Sr2FeMoO6 Thin 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.

Increased Curie temperature and magnetoresistive response by modifying Fe/Mo ratio in Sr[formula omitted]FeMoO[formula omitted] thin films

We investigated the effect of deposition distance on a set of otherwise identically grown Sr2FeMoO6 (SFMO) thin films grown by pulsed laser deposition. Based on the detailed magnetic and transport measurements, we found that the optimal properties can be realized at longer deposition distances than earlier expected. The achieved onset Curie temperature of the order of 400K with the middle transition value of 372K, which are clearly the highest presented in the literature for the SFMO thin films. In addition, the increased metallicity and magnetoresistive response is observed in films deposited at longer distances. The improvements are widely discussed in the light of discovered stoichiometric imbalance between the cations Fe and Mo, which modify the magnetic interactions and thus magnetic and electric properties. Therefore, this study shows a new approach in the deposition process to provide the SFMO thin films and multilayers of high quality for future spin valve devices working at room temperature.

Nanostructured double perovskite magnetic thin films by PLD

Semi-metallic ferromagnetic oxides, having a double perovskite structure, exhibit a variety of strange magneto transport and magnetic characteristics for spintronics applications. The double perovskites Sr2FeMoO6 (SFMO) and Sr2FeWO6 (SFWO) are known as potential magneto resistive materials that exhibit appreciable tunnel-type intrinsic magnetoresistance (TMR) at room temperature. Current research is focused on the deposition and characterization of nanocrystalline thin layers of SFMO and SFWO materials on LaAlO3 substrate with a temperature of 825 °C and pressure of 0.03 mTorr using the pulsed laser deposition (PLD) method. Films are nearly single-crystalline with a predominant c-axis orientation. A smaller peak corresponding to the plane (101) at 2θ = 19.6° was also found, indicating the arrangement of Fe, and Mo ions at B and B' locations in samples. Atomic Force Microscope (AFM) studies reflect uniform grain growth with surface roughness and average grain size of approximately 8 nm and 52.6 nm; 6 nm and 28.2 nm, respectively for SFMO and SFWO thin films. Magnetization measurement at 5 K shows that SFMO is ferromagnetic and SFWO is antiferromagnetic with a significant difference in their magnetic moments. M−T curve of the SFWO film shows a peak at 36.7 K, resembling the Neel temperature. While the M−T graph of SFMO thin films reveals that ferromagnetic transition temperature (Tc) is higher than 320 K.

Exploring the anti-site disorder and oxygen vacancies in Sr[formula omitted]FeMoO[formula omitted] thin films

To address the importance of nanoscale defects in complex magnetic oxides, we present an effective tool, variable energy positron annihilation spectroscopy, for probing the relatively small changes in anti-site disorder and oxygen vacancies of the in situ annealed double perovskite Sr2FeMoO6 thin films. By controlling the annealing conditions in wide pressure and temperature ranges and thus affecting the amount of nanoscale defects, we show that the magnetic properties of Sr2FeMoO6 thin films can be modified, particularly with the oxygen nonstoichiometry, and hence their spintronic functionality can be improved. On the basis of our findings together with proposed mechanism, we suggest that the annealing treatments can also be scaled to other complex magnetic perovskites to engineer nanoscale defects and thus improve their usability in future spintronic applications.

Refined Sr2FeMoO6 interface realized with photoemission and magnetization analysis

We investigate the effects of exsitu post-annealing on a set of identically deposited Sr2FeMoO6 (SFMO) thin films. The annealing provides a significant enhancement to ferrimagnetic exchange, evidenced by almost 40 K increase in Curie temperature. The electronic nature of the film surface is our focus of interest. The detailed analysis on oxygen 1s, strontium 3d, molybdenum 3d spectrum along with changes in the carbon contamination show purification from additional phases and significant valence fluctuation. Successful exsitu treatment provides films with higher Curie temperature and diminished surface contamination. Annealing therefore provides a possible method of reobtaining a portion of the valuable bulk attributes at interfaces in e.g. spin valve structures.

Synthesis and physical characterization of Sr2FeMoO6 and Sr3FeMoO7 thin films

Quaternary oxides based on strontium, iron, and molybdenum elements such as Sr2FeMoO6 (SFMO) and its analog n = 2 Ruddlesden-Popper (R-P) phase Sr3FeMoO7 were synthesized using chemical spray pyrolysis deposition technique. Optical band gap values, thermal conductivity and thermal diffusivity of both spray-deposited films were investigated. The results confirm that the above properties are considerable factors in forecasting the appropriate perovskite compound for future thermal barrier coating applications.

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.

Interface defects induced vertical magnetic anisotropy in Sr2FeMoO6 thin films

Pulsed laser deposition was used to fabricate high quality Sr2FeMoO6 thin films on SrTiO3 and SrLaAlO4 single crystal substrates. The focus of our research has been on the magnetic properties of Sr2FeMoO6 thin films and we have observed an outstanding shift from parallel in-plane magnetic anisotropy to perpendicular out-of-plane anisotropy, induced by the substrate. Our results also provide the first experimental evidence of magnetic stripe domain pattern in Sr2FeMoO6 thin films. Compared with previous studies for magnetoresistive thin films, we propose a conspicuous mechanism behind the shift in magnetic anisotropy. Rather than being induced directly by the lattice mismatch, we find the possible contribution to perpendicular magnetic anisotropy from over-relaxation at the film substrate interface or from other crystalline defects such as low-angle grain boundaries.

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.

Improving the surface structure of high quality Sr2FeMoO6 thin films for multilayer structures

Two sets of Sr2FeMoO6 thin films were prepared with pulsed laser deposition and the effect of the laser fluence and the deposition temperature was investigated. The Sr2FeMoO6 thin films showed clear evidence of impurity phases when the laser fluence was altered. Phase pure films resulted through the whole temperature range between 900°C and 1050°C when a proper laser fluence was used. Films fabricated at lower deposition temperatures resulted with smaller surface roughnesses around 5nm, higher Curie temperatures and with relatively high saturation magnetization values. The Curie temperature was determined from the minimum of the first order derivative and results showed the highest values of 350K and above. The films with the highest Curie temperature reached zero magnetization above 400K. The results indicate that both high microstructural and high magnetic quality Sr2FeMoO6 thin films can be obtained with a deposition temperature between 900°C and 950°C. This provides better fabrication parameters for the upcoming SFMO multilayer structures.

Toward Versatile Sr2FeMoO6-Based Spintronics by Exploiting Nanoscale Defects.

To actualize the high spintronic application potential of complex magnetic oxides, it is essential to fabricate these materials as thin films with the best possible magnetic and electrical properties. Sr2FeMoO6 is an outstanding candidate for such applications, but presently no thin film synthesis route, which would preserve the magnetic properties of bulk Sr2FeMoO6, is currently known. In order to address this problem, we present a comprehensive experimental and theoretical study where we link the magnetic and half metallic properties of Sr2FeMoO6 thin films to lattice strain, Fe-Mo antisite disorder and oxygen vacancies. We find the intrinsic effect of strain on the magnetic properties to be very small, but also that an increased strain will significantly stabilize the Sr2FeMoO6 lattice against the formation of antisite disorder and oxygen vacancies. These defects, on the other hand, are recognized to drastically influence the magnetism of Sr2FeMoO6 in a nonlinear manner. On the basis of the findings, we propose strain manipulation and reductive annealing as optimization pathways for improving the spintronic functionality of Sr2FeMoO6.

The predominance of substrate induced defects in magnetic properties of Sr2FeMoO6 thin films

A systematic study of epitaxially grown Sr2FeMoO6 thin films on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrLaAlO4 and MgO single crystal substrates were made. Transmission electron microscopy investigations showed sharp substrate/films interfaces and increased defect concentration with increased lattice mismatch, indicating defect formation such as dislocations, low angle grain boundaries and stacking faults as a strain relaxation mechanism. Large enough compressive mismatch cause the over-relaxation of the lattice parameters through reorganization or interface defects, which was observed as a tensile strain in films with compressive mismatch larger than −1.05%. All the films with compressive mismatch were phase pure and epitaxially textured while signatures of SrMoO4 parasitic particle was found only in the film grown on MgO. No correlation between the antisite disorder and other structural defects or magnetic properties were found. Instead, the saturation magnetization, Curie temperature, magnetic domain rotation etc are higly dependent on the lattice mismatch induced defects, which outshines the possible correlation with B-site ordering.

Polycrystalline Sr2FeMoO6 thin films on Si substrate by pulsed laser deposition for magnetoresistive applications

Polycrystalline Sr2FeMoO6 (SFMO) thin films were deposited at different substrate temperatures (500–800°C) on STO buffered Si (100) substrates by pulsed laser deposition (PLD). Structural analysis shows the single phase formation of polycrystalline SFMO thin films. A large (~12%) high field magnetoresistance (HFMR) was observed at 5K in polycrystalline films deposited at 800°C.. First time, we studied the MR effect in polycrystalline SFMO thin films grown on Si substrate, which make them very promising for magnetoresistive and spintronic applications and possibly open the future prospect for their possible integration in microelectronic industries.

The effect of film thickness on the magnetic and magneto-transport properties of Sr2FeMoO6thin films

Magnetoresistive Sr2FeMoO6 thin films were grown by pulsed laser deposition with three different thicknesses 150 nm, 270 nm and 500 nm. Structural, magnetic and magneto-transport properties of the films were measured. Structural properties showed that impurity phases are formed when the film thickness exceed limiting thickness over 300 nm. Otherwise no major differences were observed in structural and magnetic properties between the films. The semiconductive upturn was observed in all ρ(T) curves, but it was notably smaller for the two thickest films. At 350 K the magnetoresistive (MR) behaviour was very similar for all the films, but at 10 K the negative MR was clearly largest for the thickest film and also the shape of the curve in low fields deviated from others.

Absence of traditional magnetoresistivity mechanisms in Sr2FeMoO6 thin films grown on SrTiO3, MgO and NdGaO3 substrates

Magnetoresistive double perovskite Sr2FeMoO6 thin films were grown with two different deposition pressures on SrTiO3, MgO and NdGaO3 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, Ms, and Curie temperature, TC. 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 Sr2FeMoO6 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.

Effect of ex situ Post-annealing Treatments on Sr2FeMoO6 Thin Films

Magnetoresistive Sr2FeMoO6 thin films were grown by pulsed laser deposition at optimized deposition atmosphere and temperature. Films were then ex situ post-annealed in different atmospheres and by vacuum annealing at temperatures between 500 °C and 1100 °C. Ar and air annealed samples were destroyed by ex situ post-annealing treatment, due to formation and dominance of SrMoO4 impurity phase. X-ray diffraction showed no impurities and full texturation of vacuum and ArH2 (5%) annealed samples. Those samples showed also similar magnetic and magnetoresistive behavior like as-deposited sample. Neither magnetic, magnetotransport nor structural properties could be improved by ex situ post-annealing treatments.

Synthesis and Characterization of Thin Films of a Half-Metallic Oxide, Sr2FeMoO6, by Combinatorial Techniques

The ordered double perovskite oxide, Sr2FeMoO6, has a half-metallic property and is considered as one of the best materials for creating spin-polarized current for next-generation spintronics devices. In order to optimize the fabrication conditions for (001)-oriented single-crystalline Sr2FeMoO6 thin films, a combinatorial pulsed laser deposition (PLD) technique, together with combinatorial (high-throughput) characterization, were carried out. It is found that atmospheric pressure during the deposition affects the formation of secondary phases, thus careful control of this parameter is necessary. On the other hand, substrate temperature affects the electric transport property and surface flatness; better results are obtained at a higher substrate temperature.

Double-Perovskite Sr2FeMoO6 Thin Films Prepared by Using Pulsed Laser Deposition: Growth and Crystal, Electronic and Magnetic Structures

Double-Perovskite Sr2FeMoO6 Thin Films Prepared by Using Pulsed Laser Deposition: Growth and Crystal, Electronic and Magnetic Structures