J. Phys. Chem. B 2007, 111, 7497-7500
7497
Structural and Ferromagnetic Properties of Epitaxial SrRuO3 Thin Films Obtained by Polymer-Assisted Deposition H. M. Luo,* M. Jain, S. A. Baily, T. M. McCleskey, A. K. Burrell, E. Bauer, R. F. DePaula, P. C. Dowden, L. Civale, and Q. X. Jia* Materials Physics and Applications DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: March 6, 2007; In Final Form: May 14, 2007
Epitaxial ferromagnetic SrRuO3 thin films with a room-temperature resistivity of 300 µΩ·cm have been successfully grown on LaAlO3(001) substrates at a processing temperature in the range of 550-750 °C by a polymer-assisted deposition technique. X-ray diffraction analysis shows good epitaxial quality of SrRuO3 thin films, giving values of the full width at half-maximum (FWHM) of 0.42° from the rocking curve for the (002) reflection and 1.1° from the in-plane φ scan for the (204) reflection. Both the resistivity and the magnetization versus temperature measurements show that the SrRuO3 films are ferromagnetic with a transition temperature of 160 K. The spontaneous magnetization near the ferromagnetic transition follows the scaling law, and the low-temperature magnetization follows the Bloch law.
Introduction Ferromagnetic conductive oxide SrRuO3 (SRO), which has a Curie temperature of around 160 K, has been widely studied recently for electronic device applications because of its high electrical conductivity, crystal structure compatibility with many other technically important metal oxides, and high thermal and chemical stability. For example, epitaxial thin films of SRO have been used as bottom electrodes for ferroelectric capacitors,1 buffer layers for coated conductors,2 normal metal layers for superconductor Josephson junctions,3 and ferromagnetic metal layers in spin-polarized ferromagnetic tunnel junctions.4 SRO crystallizes in a GdFeO3-type orthorhombic structure with a space group of Pnma and lattice parameters of a ) 0.5573 nm, b ) 0.5538 nm, and c ) 0.7856 nm. The crystal structure of SRO can be also described as a slightly distorted pseudocubic perovskite with a ) 0.393 nm. The distorted perovskite structure favors its integration with a variety of other functional metal oxides such as high-temperature superconductors, ferromagnetic, and ferroelectric materials. In the past several years, great efforts have been made to grow epitaxial SRO films on different substrates by a variety of techniques. For example, pulsed laser deposition (PLD),2-14 sputtering,1,15-18 molecular-beam epitaxy,19 sol-gel,20-22 and metalorganic chemical vapor deposition (MOCVD)18,23,24 have been explored as means of depositing SRO films on LaAlO3 (LAO), SrTiO3 (STO), MgO, or Si single-crystal substrates. Chemical solution deposition, such as a sol-gel processing, provides advantages of low cost, easy setup, and coating of large areas. Recently, we have developed a new approach, polymer-assisted deposition (PAD), a wet technique, to grow both simple and complex metal oxide thin films.25 The major distinction between this PAD process and other chemical solution techniques lies in the soluble polymer that plays a significant role in preparing high-quality metal oxide films. In other words, the polymer used in the PAD
process not only controls the desired viscosity for the process but also binds the metal ions to prevent premature precipitation and formation of metal oxide oligomers. The results are a homogeneous distribution of the metal precursors in the solution and the formation of uniform metal oxide films. Here, we report our efforts to grow epitaxial ferromagnetic SRO films using the PAD process. Experimental Section The precursor solution for the deposition of SRO films was obtained from a mixture of separate solutions of Sr and Ru bound to the polymer poly(acrylic acid) (PAA). The concentration of the PAA solution was 25 wt %, and the viscosity was 416 mPa‚s. Specifically, 1 g of RuCl3 or 1 g of Sr(NO3)2 was dissolved in 8 g of PAA solution with 4 g of ethanol. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) showed final concentrations of 0.4342 M for Ru and 0.2166 M for Sr. The resulting precursor solutions with viscosities of 8-10 mPa‚ s and the desired stoichiometric molar ratio of Sr/Ru ) 1 were spin-coated on LAO(001) substrates at 2000 rpm for 30 s. Samples were then heated in flowing oxygen at temperatures in the range of 550-750 °C for 1 h. SRO films with a 100-nm thickness were obtained by one spin-coat, and the thickness of the films could be increased by multiple spin-coats. X-ray diffraction (XRD) was used to determine the crystallinity of the films and the orientation relationships between the film and the substrate. The surface morphology of the samples was characterized by scanning electron microscopy (SEM). The ferromagnetic properties of the films were evaluated with a superconducting quantum interference device (SQUID) magnetometer. The electrical resistivity (F) at temperatures in the range of 75-300 K was measured using a standard four-probe technique. Results and Discussion
* Corresponding authors: Tel.: 505-667-0239 (H.M.L.), 505-667-2716 (Q.X.J.). Fax: 505-665-3164 (H.M.L.). E-mail:
[email protected] (H.M.L.),
[email protected] (Q.X.J.).
XRD analysis shows that all of the films thermally treated in oxygen between 550 and 750 °C had the same crystal-
10.1021/jp0718451 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007
7498 J. Phys. Chem. B, Vol. 111, No. 26, 2007
Figure 1. (a) Typical θ-2θ XRD spectrum of an SRO film grown on an LAO substrate annealed at 550 °C. The inset shows the ω-rocking curve of the (002) SRO reflection. (b) φ scans from the (204) SRO and (202) LAO reflections.
lographic orientation. Figure 1a shows a typical θ-2θ XRD spectrum of a 100-nm-thick SRO film grown on a LAO(001) substrate annealed at 550 °C. The high intensity and sharp (002) and (004) reflections indicate the formation of a single-phase and preferentially oriented SRO film even at such a low processing temperature. The lattice parameters determined by X-ray analysis show a dependence on the annealing temperature. The lattice parameter is 0.393 nm for the SRO film annealed at 550 °C but 0.391 nm for the film annealed at 700 °C, indicating that the lattice mismatch between the film and substrate decreases with increasing annealing temperature. The inset in Figure 1a shows the ω-rocking curve of the (002) reflection of the same SRO film thermally annealed at 550 °C. A value of 0.42° for the full width at half-maximum (FWHM) suggests good crystallization of the film. Figure 1b shows the φ scans on reflections of SRO {204} and LAO {202}. As can be seen from the φ scans, the film is aligned in-plane as well, except for a 45° rotation with respect to the LAO lattice. Four peaks with an average FWHM value of 1.1°, as compared to 0.7° for
Luo et al. the single-crystal LAO substrate, show the film to be of good epitaxial quality. The heteroepitaxial relationship between the SRO film and the LAO substrate can be described as (001)SRO||(001)LAO and 〈110〉SRO||〈100〉LAO, consistent with films deposited by other techniques.5-11 It should be noted that relatively higher annealing temperatures usually improve the crystallinity of the films. The FWHM values for both the rocking curve on the (002) reflection and the φ scan on the (204) reflection were reduced by about 0.1° by increasing the annealing temperature from 550 to 750 °C. The surface morphology of the SRO films on LAO was investigated by SEM. Figure 2 shows SEM images of the SRO thin film annealed at 550 °C. The surface morphology is quite similar to that of a film deposited at 650 °C by PLD.5 The surface of the films is smooth, although crystalline films show some surface features such as micropits due to grain growth and incomplete grain coalescence, similarly to PLD films. A systematic study of the effect of annealing temperature on the surface morphology revealed that films annealed at 600 to 700 °C were very similar to the film annealed at 550 °C, whereas a film annealed at 750 °C showed some particulates dispersed on the surface. Figure 3a,b shows the temperature dependence of field-cooled magnetization of an SRO film annealed at 550 °C with an applied field of 100 G parallel and perpendicular to the substrate plane, respectively. The different magnetization values along the two directions indicate the anisotropic nature of epitaxial SRO films. Spontaneous magnetization (M) below 160 K indicates that the SRO film undergoes a ferromagneticparamagnetic phase transition at this temperature (TC). The spontaneous magnetization near TC follows the scaling law M ∝ (TC - T)R, with R ) 0.51 and 0.65 with the field parallel and perpendicular to the substrate surface, respectively. In comparison, the R value of SRO films deposited by magnetron sputtering is 0.43, but it is 0.5 from the mean-field behavior.17 It should be also noted that the TC value deduced from the scaling law fitting is 150-152 K, very close to that measured directly from the experiment. The insets of Figure 3a,b show plots of the reduced magnetization M(T)/M(0) versus T3/2 in the low-temperature region, where M(0) is the magnetization value at 0 K obtained by extending the straight line. The magnetization suppression behavior can be well described by Bloch’s law M(T)/M(0) ) 1 - AT3/2. In the low-temperature limit, the dominant excitation known to suppress magnetization is spinwave excitations. In this model, the spin-wave parameter can be expressed as A ) (0.0587/S)(kB/2JS)3/2, where S is the total spin of Ru4+, kB is Boltzmann’s constant, and J is the exchange interaction between two neighboring Ru4+ ions. By taking S ) 1 and applying a linear fitting to the experimental results, we
Figure 2. SEM micrographs of an SRO thin film on LAO annealed at 550 °C.
Properties of Epitaxial SrRuO3 Thin Films
J. Phys. Chem. B, Vol. 111, No. 26, 2007 7499
Figure 4. Typical temperature dependence of the normalized resistance R/R(290 K) for an SRO film annealed at 550 °C at zero field.
to that of bulk single-crystal SRO and that of PLD or sputtered SRO films.5-7,9,14,16,19 It should also be pointed out that all of the films investigated showed metallic behavior. Figure 4 shows the typical temperature dependence of the normalized resistance R/R(290 K) for an SRO film annealed at 700 °C at zero field. The resistivity decreases almost linearly with temperature from room temperature to 160 K, where a change in the slope of ∆R/∆T occurs, coinciding with the ferromagnetic transition.5,7 Conclusion In summary, high-crystallinity ferromagnetic SRO thin films with a room-temperature resistivity of 300 µΩ‚cm have been epitaxially grown on LAO substrates by polymer-assisted deposition. The spontaneous magnetization below 160 K indicates that the SRO film undergoes a ferromagneticparamagnetic phase transition at this temperature. The spontaneous magnetization near the transition temperature follows the scaling law with R values of 0.51 and 0.65 for the field parallel and perpendicular to the substrate surface, respectively. Figure 3. Temperature dependence of field-cooled magnetization of an SRO film annealed at 550 °C, where the field is (a) parallel and (b) perpendicular to the substrate surface. The solid line is the fitting by the scaling law of M ∝ (TC - T)R. The insets in a and b also show the reduced magnetization M(T)/M(0) vs T3/2 in the low-temperature region. The straight line is a fit to the Bloch law. (c) Magnetization versus magnetic field (M-H) hysteresis loops with the magnetic field perpendicular to the substrate surface at 5 and 100 K.
obtained J values of 20.08 and 17.95kB K with the magnetic field parallel and perpendicular to the substrate surface, respectively. In comparison, J values of 14.41 and 20.57kB K have been reported for SRO films deposited by sputtering.17 Figure 3c shows magnetization versus magnetic field (M-H) loops of an SRO film with the applied field perpendicular to the substrate surface measured at 5 and 100 K, respectively. The film shows a typical ferromagnetic M-H hysteresis characteristics with well-saturated behavior. Transport properties are expected to be influenced by the microstructure of the films. The electrical resistivity of the SRO films was found to be a strong function of annealing temperature: The higher the annealing temperature, the smaller the resistivity of the film. For example, a 100-nm-thick SRO film annealed at 550 °C has a room-temperature resistivity of around 1700 µΩ‚cm, whereas a film annealed at 700 °C has a resistivity of 300 µΩ‚cm at room temperature. This value is comparable
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