-
[show abstract]
[hide abstract]
ABSTRACT: La0.65Sr0.35MnO3-δ (LSMO) films (δ ≈ 0, 0.07, 0.12) were studied by hard x-ray photoelectron spectroscopy. It is found that the Mn 3d valence band states shift to higher binding energies for oxygen-deficient samples, while their overall width decreases as a result of the reduced density of charge carriers. The concurrent disappearance of the well-screened state at the Mn 2p level indicates a decrease in hybridization of the Mn 3d and the doping-induced states. The lack of clear band gap formation for oxygen-deficient LSMO above the metal/insulator transition is compatible not with polaron formation but, rather, with a Mott variable range hopping mechanism, which is also supported by our transport data. The large electron probing depth of hard x-ray photoelectron spectroscopy is crucial for this study because modifications to the electronic structure may occur in the near surface region.
Physical Review B 10/2012; 86:155102. · 3.69 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We here report on the occurrence of strong orbital order driven by off-stoichiometry in oxide manganite films. Multiple-electronic-valence Mn ions are possibly substituted into the perovskite atomic A site (namely, A-site manganites). By combining state-of-the-art synchrotron ultrafine spectroscopies, we did provide evidence of a high ferromagnetic metallic state together with a strong orbital order of Mn ions, despite that the kinetic energy of the free charge carriers should prevent long-range orbital order. By varying the total amount of Mn ions at the perovskite A site, a specific orbital order (either in-plane or out-of-plane) can be determined.
Physical Review B 09/2012; 86(11):115132. · 3.69 Impact Factor
-
S. Mercone,
C. A. Perroni,
V. Cataudella,
G. De Filippis,
C. Adamo,
M. Angeloni,
C. Aruta,
F. Miletto-Granozio,
A. Oropallo,
P. Perna, A. Yu. Petrov,
U. Scotti di Uccio,
G. Balestrino,
L. Maritato
[show abstract]
[hide abstract]
ABSTRACT: The colossal magnetoresistance (CMR) manganese oxides, have been the subject of intense studies. To elucidate the mechanisms
at work in the ferromagnetic metallic state is essential for the comprehension of the M–I transition and the associated CMR
effect. We present here comparison among resistivities behavior in Ca and Sr-doped manganite films (x = 0.3) grown by different techniques and on several substrates allowing to analyze samples with different amounts of disorder.
At low temperatures, a dominant T
2 term in the resistivity has generally been observed. Our analysis shows that for residual resistivity values larger than
a critical one, there is a substantial deviation from the T
2-like behavior and that an unusual T
2.5 one is robust. This behavior supports the theoretical proposal of single magnon scattering in the presence of minority spin
states localized by the disorder. In the high temperature insulating paramagnetic phase the resistivity shows the activated
behavior characteristic of polaronic carriers. Finally in the whole range of temperatures the experimental data are found
to be consistent with a phase separation scenario.
KEY WORDS:manganese oxides–polarons–phase separation–electric transport
Journal of Superconductivity 04/2012; 18(5):719-722.
-
P. Orgiani,
C. Adamo,
C. Barone,
A. Galdi,
S. Pagano, A. Yu. Petrov,
O. Quaranta,
C. Aruta,
R. Ciancio,
M. Polichetti,
D. Zola,
L. Maritato
[show abstract]
[hide abstract]
ABSTRACT: We report on structural, magnetic, and transport properties of La0.7Ba0.3MnO3 thin films, epitaxially grown on MgO substrates. Despite the quite similar structural features if compared to crystalline manganite films, our samples show a metal-insulator transition temperature of about 200 K, sizeably lower than the bulk-value (TMI ≃ 345 K). Moreover, the magnetotransport properties show the absence of saturation in the magnetoresistance and localization phenomena at low temperatures (T<30 K). The temperature behavior of the magnetization shows a Curie temperature Tc value above room temperature, ruling out effects due to oxygen deficiency. All these findings are analyzed in terms of possible physical mechanisms related to the growth in the presence of large mismatch between film and substrate lattice parameters.
Journal of Applied Physics 04/2008; 103(9):093902-093902-8. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have systematically studied the transport properties of colossal magnetoresistance (CMR) manganite films at temperatures of up to 550 K by changing parameters influencing the amount of disorder in the system. Independent of the type and amount of disorder, the resistivity can always be described using a Tα-power law in the ferromagnetic-metallic phase and a thermally activated behavior in the polaronic-insulating phase. A linear correlation between both the value of the α coefficient and the polaron formation energies Eg as a function of the metal-insulator transition temperature TMI has been found. Our results indicate the possibility of describing the transport properties in CMR manganites in terms of a single disorder parameter.
Phys. Rev. B. 07/2007; 76(1).
-
[show abstract]
[hide abstract]
ABSTRACT: We have fabricated La <sub>0.7</sub> Ba <sub>0.3</sub> MnO <sub>3</sub> (LBMO) thin films by a pulsed laser deposition technique using a low partial pressure (10<sup>-2</sup> mbar ) of O<sub>2</sub>+5% O<sub>3</sub> . In order to study the role of oxygen content in the transport properties, the temperature dependence of the resistivity has been measured as a function of the temperature and the time of a post-annealing process. Strain-less LBMO thin films deposited on SrTiO <sub>3</sub> substrates show a metal insulator transitions temperature T<sub> MI </sub>≃345 K (equal to that found in bulk material). A T<sup>2.5</sup>- power law has been measured in R(T) at low temperature, supporting the disorder-induced single magnon scattering scenario. At high temperature (T≫T<sub> MI </sub>) an activated behavior characteristic of polaronic carriers has been measured. Magnetotransport properties show a metal-like behavior and a negative magnetoresistance (MR) in the whole temperature range. With an external magnetic field of 70 kOe , the MR reaches the maximum value of 170% at about 320 K .
Journal of Applied Physics 03/2007; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Thin films of La0.7Sr0.3MnO3 were grown by
molecular beam epitaxy on (001)LaAlO3 crystals. High resolution X-ray
diffraction analysis proves the presence of twins in the films at room
temperature, showing that the twin structure of the substrate which forms at
the
Pm[`3]m® R[`3]cPm\bar {3}m\to R\bar {3}c
ferroelastic transition at TF = 813K
served as a template for the film microstructure. Magnetic measurements
indicate a thermomagnetic irreversibility which is ascribed to the quenched
disorder related to twinning and discussed in terms of coexisting
ferromagnetic and spin disordered regions connected with the undeformed
domain cores and strained domain walls respectively.
Physics of Condensed Matter 01/2007; 55(4):337-345. · 1.53 Impact Factor
-
S. Mercone,
C. A. Perroni,
V. Cataudella,
C. Adamo,
M. Angeloni,
C. Aruta,
G. De Filippis,
F. Miletto,
A. Oropallo,
P. Perna, A. Yu. Petrov,
U. Scotti di Uccio,
L. Maritato
[show abstract]
[hide abstract]
ABSTRACT: The resistivity of thin $La_{0.7}A_{0.3}MnO_{3}$ films ($A=Ca, Sr$) is investigated in a wide temperature range. The comparison of the resistivities is made among films grown by different techniques and on several substrates allowing to analyze samples with different amounts of disorder. In the low-temperature nearly half-metallic ferromagnetic state the prominent contribution to the resistivity scales as $T^{\alpha}$ with $\alpha \simeq 2.5$ for intermediate strengths of disorder supporting the theoretical proposal of single magnon scattering in presence of minority spin states localized by the disorder. For large values of disorder the low-temperature behavior of the resistivity is well described by the law $T^{3}$ characteristic of anomalous single magnon scattering processes, while in the regime of low disorder the $\alpha$ exponent tends to a value near 2. In the high temperature insulating paramagnetic phase the resistivity shows the activated behavior characteristic of polaronic carriers. Finally in the whole range of temperatures the experimental data are found to be consistent with a phase separation scenario also in films doped with strontium ($A=Sr$).
11/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: We have fabricated by an in situ process (La1-x
Sr
x
)
y
MnO3 epitaxial thin films by Molecular Beam Epitaxy using a low partial pressure (
@\cong
2 x 10-5 torr) of O2 + 5% ozone. Reflected High Energy Electron Diffraction analysis has been performed during the growth process to check the structural properties of the films. The samples have not been subjected to any in situ or ex situ post deposition annealing procedure. Thin films on different substrates (SrTiO3, LaAlO3, NdGaO3) and with different thickness have been fabricated to compare the transport properties and investigate the effects of the epitaxial strain. For compositions around x = 0.3, Metal-Insulator (MI) transitions at temperature as high as T
MI
= 340 K have been observed in thin films few nanometers thick. Resistivity versus temperature curves measured on samples deposited in the same run onto different substrates, have shown clear effects related to the epitaxial strain. These results are very promising for a deeper understanding of the physical mechanisms at work in manganites and in view of the future fabrication of manganite-based heterostructures for electronic applications.
Physics of Condensed Matter 06/2004; 40(1):11-17. · 1.53 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: To analyze the conducting and magnetic properties near the film and substrate interface in manganites, ultrathin films (thickness≤100 Å) of La0.7Sr0.3MnO3 were epitaxially grown by molecular-beam epitaxy on single-crystal (001) LaAlO3, (110) NdGaO3, and (001) SrTiO3 substrates. Structural, magnetic, and magnetoresistive properties were investigated. All samples exhibit a substrate-independent decrease of the c-lattice parameter for thinnest films. Highly anisotropic behavior in both transport and magnetic properties were measured along the in-plane directions parallel to the substrate crystallographic axes. In particular, for the thinnest films (60 Å), the negative magnetoresistance at about 120 K with the average current along one of the crystallographic directions, is larger than the room-temperature colossal value. In the same low-temperature range, with the current along the other in-plane crystallographic direction, the magnetoresistance changes sign (resulting to be magnetic field independent for T∼150 K). Such an in-plane anisotropy of transport and magnetic properties is investigated with respect to possible intrinsic and extrinsic physical mechanisms.
Phys. Rev. B. 74(13).
-
[show abstract]
[hide abstract]
ABSTRACT: Thin films of La 0.7 Sr 0.3 MnO 3</S ubscript> were grown by molecular beam epitaxy on (001)LaAlO 3 crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the $Pm\bar {3}m\to R\bar {3}c$ ferroelastic transition at T F =813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic irreversibility which is ascribed to the quenched disorder related to twinning and discussed in terms of coexisting ferromagnetic and spin disordered regions connected with the undeformed domain cores and strained domain walls respectively. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007
The European Physical Journal B - Condensed Matter and Complex Systems. 55(4):337-345.
-
[show abstract]
[hide abstract]
ABSTRACT: We report on measurements of current-resistance effects in La1−xSrxMnO3 ultrathin films deposited by molecular beam epitaxy. dc transport and voltage noise spectral density analyses have been performed in the temperature range of 10–300 K, and the results are compared with existing theoretical models. A possible connection between the explanation of the electrical transport properties and the two-level tunneling systems model, used for the voltage noise analysis, is proposed.
Phys. Rev. B. 75(17).
