APPLIED PHYSICS LETTERS 104, 081608 (2014)
Structural, electrical, and magnetic properties of SrRuO3 thin films Pawanpreet Kaur,1,a) K. K. Sharma,1,b) Rabia Pandit,1 R. J. Choudhary,2 and Ravi Kumar3,c) 1
Department of Physics, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177 005, India 2 UGC-DAE Consortium for Scientific Research, Indore, Madhya Pradesh 452 001, India 3 Centre for Material Science and Engineering, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177 005, India
(Received 20 September 2013; accepted 8 February 2014; published online 25 February 2014) Epitaxial thin films of SrRuO3 having thicknesses 100, 50, 25, and 12 nm have been grown on SrTiO3 (001) substrate by pulsed laser deposition technique. The thickness dependent resistivity analysis reveals the non-Fermi-liquid type behavior by obeying T1.5 temperature dependence below the transition temperature. Increase in disorder and correlation with decrease in the film thickness lead to the transition from metallic to insulating phase for 12 nm film. Magnetic studies suggest the destruction of ferromagnetism for this film. Magnetization obeys 3D mean field model C 2014 AIP Publishing LLC. for 100 nm film and 2D Ising model for 50 and 25 nm films. V [http://dx.doi.org/10.1063/1.4866775] Among ruthenium oxides, SrRuO3 (SRO) has gained much attention due to its intriguing electrical and magnetic properties. SrRuO3 is the only known ferromagnetic metal among 4d oxides. SRO crystallizes in GdFeO3 type orthorhombic structure with Pbnm space group.1 The itinerant ferromagnetism in SRO was confirmed by band structure calculation using self consistent spin density functional theory and density functional theory.2,3 The transport properties of SRO also broadened its area of interest. At low temperatures, SRO exhibits Fermi-liquid like behavior, while bad metallic itinerant ferromagnetism at higher temperature.4,5 Bulk SrRuO3 undergoes several structural phase transitions.6 As this compound permits the epitaxial growth of single crystal thin films hence they are of great interest from a technology point of view, due to which SRO thin films have been actively studied by numerous research groups for the last few years.7–9 Previous studies have established that transport and magnetic properties of this compound are very sensitive to surface morphology, growth temperature, and substrate induced strain.10–14 Besides the above mentioned factors, thickness of the film also influences its magnetic and electric properties. Toyota et al. reported the enhanced resistivity in thin films due to the energy gap at Fermi energy level.15 Transport properties are also sensitive to the disorder in the thin films.16 Xia et al. noticed the exchange bias behavior in the SRO thin films below critical thickness.17 Due to the perplexed nature of SRO thin film, its properties are not yet fully understood. So in the present study, we are focusing on the thickness dependent structural, magnetic, and electrical properties of SRO films. SRO films with 100, 50, 25, and 12 nm thickness were grown on SrTiO3 (STO) (001) substrate kept at 750 C temperature by pulsed laser deposition technique using a KrF ¼ 248 nm excimer laser, with a flux of approximately 2 J/cm2 and pulse repetition rate of 10 Hz. During deposition, a)
[email protected] [email protected] c)
[email protected]. Present address: Beant College of Engineering and Technology, Gurdaspur, Punjab 143 521, India. b)
0003-6951/2014/104(8)/081608/4/$30.00
oxygen pressure of 0.3 millibars was maintained. The films were cooled to room temperature at rate of 2 C per minute under oxygen pressure of 400 millibars. Target to substrate distance was kept at 4 cm. The good structural qualities of thin films were confirmed by using X’pert Pro Panalytical, X-Ray diffractometer, which reveals the single phase, c-axis oriented (001) growth of the films and the enhancement in relative intensity of film peak with increase in film thickness is seen. It is quite clear that the film with the lowest thickness (12 nm) reveals out of plane lattice parameter “c” (0.3977 nm) larger than that of the bulk value (0.3924 nm). With increase in film thickness, c decreases gradually (0.3949 nm for 25 nm and 0.3933 nm for 50 nm thick film) and get closer (i.e., 0.3922 nm) to the bulk value for 100 nm film. This indicates that the lowest film thickness suffers largest tensile strain in the out of plane direction, which relaxes with increase in thickness. The observed out of plane lattice parameter of SRO films grown on STO substrate is consistent with the previous reports and in-plane lattice parameter for such.10,18,19 Figure 1 shows the three-dimensional (3D) images (2 2 lm2) of the SRO thin films for 12, 100 nm thicknesses scanned by atomic force microscope (AFM) at room temperature. The surface morphology of 12 nm thin film infers that only a few high amplitude 3D islands appear, whereas for 100 nm film lesser amplitude island growth is observed. Moreover, the observed 2D AFM images (2 2 lm2) also revealed the mesh like structure due to the merging of islands in 100 nm film indicating that the film is getting smoother with the increase in film thickness. Such type of island merging has been also reported by Toyota et al.15 The electrical resistivity measurements were performed by standard four probe technique using the Keithley current source and nanovoltmeter in the temperature range of 80 K–300 K. Resistivity as a function of temperature for 100 nm, 50 nm, and 25 nm thick SRO films grown on SrTiO3 (001) substrate has been plotted in Fig. 2(a). It is clearly evident that all the films are showing metallic behavior. However, the film with thickness 12 nm exhibits insulating behavior (see Fig. 2(b)).
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FIG. 1. 3D AFM images of SRO thin films. (a) 12 nm, (b) 100 nm.
The kink in the metallic resistivity curve at a particular temperature represents the paramagnetic to ferromagnetic transition temperature (Tc) of SRO, which represents a coupled magnetic and electrical behavior. It is worth mentioning here that Tc for 100 nm film is around 159 K and resistivity value at room temperature is 256 lX cm, which are very close to the values for single crystal confirming the good quality of the film.20 To clearly observe the transition temperature, we have plotted the derivative of resistivity with respect to temperature, the corresponding transition temperature for 50 and 25 nm films are 155 K and 149 K, respectively (Fig. 2(c)). It is clear that as the film thickness decreases, transition temperature also decreases, indicative of subsidence in ferromagnetic stability. Moreover, we have observed an enhancement in resistivity value with decrease in the film thickness. This upsurge can be understood by microstructural study of these films as mentioned in AFM analysis, which revealed the merging of 3D islands providing a conductive path to the charge carriers and lowering the resistivity for 100 nm as compared to 50 and 25 nm thin films.10,15 It is observed from Fig. 2(a) that the resistivity of SRO above the Curie temperature increases almost linearly without saturation hence confirming the bad metal behavior.21
The slope of the curve does not show any significant change with a decrease in the film thickness. The origin of resistivity above the Curie temperature is still controversial. Allen described that as temperature increases above Tc, the electron phonon scattering increases while spin fluctuation scattering saturates.2 Klein et al. suggested it as the nonphononic origin of resistivity.5 However, below the Curie temperature, we can observe the resistivity upturn of SRO films rapidly with increasing temperature. It is clearly evident from Fig. 2(d) that the resistivity data below the transition temperature are well fitted with the power law q ¼ q0 þ AT 1:5 (q0 is residual resistivity, A is coefficient, and T is temperature) in 100, 50, and 25 nm thin films. There are various models proposed to explain the T1.5 behavior.22,23 The value of the coefficient A ¼ 3.87 108 X cm K1.5 obtained in the resistivity data fitting for 100 nm film is quite close to the value reported by Rivadulla et al. for strongly correlated system as well as to the value of A obtained by Wang et al. for SRO.4,24 Such type of resistivity behavior is attributed to the coexistence of fluctuations induced localized electrons and Fermi electrons in the lower Hubbard band below Tc. The values of coefficient A noticed for 50 and 25 nm thin films are 13.77 108 X cm K1.5 and 29.39 108 X cm K1.5, respectively. The increased value of A with decrease in film
FIG. 2. Temperature dependence of electrical resistivity for SRO films. (a) 100, 50, 25 nm, (b) 12 nm, where inset shows the resistivity data fit using Mott variable range hopping model, (c) temperature dependence of resistivity derivative for 50, 25 nm films, (d) T1.5 fitting below Tc for 100, 50 and 25 nm films.
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thickness explains the correlation effects which play an important role in driving metal to an insulating state.25 Furthermore, the residual resistivity increases from 120 to 355 lX cm as the film thickness changes from 100 to 25 nm. Comparatively drastic increase in the q0 in 25 nm film infers that the disorders are more prominent near the film substrate interface and for this thickness substrate induced strain is still effective. Increase in such disorder plays a direct role in enhancing the resistivity values of thin films.16 It may be concluded that the enhanced value of A and q0 both contribute to the rise of resistivity as the thickness decreases, which drive the system to the insulating state as observed for 12 nm film (see Fig. 2(b)). This is also supported by the XRD analysis where a tensile strain is noticed in the 12 nm film, which modifies Ru-O-Ru interatomic distances, resulting in resistivity enhancement.11,26 The insulating behavior induced by disorder in the system follows the variable range hooping model:27 q ¼ q0 exp
1=4 : T 0
T
(1)
Inset in Fig. 2(b) demonstrates the well fitted pattern by Mott variable range hooping theory in the whole temperature range. The magnetic studies were performed using A7 Tesla SQUID-VSM (Quantum design) in the presence of external field of 500 Oe and the temperature range from 4 K to 300 K. Figure 3(a) shows the zero field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature for 100, 50, 25 nm thin films of SRO. All the data are recorded during the warm up cycle for the ZFC and FC magnetization. All the curves show typical ferromagnetic behavior and there exists a large difference between ZFC and FC magnetization below Tc, which indicates the magnetic anisotropy in these films. The FC magnetization data near Tc are fitted with
scaling law M / ðTc TÞb for the above mentioned films, where M is the spontaneous magnetization and b is the exponent. The value of b ¼ 0.58 for 100 nm film (see Fig. 3(b)) is close to that of observed for SRO single crystal and can be explained on the basis of the mean field model.28 Whereas for 50 and 25 nm films b ¼ 0.15, which may be explained by the 2D Ising model for ferromagnets.29 From this analysis, we conjecture that there is a transition from 3D mean field model to 2D Ising model with decrease in thickness. The ferromagnetic transition temperature also decreases with decrease in thickness, which is consistent with resistivity data. Such type of decrease in Tc can be due to change in dimensionality, reducing dimensionality decreases connectivity which in turn reduces ferromagnetic coupling.25 The variation of magnetization in ferromagnetic state in the low temperature range can be explained using Bloch law given as MðT Þ=Mð0Þ ¼ 1 BT 3=2 ;
(2)
where Mð0Þ is the magnetization at absolute 3 zero temperature and coefficient B ¼ ð0:0587 S ÞðkB 2JS Þ2 for the pseudo cubic magnetic lattice, where B is spin wave parameter, S is total spin of Ru4þ, kB is the Boltzmann constant, and J is the exchange interaction between two neighboring Ru4þ ions. Inset in Fig. 3(b) shows the good fit to Eq. (2) for 100 nm film. In ZFC, magnetization curves show cusp type behavior near Tc in Fig. 3(a). Isothermal magnetization (MH) curves are shown in Fig. 3(c) at 10 K and 100 K for 100, 50, and 25 nm films. The random jump in the hysteresis curve of 100 nm film at 10 K between two regions of opposite field could be due to Barkhausen jumps, as reported by Wang et al.11 This figure also reveals a decrease in the net saturation magnetization at 100 K as compared to that of 10 K for all the samples. This may be attributed to the fact that in itinerant ferromagnets
FIG. 3. (a) FC and ZFC curves for 100, 50, 25 nm films, (b) FC curve fitting near Tc for 100 nm film using scaling law M ¼ c (TcT)b, where inset shows the Bloch law fit, (c) magnetization curves at 10 K and 100 K for 100, 50, 25 nm films, (d) FC and ZFC curve for 12 nm film and inset shows the hysteresis curves.
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the magnetization usually gets reduced with an increase in temperature both by thermally excited magnons as well as by Stoner excitation. Decrease in magnitude of coercivity from 8902.19 Oe for 100 nm to 463.54 Oe for 25 nm film at 10 K is in agreement with the ZFC peak positions.30 Coercivity of the thinner films decreases so sharply that they start resembling double step loop. The double shift magnetization loop structure can appear when the exchange bias anisotropies are present in the system. The exchange bias phenomenon reported in SRO by Pi et al. as occurring due to spin glass property, rather than the interaction between antiferromagnetic and ferromagnetic interface.31 So in the present case, uniaxial anisotropies and spin glass behavior are both contributing to the observed magnetization shapes in 50 and 25 nm films. A ZFC and FC plot of the 12 nm film plotted in Fig. 3(d) represents the collapse of ferromagnetism. The irreversibility between ZFC and FC curves in the whole temperature range indicates the spin glass behavior for this film. Moreover, the absence of long range ferromagnetism has also been verified from resistivity versus temperature (RT) graph for the same thickness, as no kink is seen (Fig. 2(b)). From the magnetization curves (inset of Fig. 3(d)), we can see S-shaped curves. This behavior can be due to local ferromagnetism. It is reported that with decrease in thickness, the density of states near the Fermi level gets reduced and also the strain in the film affects its magnetic properties.15,18,32 Hence, for 12 nm film both the drop in the density of states and strain are so much that no stoner ferromagnetism appears. To conclude, XRD study of epitaxially grown thin films reveals the presence of tensile strain in 12 nm film and the AFM images show the merging of islands resulting in film smoothness with increase in thickness. The transport measurements of 25, 50, and 100 nm confirm the non-Fermi-liquid like behavior with the dominant scattering mechanism between Fermi liquids and localized electrons below Tc. The present study also reveals the corresponding increase in disorder and correlation in thin films which is playing crucial role in driving the 12 nm film to insulating state. The drop in transition temperature for thinner films is confirmed by resistivity and magnetic studies both, establishing the complementary nature between the two properties. The dimensional crossover from 3D in 100 nm to 2D in 50, 25 nm films is also noticed. The long range ferromagnetism changes to local magnetism in 12 nm thin film. This work concludes that the film thickness has substantial effects on structural, electrical, and magnetic properties of SRO films. We are thankful to Dr. K. Asokan for providing the facility of low temperature for transport measurements at the Inter University Accelerator Centre, Delhi, India. Authors
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are also thankful to Dr. V. R. Reddy for providing phi scan facilities at UGC-DAE Consortium for Scientific Research, Indore, Madhya Pradesh 452 001, India
1
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