1 Introduction

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) is an environment-friendly cooling technology with low energy consumption [1,2,3]. Its application has drawn attention not only at room temperature but even lower (e.g., gas liquefaction as helium, hydrogen, and natural gas). The MCE is an inherent property of magnetic materials, and it is the result of exposing the magnetic material to an external magnetic field or to removing it [4]. This property enables us to determine whether a magnetic material can be regarded as a good magnetic refrigerant or not. MCE can be evaluated through the calculation of magnetic entropy changes (ΔSM) based on the isothermal magnetization data and using the Maxwell equation [5].

$${\Delta S}_{M}\left(T,H\right)={S}_{M}\left(T,{H}_{1}\right)-{S}_{M}\left(T,{H}_{2}\right)={\int }_{{H}_{1}}^{{H}_{2}}\left(\frac{\partial M}{\partial T}\right)dH.$$
(1)

For industrial applications, the efficiency of a good magnetocaloric material destined for magnetic refrigeration is evaluated in terms of a high value of magnetic entropy change (− ΔSMmax) and a large relative cooling power (RCP) [6]. RCP is the heat transfer between the hot and cold reservoirs in the ideal refrigeration cycle, which is generally assessed using the following relation [7]:

$$RCP=\left|{(-\Delta S}_{M}^{max})\right|\times {\delta T}_{FWHM},$$
(2)

where \(\left|{(-\Delta S}_{M}^{max})\right|\) is the absolute value of the maximum magnetic entropy change and \({\delta T}_{FWHM}\) is the temperature difference at the full width at half maximum (FWHM) of (ΔSM) curve, with \({\delta T}_{FWHM}={T}_{2}-{T}_{1}.\).

A lot of research work has been done to find the best refrigerants with potential applications at RT magnetic refrigeration, such as the study of Gd5(Ge1-xSix) [8], MnAs1-xSbx [9], MnFeP1-xAsx, [10], LaFe13-xSix [11], and Ni–Mn–Ga [12]. Currently, the perovskite manganite oxide with the chemical formula Ln1−xAxMnO3 (Ln = trivalent rare earth, A = divalent alkaline earth) has attracted much attention due to its wide range of properties such as (MCE) [7]. The principal advantages of this type of compounds over Gd and GdSiGe alloys are low cost, high chemical stability (no oxidation), tunable TC, low coercivity, and high electric resistance [13]. Consequently, manganites oxides have been considered as promising candidates for MR [14]. Several reports are addressing magnetic and MCE properties of a perovskite with A-site substitutions such as Pr1/2Sr1/2MnO3 [15], La0.6Pb0.4MnO3 [16], Nd1-xSrxMnO3 [17], La1-xCaxMnO3 [18], Ln0.67Sr0.33MnO3 (Ln = La, Pr and Nd) [19], and Pr-doped La0.67Ca0.33MnO3 nanoparticles [20]. Among these materials, Lanthanum manganites have attracted the attention of many researchers, thanks to the very rich phase diagram observed in La1−xCaxMnO3 and La1−xSrxMnO3 samples. Lanthanum manganites are characterized by multi-phase transitions [21,22,23]. In particular, La1-xSrxMnO3 perovskite manganite is an important member in the manganite family due to its significant physical properties and potential applications in high-performance magneto-resistive sensors and magnetic heads [24]. The highest Curie temperature was detected in La0.7Sr0.3MnO3 which was about 360 K. To be used for magnetic refrigeration near room temperature, it is important to use appropriate alternative methods to reduce its TC. In general, a partial replacement of non-magnetic ion La3+ with a smaller sized rare-earth magnetic ion (Sm, Nd, Gd…) leads to a decrease in the Curie temperature [25]. In other hand, research has also been carried out on the replacement of Mn site with transition elements (eg Fe, Co, Ga, Cr, etc.) [26,27,28,29,30,31,32,33]. The Cr substitution is notably interesting, since Cr3+ is isoelectronic with Mn4+ (\({t}_{2g}^{3}{e}_{g}^{0}\)) and is a non-Jahn–Teller ion [34]. A literature overview highlights that the ionic radius of Cr3+ is closely the same as that of the Mn3+; therefore, it can easily substitute Mn in the manganites system. Cr substitution in the B-site of manganites affects the Mn3+ –O– Mn4+ network because it will influence the double exchange, which leads to change the physical properties of the material [35]. For an industrial achievement, a major interest is focusing on the design of simple, greener, economic, and scalable synthesis process of magnetic materials [36]. Magnetic materials have been prepared using several synthesis techniques (e.g., solid-state reaction, ball milling, sol–gel). In the recent time, auto-combustion method of glycine-nitrate precursors is considered to be the easiest, the fastest process for formation of homogeneous crystalline particles, and low external energy consumption. We study in this article the effect of Cr doping on the structural, magnetic, and magnetocaloric properties of La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15, and 0.20) prepared by auto-combustion method.

2 Experimental procedure

2.1 Synthesis method

La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15 and 0.20) powders were prepared using the auto-combustion method (See Fig. 1). Stoichiometric amount of La(NO3)3, 6 H2O, Sm(NO3)3, 6 H2O, Sr(NO3)2, Mn(NO3)2, 4 H2O, Cr(NO3)3, and 9 H2O (99.995% in purity, “Sigma Aldrich”) were dissolved in distilled water. Then we add the Glycine nitrate (C2H5NO2, ~ 99.995% “Sigma Aldrich”) to the solution which is used as a fuel. The molar ratio of glycine-nitrate was fixed on 1 in the present work. The solution was heated on a hot plate at 100 °C for 1 h with stirring. The water evaporation converted the solution to a viscous transparent gel. The auto-ignition started when the temperature reached 350 °C, the burning lasted only a few seconds, and a large amount of fine "ash" was produced. The obtained ‘‘ash’’ was preheated at 350 °C for 4 h to eliminate any carbon remainder in the powder and to form a pure-crystal structure. The calcined powder was pressed at 4 tons/cm2 to form pellets with a thickness of 1 mm and then sintered at 600 and 800° C for 12 h.

Fig. 1
figure 1

Details of the auto-combustion process

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2.2 Characterization techniques

2.2.1 X-ray powder diffraction

The phase purity, crystal structure, and lattice parameters have been examined using the X-ray powder diffraction (XRD) at room temperature. XRD patterns were collected by a Phillips powder diffractometer PW 1710 using Cu Kα radiation (λ = 1.5405Ǻ).

The diffraction data were analyzed by the Rietveld method using the FULLPROF software [37].

2.2.2 FTIR—Fourier Transformation Infrared—spectroscopy analysis

To identify the functional group of the prepared samples, we use the FTIR analysis. Nicolet Impact 400 Spectrophotometer was used to record FTIR spectra in the wavenumber range of 400–4000 cm−1 at RT.

2.2.3 Vibrating sample magnetometer (VSM)

A vibrating sample magnetometer (VSM) was used to record the magnetic measurements value with a constant applied magnetic field of 0.1 T under the conditions of zero-field cooled (ZFC) and field cooled (FC) in the temperature range of 5–300 K.

Isothermal measurements M (H) were carried out by varying H up to 5 T at different Ts.

3 Results and discussions

3.1 Structural analysis

The Rietveld refinement of X-ray diffraction patterns confirms that all compounds are single phase with orthorhombic structural, Pnma space group (See Figs. 2 and 3). Table 1 shows information about the relevant structural parameters obtained by Rietveld refinements. A reduction of the unit cell volume and the lattice parameters is expected upon substitution manganese by chromium because the radius of Cr3+ (0.615 Å) is a little smaller than that of Mn3+ (0.645 Å) [38]. The bond lengths dMn–O and the bond angle Mn–O–Mn change slightly with increasing Cr3+ content.

Fig. 2
figure 2

X-ray diffraction pattern of La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15, and 0.20)

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Fig. 3
figure 3

Observed and calculated X-ray diffraction data and Rietveld refinement for La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15, and 0.20). Vertical bars are the Bragg reflections for the space group Pnma. The difference pattern between the observed data and fits are shown at the bottom

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Table 1 Refinement results for the samples La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15 and 0.20) determined from XRD patterns measured at room temperature
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3.2 Fourier transformation infrared spectroscopy

In order to detect the absorption bands in the prepared samples, Fourier transformation infrared spectroscopy analysis was performed at room temperature in the wave number range of 400–4000 cm−1. The FTIR spectrums are presented in Fig. 4. The main absorption bands are observed at around 600 cm−1. These absorption bands can be attributed to Mn–O and O–Mn–O bond vibrations of the octahedral MnO6 in our perovskite structure samples [39, 40].

Fig. 4
figure 4

FTIR spectra of La0.5Sm0.2Sr0.3 Mn1-xCrxO3 (x = 0.00, 0.10, 0.15 and 0.20) compounds

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3.3 Magnetic properties

Figure 5 displays the temperature dependence of magnetization (M) of La0.5Sm0.2Sr0.3Mn1-xCrxO3 samples in the (ZFC) and (FC) processes measured with the application of a magnetic field of 0.1 T. It can be seen that the FC curves exhibit a single (PM-FM) behavior transition with decreasing T. At an irreversibility temperature (Tirr), a bifurcation between the FC and ZFC curves and a drop in ZFC magnetization (λ shape) are observed which can be associated to super para-magnetism or spin glass behavior [41,42,43]. Similar behavior was observed in La0.5Sm0.2Sr0.3Mn1-xFexO3 compounds reported by Kh. Abdouli et al. [29].

Fig. 5
figure 5

Variation of the magnetization (M) vs. temperature (T) of La0.5Sm0.2Sr0.3 Mn1-xCrxO3 (x = 0.00, 0.10, 0.15 and 0.20) compounds measured at an applied magnetic field of 0.1 T. Inset: The variation of dM/dT as a function of temperature (T)

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Curie temperature (TC) is the temperature where the (PM-FM) transition occurs and is estimated from the peak of dM/dT (T) curves (inset of Fig. 5). The TC values for the samples (x = 0.00, 0.10, 0.15, and 0.20) are given in Table 2. Substitution of Cr at the Mn site causes a gradually reduction in Curie temperature from 278 to 205 K for x = 0.00 and x = 0.20.

Table 2 Magnetic parameters for La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15, and 0.20) compounds
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The decrease of TC is related to the decrease of Mn ion rate and the increase of Cr ion concentration. Cr3+ ions can basically replace Mn3+ ions. Indeed, \({r}_{<{Cr}^{3+}>}\) = (0.615 Å) is smaller than \({r}_{<{Mn}^{3+}>}\)= (0.645 Å) and bigger than \({r}_{<{Mn}^{4+}>}\) = (0.53 Å). Furthermore, Cr3+ ion has an electronic configuration similar to Mn4+ ion (\({t}_{2g}^{3}{e}_{g}^{0}\)) which lead to the appearance of an antiferromagnetic super exchange interaction between Cr3+ and Mn4+ [44]. The increase of the content of Cr3+ ion significantly decreases Mn3+/ Mn4+ ratio which means that the double-exchange interactions between Mn3+ and Mn4+ ions was partially destroyed, while the antiferromagnetic interactions between Cr3+ and Mn4+ are enhanced. Also, the antiferromagnetic coupling between Mn4+-Mn4+ and Cr3+-Cr3+ should be considered. To sum up, ferromagnetic double-exchange interactions are weakened when Mn is substituted by Cr, while the antiferromagnetic interactions are reinforced.

Furthermore, in order to know the magnetic behavior in the paramagnetic range, we studied the temperature dependence of the inverse magnetic susceptibility (χ), which is plotted from the M (T) data and shown in (Fig. 6). The fit of (1/χ) as a function of temperature reveals that at high temperature, the four samples follow the Curie–Weiss behavior:

$$\chi = \frac{C}{{T - \theta _{p} }}$$
(3)

where χ is the susceptibility, θp is the Curie–Weiss temperature and C is the Curie constant.

Fig. 6
figure 6

Variation of the inverse magnetic susceptibility (χ−1) as function of temperature, and the red lines are the fits according to the Curiee-Weiss law

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It can be noted that θp values are slightly higher than that of TC, their positive values indicate that there is a ferromagnetic exchange interaction between the nearest neighbors. In general, this difference between θp and TC depends on the material and is related with the existence of short range ordered slightly above TC, which is associated to the existence of a magnetic inhomogeneity.

According to the determined Curie constant (C), the known relation

$$(\mu _{{eff}}^{{\exp }} )^{2} = \frac{{3K_{B} M_{m} }}{{N_{A} \mu _{B}^{2} }}C$$
(4)

is used to determine the experimental effective paramagnetic moments \((\mu _{{eff}}^{{\exp }} ),\). where NA is the Avogadro’s number (6.023 × 1023 atoms mol−1), μB is the Bohr magneton (9.274 × 10–21), Mm is the molecular weight, and KB is the Boltzmann constant (1.38016 × 10–16 erg K−1).

According to La0.5Sm0.2Sr0.3Mn1-xCrxO3 composition, the theoretical effective paramagnetic moment is determined via the following relation:

$$\mu _{{eff}}^{{th}} = \sqrt {0.2[\mu _{{eff}}^{{th}} (Sm^{{3 + }} )]^{2} + (0.7 - x)[\mu _{{eff}}^{{th}} (Mn^{{3 + }} )]^{2} + x[\mu _{{eff}}^{{th}} (Cr^{{3 + }} )]^{2} + 0.3[\mu _{{eff}}^{{th}} (Mn^{{4 + }} )]^{2} } ,$$
(5)

where μeff = 0.85μB for Sm3+, μeff = 4.9μB for Mn3+, μeff = 3.87μB for Mn4+, and μeff = 3.87μB for Cr3+.

θP, \(\mu _{{eff}}^{{\exp }} ,\) and \({\mu }_{eff}^{th}\) values are registered in Table 2.

The dissimilarity between \({\mu }_{eff}^{exp}\) and \({\mu }_{eff}^{th}\) values may be related to the existence of the ferromagnetic polarons in the paramagnetic state [45].

3.3.1 Isothermal magnetization studies

To understand the magnetism and to approve the ferromagnetic comportment of the samples at low temperatures, we report in Fig. 7 the relationship between the magnetization (M) and the applied magnetic field (H) at several temperatures near to TC (up to 5 T). It can be observed that M (H) curves, near the FM-PM transition, gradually increase with increasing H values. Below TC, a typical behavior of ferromagnetic material is detected, after applying a magnetic field up to 1 T, the magnetization rises sharply and then reaches saturation. Above TC, the magnetization M increases more steadily, and the M (H) curves gradually become linear, which is common in paramagnetic materials. It can be clearly seen from Fig. 7 that an increase in temperature will cause a slight decrease in magnetization. This reduction is mainly due to the magnetic moment disorder caused by thermal stirring.

Fig. 7
figure 7

Isothermal magnetization for La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15, and 0.20) samples measured at different temperatures around TC

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3.3.2 Arrott curves

In order to further understand the nature of the FM-PM phase transition, we derive the Arrott plots (H/M vs. M2) from M(H) plots [46] for the four samples as shown in Fig. 8. Arrott plots are usually used to determine the sequence of magnetic phase transitions. Indeed, as reported by the criterion suggested by Banerjee [47], the negative slope corresponds to the first-order magnetic transition, and the positive slope corresponds to the second-order magnetic transition. It can be clearly seen from Fig. 8 that the Arrott plots show a positive slope, confirming the second-order nature of the phase transition from FM to PM. This approves the results already seen in the discussion section of M (T).

Fig. 8
figure 8

Arrott plots (M2 vs. μ0H/M) of La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15 and 0.20) samples around TC

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3.4 Magnetocaloric effect

Figure 9 shows the temperature dependence of the magnetic entropy changes (− ΔSM) for our four samples when the magnetic field change is at most 5 T. It is found that (− ΔSM) rises with the rising of temperature, thus, reaching the maximum value at TC. The characteristics of the magnetic transition can be concluded from the sign of (− ΔSM) (T, H). For materials presenting a FM transition, we found positive values of (− ΔSM) (T, H) due to the formation of a magnetically ordered configuration when an external magnetic field is applied [48]. As the magnetic field increases, the magnetization and spin alignment increase, followed by the increase in the maximum magnetic entropy change value \(\left(-\Delta {S}_{M}^{max}\right)\) of all compounds. However, due to the secondary nature of the ferromagnetic transition in these compounds, the peak position is hardly affected. It is found that \(\left(-\Delta {S}_{M}^{max}\right)\) is influenced by the Cr doping. In fact, under H = 5 T \(\left(-\Delta {S}_{M}^{max}\right)\) is 3.06 J kg−1 K−1 at 277 K, 2.44 J kg−1 K−1 at 269 K, 2.30 J kg−1 K−1 at 254 K, and 1.92 J kg−1 K−1 at 204 K for x = 0.00, 0.10, 0.15, and 0.20 (Fig. 9).

Fig. 9
figure 9

Magnetic entropy change (− ΔSM) as a function of temperature in various magnetic fields between 1 and 5 T of the compounds La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15, and 0.20)

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The values of our entropy change decrease with Cr3+ substitution increases. We can explain this result by the decreasing number of hopping electrons and the reduction of DE interactions of Mn3+-O-Mn4+. As previously mentioned, such as Cr3+ replaces Mn3 + , the replacement of Mn site by trivalent ions reduces Mn3+/Mn4+ rate and the lattice will be affected due to the presence of Cr3+ in Mn–O–Mn chains. The chromium substitution reduces the DE interaction and the magnitude of the \(\left(-\Delta {S}_{M}^{max}\right)\).

We list the \(\left(-\Delta {S}_{M}^{max}\right)\) and RCP values of all samples in Table 3 and compare them with other results reported in the literature. It is noticed that RCP values increase with rising the applied magnetic field. This can be the result of spin coupling, which is weaker when a high magnetic field is applied.

Table 3 Summary of the maximum magnetic entropy change (− ΔSMmax), δTFWHM and RCP values, under a magnetic field (H) for some magnetocaloric materials
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At 5 T, RCP values are 268 J Kg−1, 243 J Kg−1, 244 J Kg−1, and 190 J Kg−1 for x = 0.00, 0.10, 0.15, and 0.20, respectively. The RCP values of our compounds are about 50% of the RCP value of pure Gd, indicating that our compounds may be candidate materials for magnetic refrigeration.

4 Conclusion

In this work, the effect of partial replacement of Manganese by Chromium on structure, magnetic, and magnetocaloric properties of La0.5Sm0.2Sr0.3Mn1-xCrxO3 (x = 0.00, 0.10, 0.15, and 0.20) compounds is studied. The four prepared compounds are crystallized at room temperature in the orthorhombic structure with Pnma space group. Magnetization measurement results show a second-order paramagnetic–ferromagnetic transition. TC value decreases from 278 K (x = 0.00) to 205 K (x = 0.20). The magnetocaloric properties of these compounds are checked based on the relationship between the isothermal magnetization and the magnetic field. These data are measured at different temperatures around TC. The maximum magnetic entropy change \(\left(-\Delta {S}_{M}^{max}\right)\) of our samples decreases with the increase of added chromium’s rate. The RCP values are also studied and are sufficiently high to make the present compounds’ good candidates for near-room magnetic refrigeration.