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ABSTRACT: We report on the effect of oxygen on the Jahn-Teller distortion and dynamic magnetic properties of low hole-doped Pr(0.9)Ca(0.1)MnO(3) thin films, using micro-Raman spectroscopy and the temperature dependent ac susceptibility, as a function of the frequency, dc field bias, and thermal cycles. The as-grown and vacuum annealed thin films show a high amount of magnetic frustration and inferior ferromagnetic ordering compared with the oxygen annealed thin films. It has been found that the amount of magnetic frustration in the film is interlinked with the Jahn-Teller distortion and domain wall dynamics. An attempt has been made to understand the origin and nature of the magnetic frustration.
Journal of Physics Condensed Matter 01/2013; 25(6):066005. · 2.55 Impact Factor
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ABSTRACT: Magnetotransport of La0.7Sr0.3MnO3 (LSMO)/regioregular poly3-hexylthiophene (rr-P3HT) interfaces were studied at 5–300 K to gain insight of spin transport in polymer coated LSMO. LSMO films on SrTiO3 (STO), MgO, and quartz substrates were characterized in pristine state, after depositing rr-P3HT and after removing rr-P3HT. Application and removal of rr-P3HT caused the disappearance of colossal magneto resistance and the emerging of low-field magnetoresistance (LFMR) in STO/LSMO, while the same treatment on MgO and quartz showed only a large LMFR signal with no significant changes during application and removal of rr-P3HT. This result signifies that epitaxial thin films of LSMO do not maintain their transport characteristics when coated with organic semiconductors, posing a limitation for efficient spin polarized injection at such interfaces.
AIP Advances. 01/2013; 3:042102.
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ABSTRACT: Magnetoresistive double perovskite Sr(2)FeMoO(6) thin films were grown with two different deposition pressures on SrTiO(3), MgO and NdGaO(3) substrates by pulsed laser deposition and thorough structural, magnetic and magneto-transport characterization was made. According to x-ray diffraction, all the films were phase pure and fully textured. Indication of substrate dependent strain and low angle grain boundaries was found, especially in films on MgO. Both the deposition pressure and the choice of the substrate have a strong influence on the saturation magnetization, M(s), and Curie temperature, T(C). The structural and magnetic data indicate the presence of anti-site disorder (ASD) in the films. The temperature dependence of resistivity showed semiconductive behaviour at temperatures below 100 K and metallic behaviour at higher temperatures. The semiconductive behaviour was found to increase with increasing ASD. In good quality films, up to 12% negative magnetoresistance (MR) was observed and films grown on MgO and NGO substrates also showed low field MR. However, the most significant observation of this study was that the magnetoresistivity of these Sr(2)FeMoO(6) thin films could not be explained with any traditional MR mechanism, but carried the clear signature of superposition of different mechanisms, in particular low angle grain boundary tunnelling and suppression of antiferromagnetically ordered domains under a magnetic field.
Journal of Physics Condensed Matter 08/2012; 24(36):366003. · 2.55 Impact Factor
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ABSTRACT: The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr(1-x)Ca(x)MnO(3) (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.
Journal of Physics Condensed Matter 04/2012; 24(21):216002. · 2.55 Impact Factor
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ABSTRACT: The effects of ex situ vacuum and oxygen annealing treatments on thin films of the low-bandwidth compound Pr(1-x)Ca(x)MnO(3) (PCMO) are investigated. Structural and magnetic measurements reveal that increased ferromagnetism can be achieved by oxygen annealing treatment, which is linked to the increased Mn(4+) ion content, as observed from x-ray photoelectron spectroscopy (XPS) measurements, as well as relaxation of the substrate-induced tensile strain of the PCMO unit cell. The increased number of Mn(4+) ions and partial release of strain lead to stronger double-exchange interaction in the system. Vacuum annealing increases the ferromagnetic (FM) interaction as well; however, the increased FM ordering is not directly related to the improved double-exchange interaction, as XPS measurement reveals an indication of a slight increase in Mn(3+) ions in this case. Trapping of carriers in the oxygen vacancies and formation of magnetic polarons have been suggested as the causes of the increase in ferromagnetic ordering, and this is also supported by the large coercivity and longer spin memory in the vacuum annealed PCMO.
Journal of Physics Condensed Matter 04/2012; 24(20):206002. · 2.55 Impact Factor
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ABSTRACT: The persistent photoinduced magnetization (PPM) in the low bandwidth material Pr(1-x)Ca(x)MnO₃ at the low hole doping level of x = 0.1 is reported. Upon zero-field cooling under photoexcitation, significant improvement of the ferromagnetic (FM) ordering was observed in the low temperature spin-glass phase. However, upon field cooling, the FM ordering was found to be suppressed due to weakening of the double-exchange interaction. High kinetic energy x-ray photoelectron spectroscopy measurements indicated a slight increase in the Mn³⁺ peak under photoexcitation which clarifies the weakening of the FM interaction. The fast relaxation of the PPM is discussed in view of localization of spin polarons in sites of magnetic disorders and the results are compared with previous reports of PPM in intermediate bandwidth Pr₀.₉Ca₀.₁MnO₃ samples.
Journal of Physics Condensed Matter 11/2011; 23(46):466002. · 2.55 Impact Factor
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ABSTRACT: Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750 °C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.
Journal of Physics Condensed Matter 09/2011; 23(38):386005. · 2.55 Impact Factor
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ABSTRACT: We have studied aging in laser ablated 2 wt% BaZrO<sub>3</sub> doped GdBaCuO (GdBCO) thin films. The films were coated with gold, silver, copper, photoresist and stycast. One film was left without coating for reference. The superconducting and structural properties were followed for several months. The critical temperature Tc and current density Jc were determined from magnetization measurements. The changes in structure were investigated with x-ray diffraction (XRD). It was found that in all the films the superconducting properties deteriorated more than can be accepted during the aging time.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: A series of studies has been carried out on thin films prepared by pulsed laser deposition (PLD) from micro-grained targets of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-</sub> x (YBCO) to determine the effect of doping them with BaCeO<sub>3</sub> (BCO) in concentrations between 0-10 wt%. BCO has been found to increase J <sub>c</sub> in e.g. melt-textured and bulk samples, but no systematic study with PLD thin films has been performed. Magnetometric measurements up to 8 T and down to 10 K as well as structural measurements with XRD and AFM were performed. J <sub>c</sub> increased with doping and the highest J <sub>c</sub> 's were achieved with low dopant levels (2% and 4%), which varied according to the external field and temperature. T <sub>c</sub> decreased when the dopant concentration was increased, similarly as in BaZrO<sub>3</sub> (BZO) doped films. However, the BCO growth process in the films was found to be different from the BZO and, furthermore, an interesting effect where the YBCO twin structure is altered was found around 6 wt% of BCO. The J <sub>c</sub> results and a comparison with structural properties with increasing dopant level and optimally BZO-doped YBCO thin films are presented.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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ABSTRACT: The influence of the BaZrO<sub>3</sub> (BZO) dopant concentration on structural and superconducting properties of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6+</sub> x (YBCO) thin films deposited on single crystal SrTiO<sub>3</sub> (STO) and buffered NiW metal substrates is investigated. Depending on the applied temperature and external magnetic field, the optimal BZO content n <sub>opt</sub> is found to vary so that irrespective of substrate used, in higher magnetic fields also higher n <sub>opt</sub> is needed. Moreover, the n <sub>opt</sub> of 4wt.% in YBCO films grown on STO is increased up to 7.5wt.% in films on NiW, especially in fields above 1 T. This phenomenon is qualitatively explained by the difference in size, shape and the distribution of BZO pinning centers and other structural disorders in the YBCO matrix. The results substantially affect to the development of the optimal flux pinning structure in future applications of high-temperature superconductors.
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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06/2011: pages 45 - 56; , ISBN: 9781118144121
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ABSTRACT: An aging effect investigation was made for GdBa2Cu3O7 (GdBCO) thin films grown on SrTiO3 (001) substrates with pulsed laser deposition (PLD) method from nanograined targets. The films were cut into two pieces where one piece was coated with gold cap layer while the other was left without coating. Both pieces were kept in ambient air during the half year measurement period. Magnetization measurements as well as phase purity, lattice parameter, oxygen effect and depth structure determination with x-ray diffraction (XRD) were made in one month interval. For structure and oxygen content, x-ray photoelectron spectroscopy measurements (XPS) were done in the beginning and in the end of the period. A reduction of the critical temperature and the critical current density, Jc, was found in the gold coated GdBCO film in ambient air in course of time. A smaller decrease of Jc was detected in uncoated GdBCO. No development of impurity phase, increase of a-orientation or reduction of the pinning structure was detected in uncoated GdBCO. However, a small development of impurity phase was found in gold coated GdBCO. The diminution of Tc and Jc is concluded to originate from oxygen release. No such a phenomenon was found in YBa2Cu3O7. We conclude that gold is not a proper cap layer at least for some applications.
Journal of Physics Conference Series 07/2010; 234(1):012036.
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ABSTRACT: The effect of BaZrO <sub>3</sub> (BZO) dopant concentration on superconducting properties in YBa <sub>2</sub> Cu <sub>3</sub> O <sub>6+x</sub> thin films is investigated in a wide magnetic field range up to 30 T. Based on the magnetization and resistivity measurements, the optimal BZO concentration for flux pinning is found to be 4% up to 20 T and increasing up to around 8% in higher fields. This result is qualitatively explained by a model where more BZO is needed in order to organize optimal columnar defect structure at high fields because the amount of single BZO particles acting as isotropic point pinning centers is increased, especially in high BZO concentrations. This means that for applications of thin film or coated conductor structures carried out at high magnetic fields, even higher dopant concentration than used in a low-field range should be produced in order to attain the optimal flux pinning conditions.
Journal of Applied Physics 04/2010; · 2.17 Impact Factor
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ABSTRACT: The resistivities of undoped and 4 wt% BaZrO3-doped (BZO) GdBa2Cu3O7 − δ (GdBCO) thin films were measured in a temperature activated flux-flow regime (TAFF). In addition, resistivity versus rotation angle of magnetic field, ρ(Θ), measurements were done near the critical temperature, Tc. The results of undoped and doped GdBCO were compared with ones of YBCO, and they showed that YBCO has better intrinsic pinning of CuO-planes than GdBCO. This is explained by the extra stacking faults distorting the ab-planes in GdBCO. The BZO-doping increased activation energy, U0, in geometry at fields higher than 1 T in YBCO and 3 T in GdBCO, but reduced U0 in the direction in the whole measured magnetic field range. Also, the irreversibility field, Birr, was enhanced in BZO-doped GdBCO at fields higher than 3 T in , but was reduced in the direction. The reduction of U0 and Birr in the direction in BZO-doped films is explained by BZO nanorods distorting the ab-planes, too. The distortion may explain the more isotropic ρ(Θ) in GdBCO and BZO-doped material. The 4 wt% BZO-doping seems to be more effective on GdBCO, and we suggest that either cation substitution, excess strain caused by the interaction of the extra stacking faults and BZO nanorods, or oxygen deficiency result in nanosized regions whose superconducting properties are reduced and which act as extra pinning sites. According to the resistivity measurements near Tc, the undoped YBCO should be used if high pinning of the ab-planes is needed. Further, from these materials 4 wt% BZO-doped GdBCO is best suited for applications where high magnetic fields are needed at high temperatures.
Superconductor Science and Technology 03/2010; 23(5):055010. · 2.66 Impact Factor
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ABSTRACT: We report the effect of 4 wt% BZO-doping in GdBCO thin films on (100) STO substrates with novel nanograined target materials deposited by pulsed laser deposition (PLD) method. X-ray diffraction, atomic force microscopy and magnetization measurements, which were made in particularly wide temperature and magnetic field range, were used to determine the structural and superconducting properties. The results were compared with ones of YBCO. The deposition temperature, T <sub>s</sub>, is not very critical for GdBCO and it is found to be much lower in undoped and BZO-doped GdBCO than in YBCO. The BZO-doping of GdBCO enhanced the field dependence of critical current density, j <sub>c</sub>, at low temperatures. Furthermore, BZO-doping enhances the accommodation field, B* , below 60 K being clearly higher at 10 K in doped GdBCO than in YBCO. The pinning mechanism in GdBCO was concluded to be similar to that of YBCO. Our results clearly show that undoped and BZO-doped nanophase GdBCO is a very promising target material for applications as well as for coated conductors.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: The influence of thermal annealings on La(0.9)Ca(0.1)MnO(3) (LCMO) films in oxygen and in vacuum with low hole doping is investigated in the phase separation region where competition between AFM and FM phases is high. Measurements by x-ray diffractometry, atomic force microscopy and magnetometry reveal changes in the lattice parameters and magnetic properties of the films, depending on the oxygen content. All films show magnetic cluster glass properties with similar freezing temperatures of around 45 K. Clearly the highest increase of the magnetization is observed in the films annealed in vacuum. We attribute this effect to trapping of unpaired electrons at oxygen vacancies where they can form rigid self-trapped magnetic polarons in potential wells of local moments. As a result long-range spin distortions with local ferromagnetic order may be realized. In conformity with these results, photoinduced persistent magnetization showing different mechanisms of generation, depending on the method of thermal annealing, is observed.
Journal of Physics Condensed Matter 07/2009; 21(26):266001. · 2.55 Impact Factor
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ABSTRACT: A systematic investigation of superconducting multilayered films composed of alternating nanostructured YBa2Cu3O6+x (YBCO) and secondary phase BaZrO3 (BZO) layers is carried out. Detailed growth mechanism studies of these structures as well as superconducting properties in a wide temperature and magnetic field range are investigated by varying the thicknesses of layers grown by pulsed laser deposition. Structural analysis shows that multilayering changes clearly the growth mechanism of YBCO. Alternating thin BZO layers smoothen the film surface, create in-plane mosaic spread structure of the YBCO crystals, produce stress in the BZO interface region and moreover improve the out-of-plane alignment of YBCO. Magnetic measurements of superconducting properties demonstrate that the critical temperature decreases slightly when the thickness of the BZO layers increases relative to the YBCO layers. This indicates a growing strain effect at the layer interfaces. At the same time, critical current densities of multilayers in the whole used temperature and magnetic field range are increased to the same level as in conventionally BZO-doped YBCO films when very thin alternating YBCO/BZO layer structures are deposited. This thickness dependent result is explained by the dense flux pinning centre network in layered superconductors, giving the opportunity to increase the overall thickness of a film and further enhance the current-carrying capability.
Superconductor Science and Technology 06/2009; 22(7):075019. · 2.66 Impact Factor
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ABSTRACT: The influence of thermal annealings on La0.9Ca0.1MnO3 (LCMO) films in oxygen and in vacuum with low hole doping is investigated in the phase separation region where competition between AFM and FM phases is high. Measurements by x-ray diffractometry, atomic force microscopy and magnetometry reveal changes in the lattice parameters and magnetic properties of the films, depending on the oxygen content. All films show magnetic cluster glass properties with similar freezing temperatures of around 45 K. Clearly the highest increase of the magnetization is observed in the films annealed in vacuum. We attribute this effect to trapping of unpaired electrons at oxygen vacancies where they can form rigid self-trapped magnetic polarons in potential wells of local moments. As a result long-range spin distortions with local ferromagnetic order may be realized. In conformity with these results, photoinduced persistent magnetization showing different mechanisms of generation, depending on the method of thermal annealing, is observed.
Journal of Physics Condensed Matter 06/2009; 21(26):266001. · 2.55 Impact Factor
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ABSTRACT: The resistivity of YBa2Cu3O7−δ (YBCO) films deposited by a pulsed laser from nanograined targets doped with various amounts of BaZrO3 (BZO) is investigated in the thermally activated flux-flow (TAFF) regime, where the activation energy, U, has a linear temperature dependence. It is found that the magnetic field dependence of the irreversibility temperature, Tirr(B), has a maximum at some optimal BZO concentration, which depends on the applied magnetic field. Our results show that the superconducting materials used in different magnetic fields need optimization of the dopant concentration to achieve the best properties for various applications.
Superconductor Science and Technology 04/2009; 22(6):065006. · 2.66 Impact Factor
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ABSTRACT: The phonon echo excited by radio-frequency pulses in superconducting MgB2 in external magnetic field was studied. Using the echo technique two contributions to ultrasound attenuation are observed: low-temperature relaxation, which depends on the magnetic field, and a decay connected to the superconducting energy gap at temperatures close to Tc. The value of about 7kBTc for the energy gap was extracted from the experimental data.
EPL (Europhysics Letters) 03/2009; 85(6):67001. · 2.17 Impact Factor
