Sr2FeMoO6 Films Research Articles

Enhanced cation ordering, electron-spin-phonon interactions and Fano resonance in half-metallic Sr2FeMoO6 thin films

We report the presence of electron-spin-phonon interactions in half-metallic ferromagnetic Sr2FeMoO6 (SFMO) double perovskite thin films using temperature-dependent Raman spectroscopy. A series of SFMO thin films have been prepared on LaAlO3 (001) single-crystal substrate using the Pulsed Laser Deposition technique. These depositions have been made in different gas-conditions such as in vacuum, and under nitrogen and oxygen gas pressures. At room temperature, Raman spectra manifest a Fano feature which indicates the presence of electron-phonon coupling in the films. The electron-phonon coupling strength further changes with a change in deposition conditions. Magnetization results show that the SFMO film grown in vacuum has the highest saturation magnetization which suggests better cation ordering as compared to the other films. For enhanced understanding, Raman spectra were recorded at varied temperatures and the data were analyzed by theoretical model fittings. A parameter quantifying temperature-dependent anharmonic nature of phonons has been derived using Balkanski model fits. This parameter shows a drastic deviation in the vicinity of Curie temperatures, manifesting a spin-phonon coupling in SFMO films. We further show that the spin-phonon coupling strengthens with improved Fe–Mo ordering. Any experimental observation of spin-phonon coupling has not been reported for SFMO systems till date. The magnetization data corroborate well with these observations made by Raman measurements. Our results of Raman spectroscopy, magnetization and resistivity collectively suggest that the SFMO films exhibit electron-spin-phonon interactions, which are influenced by the cation ordering. We also devised out the method of relating the anharmonic nature of Raman modes with the degree of Fe–Mo ordering and spin-phonon coupling in double-perovskite materials.

Process-Gas-Influenced Anti-Site Disorder and Its Effects on Magnetic and Electronic Properties of Half-Metallic Sr2FeMoO6 Thin Films

Anti-site disorder, arising due to the similar size of Fe and Mo ions in Sr2FeMoO6 (SFMO) double perovskites, hampers spintronic applicability by deteriorating the magnetic response of this double perovskite system. A higher degree of anti-site disorder can also completely destroy the half-metallicity of the SFMO system. To study the effects of different process gas conditions on the anti-site disorder, we have prepared a series of SFMO thin films on SrTiO3 (001) single-crystal substrate using a pulsed laser deposition (PLD) technique. The films are grown either under vacuum or under N2/O2 partial gas pressures. The vacuum-grown SFMO film shows the maximum value of saturation magnetization (MS) and Curie temperature (TC), signaling the lowest anti-site disorder in this series. In other words, there is a long-range Fe/Mo-O-Mo/Fe ferrimagnetic exchange in the vacuum-grown thin film, thereby enhancing the magnetization. Further, all the SFMO films show a semiconducting state with a systematic increase in overall resistivity with the increased anti-site disorder. The electrical conduction mechanism is defined by the variable-range hopping model at low temperatures. Raman spectra show a weak Fano peak, suggesting the presence of electron–phonon coupling in SFMO thin films. These results show the significance of the process gas in causing anti-site disorder, tuning the degree of this disorder and therefore its influence on the structural, magnetic, electrical, and vibrational properties of SFMO thin films.

Sr2FeMoO6 thin films: Effect of gas flow during deposition

Abstract The effect of gas flow on Sr2FeMoO6 thin films during pulsed laser deposition was investigated by characterizing their structural and magnetic properties. Crystallographic results reveal a great amount of structural distortion in films fabricated with high Ar or Ar + 5%H2 flow rate. The films with structural distortion have also larger coercive field, demonstrating domain pinning. The same films show also higher Curie temperature and saturation magnetization. Our results indicate that, in addition to other optimal deposition parameters, adjusting gas flow can be used for fine adjustment of Sr2FeMoO6 films for spintronic applications.

Surface topography and electrical properties in Sr2FeMoO6 films studied at cryogenic temperatures

Pulsed laser deposited Sr2FeMoO6 thin films were investigated for the first time with scanning tunneling microscopy and spectroscopy. The results confirm atomic scale layer growth, with step-terrace structure corresponding to a single lattice cell scale. The spectroscopy research reveals a distribution of local electrical properties linked to structural deformation in the initial thin film layers at the film substrate interface. Significant hole structure giving rise to electrically distinctive regions in thinner film also seems to set a thickness limit for the thinnest films to be used in applications.

Pulsed laser deposition of Sr2FeMoO6 thin films grown on spark plasma sintered Sr2MgWO6 substrates

Sr2FeMoO6 (SFMO) films were deposited on polycrystalline spark plasma synthesized Sr2MgWO6 (SMWO) substrates. Films were grown using pulsed laser deposition at temperatures (Tdep) between 720 °C and 820 °C in a vacuum environment of pressure mbar (0.1 mPa); after deposition they were cooled either in a pressure or mbar (10 mPa) O2. Despite the use of an isostructural substrate, the growth and cooling conditions play the primary role in determining details of the films’ structures and properties, similarly to single-crystals. Grazing x-ray and electron back-scatter diffraction indicate that vacuum-cooled films were pure perovskite-structured SFMO exhibiting grain-over-grain growth that aligned the perovskite sub-cells. SrMoO4 impurities were observed in the x-ray patterns for the oxygen-cooled films similarly to single-crystal substrates. Magnetic, electronic and magnetoresistive properties were all a function of growth and cooling environments. The Curie temperature and magnetization of the films increased with Tdep up to 800 °C. The vacuum-cooled films had low-resistivities with essentially metallic conductivity (small resistivity increases occurred at low-T), while the conductivity of oxygen-cooled films were consistent with variable range hopping. The oxygen-cooled films had higher low-field magnetoresistance effects at 5 K than the vacuum-cooled films, which seems consistent with SrMoO4 forming at grain boundaries. This work opens the route to tailor the electronic properties by engineering the grain boundaries in thin films.

Coherent Fe-rich nano-scale perovskite oxide phase in epitaxial Sr2FeMoO6 films grown on cubic and scandate substrates

We report the growth of high-quality epitaxial Sr2FeMoO6 (SFMO) thin films on various unconventional oxide substrates, such as TbScO3, DyScO3, and Sr2Al0.3Ga0.7TaO6 (SAGT) as well as on the most commonly used one, SrTiO3 (STO), by pulsed laser deposition. The films were found to contain a foreign nano-scale phase coherently embedded inside the SFMO film matrix. Through energy dispersive X-ray spectroscopy and scanning transmission electron microscopy, we identified the foreign phase to be Sr2−xFe1+yMo1−yO6, an off-stoichiometric derivative of the SFMO compound with Fe rich content (y ≈ 0.6) and a fairly identical crystal structure to SFMO. The films on STO and SAGT exhibited very good magnetic properties with high Curie temperature values. All the samples have fairly good conducting behavior albeit the presence of a foreign phase. Despite the relatively large number of items of the foreign phase, there is no significant deterioration in the properties of the SFMO films. We discuss in detail how magneto-transport properties are affected by the foreign phase.

Cantilever detected ferromagnetic resonance in thin Fe50Ni50, Co2FeAl0.5Si0.5 and Sr2FeMoO6 films using a double modulation technique

In this work we introduce a new method, which employs commercial piezo-cantilevers, for a ferromagnetic resonance (FMR) detection from thin, nm-size, films. Our setup has an option to rotate the sample in the magnetic field and it operates up to the high microwave frequencies of 160GHz. Using our cantilever based FMR spectrometer we have investigated a set of samples, namely quasi-bulk and 84nm film Co2FeAl0.5Si0.5 samples, 16nm Fe50Ni50 film and 150nm Sr2FeMoO6 film. Low frequency and room temperature test of our setup using 84nm Co2FeAl0.5Si0.5 film yielded a result identical to a standard X-Band spectrometer, namely a single line with quite small linewidth. Our measurements at low temperatures and high frequencies revealed a quite strong FMR response detected in all samples. The FMR spectra share common features, such as the emergence of the second line with an opposite angular dependence, and a drastic increase of the linewidths with increasing microwave frequency. We believe that these findings are results of the complicated dynamics of the magnetization at low temperatures and high frequencies, which we were able to probe using our cantilever based FMR setup.

Effect of Annealing Temperature to Oxidation State of Sr<sub>2</sub>FeMoO<sub>6</sub> Double Perovskite Film

We report our investigation on structural and magnetic properties of double perovskite Sr2FeMoO6 (SFMO) films fabricated at different annealing temperatures. Structural properties have been investigated by means of X-ray absorption near-edge spectra and extended X-ray fine structure, where a change of oxidation state has been observed depending on the annealing temperature. It is found that the oxidation state becomes more deficient for a lower annealing temperature, indicating a lower number of unoccupied Fe states with decreasing Mo coordination number. Ordering degree of Fe and Mo ions in SFMO films is found to strongly correlate to the oxidation state, resulting in the subsequent magnetic property change.

Microstructure and properties of epitaxial Sr2FeMoO6 films containing SrMoO4 precipitates

Thin films of Sr2FeMoO6 (SFMO) were grown by pulsed laser deposition in non-optimized argon ambient pressures. The films were found to contain a high number of precipitates of foreign phases. The nature and microstructure of these phases were investigated in detail by high-resolution scanning transmission electron microscopy (STEM) and X-ray diffractometry (XRD). We found out that the dominant foreign phase embedded in the SFMO film matrix was SrMoO4 (SMO). Through STEM and XRD analysis, we determined that the SMO phase grows epitaxially with respect to the surrounding SFMO matrix and has a fairly good crystallinity. Although the SFMO films include many foreign precipitates, they still exhibit good conducting properties and moderate magnetization values. Tuning the growth of the SMO phase on top of SFMO films to obtain a natural tunnel barrier might pave the way for future applications of SFMO in spintronic devices.

Thickness Dependent Properties of Sr2FeMoO6 Thin Films Grown on SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 Substrates

Pure, fully textured and c-axis oriented Sr2FeMoO6 films were deposited on SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates with different thicknesses. A decrease in substrate induced strain was observed in films on SrTiO3 with increasing thickness, but the strain in the films on (LaAlO3)0.3(Sr2AlTaO6)0.7 was nearly constant within the whole film thickness range. Despite the differences in the strain, the magnetic properties of the films showed similar thickness dependence on both substrates. The saturation magnetization and Curie temperature increased until around 150nm thickness was reached. Semiconducting low temperature upturn in resistivity was observed in all the films and it was enhanced in the thinnest films. Thus, the band gap energy increases with increasing film thickness. According to these results, at least 150nm thickness is required for high quality Sr2FeMoO6 films.

Double perovskite Sr(2)FeMoO(6) films prepared by electrophoretic deposition.

The present work reports on the new approach to create metal-supported Sr2FeMoO6 (SFMO)-based electrodes that have high potential to be applied in solid oxide fuel cells. The SFMO films were formed on stainless steel substrates by electrophoretic deposition (EPD) method. Ethyl alcohol with phosphate ester as a dispersant and isopropyl alcohol with I2-acetone mixture as a charge additive were considered as an effective medium for EPD of SFMO particles. The synthesis of SFMO powder as well as suspension preparation and deposition kinetics were systematically studied. The effect of applied voltage on the thickness and morphology of SFMO films was established. The microstructure of the deposits was examined by electron microscopy. The thickness, morphology and porosity of the SFMO layers can be fine-tuned by varying solvent, charging additives, deposition time, and applied voltage. According to X-ray photoelectron spectroscopy analysis, it was found that Fe(3+)-Mo(5+) and Fe(2+)-Mo(6+) pairs coexist, whereas the valent balance shifts toward an Fe(2+)-Mo(6+) configuration.

Cation non-stoichiometry in pulsed laser deposited Sr2+yFe1+xMo1-xO6 epitaxial films

Sr2FeMoO6 (SFMO) films were grown on SrTiO3 (100)- and (111)-oriented substrates via pulsed laser deposition (PLD). In order to study the fundamental characteristics of deposition, films were grown in two different PLD chambers. In chamber I, the best films were grown with a relatively long substrate-to-target distance (89 mm), high substrate temperature (850 °C), and low pressure (50 mTorr) in a 95% Ar/5% H2 atmosphere. Although X-ray diffraction (XRD) measurements indicate these films are single phase, Rutherford Backscattering (RBS) measurements reveal considerable non-stoichiometry, corresponding to a Sr2Fe1−xMo1+xO6 composition with x ≅ 0.2–0.3. This level of non-stoichiometry results in inferior magnetic properties. In chamber II, the best films were grown with a much shorter substrate-to-target distance (38 mm), lower temperature (680 °C), and higher pressure (225 mTorr). XRD measurements show that the films are single phase, and RBS measurements indicate that they are nearly stoichiometric. The degree of ordering between Fe and Mo was dependent on both the temperature and pressure used during deposition, reaching a maximum order parameter of 85%. The saturation magnetization increases as the Fe/Mo ordering increases, reaching a maximum of 2.4 μB/f.u. Based on prior studies of bulk samples, one would expect a higher saturation magnetization for this degree of Fe/Mo order. The presence of extra strontium oxide layers in the form of Ruddlesden-Popper intergrowths appears to be responsible for the lower than expected saturation magnetization of these films.

Effect of Thickness on Crystal Structure and Magnetic Property of Sr<sub>2</sub>FeMoO<sub>6</sub> Film

Double perovskite Sr2FeMoO6 film with different thickness was prepared by electrophoresis method on single crystal Si substrate. The post-annealing treatment was carried out at 1100°C with 5% H2/Ar flow. XRD result shows that all the films are of single phase and belong to the I4/m lattice. All the films have no preferred orientation and exhibit typical polycrystal property. Measurement on scanning electron microscope shows that all the three films exhibit similar surface morphology. Different from crystal structure and surface morphology, Curie temperature (TC) shows a non-linear evolution with the thickness of the film. TC of the film increases firstly and then decreases, the highest TC is 300 K. This result indicates that ferromagnetism of the film is strengthened at the thickness about 35μm.

Structural and magnetic properties of Sr2FeMoO6 film prepared by electrophoresis technique

Double perovskite Sr2FeMoO6 film was fabricated by electrophoresis method on single crystal Si substrate. The post-annealing treatment was carried out at 900°C, 960°C, 1060°C and 1100°C with 5% H2/Ar atmosphere. Surface micrograph, structural and magnetic properties of the film have been investigated. It is found that the annealing temperature plays an important role on the phase formation and magnetic properties. According to X-ray diffraction (XRD), single phase Sr2FeMoO6 film was obtained at annealing temperature 1100°C, the film shows typical polycrystal property. However, the superstructure reflection assigning the ordering arrangement of Fe and Mo atoms in the perovskite structure disappears in our XRD pattern. Raman detection finds two peaks at around 440cm−1 and 620cm−1, which belongs to Sr2FeMoO6 phase. Consistent with XRD results, typical impurity like SrMoO4 appears at around 880cm−1 in the film annealed at lower temperature. Microstructure investigation shows that the surface of the film is homogeneous and the grain size of particles increases with annealing temperature. Temperature dependence of magnetization reveal that the highest Curie temperature (TC=282K) is obtained from the film annealed at 1100°C. Owing to the large amount of anti-site defect, the highest TC is still lower than that of parent Sr2FeMoO6 powder.

HR-STEM Imaging and EELS Characterizing of Nano-Scale Defects in Sputter Deposited Thin Films of Double-Perovskite Sr2FeMoO6 (SFMO) and Sr2CrReO6 (SCRO)

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

X-ray photoemission spectroscopy of Sr2FeMoO6 film stoichiometry and valence state

The authors have used a combination of monochromatized x-ray photoemission spectroscopy and Rutherford backscattering spectrometry to develop a protocol for determining bulk film stoichiometry and charge state of Sr2FeMoO6 epitaxial films These studies identify an optimum ion sputtering process for removing surface contaminants while avoiding preferential sputtering of film constituents or alteration of their characteristic valence states. For Sr2FeMoO6, low energy (500 eV), glancing incidence Ar+ sputtering for short (tens of seconds) periods is successful in achieving stoichiometric compositions and characteristic charge states of the film constituents. The evolution of composition and valence state with sputtering provides a guide to measure stoichiometry and charge state of complex oxide thin films in general.

Spin-polarized electron tunneling in polycrystalline Sr2FeMoO6 thin films

We report a comparative study of magnetotransport in high quality epitaxial and polycrystalline thin films of Sr2FeMoO6 deposited on single crystal substrates of SrTiO3 (STO) and Yttria-stabilized zirconia (YSZ) respectively. The polycrystallinity of the films on YSZ affects their magnetotransport significantly. The resistivity ρ(T) of such films is thermally activated in contrast to metallic behavior of ρ(T) of the thin films on STO. A large (≈ 13 %) low-field magnetoresistance (MR) at 10 K is seen in the polycrystalline samples compared to the negligibly small MR (≈ 2.5 %)of their epitaxial counterparts. A detail analysis of the temperature and magnetic-field-dependent resistivity and magnetization provide strong evidence for tunneling of spin-polarized carriers through a magnetic intergranular material of insulating ground state.

Formation of nanocrystalline films of Sr2FeMoO6 on Si(100) by pulsed laser deposition: Observation of preferential oriented growth

Nanocrystalline Sr2FeMoO6 films have been grown on a Si(100) substrate by pulsed laser deposition under different growth conditions including deposition temperature and time. A nanocrystalline single-phase Sr2FeMoO6 film was obtained at a temperature as low as 600 °C. This high-quality ferromagnetic film was found to have a saturation magnetic moment of 3.4μB per formula unit and a coercive field of 1.5 kOe at 77 K with micrometer-sized magnetic domains. By using glancing-incidence x-ray diffraction with different incident beam angles, the crystal structure of the film was sampled as a function of depth. For the as-grown Sr2FeMoO6 films thicker than 60 nm, a preferential orientation of the nanocrystals in the film was observed, despite the lack of good lattice matching with the Si substrate. At a higher deposition temperature of 800 °C, the as-grown film exhibited the same saturation magnetic moment but with a discernibly lower coercive field of 0.8 kOe, consistent with the larger grain size obtained at a higher growth temperature.

Angular dependence of spin-wave resonance and relaxation in half-metallic Sr2FeMoO6 films

We investigated the magnetic anisotropic parameters and spin-wave relaxation of thin films of the ferromagnetic half-metallic Sr2FeMoO6 by ferromagnetic resonance technique. The resonance field and linewidth were recorded as a function of relative angle between applied magnetic field and crystallographic axes of the sample. The resonance field varies sinusoidally and considerable linewidth broadening occurs when the applied field is rotated parallel to the sample plane. The results are described using higher order components of anisotropy fields. We obtain the values of the cubic anisotropic field, 2K4∥∕M=0.09611T, the effective demagnetization field, 4πM−2K2⊥∕M=0.1216T, and the planar anisotropic field, 2K4⊥∕M=−0.108T. Moreover, we estimated the spin relaxation time (damping factor) from the analysis of the angular dependence of peak-to-peak linewidth, leading to an intrinsic value of α∼0.00025 (Gilbert damping).

Double-perovskite Sr2FeMoO6 epitaxial films with ordered cation structure grown in mixture gas of hydrogen and argon

Epitaxial double-perovskite Sr2FeMoO6 films have been grown by pulsed laser deposition (PLD) on SrTiO3 substrates. High-quality metallic Sr2FeMoO6 thin films with low resistivity were obtained by deposited in a mixture gas of hydrogen and argon. Resistivity versus temperature measurements, magnetic measurement, and x-ray diffraction indicate that using a mixture forming gas of hydrogen and argon in PLD is a flexible method for preparing high-quality epitaxial Sr2FeMoO6 films. In (011) cross-section high-resolution transmission electron microscopy images of epitaxial Sr2FeMoO6 films, the clear superlattice structure of ordered cations of Fe and Mo is shown.