1 Introduction

Perovskite oxides of general formula A2BBʹO6, where A site is occupied by rare-earth or an alkaline earth cation, B and Bʹ sites occupied by transition metals are called as double perovskites [1]. These oxides have been known from many decades, while the first studies on these compounds have initiated in about 1950s. Since then hundreds of compounds belonging to this family have been synthesized and studied as they display interesting and diverse structural, magnetic and electronic properties [2]. The practical diversity in the physical properties exhibited by these compounds is because of the fact that there is considerable scope for formation of new compounds by just choosing the A site, B and Bʹ site ions among the alkaline earth metals/ rare earth and transition metals respectively. Physical properties exhibited by the double perovskites are mostly determined by the B and Bʹ transition metal cations. In last many years, there has been a surge of interest in the synthesis and studies on the double perovskite oxides as some of the compounds belonging to this family like Sr2FeMoO6, La2VRuO6, La2NiMnO6, Gd2NiMnO6, La2CoMnO6 etc. display remarkable physical properties such as colossal magnetoresistance, half-metallicity, ferromagnetism etc. [3, 4]. The other technologically important applications exhibited by these double perovskites include solid-state Peltier coolers [5], magnetodielectric capacitors [6, 7], spintronic devices [8,9,10,11,12] tunnel junctions [13, 14] and most importantly solar cell applications [15]. Due to these versatile properties and potential applications, these oxides figure among the most investigated compounds by the researchers.

Out of this family of double perovskites, the manganite oxides R2MMnO6, where M = (Ni, Co) by virtue of being ferromagnetic semiconductors and possessing vast magnetic and electronic properties [9, 10, 16,17,18,19,20] are of highest interest. Particularly, La2NiMnO6, having potential applications in spintronics [10, 21] has grabbed more focus. Also, the fundamental laws governing the physical properties of La2NiMnO6 are interesting [22, 23], since most of these oxides are antiferromagnetic owing to super-exchange interactions, however, is ferromagnetic. The magnetic properties being governed by Kanamori & Goodenough rules [24] wherein the ordered arrangement of MnO6 & NiO6 octahedra leads to ferromagnetic interactions between Mn4+ and Ni2+cations. Thus, this oxide possesses remarkable ferromagnetic properties. Besides, significant changes are possible in the physical properties of La2NiMnO6 when some suitable dopant is added at the La site [25]. The doping in La2NiMnO6 compounds carries a great significance owing to the applications especially in magnetoresistance, solar cells and spintronics [15, 26,27,28,29]. The studies on the bulk as well as thin films of La2NiMnO6 has attracted a lot of research because of its applications in solar cells [15], spintronic devices, magnetic memory devices [10, 29,30,31,32,33,34,35], photovoltaic cells [36] and hydrogen storage [37]. Not enough work has yet been performed on the thin films of La2NiMnO6 though the studies on bulk have been many. Thin films of La2NiMnO6 are commonly deposited by chemical solution deposition [38] or pulsed laser deposition [33, 39]. The deposition parameters, however, play a crucial part in defining the properties of the deposited films. Therefore, to achieve better properties, it is imperative to deposit the films in optimized circumstances. The optimized film fabrication besides doping is therefore highly important for the device applications.

This work presents the optimized fabrication of La1.9Sr0.1NiMnO6-δ thin film on the substrate of Si (1 0 0) synthesized by pulsed laser deposition. This technique was chosen as it precisely retains the target-stoichiometry on the substrate. Sr was substituted at the La site as a dopant. Mainly the structure analysis, morphology and the magnetic studies performed on La1.9Sr0.1NiMnO6-δ are explained in this manuscript and the parameters of interest like structural phase, grain size, roughness and magnetic parameters including coercivity, remanence, Curie temperature etc. are discussed.

2 Experimental details

The thin film of La1.9Sr0.1NiMnO6-δ (hereafter referred to as LSNMO) was deposited by pulsed laser deposition (PLD) technique. A fine powder of LSNMO, prepared by the solid-state reaction was hydraulically pelletized and made into pellets of 15 mm diameter and 2 mm thickness. This pellet was used as the target for the deposition of a thin film of LSNMO on Si (1 0 0) substrate. Before the start of deposition, the substrate was cleaned with an ultrasonic bath in acetone and the deposition chamber was cleaned by using ethanol. The target was placed on the target holder and the substrate was placed on the substrate holder. Both of them were aligned with each other and separated by a distance of 40 mm. Later the heating was started and the substrate temperature was raised to 700-degree centigrade. The chamber was filled with 99.8% pure oxygen and the deposition was done against the oxygen background pressure of 500 mTorr. Lambda-Physik LPX210 an excimer laser source having a wavelength of 248 nm was used for the deposition. The laser source of incident energy 270 mJ was focussed on the target and the deposition was started and lasted for 40 min at a frequency of 10 pulses per second. This incident laser ablated the material from the target and the stoichiometric ablated material was deposited on the substrate kept at a temperature of 700 degrees. After finishing deposition, the substrate temperature was decreased to 550 degrees and the deposited film was annealed for 30 min by filling the chamber with 760 mTorr oxygen. Afterwards, the annealed film was cooled at a rate of 10 degrees per minute down to the room temperature and the LSNMO film was taken out.

The phase identification of the as-prepared film was done by performing the grazing incidence X-ray diffraction with a grazing angle of 1°, using the Bruker AXS D8 Discover X-ray diffractometer. The scan was carried out in the 2θ range of 15°–80°. The surface morphology of the film was studied with the help of atomic force microscopy (AFM), using Nanoscope E digital instrument in tapping mode. Renishaw in via microscope having 520 nm as wavelength was used to record the Raman spectra of the film. To avoid any damage to the film the laser power was minimized to 0.3 mW. The magnetic measurements were performed on the 7 T MPMS, SQUID. The MH, hysteresis measurement was done in the field − 3 T to + 3 T at a cryogenic temperature of 5 K. Moreover, the MT, magnetisation versus temperature in ZFC and FC condition was recorded in the temperature ranging from 5 to 300 K in a coercive field of 300 Oe.

3 Results and discussion

3.1 GIXRD analysis

The GIXRD pattern of La1.9Sr0.1NiMnO6-δ showing the polycrystalline nature of the film is shown in Fig. 1. All the diffraction peaks corresponding to monoclinic phase like (110), (112), (202), (220), (310), (312), (204) and (224) are present in the film, confirming the single phase that is the monoclinic phase with P21/n space group as also reported in bulk La2NiMnO6 target [28, 40]. The extra peak around 38 degrees is from the precursor Mn2O3 [41] (JCPDS Card No. 78-0390), that might have remained unmixed. The lattice parameters calculated on the basis of the monoclinic structure are presented in Table 1. Scherrer’s equation was used to obtain the crystallite size:

$$D = K\lambda / \beta cos \theta$$
Fig. 1
figure 1

Grazing incidence XRD pattern of double perovskite La1.9Sr0.1NiMnO6-δ film

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Table 1 Lattice parameters, crystallite size and value of strain for the La1.9Sr0.1NiMnO6-δ film
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In the above equation, D represents the crystallite size, K is a shape factor mostly taken as 0.9, λ, the X-ray wavelength (for CuKα source = 1.54 A°), β, the full width at half maximum intensity for the chosen Bragg peak. The obtained value of the crystallite size from the above equation is presented in Table 1.

The lattice strain present in the film was calculated from the Williamson Hall equation.

$$\beta_{hkl} \cos \theta = {\raise0.7ex\hbox{${K\lambda }$} \!\mathord{\left/ {\vphantom {{K\lambda } D}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$D$}} + 4\upvarepsilon \sin \theta$$

Again here \((\beta_{hkl} )\) is full width at half maximum of a chosen Bragg peak, \(K\) = 0.9 is a shape factor constant,\(\lambda\), the source X-ray wavelength, \(D\) represents the crystallite size and the term ɛ in the above equation of Williamson and hall is the magnitude of strain. The term \(\left( {\beta_{hkl} \cos \theta } \right)\) along the Y-axis is plotted against \(4\upvarepsilon \sin \theta\) along X-axis. The slope of the linear fit of this plot yields the strain and the intercept on the Y-axis of this linear fit line represents the Crystallite size. The values of strain (ɛ) and crystallite size (D) calculated by this Williamson hall analysis are reported in Table 1.

3.2 AFM analysis

The morphology, grain diameter and surface roughness of the film La1.9Sr0.1NiMnO6-δ were studied from the AFM micrographs. The AFM micrographs are shown in Fig. 2. The morphology of the deposited film is more or less spherical with some flattened spherical grains. Different parameters like grain diameter and roughness were calculated from the software Nanoscope. The grain diameter, taken as the root mean square value came around to be 171.41 nm. Also, the roughness was estimated to be about 10.32 nm. The values are reported in Table 2.

Fig. 2
figure 2

Surface morphology of La1.9Sr0.1NiMnO6-δ thin-film represented by AFM images

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Table 2 The surface roughness, grain size and magnetic parameters of La1.9Sr0.1NiMnO6-δ film
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3.3 Raman analysis

Figure 3 represents the Raman spectra of La1.9Sr0.1NiMnO6-δ thin film on Si (1 0 0) taken at room temperature. The spectra show dominant modes at the wavenumbers (530 and 657) cm−1. The intensity and the number of peaks appearing in Raman spectra are closely associated with the deviation from the ideal cubic structure of La2NiMnO6. These modes respectively correspond to the asymmetrical stretching (AS) and symmetrical stretching (S) of MnO6/NiO6 octahedra. Also, these modes are characteristic of the monoclinic structure with space group as P21/n [28], accomplishing the Ni/Mn cation ordering and hence consistent with the XRD results [30]. The FWHM of the corresponding Raman peaks is not much broad, which implies that the crystallization of the film is better [42].

Fig. 3
figure 3

Raman spectra of La1.9Sr0.1NiMnO6-δ film showing characteristic symmetric and asymmetric stretching modes

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3.4 Magnetic study

Figure 4 represents the isothermal, magnetization vs applied field behaviour (MH curve) of La1.9Sr0.1NiMnO6 taken at cryogenic temperature of 5 K. The inset shows the presence of hysteresis loop which is ferromagnetic in nature and having a coercivity (Hc) of 371 K. The MH curve was obtained to study the role of magnetic domains giving rise to all the magnetic properties of La1.9Sr0.1NiMnO6-δ and thus attain the magnetic parameters like, Hc, Mr and Ms that is coercivity, remnant magnetization and saturation magnetization respectively. The values of all these parameters are listed in Table 2. The saturated magnetic moment, \(\mu_{B} /{\text{f}}.{\text{u}}.\) for the LSNMO/Si (100), at the 5 K temperature has been calculated by the formula [43];

$$\mu_{B} /f.u. = \left( {M_{s} *M_{w} } \right)/5585$$

where \(M_{s}\) is the maximum value of saturation magnetization and \(M_{w}\) is the molecular weight of the sample in grams. The value came out to be (4.96 \(\mu_{B} /{\text{f}}.{\text{u}}.\)), as compared to the value of 4.63 \(\mu_{B} /{\text{f}}.{\text{u}}.\) obtained by H Guo et.al [39]. in the epitaxial thin films of La2NiMnO6 on SrTiO3 (STO) substrate.

Fig. 4
figure 4

The MH hysteresis curve of La1.9Sr0.1NiMnO6-δ thin-film recorded at 5 K. Inset presents the zoomed view to highlight the ferromagnetic loop

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Figure 5 represents the temperature-dependent magnetization that is MT-curve of La1.9Sr0.1NiMnO6-δ taken within the 5 K to 300 K temperature range. The MT curve has been obtained by the field applied parallel to the film surface with a coercive field/ applied field of 500 Oe. The sample was first cooled in SQUID chamber down to 5 K without applying any external field.

Fig. 5
figure 5

Field cooled and zero field cooled MT-curves of La1.9Sr0.1NiMnO6-δ film. Inset presents the value of Tc calculated from the minima of \(dM/dT\)

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After that zero-field cooled (ZFC), magnetization versus temperature (MT) was taken under a constant coercive magnetic field of 500 Oe. The ZFC-MT was recorded during heating in the temperature range of 5–300 K. Subsequently, field cooled (FC), magnetization versus temperature (MT) was taken under the same constant coercive field. Also, the FC-MT was recorded while cooling the sample in the temperature range of 300–5 K. From the MT curves in Fig. 5, it is clearly seen that the magnetization decreases with the corresponding increase in the temperature. The first derivative of Magnetization with respect to the temperature that is \(dM/dT\) was taken and the minima of this curve represents the ferromagnetic curie temperature, \(T_{c}\). The value of curie temperature was found to be 186.9 K. An enhancement in value of \(T_{c}\) is clearly seen from that of the pure LNMO when compared with our previous report [40]. The enhancement in \(T_{c}\) may possibly be due to the change of the valency of Ni from + 2 to + 3 upon Sr doping to maintain charge neutrality in the compound. Moreover, LNMO has been reported to exhibit more than one magnetic transition, largely depending on the synthesis conditions. Two FM transitions at \(T_{c} 1\) ~ 280 K and \(T_{c} 2\) ~ 150 K have also been reported in some reports [17, 30, 32, 44]. Besides, in MT curves, the point where ZFC and FC curves segregate from each other is called the temperature of irreversibility and may also be called as the magnetic-ordering temperature. The permanent difference at a lower temperature as seen from the MT curves between ZFC and FC curves clearly indicates that the deposited film has well magnetic ordering.

4 Conclusion

Thin-film of La1.9Sr0.1NiMnO6-δ was successfully deposited from the bulk target on Si (1 0 0) substrate using pulsed laser ablation technique. Polycrystalline nature of the grown film was revealed from the GIXRD patterns. X-ray analysis confirmed the monoclinic structure of the film having P21/n space group. The tensile strain was noted in the film and the strain value was calculated from the Williamson-Hall equation. The morphology showed the spherical grains with average grain size and roughness calculated by the Nanoscope software. The asymmetric and symmetric stretching modes of the NiO6/MnO6 octahedra were revealed by the wavenumbers corresponding to 530 cm−1 and 657 cm−1 in the Raman spectra of the film. Moreover, the ferromagnetic nature of the film at a cryogenic temperature of 5 K was revealed from the magnetic studies as depicted by the MH loop. Also, the ferromagnetic curie temperature Tc calculated from the MT plots was found to be 186.9 K for this film.