1 Introduction

In the past few decades, the rare-earth perovskites La1−xAxMnO3, with A being Ca, Sr and Ba etc. divalent alkali element, has attracted a considerable attention due to its extraordinary magnetic and electronic properties, as well as the promise of potential applications in photo-electronic (uncooled infrared or bolometer detectors) and magnetic devices (uncooled magnetic detectors) [1,2,3]. In particular, fascinating colossal magnetoresistance (CMR) effect [1, 4,5,6,7,8], ferromagnetic (FM)-paramagnetic (PM) transition, metal-insulator (MI) transition [6], and several unusual properties, could be interpreted by the double exchange (DE) mechanism [9,10,11,12], Jahn-Teller (JT) effects [13, 14], and the grain boundaries (GBs) effect [15]. It is well known that there are two CMR effects have been found in those materials [16]. The first one is the intrinsic CMR, which was proposed by Zener in 1951 [10] and is due to DE, whereas the second one is the extrinsic CMR, which is related to the GBs and can be explained by spin-polarized tunneling [9, 17]. In addition, the intrinsic CMR effect can be only activated at high magnetic fields of several Tesla and ultra-low temperature, which limit its application. Therefore, it is necessary to adjust the insulator-metal transition temperature (Tp) and magnetoresistance (MR) to the room temperature by changing the component in La1−xAxMnO3 ceramics for the practical application. For another, the large temperature coefficient of resistance (TCR) is essential for practical applications as well [18]. The sharp drop of the resistance implies high TCR, which is useful for applications, such as temperature controlling resistance devices or bolometers detectors, etc. [9]. Unfortunately, high Tp usually corresponds to low TCR and vice versa.

For the above reasons, La1−xSrxMnO3 (LSMO) manganites have attracted extensive interests due to their large CMR and interesting room temperature physical properties. Numerous study have been focused on LSMO with the Sr content at x = 0.33 since an LSMO with that Sr content exhibited high-spin polarization [9], low resistivity (ρ), strong DE interaction, and a wide range of MR. However, the Curie temperature (TC) and Tp of these manganites are still away from room temperature (295 K). In addition, these materials have low TCR and MR values, undoubtedly restricting their practical application [19, 20]. The magnetic phase diagram of LSMO is displayed in Fig. 1. It can be seen that the Curie temperature achieves the room temperature as the content of Sr varies from 0.1 to 0.2 [3, 4]. Therefore, in this work, the LSMO polycrystalline ceramics were fabricated via the optimized sol–gel method and with the Sr addition in the range of 0.1–0.2. By modulating the Sr content of the LSMO polycrystalline ceramics, the Tp and MR at room temperature were obtained. The results provide new insights into the potential applications in different fields, including uncooling bolometer and infrared detectors, magnetic sensors and magnetic recording devices at room temperature.

Fig. 1
figure 1

The magnetic phase diagram of La1−xSrxMnO3 [4]. AFM phase is an A−type AF metal with uniform barbital order. PM, PI, FM, FI, and CI is paramagnetic metal, paramagnetic insulator, FM metal, FM insulator, and spin−canted insulator states, respectively. Tc is the Curie temperature, whereas TN is the Neel temperature

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2 Materials and methods

A series of LSMO polycrystalline ceramics with x = 0.1, 0.125, 0.15, 0.16, 0.175, and 0.2 were synthesized by using the two-steps method. Compared with other methods, the sol–gel method has several advantages such as relatively homogenous size of sample granules, low temperature, and short reaction time, and so on. In this study, the relatively homogenous size of sample granules is the most important advantages to prepare high quality of LSMO polycrystalline ceramics. Firstly, LSMO powders were fabricated via the optimized sol–gel method. The solution were composed of chemical constitutions of analytical grade reagents La(NO3)3·nH2O, Mn(NO3)2 aqueous solutions (50% by mass), Sr(NO3)2 and deionized water. Complexing agents including ethylene glycol and citric acid were added to the nitrate solution. Then, undergone a continuous magnetic stirring and heating, citric acid, used as polymerizing, were added to the mixture. Until the mixture became a brown gel, put it into the drying oven at 140 °C for 24 h. Secondly, after pressing at 16 MPa, the LSMO powder became the pellets and sintered at 1450 °C for 12 h under flowing oxygen at 0.02 MPa [21].

The crystal structure and surface morphology of LSMO ceramics were characterized by X-ray diffraction (XRD, ULTIMA IV) and scanning electron microscopy (SEM, SU8010), respectively. By using the conventional four-probe method in the temperature range of 100–360 K to measure the electrical and magnetic transport behaviors of the samples under an external magnetic field of 1 T with KEITHLEY Instruments.

3 Results and discussion

Figure 2a displays the XRD of the as-prepared LSMO polycrystalline samples with x = 0.1, 0.125, 0.15, 0.16, 0.175, and 0.2. The inset graph illustrates the enlarged views of the main peaks in the 2θ ranging from 31.6 to 33.6°. All the samples are a single phase with the crystal structure of rhombohedral phase (R\(\bar 3\)c space group). Figure 2b displays that all the samples have the (110/114) double peaks and move towards low angles with x increasing. Since the ion radius of Sr (1.26 Å) is bigger than the ion radius of La (1.16 Å), the lattice spacing of LSMO polycrystalline material increases when the content of Sr increases. Changes of the lattice parameters are listed in Table 1. Nevertheless, this substitution has not produced a modification in the crystal structure of the LSMO samples.

Fig. 2
figure 2

a XRD patterns of LSMO with x = 0.1, 0.125, 0.15, 0.16, 0.175, and 0.2; b The enlarged views of the main peaks (110/104) in the 2θ range of 31.6–33.6°; c Rietveld refinement plots of the typical LSMO (x = 0.16) samples; d Typical crystal structure of La1−xSrxMnO3 ceramics

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Table 1 The structure and refinement parameters of the LSMO samples
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Figure 2c illustrates the Rietveld XRD of the La0.84Sr0.16MnO3 ceramics. The fitting result is reported by the red line, while the diffraction data is represented by a solid sphere; green lines define the fitting diffraction peak of position, whereas the bottom line represents the difference between the fitted and the measured values. Table 1 contains the structural parameters, including space group, lattice constants (Re, Rp, and Rb), and agreement factor (χ) describing the goodness of fit. The increment of Sr leads to an increase in the A-site ion radius, then increasing the cell volume V [22, 23]. In addition, as x increases, the a and c lattice constant also expands, which results in the change of the Mn–O bond distance (dMn–O) and the Mn–O–Mn bond angle (θMn–O–Mn). This could also explain why the reason the main peaks (110/104) have shifted to a lower angle. The Rietveld spectrum diffraction peak of the LSMO polycrystalline samples are in agreement with the experimental peak, which further confirms that all the samples contain a single phase. Moreover, the χ value is small enough to assure the reliability of the fitting results.

The SEM micrographs of the samples are shown in Fig. 3a–d. The size of the crystalline grains varies dramatically with different Sr2+ doping, which means the GBs has been reduced. Indeed, the grain-size increase causes the GBs scattering to decrease, which leads to the reduction of the sample resistivity. The substitution of Ca2+ ion (1.12 Å) by the large Sr2+ ion (1.26 Å) improve the catalytic activity and promote the crystal quality of the LSMO polycrystalline samples, in turn, which degrade the quantity of GBs and increase the grain size.

Fig. 3
figure 3

SEM images of the LSMO samples with x = 0.1 a, 0.125 b, 0.15 c, and 0.2 d

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Figure 4 illustrates the dependence of resistivity within a temperature range of 100–360 K for LSMO polycrystalline material at different contents of x. With the temperature rising, all of the samples show translation from low-temperature FM to high-temperature PM insulation for both sides of Tp. Moreover, it can be seen that the resistivity (ρ) of the LSMO ceramics reduces as the content of Sr2+ increases. The increase of the grain sizes results in the decrease of the resistivity values. In previous work, it was demonstrated that several factors like doping additions, sintering quality, and preparation method could control the grain sizes. Here, it was doping condition that is more important in increasing the grain size of LSMO ceramics. Moreover, the Tp increased as the content of Sr2+, which can be explained via the DE mechanism. The increase of Sr2+ ion concentration increases Mn4+ ion concentration inducing the changes in both Mn–O–Mn bond angles, which facilitates the DE mechanism [9].

Fig. 4
figure 4

Resistivity versus temperature plots of LSMO with x = 0.1, 0.125, 0.15, 0.16, 0.175, and 0.2

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Figure 5 displays the TCR curves of LSMO (x = 0.125, 0.15, 0.16, 0.175, and 0.2) ceramics, TCR defined as: TCR (%) = [(1/ρ) × (dρ/dT)] × 100% [21], where T and ρ are the temperature and resistivity, respectively. As x rises, the peak TCR increases from 1.6 to 6.4%, whereas the peak-TCR temperature (TK) values increases from 172.4 to 329.4 K. The peak TCR are 1.6%, 5.1%, 2.7%, 3.4%, and 6.4% K−1 for x = 0.125, 0.15, 0.16, 0.175, and 0.2, respectively. In addition, TK are 172.4, 280.1, 292.5, 311.2, and 329.4 K for x = 0.125, 0.15, 0.16, 0.175, and 0.2, respectively. The TK values have a similar variation trend to that of Tp. At x= 0.16, TK = 292.5 K, approaching the room temperature, while the peak TCR reaches 2.7% K−1. The peak TCR is impacted by several factors, such as ρ, tolerance factor (τ) [3], preparation method [24], and sintering quality [1], and so on. The transport point of FM–PM corresponds to the maximum value of TCR and has great influence to the ρ, which is the most important factor to affect TCR.

Fig. 5
figure 5

TCR versus temperature plots of LSMO at x = 0.125, 0.15, 0.16, 0.175, and 0.2

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Figure 6a–f shows the influence of temperature on the MR value to clarify the CMR effect. MR is defined by the following equation: MR (%) = [(ρ0 – ρH)/ρ0] × 100% [21], where ρ0 and ρH are the resistivities at the magnetic field of 0 and 1 T, respectively. From Fig. 6a–f, the ρ curves of all the samples at zero magnetic field are very close to the curves at 1 T. Furthermore, it is obvious that the value of ρ at zero is higher than the value measured under an external magnetic field of 1 T. The reason is that the external magnetic field could align the spin direction and increase the tunneling effects on the GBs to reduce resistivity [17, 25, 26]. It can be seen that the temperature of peak MR (TM) values are 188.4, 233.5, 285.7, 300.2, 328.5, and 341.6 K for x = 0.1, 0.125, 0.15, 0.16, 0.175, and 0.2, respectively. Moreover, the peak MR are 12.8%, 15.7%, 31.0%, 17.9%, 15.9%, and 17.2% for x = 0.1, 0.125, 0.15, 0.16, 0.175, and 0.2, respectively. At x= 0.16 and the maximum value of MR is 17.9%, the peak MR temperature (300.2 K) approaches the room temperature.

Fig. 6
figure 6

ρ-T curves of the LSMO ceramics under applying external magnetic fields of 0 and 1 T, and MR versus temperature plots of LSMO with x = 0.1 a, 0.125 b, 0.15 c, 0.16 d, 0.175 e, and 0.2 f

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Table 2 defined the detailed parameters for the transport properties of the LSMO ceramics. The increase of the content of Sr promotes the crystallization, which is responsible for the decrease of ρ. Moreover, the replacement of the A sites leads to a lattice distortion. Because the radius of Sr2+ ion is larger than the radius of La3+ ion, this substitution results in the τ and average radius of A sites (rA) to increase, which lead to a stronger Mn–O–Mn exchange interaction. According to the DE theory, this interaction between Mn ions would cause a raise in Tp, TK, and TM. Furthermore, on the GBs, an external magnetic field could align the spin direction and increase the tunneling effects to reduce resistivity, which is the reason ρ at 1 T is lower than that at zero. In addition, many other factors can influence the TCR, such as sintering time, temperature, rA, τ, preparation method, and resistivity of samples.

Table 2 Detailed parameters for the transport properties of LSMO
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Table 3 shows the obtained TCR and MR peak in comparison with reported values. In previous work, the peak TCR were 3.4% K−1 in LSMO (x = 0.33) ceramics. However, the peak TCR temperature (325 K) was above the room temperature, a condition that limits its application. However, in this work, we obtained the peak TCR (2.7% K−1) with TK at 292.5 K for x = 0.16, as well as the maximum MR (17.9%) at 300.2 K. Furthermore, the peak TCR (6.4% K−1) in the La0.8Sr0.2MnO3 polycrystalline ceramics is significantly larger than other previous reports [1, 27,28,29]. Our findings suggest that the La0.84Sr0.16MnO3 ceramics have broad applications prospects in the room temperature uncooled infrared Bolometer and magnetic devices.

Table 3 The peak TCR (%) and MR (%) obtained in previous and this work
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4 Conclusions

The LSMO polycrystalline ceramics with x = 0.1, 0.125, 0.15, 0.16, 0.175, and 0.2 were prepared via the optimal sol–gel methods. XRD showed all the bulk samples crystallized in single phase and with the crystal structure of rhombohedral phase (R\(\bar 3\)c space group). As the x content increased, the main diffraction peaks (110/114) of XRD shifted downwards to low angles, increasing the lattice structure parameters (a, c, and V) of the LSMO ceramics as well. This was attributed to the Sr ion radius (1.26 Å) being bigger than the La ion radius (1.16 Å) at the A sites. In addition, with x content further increased, Tp increased gradually, while the peak value of TCR fluctuated and reached a maximum value. By applying an external magnetic field of 1 T, the resistivity of the LSMO ceramics was lower than the samples without the magnetic field, which could be interpreted by spin-polarized tunneling occurring with the function of spin disorder in the GBs. For the sample of x = 0.16, the peak values of TCR and MR attained 2.7% K−1 (292.5 K) and 17.9% (300.2 K), respectively. At the same time, the large peak TCR was observed with the highest value of 6.4% K−1 at 329.4 K for the LSMO (x= 0.2) sample. These properties were related to DE mechanism and JT effects, which suggested that the LSMO ceramics (x = 0.16) may have underlying applications in uncooling infrared detectors or bolometers, magnetic sensors, and magnetic recording devices at room temperature.