- Email: [email protected]
Sr2FeMoO6 thin films: Effect of gas flow during deposition
Accepted Manuscript Sr2FeMoO6 thin films: effect of gas flow during deposition I. Angervo, R. Siekkinen, M. Saloaro, H. Huhtinen, P. Paturi PII: DOI: Reference:
S0304-8853(18)32752-5 https://doi.org/10.1016/j.jmmm.2018.11.098 MAGMA 64657
To appear in:
Journal of Magnetism and Magnetic Materials
Please cite this article as: I. Angervo, R. Siekkinen, M. Saloaro, H. Huhtinen, P. Paturi, Sr2FeMoO6 thin films: effect of gas flow during deposition, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/ j.jmmm.2018.11.098
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sr2FeMoO6 thin films: effect of gas flow during deposition I. Angervoa,, R. Siekkinena,, M. Saloaroa , H. Huhtinena , P. Paturia a
Wihuri Physical Laboratory, Department of Physics and Astronomy, FI-20014 University of Turku, Finland
Abstract The effect of gas flow on Sr2 FeMoO6 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 Sr2 FeMoO6 films for spintronic applications. Keywords: Pulsed laser deposition, Sr2 FeMoO6 thin film, 1. Introduction The desire for novel spintronic applications has drawn a lot of attention to certain complex magnetic oxides, with valuable attributes, like high Curie temperature (TC ) and high spin polarization. One of these materials is Sr2 FeMoO6 (SFMO), a double perovskite with Curie temperature around 420 K and with 100% spin polarization linked to large magnetoresistance [1]. However, to harness this material for great variety of applications, high quality thin films are required. Fabrication of high-class SFMO thin films has proven to be, and continues to be, a challenging task. Pulsed laser deposition (PLD) is an effective Email address: [email protected] (I. Angervo)
Preprint submitted to Journal of Magnetism and Magnetic Materials November 14, 2018
fabrication technique which has successfully been used to produce high quality SFMO thin films [2–6]. The fabrication involves free parameters such as temperature, pressure, atmosphere, pulse energy, film thickness etc., which can vary highly between or within research projects. Large amount of work has taken place to optimize and study the effects of deposition parameters. No optimal parameters to fabricate SFMO films have been found. In this, study we investigate the effects of gas flow. We have previously studied the effects of Ar and Ar+5%H2 gas pressure on SFMO thin films [4]. The current study differs from studies conducted on deposition pressure, since gas pressure during deposition in our study remains constant and only gas flow has been altered. In our case, we investigate the possibility to optimize SFMO films to be more suitable for room temperature applications and focus on the effects of the gas flow. The effects of PLD deposition gas, pressure and gas flow on SFMO films have been investigated before by other groups [3, 6–10]. The reports highlight the ease of the deposition resulting with parasitic impurity phases depending on the elements of the gas and pressure [6–9]. The properties of the gas also contribute to the structural and magnetic properties of SFMO. Fix et.al. hypothise the pressure having an effect on deposition rate and antisite disorder (ASD) [7]. ASD refers to the misplacement between Mo and Fe ions in SFMO lattice. This investigation was extended to cover two atmospheric gases: Ar and Ar+5%H2 mixture. We report structural and magnetic properties of the films and analyze the influence of the gas flow and gas nature on these properties. 2. Experimental details Two sets of SFMO thin films were fabricated with pulsed laser deposition (PLD) on SrTiO3 (100) single crystal substrates. The first set was fabricated using pure Ar gas and the second Ar+5%H2 mixture. Each set contains a film fabricated with high and low gas flow, which were achieved by higher, ≈ 0.6 Pa, and lower, ≈ 0.025 Pa, background pressures in the preparation chamber. Before the deposition, the sealed deposition chamber was pumped with a scroll pump, without external Ar or Ar+5%H2 flow, to reach the background pressure. For higher flow rate turbomolecular high-vacuum pump was used. Once the background pressure was reached, Ar or Ar+5%H2 was introduced, while maintaining the pumping used to achieve the background pressure. The pressure during deposition was set to previously optimized value of 9 Pa [4]. 2
The temperature during the deposition was 1050◦ C and the heating rate was 25◦ C/min. 2000 laser pulses were used, which corresponds to approximately 120 nm thickness [11, 12]. The SFMO films were labeled as presented in table 1. The sample quality and structural properties were measured with X-ray diffraction. θ − 2θ measurements were performed to detect possible impurity phases or peaks arising from unoriented SFMO texture. φ − ψ measurements for (204) (2θ = 57.106◦ ) peak were used to confirm that the SFMO films were c-axis oriented and fully textured. φ − ψ measurements for Fe (110) (2θ = 44.98◦ ) and SrMoO4 (112) (2θ = 27.68◦ ) were included in the impurity analysis. For more detailed structural analysis 2θ − φ-scans for SFMO (101) (2θ = 19.48◦ ) and (204) peaks were performed along with ω-scans for SFMO (004) peak (2θ = 45.96◦ ). The magnetic measurements include zero field cooled (ZFC) and field cooled (FC) magnetization measurements at temperatures between 10 K and 400 K in 100 mT field. The minimum of the first order derivative in the FC measurements was used as the Curie temperature. Magnetization hysteresis loops were measured between ±500 mT at 10 K and 400 K. The magnetization value in 400 mT was used as saturation magnetization, Msat . Using hysteresis loops measured in 10 K, the coercive field, Bc , was calculated as the average of absolute field values in zero magnetization at 10 K. All of the performed magnetic measurements were done with Quantum Design MPMS SQUID magnetometer. Table 1: SFMO films fabricated in Ar or in Ar+5%H2 mixture atmosphere both in high or low gas flow rate.
SFMO thin film A1 A2 AH1 AH2
Gas flow Low Ar High Ar Low Ar+5%H2 High Ar+5%H2
3
★ ★
★
Counts Logscale
■
■
★
■
● ●
SFMO (204)
Low Ar flow
High Ar flow
Low Ar+5%H2 flow
High Ar+5%H2 flow
20
40
60
2θ(°)
80
100
Figure 1: X-ray diffraction results of θ − 2θ of our SFMO films and φ − ψ measurements obtained for A1. The symbol , ? labels SFMO, STO (00l) peaks, respectively. • refers to STO (00l) which arise from small amount Cu Kβ radiation.
3. Results 3.1. Structural characterization According to θ − 2θ and texture measurements, presented in figure 1 for the film deposited in low Ar flow, all SFMO films are fully textured, c-axis oriented and phase pure. Essentially, there were no differences between films regarding θ − 2θ and φ − ψ XRD characterization. Figure 2 presents the results for more detailed structural characterization. The results presented are for A1 and A2, the films fabricated during the low and high Ar flow. Figure 2 (a) and (b) show the 2θ − φ 2D-maps for SFMO (101) peak. Figure 2 (c) and (d) shows ω-scan for SFMO (004) peak. Figures show clear peaks arising from SFMO (101) and (004) accompanied by substrate peaks. SFMO peaks were fitted with a symmetric Gaussian distribution function to depict the variations in the films. Table 2 lists the results obtained for FWHMvalues from the Gaussian fitting. We refer to FWHM as ∆. The results for films fabricated during low Ar gas or Ar+5%H2 gas mixture show relatively similar values for ∆. However, when comparing films deposited during high flow to the films deposited during low flow, using the same atmospheric gas, the values for ∆ are from close 50% to 100% larger. The values of full width at half maximum describe the scale of deviation 4
20.4
20.4
SFMO(101)
(a)
2θ(°)
2θ(°)
20
19.6 19.2 18.8
SFMO(101)
(b)
20
19.6 19.2
Low Ar flow 44
45
18.8
46
High Ar flow 44
45
φ(°)
φ(°)
STO(002)
High Ar flow Counts Logscale
Counts Logscale
Low Ar flow SFMO(004)
(c)
21
22
46
23
24
(d)
21
ω(°)
STO(002)
SFMO(004)
22
23
24
ω(°)
Figure 2: X-ray diffraction results obtained for A1 and A2 used for detailed structural characterization, (a) 2θ − φ (101) for A1, (b) 2θ − φ (101) for A2, (c) ω (004) for A1 and (d) ω (004) for A2.
from the ideal perfect atomic lattice. ∆φ-values obtained from the 2θ − φscans for (204) and (101) peaks describe the level of distortion of the unit cells in the rotational film plane orientation. High values are evidences for low angle grain boundaries. ∆ω-values for SFMO (004) peak is essentially a result from tilt of the lattice c-axis orientation. As a summary, the film deposited during the high gas flow rate show greater amount of low angle grain boundaries and tilt in the c-axis. When we compare the films fabricated during similar flow rate in different atmosphere, the structural variations between films are less significant. However, the films fabricated in Ar+5%H2 gas mixture show slightly larger ∆φ- and ∆ω-values almost in every case when compared to the films fabricated in Ar gas.
5
3.2. Magnetic measurements and discussion Figure 3 presents the magnetic measurement results for SFMO films, (a) temperature dependent FC and (b) magnetic hysteresis loops, fabricated in Ar gas with low (blue ) and high gas flow (red •). The inset of figure 3 (a) presents the first order derivative, between 270 K and 350 K, obtained from the FC magnetization. The minimum in the derivative curve was used to determine the Curie temperature, which is shown together with saturation magnetization and coercive field in table 3. The films fabricated in low gas flow show lower TC , approximately 308 K, when compared to the films fabricated in high gas flow, showing TC close to 325 K. The values for TC are slightly lower than our highest reported values, around 360 K, but still well comparable with our other results [13, 14]. Our preceding publication shows rather high TC for films fabricated with similar deposition parameters [4]. This is partially due to the fact that TC was previously determined from the deviation point of ferri-paramagnetic transition where ferrimagnetic ordering begins in FC measurements. This results with artificially higher Curie temperature value. Considering the effects arising from the nature of the gas, there were no significant differences between the films fabricated in pure Ar or in Ar+5%H2 , when the gas flow, either low or high, was kept the same. In addition to just having similar TC values, ZFC/FC curves appear similar to results shown in figure 3 (a), with similar magnetizations values and magnetization decaying as the temperature is increased. The hysteresis results were used to determine Msat and Bc , presented in table 3. The results reveal that the films fabricated in low gas flow show over 35% increase in Msat and close to 35% decrease in Bc , when compared to the films fabricated in high gas flow. Again, there were no significant differences in the results between films fabricated in pure Ar or in Ar+5%H2 as long Table 2: Structural results obtained from the 2θ − φ and ω scans.
Sample
∆φ (101)
∆φ (204)
∆ω (004)
A1 A2 AH1 AH2
0.481 0.970 0.654 1.204
0.467 0.692 0.573 0.830
0.243 0.535 0.295 0.521
6
2
1.2
M(µB/f.u.)
dM/dT
1.6 M(µB/f.u.)
low flow 2 high flow
280 300 320 340
(a)
0.8
1 0
(b)
-1 0.4 low flow high flow
0 0
10 K
100 mT
-2
50 100 150 200 250 300 350 400
-0.2
T (K)
-0.1
0 B (T)
0.1
0.2
Figure 3: (a) ZFC/FC measurement results for A1 and A2. The inset presents the first order derivative, used to determine the TC , obtained from the FC-curves. (b) hysteresis loops for A1 and A2.
as the rate of gas flow remained the same. The films fabricated in low gas flow, A1 and AH1, showed Msat and Bc values close to 2.4 µB /f.u. and 33 mT, respectively. The films fabricated during high gas flow, A2 and AH2, showed Msat and Bc values close to 1.6 µB /f.u. and 50 mT, respectively. Saturation magnetization and coercive field values are close to the results we have previously obtained from magnetization easy axis hysteresis loops [13, 14]. The analysis showed that the SFMO films fabricated during high Ar or Ar+5%H2 mixture gas flow showed high levels of structural distortion. This was evidenced by large ∆φ- and ∆ω-values. The structural variations have a clear effect on magnetic results. Considering the domain dynamics it is evident from the hysteresis loops, especially the coercive fields, that the films with apparently higher structural deformation experience significantly stronger domain pinning. Other studies have also reported structural distortion having resulted with stronger domain pinning [21], higher resistance [22] and films with low angle grain boundaries exhibiting spin-dependent tunneling magnetoresistance [14, 22, 23]. Differences in saturation magnetization, Curie temperature and coercive field all arise essentially from structural characteristics in our SFMO samples. Lattice site defects like oxygen vacancies and anti-site disorder (ASD) have a significant influence on Msat and TC . Oxygen vacancies and ASD have been shown to decrease saturation magnetization and the latter also decreasing Curie temperature [5, 15–19], while oxygen vacancies have been shown to increase Curie temperature [5, 19]. The decreased Msat and increased TC , seen 7
in high gas flow films, can be argued through increase of number of oxygen vacancies. Bc describes a part of the magnetic domain dynamics. Due to stronger domain pinning, change in the magnetization requires higher field to turn the magnetization along the magnetic field. Structural defects can act as pinning centers for domain movement [20]. Besides magnetic pinning, structural deformation has more to do with magnetic results. Magnetic results, including the increase of TC and the decrease of Msat , suggest the increase of oxygen vacancies. However, direct evidence of oxygen vacancies or ASD cannot be achieved with conducted structural characterization. An accurate decription of the dynamics taking place during the PLD fabrication is extremely difficult. The results suggests that high gas flow induces deoxidation of the films and simultaneously, in terms of general structural integrity, resulting with more unstable film growth ensuring structural distortion. Further research to reveal the detailed role of the lattice defects would require methods of obtaining direct evidence e.g. from oxygen vacancies. This could be attempted with positron annihilation spectroscopy [24]. One important topic, which is not covered here, would be to explore the magnetoresistance of the films, because we should expect structural distortion having a role in magnetoresistive properties. 4. Conclusions We have studied the effects of gas flow and gas nature on SFMO thin films fabricated with PLD. Structural analysis shows greater distortion in the films fabricated during high gas flow. In magnetic results this is seen as domain pinning. Magnetic results indicate possible increase of oxygen vacancies, causing the drop in saturation magnetization and increase of Curie Table 3: Magnetic results obtained from FC measurements and hysteresis loops.
Sample
TC (K)
Msat (µB /f.u.)
Bc (mT)
A1 A2 AH1 AH2
308 326 307 323
2.303 1.701 2.400 1.574
32.65 49.55 34.55 53.85
8
temperature in the same films deposited with high gas flow. Changes between the films fabricated during same flow rate under different gas atmosphere show no significant change and flow rate appears to be a more significant deposition parameter than the nature of the gas. Considering the goal of enhancing the properties of the films to be used in applications, no significant advantage over previous work was obtained. However, our report provides our first results regarding the effects of gas flow during PLD deposition and our results show that optimization could be used as a valuable part of more thorough investigation and optimization of the films. Acknowledgments The Jenny and Antti Wihuri foundation and the University of Turku Graduate School are acknowledged for financial support. References [1] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Room-temperature magnetoresistance in an oxide material with ordered double-perovskite structure, Nature 395 (1998) 677. [2] T. Manako, M. Izumi, Y. Konishi, K.-I. Kobayashi, M. Kawasaki, Y. Tokura, Epitaxial thin films of ordered double perovskite Sr2 FeMoO6 , Appl. Phys. Lett. 74 (1999) 2215. [3] J. Santiso, A. Figueras, J. Fraxedas, Thin films of Sr2 FeMoO6 grown by pulsed laser deposition: Preparation and characterization, Surf. Interface Anal. 33 (2002) 676. [4] P. Paturi, M. Mets¨anoja, H. Huhtinen, Optimization of deposition temperature and atmosphere for pulsed laser deposited Sr2 FeMoO6 thin films, Thin Solid Films 519 (2011) 8047. [5] M. Saloaro, M. Hoffmann, W. A. Adeagbo, S. Granroth, H. Deniz, H. Palonen, H. Huhtinen, S. Majumdar, P. Laukkanen, W. Hergert, A. Ernst, P. Paturi, Toward versatile Sr2 FeMoO6 -based spintronics by exploiting nanoscale defects, ACS Appl. Mater. Interfaces 8 (2016) 20440.
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[6] A. Venimadhav, F. Sher, J. P. Attfield, M. G. Blamire, Oxygen assisted deposition of Sr2 FeMoO6 thin films on SrTiO3 (100), J. Magn. and Magn. Mater. 269 (2004) 101. [7] T. Fix, G. Versini, J. L. Loison, S. Colis, G. Schmerber, G. Pourroy, A. Dinia, Pressure effect on the magnetization of Sr2 FeMoO6 thin films grown by pulsed laser deposition, J. Appl. Phys. 97 (2005) 024907. [8] S. R. Shinde, S. B. Ogale, R. L. Greene, T. Venkatesan, K. Tsoi, S.W. Cheong, A. J. Millis, Thin films of double perovskite Sr2 FeMoO6 : Growth, optimization and study of the physical and magnetotransport properties of films grown on single-crystalline and polycrystalline SrTiO3 substrates, J. Appl. Phys. 93 (2003) 1605. [9] W. Westerburg, D. Reisinger, G. Jakob, Epitaxy and magnetotransport of Sr2 FeMoO6 , Phys. Rev. B 62 (2000) R767. [10] M. Besse, F. Pailloux, A. Barthelemy, K. Bouzehouane, A. Fert, J. Olivier, O. Durand, F. Wyczisk, R. Bisaro, J.-P. Contour, Characterization methods of epitaxial Sr2 FeMoO6 thin films, J. Crystal Growth 241 (2002) 448. [11] I. Angervo, M. Saloaro, J. Tikkanen, H. Huhtinen, P. Paturi, Improving the surface structure of high quality Sr2 FeMoO6 thin films for multilayer structures., Appl. Surf. Sci. 396 (2017) 754. [12] I. Angervo, M. Saloaro, , H. Huhtinen, P. Paturi, Interface defects induced vertical magnetic anisotropy in Sr2 FeMoO6 thin films., Appl. Surf. Sci. 422 (2017) 682. [13] M. Saloaro, S. Majumdar, H. Huhtinen, P. Paturi, The effect of film thickness on the magnetic and magneto-transport properties of Sr2 FeMoO6 thin films, EPJ Web of Conferences 40 (2013) 15012. [14] M. Saloaro, H. Deniz, H. Huhtinen, H. Palonen, S. Majumdar, P. Paturi, The predominance of substrate induced defects in magnetic properties of SFMO thin films, J. Phys. Cond. Mat. 27 (2015) 386001. [15] S. Ray, D. Sarma, Sr2 FeMoO6 : A prototype to understand a new class of magnetic materials, Hyperfine interactions 160 (2005) 67. 10
[16] J. Navarro, L. Balcells, F. Sandiumenge, M. Bibes, A. Roig, B. Martinez, J. Fontcuberta, Antisite defects and magnetoresistance in Sr2 FeMoO6 double perovskite, J. Phys: Condens. Matter 13 (2001) 8481. [17] Q. Zhang, G. H. Rao, X. M. Feng, G. Y. Liu, Y. G. Xiao, Y. Zhang, J. K. Liang, Influence of v substitution for fe on the transport and magnetic properties of Sr2 FeMoO6 , Solid State Communications 133 (2005) 223. [18] B. J. Park, H. Han, J. Kim, Y. J. Kim, C. S. Kim, B. W. Lee, Correlation between anti-site disorder and magnetic properties in ordered perovskite Sr2 FeMoO6 , J. Magn. and Magn. Mater. 272-276 (2004) 1851. [19] M. Hoffmann, V. N. Antonov, L. V. Bekenov, K. Kokko, W. Hergert, A. Ernst, Variation of magnetic properties of Sr2 FeMoO6 due oxygen vacancies , J. Phys. Condens. Matter 30 (2018) 305801. [20] D. Jiles, D. Atherton, Theory of ferromagnetic hysteresis, J. Magn. Magn. Mater. 61 (1986) 48. [21] I. Angervo, M. Saloaro, H. Palonen, S. Majumdar, H. Huhtinen, P. Paturi, Thickness dependent properties of Sr2 FeMoO6 thin films grown on SrTiO3 and (LaAlO3 )0.3 (Sr2 AlTaO6 )0.7 substrates, 20th International Conference on Magnetism 75 (2015) 1011. [22] J. H. Song, J.-H. Park, Y. H. Jeong, Achieving large magnetoresistance in Sr2 FeMoO6 thin films, J. Appl. Phys. 97 (2005) 46105–1. [23] D. Kumar, D. Kaur, Substrate-dependent structural and magnetic properties Sr2 FeMoO6 nanostructured double perovskite thin films, Physica B 405 (2010) 3259. [24] A. Uedono, K. Shimayama, M. Kiyohara, Z. Q. Chen, K. Yamabe, Study of oxygen vacancies in SrTiO3 by positron annihilation, J. Appl. Phys. 92, 2697 (2002)
11
S0304-8853(18)32752-5 https://doi.org/10.1016/j.jmmm.2018.11.098 MAGMA 64657
To appear in:
Journal of Magnetism and Magnetic Materials
Please cite this article as: I. Angervo, R. Siekkinen, M. Saloaro, H. Huhtinen, P. Paturi, Sr2FeMoO6 thin films: effect of gas flow during deposition, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/ j.jmmm.2018.11.098
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sr2FeMoO6 thin films: effect of gas flow during deposition I. Angervoa,, R. Siekkinena,, M. Saloaroa , H. Huhtinena , P. Paturia a
Wihuri Physical Laboratory, Department of Physics and Astronomy, FI-20014 University of Turku, Finland
Abstract The effect of gas flow on Sr2 FeMoO6 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 Sr2 FeMoO6 films for spintronic applications. Keywords: Pulsed laser deposition, Sr2 FeMoO6 thin film, 1. Introduction The desire for novel spintronic applications has drawn a lot of attention to certain complex magnetic oxides, with valuable attributes, like high Curie temperature (TC ) and high spin polarization. One of these materials is Sr2 FeMoO6 (SFMO), a double perovskite with Curie temperature around 420 K and with 100% spin polarization linked to large magnetoresistance [1]. However, to harness this material for great variety of applications, high quality thin films are required. Fabrication of high-class SFMO thin films has proven to be, and continues to be, a challenging task. Pulsed laser deposition (PLD) is an effective Email address: [email protected] (I. Angervo)
Preprint submitted to Journal of Magnetism and Magnetic Materials November 14, 2018
fabrication technique which has successfully been used to produce high quality SFMO thin films [2–6]. The fabrication involves free parameters such as temperature, pressure, atmosphere, pulse energy, film thickness etc., which can vary highly between or within research projects. Large amount of work has taken place to optimize and study the effects of deposition parameters. No optimal parameters to fabricate SFMO films have been found. In this, study we investigate the effects of gas flow. We have previously studied the effects of Ar and Ar+5%H2 gas pressure on SFMO thin films [4]. The current study differs from studies conducted on deposition pressure, since gas pressure during deposition in our study remains constant and only gas flow has been altered. In our case, we investigate the possibility to optimize SFMO films to be more suitable for room temperature applications and focus on the effects of the gas flow. The effects of PLD deposition gas, pressure and gas flow on SFMO films have been investigated before by other groups [3, 6–10]. The reports highlight the ease of the deposition resulting with parasitic impurity phases depending on the elements of the gas and pressure [6–9]. The properties of the gas also contribute to the structural and magnetic properties of SFMO. Fix et.al. hypothise the pressure having an effect on deposition rate and antisite disorder (ASD) [7]. ASD refers to the misplacement between Mo and Fe ions in SFMO lattice. This investigation was extended to cover two atmospheric gases: Ar and Ar+5%H2 mixture. We report structural and magnetic properties of the films and analyze the influence of the gas flow and gas nature on these properties. 2. Experimental details Two sets of SFMO thin films were fabricated with pulsed laser deposition (PLD) on SrTiO3 (100) single crystal substrates. The first set was fabricated using pure Ar gas and the second Ar+5%H2 mixture. Each set contains a film fabricated with high and low gas flow, which were achieved by higher, ≈ 0.6 Pa, and lower, ≈ 0.025 Pa, background pressures in the preparation chamber. Before the deposition, the sealed deposition chamber was pumped with a scroll pump, without external Ar or Ar+5%H2 flow, to reach the background pressure. For higher flow rate turbomolecular high-vacuum pump was used. Once the background pressure was reached, Ar or Ar+5%H2 was introduced, while maintaining the pumping used to achieve the background pressure. The pressure during deposition was set to previously optimized value of 9 Pa [4]. 2
The temperature during the deposition was 1050◦ C and the heating rate was 25◦ C/min. 2000 laser pulses were used, which corresponds to approximately 120 nm thickness [11, 12]. The SFMO films were labeled as presented in table 1. The sample quality and structural properties were measured with X-ray diffraction. θ − 2θ measurements were performed to detect possible impurity phases or peaks arising from unoriented SFMO texture. φ − ψ measurements for (204) (2θ = 57.106◦ ) peak were used to confirm that the SFMO films were c-axis oriented and fully textured. φ − ψ measurements for Fe (110) (2θ = 44.98◦ ) and SrMoO4 (112) (2θ = 27.68◦ ) were included in the impurity analysis. For more detailed structural analysis 2θ − φ-scans for SFMO (101) (2θ = 19.48◦ ) and (204) peaks were performed along with ω-scans for SFMO (004) peak (2θ = 45.96◦ ). The magnetic measurements include zero field cooled (ZFC) and field cooled (FC) magnetization measurements at temperatures between 10 K and 400 K in 100 mT field. The minimum of the first order derivative in the FC measurements was used as the Curie temperature. Magnetization hysteresis loops were measured between ±500 mT at 10 K and 400 K. The magnetization value in 400 mT was used as saturation magnetization, Msat . Using hysteresis loops measured in 10 K, the coercive field, Bc , was calculated as the average of absolute field values in zero magnetization at 10 K. All of the performed magnetic measurements were done with Quantum Design MPMS SQUID magnetometer. Table 1: SFMO films fabricated in Ar or in Ar+5%H2 mixture atmosphere both in high or low gas flow rate.
SFMO thin film A1 A2 AH1 AH2
Gas flow Low Ar High Ar Low Ar+5%H2 High Ar+5%H2
3
★ ★
★
Counts Logscale
■
■
★
■
● ●
SFMO (204)
Low Ar flow
High Ar flow
Low Ar+5%H2 flow
High Ar+5%H2 flow
20
40
60
2θ(°)
80
100
Figure 1: X-ray diffraction results of θ − 2θ of our SFMO films and φ − ψ measurements obtained for A1. The symbol , ? labels SFMO, STO (00l) peaks, respectively. • refers to STO (00l) which arise from small amount Cu Kβ radiation.
3. Results 3.1. Structural characterization According to θ − 2θ and texture measurements, presented in figure 1 for the film deposited in low Ar flow, all SFMO films are fully textured, c-axis oriented and phase pure. Essentially, there were no differences between films regarding θ − 2θ and φ − ψ XRD characterization. Figure 2 presents the results for more detailed structural characterization. The results presented are for A1 and A2, the films fabricated during the low and high Ar flow. Figure 2 (a) and (b) show the 2θ − φ 2D-maps for SFMO (101) peak. Figure 2 (c) and (d) shows ω-scan for SFMO (004) peak. Figures show clear peaks arising from SFMO (101) and (004) accompanied by substrate peaks. SFMO peaks were fitted with a symmetric Gaussian distribution function to depict the variations in the films. Table 2 lists the results obtained for FWHMvalues from the Gaussian fitting. We refer to FWHM as ∆. The results for films fabricated during low Ar gas or Ar+5%H2 gas mixture show relatively similar values for ∆. However, when comparing films deposited during high flow to the films deposited during low flow, using the same atmospheric gas, the values for ∆ are from close 50% to 100% larger. The values of full width at half maximum describe the scale of deviation 4
20.4
20.4
SFMO(101)
(a)
2θ(°)
2θ(°)
20
19.6 19.2 18.8
SFMO(101)
(b)
20
19.6 19.2
Low Ar flow 44
45
18.8
46
High Ar flow 44
45
φ(°)
φ(°)
STO(002)
High Ar flow Counts Logscale
Counts Logscale
Low Ar flow SFMO(004)
(c)
21
22
46
23
24
(d)
21
ω(°)
STO(002)
SFMO(004)
22
23
24
ω(°)
Figure 2: X-ray diffraction results obtained for A1 and A2 used for detailed structural characterization, (a) 2θ − φ (101) for A1, (b) 2θ − φ (101) for A2, (c) ω (004) for A1 and (d) ω (004) for A2.
from the ideal perfect atomic lattice. ∆φ-values obtained from the 2θ − φscans for (204) and (101) peaks describe the level of distortion of the unit cells in the rotational film plane orientation. High values are evidences for low angle grain boundaries. ∆ω-values for SFMO (004) peak is essentially a result from tilt of the lattice c-axis orientation. As a summary, the film deposited during the high gas flow rate show greater amount of low angle grain boundaries and tilt in the c-axis. When we compare the films fabricated during similar flow rate in different atmosphere, the structural variations between films are less significant. However, the films fabricated in Ar+5%H2 gas mixture show slightly larger ∆φ- and ∆ω-values almost in every case when compared to the films fabricated in Ar gas.
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3.2. Magnetic measurements and discussion Figure 3 presents the magnetic measurement results for SFMO films, (a) temperature dependent FC and (b) magnetic hysteresis loops, fabricated in Ar gas with low (blue ) and high gas flow (red •). The inset of figure 3 (a) presents the first order derivative, between 270 K and 350 K, obtained from the FC magnetization. The minimum in the derivative curve was used to determine the Curie temperature, which is shown together with saturation magnetization and coercive field in table 3. The films fabricated in low gas flow show lower TC , approximately 308 K, when compared to the films fabricated in high gas flow, showing TC close to 325 K. The values for TC are slightly lower than our highest reported values, around 360 K, but still well comparable with our other results [13, 14]. Our preceding publication shows rather high TC for films fabricated with similar deposition parameters [4]. This is partially due to the fact that TC was previously determined from the deviation point of ferri-paramagnetic transition where ferrimagnetic ordering begins in FC measurements. This results with artificially higher Curie temperature value. Considering the effects arising from the nature of the gas, there were no significant differences between the films fabricated in pure Ar or in Ar+5%H2 , when the gas flow, either low or high, was kept the same. In addition to just having similar TC values, ZFC/FC curves appear similar to results shown in figure 3 (a), with similar magnetizations values and magnetization decaying as the temperature is increased. The hysteresis results were used to determine Msat and Bc , presented in table 3. The results reveal that the films fabricated in low gas flow show over 35% increase in Msat and close to 35% decrease in Bc , when compared to the films fabricated in high gas flow. Again, there were no significant differences in the results between films fabricated in pure Ar or in Ar+5%H2 as long Table 2: Structural results obtained from the 2θ − φ and ω scans.
Sample
∆φ (101)
∆φ (204)
∆ω (004)
A1 A2 AH1 AH2
0.481 0.970 0.654 1.204
0.467 0.692 0.573 0.830
0.243 0.535 0.295 0.521
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2
1.2
M(µB/f.u.)
dM/dT
1.6 M(µB/f.u.)
low flow 2 high flow
280 300 320 340
(a)
0.8
1 0
(b)
-1 0.4 low flow high flow
0 0
10 K
100 mT
-2
50 100 150 200 250 300 350 400
-0.2
T (K)
-0.1
0 B (T)
0.1
0.2
Figure 3: (a) ZFC/FC measurement results for A1 and A2. The inset presents the first order derivative, used to determine the TC , obtained from the FC-curves. (b) hysteresis loops for A1 and A2.
as the rate of gas flow remained the same. The films fabricated in low gas flow, A1 and AH1, showed Msat and Bc values close to 2.4 µB /f.u. and 33 mT, respectively. The films fabricated during high gas flow, A2 and AH2, showed Msat and Bc values close to 1.6 µB /f.u. and 50 mT, respectively. Saturation magnetization and coercive field values are close to the results we have previously obtained from magnetization easy axis hysteresis loops [13, 14]. The analysis showed that the SFMO films fabricated during high Ar or Ar+5%H2 mixture gas flow showed high levels of structural distortion. This was evidenced by large ∆φ- and ∆ω-values. The structural variations have a clear effect on magnetic results. Considering the domain dynamics it is evident from the hysteresis loops, especially the coercive fields, that the films with apparently higher structural deformation experience significantly stronger domain pinning. Other studies have also reported structural distortion having resulted with stronger domain pinning [21], higher resistance [22] and films with low angle grain boundaries exhibiting spin-dependent tunneling magnetoresistance [14, 22, 23]. Differences in saturation magnetization, Curie temperature and coercive field all arise essentially from structural characteristics in our SFMO samples. Lattice site defects like oxygen vacancies and anti-site disorder (ASD) have a significant influence on Msat and TC . Oxygen vacancies and ASD have been shown to decrease saturation magnetization and the latter also decreasing Curie temperature [5, 15–19], while oxygen vacancies have been shown to increase Curie temperature [5, 19]. The decreased Msat and increased TC , seen 7
in high gas flow films, can be argued through increase of number of oxygen vacancies. Bc describes a part of the magnetic domain dynamics. Due to stronger domain pinning, change in the magnetization requires higher field to turn the magnetization along the magnetic field. Structural defects can act as pinning centers for domain movement [20]. Besides magnetic pinning, structural deformation has more to do with magnetic results. Magnetic results, including the increase of TC and the decrease of Msat , suggest the increase of oxygen vacancies. However, direct evidence of oxygen vacancies or ASD cannot be achieved with conducted structural characterization. An accurate decription of the dynamics taking place during the PLD fabrication is extremely difficult. The results suggests that high gas flow induces deoxidation of the films and simultaneously, in terms of general structural integrity, resulting with more unstable film growth ensuring structural distortion. Further research to reveal the detailed role of the lattice defects would require methods of obtaining direct evidence e.g. from oxygen vacancies. This could be attempted with positron annihilation spectroscopy [24]. One important topic, which is not covered here, would be to explore the magnetoresistance of the films, because we should expect structural distortion having a role in magnetoresistive properties. 4. Conclusions We have studied the effects of gas flow and gas nature on SFMO thin films fabricated with PLD. Structural analysis shows greater distortion in the films fabricated during high gas flow. In magnetic results this is seen as domain pinning. Magnetic results indicate possible increase of oxygen vacancies, causing the drop in saturation magnetization and increase of Curie Table 3: Magnetic results obtained from FC measurements and hysteresis loops.
Sample
TC (K)
Msat (µB /f.u.)
Bc (mT)
A1 A2 AH1 AH2
308 326 307 323
2.303 1.701 2.400 1.574
32.65 49.55 34.55 53.85
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temperature in the same films deposited with high gas flow. Changes between the films fabricated during same flow rate under different gas atmosphere show no significant change and flow rate appears to be a more significant deposition parameter than the nature of the gas. Considering the goal of enhancing the properties of the films to be used in applications, no significant advantage over previous work was obtained. However, our report provides our first results regarding the effects of gas flow during PLD deposition and our results show that optimization could be used as a valuable part of more thorough investigation and optimization of the films. Acknowledgments The Jenny and Antti Wihuri foundation and the University of Turku Graduate School are acknowledged for financial support. References [1] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Room-temperature magnetoresistance in an oxide material with ordered double-perovskite structure, Nature 395 (1998) 677. [2] T. Manako, M. Izumi, Y. Konishi, K.-I. Kobayashi, M. Kawasaki, Y. Tokura, Epitaxial thin films of ordered double perovskite Sr2 FeMoO6 , Appl. Phys. Lett. 74 (1999) 2215. [3] J. Santiso, A. Figueras, J. Fraxedas, Thin films of Sr2 FeMoO6 grown by pulsed laser deposition: Preparation and characterization, Surf. Interface Anal. 33 (2002) 676. [4] P. Paturi, M. Mets¨anoja, H. Huhtinen, Optimization of deposition temperature and atmosphere for pulsed laser deposited Sr2 FeMoO6 thin films, Thin Solid Films 519 (2011) 8047. [5] M. Saloaro, M. Hoffmann, W. A. Adeagbo, S. Granroth, H. Deniz, H. Palonen, H. Huhtinen, S. Majumdar, P. Laukkanen, W. Hergert, A. Ernst, P. Paturi, Toward versatile Sr2 FeMoO6 -based spintronics by exploiting nanoscale defects, ACS Appl. Mater. Interfaces 8 (2016) 20440.
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[16] J. Navarro, L. Balcells, F. Sandiumenge, M. Bibes, A. Roig, B. Martinez, J. Fontcuberta, Antisite defects and magnetoresistance in Sr2 FeMoO6 double perovskite, J. Phys: Condens. Matter 13 (2001) 8481. [17] Q. Zhang, G. H. Rao, X. M. Feng, G. Y. Liu, Y. G. Xiao, Y. Zhang, J. K. Liang, Influence of v substitution for fe on the transport and magnetic properties of Sr2 FeMoO6 , Solid State Communications 133 (2005) 223. [18] B. J. Park, H. Han, J. Kim, Y. J. Kim, C. S. Kim, B. W. Lee, Correlation between anti-site disorder and magnetic properties in ordered perovskite Sr2 FeMoO6 , J. Magn. and Magn. Mater. 272-276 (2004) 1851. [19] M. Hoffmann, V. N. Antonov, L. V. Bekenov, K. Kokko, W. Hergert, A. Ernst, Variation of magnetic properties of Sr2 FeMoO6 due oxygen vacancies , J. Phys. Condens. Matter 30 (2018) 305801. [20] D. Jiles, D. Atherton, Theory of ferromagnetic hysteresis, J. Magn. Magn. Mater. 61 (1986) 48. [21] I. Angervo, M. Saloaro, H. Palonen, S. Majumdar, H. Huhtinen, P. Paturi, Thickness dependent properties of Sr2 FeMoO6 thin films grown on SrTiO3 and (LaAlO3 )0.3 (Sr2 AlTaO6 )0.7 substrates, 20th International Conference on Magnetism 75 (2015) 1011. [22] J. H. Song, J.-H. Park, Y. H. Jeong, Achieving large magnetoresistance in Sr2 FeMoO6 thin films, J. Appl. Phys. 97 (2005) 46105–1. [23] D. Kumar, D. Kaur, Substrate-dependent structural and magnetic properties Sr2 FeMoO6 nanostructured double perovskite thin films, Physica B 405 (2010) 3259. [24] A. Uedono, K. Shimayama, M. Kiyohara, Z. Q. Chen, K. Yamabe, Study of oxygen vacancies in SrTiO3 by positron annihilation, J. Appl. Phys. 92, 2697 (2002)
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