin films

Thin films of the charge-ordered (CO) compound Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ have been deposited onto (100)-oriented SrTiO$_{3}$ substrates using the Pulsed Laser Deposition technique. Magnetization and transport properties are measured when the thickness of the film is varied. While the thinner films do not exhibit any temperature induced insulator-metal transition under an applied magnetic field up to 9T, for thickness larger than 1100\UNICODE{0xc5} a 5T magnetic field is sufficient to melt the CO state. For this latest film, we have measured the temperature-field phase diagram. Compared to the bulk material, it indicates that the robustness of the CO state in thin films is strongly depending on the strains and the thickness. We proposed an explanation based on the distortion of the cell of the film.Comment: 9 pages, 6 figures, submitted to Phys. Rev.

Effect of Strain Relaxation on Magnetotransport properties of epitaxial La_0.7Ca_0.3MnO_3 films

In this paper, we have studied the effect of strain relaxation on magneto-transport properties of La_0.7Ca_0.3MnO_3 epitaxial films (200 nm thick), which were deposited by pulsed laser deposition technique under identical conditions. All the films are epitaxial and have cubic unit cell. The amount of strain relaxation has been varied by taking three different single crystal substrates of SrTiO_3, LaAlO_3 and MgO. It has been found that for thicker films the strain gets relaxed and produces variable amount of disorder depending on the strength of strain relaxation. The magnitude of lattice relaxation has been found to be 0.384, 3.057 and 6.411 percent for film deposited on SrTiO_3, LaAlO_3 and MgO respectively. The films on LaAlO_3 and SrTiO_3 show higher T_{IM} of 243 K and 217 K respectively as compared to T_{IM} of 191 K for the film on MgO. Similarly T_C of the films on SrTiO_3 and LaAlO_3 is sharper and has value of 245 K and 220 K respectively whereas the TC of the film on MgO is 175 K. Higher degree of relaxation creates more defects and hence TIM (T_C) of the film on MgO is significantly lower than of SrTiO_3 and LaAlO_3. We have adopted a different approach to correlate the effect of strain relaxation on magneto-transport properties of LCMO films by evaluating the resistivity variation through Mott's VRH model. The variable presence of disorder in these thick films due to lattice relaxation which have been analyzed through Mott's VRH model provides a strong additional evidence that the strength of lattice relaxation produces disorder dominantly by increase in density of defects such as stacking faults, dislocations, etc. which affect the magneto-transport properties of thick epitaxial La_0.7Ca_0.3MnO_3 films

Enhanced Room Temperature Coefficient of Resistance and Magneto-resistance of Ag-added La0.7Ca0.3-xBaxMnO3 Composites

In this paper we report an enhanced temperature coefficient of resistance (TCR) close to room temperature in La0.7Ca0.3-xBaxMnO3 + Agy (x = 0.10, 0.15 and y = 0.0 to 0.40) (LCBMO+Ag) composite manganites. The observed enhancement of TCR is attributed to the grain growth and opening of new conducting channels in the composites. Ag addition has also been found to enhance intra-granular magneto-resistance. Inter-granular MR, however, is seen to decrease with Ag addition. The enhanced TCR and MR at / near room temperature open up the possibility of the use of such materials as infrared bolometric and magnetic field sensors respectively.Comment: 22 pages of Text + Figs:comments/suggestions([email protected]

Electronic phase transitions in Pr0.5_{0.5}Ca0.5_{0.5}MnO3_3 epitaxial thin films revealed by resonant soft x-ray scattering

We report the study of magnetic and orbital order in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films grown on (LaAlO$_3$)$_{0.3}$-(SrAl$_{0.5}$Ta$_{0.5}$O$_3$)$_{0.7}$ (LSAT) (011)$_c$. In a new experimental approach, the polarization and energy dependence of resonant soft x-ray scattering are used to reveal significant modifications of the magnetic order in the film as compared to the bulk, namely (i) a different magnetic ordering wave vector, (ii) a different magnetic easy axis and (iii) an additional magnetic reordering transition at low temperatures. These observations indicate a strong impact of the epitaxial strain on the spin order, which is mediated by the orbital degrees of freedom and which provides a promising route to tune the magnetic properties of manganite films. Our results further demonstrate that resonant soft x-ray scattering is a very suitable technique to study the magnetism in thin films, to which neutron scattering cannot easily be applied due to the small sample volume.Comment: 5 pages, 3 figure

High magnetic field transport measurement of charge-ordered Pr0.5_{0.5}Ca0.5_{0.5}MnO3_3 strained thin films

We have investigated the magnetic-field-induced phase transition of charge-ordered (CO) Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ thin films, deposited onto (100)-oriented LaAlO$_3$ and (100)-oriented SrTiO$_3$ substrates using the pulsed laser deposition technique, by measuring the transport properties with magnetic fields up to 22T. The transition to a metallic state is observed on both substrates by application of a critical magnetic field ($H_C>10T$ at 60K). The value of the field required to destroy the charge-ordered insulating state, lower than the bulk compound, depends on both the substrate and the thickness of the film. The difference of the critical magnetic field between the films and the bulk material is explained by the difference of in-plane parameters at low temperature (below the CO transition). Finally, these results confirm that the robustness of the CO state, depends mainly on the stress induced by the difference in the thermal dilatations between the film and the substrate.Comment: 10 pages, 6 figures. To be published in Phys. Rev.

Lattice dynamical study of optical modes in Tl2Mn2O7 and In2Mn2O7 pyrochlores

The Raman, IR and force field have been investigated for A2Mn2O7 (A= Tl, In) by means of a short-range force constant model which includes four stretching and four bending force constants. Unusual spectral and force field changes are observed and analyzed. The stretching force constant Mn-O and A-O, are found to be relatively higher than those of other pyrochlore oxides of the A2Mn2O7 family, while the remaining force constant values are significantly smaller, especially for Tl2Mn2O7. This trend may be due to strong hybridization of the Tl (6s) orbital with O (2p) and Mn (3d). The assignment of all the modes has been proposed and potential energy distribution is also reported. The evaluated frequencies are close to the available observed infrared and Raman frequencies, giving further support to the present assignments.Comment: To be published in PRB, 17 page

Nanopillar spin filter tunnel junctions with manganite barriers.

The potential of a manganite ferromagnetic insulator in the field of spin-filtering has been demonstrated. For this, an ultrathin film of Sm0.75Sr0.25MnO3 is integrated as a barrier in an epitaxial oxide nanopillar tunnel junction and a high spin polarization of up to 75% at 5 K has been achieved. A large zero-bias anomaly observed in the dynamic conductance at low temperatures is explained in terms of the Kondo scattering model. In addition, a decrease in spin polarization at low bias and hysteretic magneto-resistance at low temperatures are reported. The results open up new possibilities for spin-electronics and suggest exploration of other manganites-based materials for the room temperature spin-filter applications.This work was partially supported by the ERC Advanced Integrators Grant “SUPERSPIN”. B.P. was funded by the Nehru Trust for Cambridge University and St John’s College. The TEM work at Texas A&M was supported by the U.S. National Science Foundation (NSF-DMR 0846504). The authors wish to thank Prof. J. Kumar (IIT Kanpur, India) for help in improving the manuscript.This document is the Accepted Manuscript version of a Published Work that appeared in final form in Nano Letters, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/nl500798

Détermination des contraintes résiduelles et de la microstructure intra-granulaire dans des films minces de W déposés par pulvérisation magnétron

Cet article concerne l'étude des propriétés mécaniques et microstructurales de films minces de tungstène (700 nm sur Si (100)) élaborés par pulvérisation magnétron en fonction de la puissance R.F. Les contraintes totales présentes dans les films (déterminées par la méthode de la flèche) sont très fortement influencées par la valeur de la puissance R.F. Elles varient de +1,2 GPa à -1,5 GPa pour des puissances de 350 W à 400 W. L'analyse par diffraction des rayons X a révélé la présence de phase β-W dans certains dépôts. Pour ces films, nous avons déterminé les contraintes dans chaque phase par la méthode des sin$^2~\psi$. Les mesures d'intensité intégrée des pics de diffraction nous ont permis d'évaluer la proportion des phases dans chaque film. Nous avons vérifié le bon accord, en appliquant la règle des mélanges, entre les valeurs des contraintes totales et celles calculées à partir des résultats de diffraction X. Nous avons montré que la présence de la phase β-W semble favorisée par un état de tension des films (jusqu'à 60 % en volume pour des dépôts en tension de +1,2 GPa). Le paramètre de maille libre de contrainte varie de façon spectaculaire. Il est très supérieur à celui du W massif pour les dépôts en compression (0,3173 nm au lieu de 0,3165 nm), et légèrement inférieur pour ceux en tension (0,3164 nm)

X-ray lithography : an overview and recent activities at super-ACO

In this paper, the current status of synchrotron based XRL is discussed in view to present this technic as an alternative path to ultra-large-scale-integrated circuit (ULSI) with feature sizes of quarter micrometer and below. Also, are described the L2M laboratory activities in nanomask fabrication and nanolithography using the synchrotron radiation of Super-ACO. Results show that the range of 50 nm can be currently achieved. Examples of device nano-fabrication are given