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Effect of Sn Doping on Magnetocaloric Effect and Phase Transition Properties of (La, Ce)(Fe, Al, Si)13 Alloys  PDF

  • Song Boyu 1,2,3 ✉
  • Han Yongquan 1,3 ✉
  • Cheng Juan 4
  • Gao Lei 4
  • Liu Cuilan 4
  • Huang Jiaohong 4
1. Inner Mongolia University of Technology, Hohhot 010051, China； 2. Baotou Vocational and Technical College, Baotou 014030, China； 3. Engineering Research Center of Development and Processing Protection of Advanced Light Metals, Ministry of Education, Hohhot 010051, China； 4. State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization, Baotou 014030, China

Updated：2023-07-31

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Abstract

The magnetocaloric effect and phase transition properties of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2, wt%) alloys were investigated. X-ray diffraction results show that with increasing the Sn doping content, the content of 1:13 main phase is decreased, and the content of α-Fe and LaFeSi phases is increased. By combining the density functional theory with experimental results, it is found that increasing the Sn doping content can increase the lattice constant and enhance the exchange interactions between adjacent atoms, thereby increasing the Curie temperature. When the magnetic field is 2 T, the maximum magnetic entropy change in the system is 13.59 J·kg-1·K-1 and its relative cooling power is 154 J/kg, showing great potential as magnetic refrigeration material. Based on the Banerjee criterion and the field dependence index n of isothermal entropy change, it is concluded that when the Sn doping content is 0.05wt%, the first-order phase transition changes to the second-order phase transition of the alloy. The phase transition behavior of the alloy is very sensitive to the Sn content, and multi-stage series refrigeration can be achieved by adjusting the doping content.

Because gas-compression refrigeration suffers from the non-environmentally friendly refrigeration process and low refrigeration efficiency, magnetic refrigeration technology has been proposed based on the magnetocaloric effect (MCE), which has the advantages of energy saving, high efficiency, environment protection, stability, and reliability, showing great potential in the refrigeration industry[

1–3]. In the MCE-based magnetic refrigeration technology, the magnetic moment of the magnetocaloric material remains ordered or disordered along the direction of magnetic field with or without an external magnetic field, so the entropy of the system changes, resulting in the heat absorption and discharge[4–5]. MCE is related to the first-order magnetic transition (FOMT) and the second-order magnetic transition (SOMT). According to Ehrenfest classification, FOMT feature is the discontinuity of the first derivative of free energy related to the thermodynamic parameters, and the discontinuity of the second derivative of free energy corresponds to SOMT feature[6]. Generally, magnetocaloric materials with FOMT characteristics have large MCE, relatively narrow temperature range, and thermal or magnetic hysteresis. Conversely, the magnetocaloric materials with SOMT characteristics exhibit small MCE and relatively wide temperature range without thermal or magnetic hysteresis.

Magnetocaloric materials include Gd[

7], La(Fe, Si)13[8], GdSiGe[9], MnAs[10], Heuslers alloys[11], and FeRh[12]. As a typical second-order magnetic phase transition material, Gd is usually used in the magnetic refrigerators. However, the magnetocaloric properties of Gd are sensitive to the purity. High-purity Gd is expensive, and its Curie temperature is not adjustable, which seriously restricts its application. Among the first-order magnetic phase transition materials, the La(Fe, Si)13 alloy is non-toxic and exhibits large magnetic entropy change and adjustable Curie temperature. In addition, the preparation cost of La(Fe, Si)13 alloy is relatively small, presenting great application potential[13]. MCE and phase transition properties of La(Fe, Si)13 alloy can be greatly changed by adjusting the element proportion or doping elements. For the La(Fe, Si)13 alloy, when Si content≤1.6wt%, the alloy exhibits the first-order phase transition (FOPT) characteristics; when Si content≥2wt%, the alloy exhibits the second-order phase transition (SOPT) characteristics; the NaZn13 cubic structure remains unchanged[14–15]. Lin et al doped Mn into the La0.8Ce0.2-Fe11.4-xMnxSi1.6 alloy. With increasing the Mn content from 0wt% to 0.3wt%, the alloy shows FOPT, the transition temper-ature is decreased from 177 K to 121 K, and the relative cooling power (RCP) remains as a large constant under mag-netic field of 1 T[16]. After partially replacing Fe with Co at-oms, the phase transition changes from FOPT to SOPT when Co content=0.4wt%, and the Curie temperature (TC) shifts to higher temperatures with maintaining the large MCE[17–18]. Nb[19] and Gd[20] have been doped into the La(Fe, Si)13 alloy. With increasing their doping content, the magnetic entropy change is decreased, TC is increased, and the hysteresis loss is eliminated. When Nb content is 4wt% and Gd content is 7wt%, the phase transition changes from FOPT to SOPT. Hu et al[21] prepared La(FexAl1-x)13 alloys and found that when x=0.86, the alloy is soft ferromagnetic, and the phase transition from paramag-netic state to the ferromagnetic state is SOPT. FOPT occurs at x=11.18–11.96. Additionally, the density functional theory (DFT) has been proposed and it is common-ly used to analyze the effect of doping elements in La(Fe, Si)13 alloys. Dai et al[22] used DFT to reconstruct the LaFe11.6Si1.4 alloy doped with B and C, and found that the magnetic-transition behavior is related to the Fe-Fe bond. Moreno-Ramírez et al[23–24] studied MCE and phase transition sequence of the La(Fe, Si)13 alloy doped with Ni and Cr of different contents by DFT and experiments. DFT results are consistent with the experimental ones. Doping Ni and Cr cannot signi-ficantly change the crystal structure and total magnetic mo-ment. Besides, when Ni>0.21wt% and Cr>0.53wt%, the phase transition changes from FOPT to SOPT. Therefore, it is very important to evaluate the effects of different doping elements on MCE and phase transition properties of alloys, providing application guidance for the magnetocaloric materials.

Doping or substituting elements with similar chemical properties is a common method to improve the comprehensive properties of materials[

25]. Sn and Si belong to the same group of elements in the periodic table, thereby presenting similar chemical properties. The Sn doping in Gd5Si2Ge2 magnetic refrigeration materials increases the Curie temperature and expands the refrigeration temperature range while maintaining the magnetocaloric effect[26]. However, the effect of Sn doping in La(Fe, Si)13 alloy is rarely investigated. In this research, Sn was doped in the (La, Ce)(Fe, Al, Si)13 alloy[27] with high magnetic entropy change and refrigeration capacity. DFT cal-culations were conducted to investigate the exchange interac-tion between the magnetic atoms, and the Curie temperature, isothermal magnetic entropy change, and phase transition properties of the alloy were investigated through experiments.

1 Experiment

The industrially pure raw materials (99.5wt% La, 99.5wt% Ce, 99wt% Fe, 99.7wt% Al, 99.9wt% Si, and 99.7wt% Sn) were used. La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2, wt%) alloys were prepared by melting under the protection of argon gas in a high-frequency induction furnace. Each raw metal of 70 g was used for melting (excess 5wt% La and 5wt% Ce were added to compensate for the volatilization of La and Ce during melting). To ensure the uniform composition of alloy specimens, each specimen was turned and melted three times. Then, they were placed in a sintering furnace, heat-treated at 1090 °C for 144 h under argon protection, and water quenched. X-ray diffractometer (XRD, X'Pert PRO diffractometer) was used to analyze the phases of crushed specimens, and XRD patterns were treated by Rietveld refinement through Maud software. The magnetic properties of the specimens of approximately 20 mg were measured by LakeShore7407 vibrating specimen magnetometer. The magnetic entropy (ΔSM) values of the alloys were calculated by Maxwell equation, and the magnetocaloric properties were analyzed.

2 Results and Discussion

2.1 Structural characterization

XRD patterns of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys are shown in Fig.1.

Fig.1  XRD patterns of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys

The La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloy with x=0 is mainly composed of the 1:13 phase and a small amount of α-Fe phase (2θ=45°). With increasing the Sn doping content, the peak of α-Fe phase gradually becomes more obvious, and the corres-ponding peak of LaFeSi also appears. The analysis results based on XRD patterns are shown in Table 1. With increasing the Sn doping content, the content of 1:13 main phase is gradually decreased, and the contents of α-Fe and LaFeSi phases are gradually increased. Additionally, because the Sn radius (0.141 nm) is larger than that of Fe atom (0.117 nm), the lattice constant is monotonically increased with increasing the Sn doping content, indicating that the Sn atoms replace the Fe atoms. The variation in lattice constant depends on the content of doping element in the alloy.

Table 1  Lattice constant, phase proportions, R-weighted pattern factor (Rwp), R-Bragg factor (Rb), R-expected factor (Rexp), and goodness of fit (GOF) of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys after Rietveld refinement
Parameterx=0x=0.05x=0.1x=0.2
Experimental lattice constant, aexp/nm 1.1471 1.1475 1.1477 1.1483
DFT-simulated lattice constant, aDFT/nm 1.1428 1.1550 1.1639 1.1798
Lattice constant error/% 0.37 0.65 1.41 2.74
1:13 phase content/wt% 96.4 90.7 89.7 89.5
α-Fe phase content/wt% 3.6 6.6 7.7 7.9
LaFeSi phase content/wt% 0.0 2.7 2.5 2.6
R-weighted pattern factor, Rwp/% 2.582 2.594 2.604 2.833
R-Bragg factor, Rb/% 1.926 1.955 1.956 2.103
R-expected factor, Rexp/% 1.899 1.995 1.995 1.993
GOF 1.359 1.300 1.305 1.421

2.2 Lattice parameter and Curie temperature

CASTEP module of Materials Studio was used to calculate the electronic structures of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys based on DFT framework by Ceperley, Alder, Perdew, and Zunger exchange-correlation functions[

28–31] with the local density approximation and plane wave cutoff energy of 335 eV[32] in 7×7×7 k-point mesh. The self-consistent field precision is less than 1×10-7 eV/atom, the atomic interaction force is less than 0.05 eV/nm, and all calculations include the collinear spin polarization. Cell construction was based on the ideal cell of NaZn13-type La(Fe, Si)13 (space group Fm3c), where the La atom occupies the Wyckoff position at 8a(1/4, 1/4, 1/4) sites, the FeI atom is located at 8b(0, 0, 0) sites, and the FeII or Si atom is located at 96i(0, y, z) sites. The lattice constant of intrinsic LaFe11.5Si1.5 alloy is 1.1411 nm, and the error is only 0.46%, compared with the experimental value[33]. Construction of Sn-doped structures was then performed. Based on the assumption that the doped Sn atom in La(Fe, Si)13 alloy is at the 96i or 8b atomic position, the separate energy calculations were conducted. It is found that the Sn atom prefers to occupy the 96i position. According to Ref.[34], the preferred position of Al atom in the cell is 96i. Therefore, not only FeII and Si atoms, but also Al and Sn atoms exist at the 96i position. The proportion of different atoms at 96i position can be calculated by dividing the total mass ratio of an element by its relative atomic mass ratio. For example, the LaFe11.5Si1.5 alloy is composed of cells of 87.5wt% FeII and 12.5wt% Si. The lattice constants of LaFe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) simulated cells corresponding to the La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys were then calculated. The lattice constants of the alloys with different Sn contents obtained by Rietveld refinement and DFT calculation are shown in Fig.2. The change trends of lattice constant from DFT calculation and Rietveld refinement are in good agreement. With increasing the Sn content, the lattice constant calculated by DFT is increased, which leads to the narrowing of 3d band, the increase in interactions between the ferromagnetism, and the increase in Curie temperature[35].

Fig.2  Lattice constants of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys obtained by Rietveld refinement and LaFe11.5-xAl0.2Si1.3Snx alloys obtained by DFT calculations

It is widely known that the variation of Curie temperature not only influences the lattice constant, but also changes the magnetic interaction caused by expansion or contraction of the bonds between the atoms in crystal structure[

36–37]. The bond lengths of Fe-Fe atom pairs at five positions of the unit cell are shown in Fig.3, where B1 is the FeI-FeII/Si/Al/(Sn) atom pair and B2–B5 are FeII/Si/Al/(Sn)-FeII/Si/Al/(Sn) atom pairs. The weighted average value of the Fe-Fe bond lengths represents the average Fe-Fe bond length[38]. If the elongation of the average Fe-Fe bond length is less than 0.255 nm or the shortening of average Fe-Fe bond length is greater than 0.255 nm, the exchange interaction will be strengthened and the exchange integral JFe-Fe will be increased. Conversely, if the elongation of average Fe-Fe bond length is greater than 0.255 nm or the shortening of average Fe-Fe bond length is less than 0.255 nm, the exchange interaction will be weakened and the

Fig.3  Fe-Fe bonds at different positions in LaFe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys

exchange integral JFe-Fe will be decreased. The competition between the two opposite effects determines the variation of total JFe-Fe[

39]. The measured results are shown in Fig.4. With increasing the Sn doping content, the B1 and B5 bonds elongate by less than 0.255 nm, the B3 and B4 bonds elongate by greater than 0.255 nm, and the B2 bond contracts by greater than 0.255 nm. In addition, the variation of B1–B5 bonds is 0.0045, -0.0001, 0.006, 0.0157, and 0.0061 nm, respectively. Therefore, the average Fe-Fe bond length increases, which can enhance the exchange interaction between adjacent atoms. Besides, it is inferred that the total JFe-Fe is increased with increasing the Sn doping content. With larger total JFe-Fe, the bonds are easily parallel to each other after spin, which normally requires more thermal energy to disrupt the regular arrangement within the magnet, thereby increasing the Curie temperature.

Fig.4  Relationship between Fe-Fe average bond length and Sn doping content of LaFe11.5-xAl0.2Si1.3Snx alloys

2.3 Magnetocaloric properties and phase transition char-acteristics

The heating (cooling) curves of magnetic moment (M)-temperature (T) of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys under applied magnetic field of 0.05 T are shown in Fig.5.

Fig.5  M-T curves of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys during heating (cooling) at magnetic field of 0.05 T

The M-T curve of the alloy with x=0 does not overlap with the front end of the M-T curve of cooling, and the thermal hysteresis occurs. Conversely, the M-T curves for the Sn-doped alloys overlap with those of cooling process, and almost no thermal hysteresis occurs. The Curie temperature is the critical temperature for the transition between the ferromagnetic and paramagnetic states, and it is defined as the minimum value from the relationship plot between the first derivative of magnetization and T. At x=0, 0.05, 0.1, and 0.2, the Curie temperature is 177, 184, 190, and 204 K, respectively, indicating that the Curie temperature is increased with increasing the Sn doping content. This result agrees well with the trend predicted by DFT calculations and the rule of periodic table: the elemental transition temperature is lower for the left-side elements of Fe (anti-ferromagnetic coupling) and higher for the right-side elements of Fe (ferromagnetic coupling)[

40].

To measure the magnetocaloric properties of La0.75Ce0.25-Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys, the isothermal magnetic entropy change (∆SM) is calculated, as follows:

ΔSM=∫0H(∂M∂T)HdH (1)

where H is the applied magnetic field, M is the magnetization, and T is the temperature.

The isothermal magnetic entropy change curves of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys under magnetic field of 2 T are shown in Fig.6. When the Sn doping content changes from 0wt% to 0.05wt%, ∆SM is decreased from 13.59 J/(kg·K) to 6.08 J/(kg·K), the corresponding Curie temperature shifts towards the high-temperature region, and the curve width is significantly increased. With 0.1wt% Sn doping, the Curie temperature remains constant and ΔSM slightly increases to 7.38 J/(kg·K). With 0.2wt% Sn doping, the Curie temperature increases to 204 K, the full width at half maximum (δTFWHM) of curve significantly increases, but ΔSM decreases to 3.69 J/(kg·K).

Fig.6  Relationship between isothermal magnetic entropy change and temperature of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys under magnetic field of 2 T

In addition, RCP of the La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys is calculated by Eq.(2) [

41], as follows:

RCP=-ΔSMδTFWHM (2)

where RCP represents the heat transfer by the magnetic refrigerant between the hot and cold ends during the magnetic refrigeration cycle; ΔSM is the peak value of isothermal magnetic entropy change; δTFWHM is calculated by integrating the M-T curve under a certain magnetic field. The relative cooling capacities of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys with x=0, 0.05, 0.1, and 0.2 are 154, 96, 119, and 123 J/kg, respectively. The MCE properties of La0.75Ce0.25Fe11.5-xAl0.2Si1.3-Snx alloys are shown in Table 2, and they are compared with the results from different reports. RCP value of the La0.75Ce0.25Fe11.3Al0.2Si1.3Sn0.2 alloy with low MCE and large δTFWHM reaches 80% of that of the La0.75Ce0.25Fe11.5Al0.2Si1.3 alloy with high MCE and small δTFWHM. In addition, the Curie temperature of the La0.75Ce0.25Fe11.5Al0.2Si1.3Sn0.2 alloy increases by 15%, the δTFWHM increases by nearly three times, and the thermal hysteresis decreases.

Table 2  Comparison of isothermal magnetic entropy change, Curie temperature, thermal hysteresis, δTFWHM, and RCP of different alloys
Alloy

Isothermal magnetic

entropy change,

ΔSM/J·kg-1·K-1

Curie

temperature,

TC/K

Thermal

hysteresis,

ΔThyst/K

Full width at

half maximum,

δTFWHM/K

Relative cooling

power,

RCP/J·kg-1

Ref.
La0.75Ce0.25Fe11.5Al0.2Si1.3 13.6 177 1 11.32 154 -
La0.75Ce0.25Fe11.45Al0.2Si1.3Sn0.05 6.08 184 0 15.74 96 -
La0.75Ce0.25Fe11.4Al0.2Si1.3Sn0.1 7.38 190 0 16.18 119 -

La0.75Ce0.25Fe11.3Al0.2Si1.3Sn0.2

La0.95Gd0.05Fe11.5Si1.5

3.69

15.3

204

196

0

-

33.29

9.2

123

140.2

-

[

20]

La0.85Gd0.15Fe11.5Si1.5 9.5 198 - 11.6 110.2 [20]
La1.3Fe11.5Si1.4Ga0.1 16.36 188 1.41 - 178.4 [42]

La1.3Fe11.3Si1.4Ga0.3

LaFe11.4Si1.6

5

14.3

200

208

0.71

-

-

-

125.3

70

[

42]

[

43]

The isothermal magnetization curves of La0.75Ce0.25Fe11.5-x-Al0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys at different temper-atures with temperature interval of 2 K are shown in Fig.7.

Fig.7  Isothermal magnetization curves of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys at different temperatures with temperature interval of 2 K: (a) x=0, (b) x=0.05, (c) x=0.1, and (d) x=0.2

The magnetization of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys below the Curie temperature rapidly reaches saturation with increasing applied magnetic field, showing typical ferromagnetic-state characteristics. With increasing the temperature, the magnetization of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys slowly is increased, presenting the transition from ferromagnetic state to paramagnetic state. An obvious S-shaped curve can be observed at x=0 (Fig.7a), indicating that this alloy has a first-order itinerant electron variable magnetic transition. At x=0.05, 0.1, and 0.2, the main part of isothermal magnetization curves is basically a straight line, and no first-order itinerant electron variable magnetic transition can be observed, as shown in Fig.7b–7d.

The Belov-Arrott plots of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys at the corresponding transition temperatures near Curie temperature are shown in Fig.8. The magnetic field is represented by μ0H.

Fig.8  Belov-Arrott plots of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx (x=0, 0.05, 0.1, 0.2) alloys at corresponding transition temperatures around Curie temperature

According to the Banerjee criterion, the magnetic phase transition of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloy can be determined by the slope of M2-μ0H/M curve[

44]. If the curve has an inflection point near the Curie temperature or the slope of the curve is negative, it is suggested that the phase transition is FOPT, and vice versa. The Belov-Arrott plot at x=0 has one inflection point, indicating that this alloy exhibits a strong primary phase transition near the Curie temperature. Additionally, the Belov-Arrott plots of other alloys show positive slopes, indicating that the variable magnetic-transition behavior of the alloys weakens near the Curie temperature and the secondary phase transition occurs. Therefore, it is concluded that Sn element can induce the secondary phase transitions in the (La, Ce)(Fe, Al, Si)13 alloys with significantly low doping contents, compared with Co (0.4wt%)[18], Ni (0.21wt%)[23], Cr (0.53wt%)[24], and Ga (0.3wt%)[42].

Because the Banerjee criterion is based on the assumptions of mean-field model, there are certain restrictions. Therefore, the Law method was proposed to confirm the phase transformation through the field dependence of MCE, and the phase-transformation order of the Sn-doped alloys can be obtained[

45]. This approach does not rely on the fitting data of any particular magnetization model or any specific equation of state, and it allows a more objective judgment of phase changes, which has been applied to La(Fe,Si)13 [23], Heusler alloys[46], and Ni-Mn-Ga/Al[47] alloys[48].

The relationship between the isothermal magnetic entropy change and the magnetic field obeys the power law of the field, as follows:

ΔSM∝Hn (3)

where the exponent n represents the function depending on the temperature and field.

n=dlnΔSMdlnH (4)

When n>2, the phase transition is FOPT. Otherwise, the phase transition is SOPT. The relationship between μ0H and T is shown in Fig.9, where red represents the region with n>2, green represents the region with n=[1, 2], and blue represents the region with n=[0, 1). At x=0, there is a red region, indicating that the n value of La0.75Ce0.25Fe11.5Al0.2Si1.3 alloy is above 2 and the alloy undergoes FOPT.

Fig.9  Variation of exponent n of La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys under different magnetic fields and temperatures: (a) x=0, (b) x=0.05, (c) x=0.1, and (d) x=0.2

With increasing the Sn content from 0.05wt% to 0.2wt%, no red areas (overshoot zones) can be observed (Fig.9b–9d), indicating that only SOPT occurs in these alloys. According to the Banerjee criterion and the criterion of phase-transition sequence, when the Sn doping content is 0.05wt%, the alloy changes from FOPT to SOPT. Therefore, the doping content of Sn element has a significant effect on the phase transition properties of (La, Ce)(Fe, Al, Si)13 alloys. By adjusting the doping content of Sn element, the phase transformation and magnetocaloric properties of the alloy can be adjusted to achieve multi-stage series refrigeration.

3 Conclusions

1) According to the density functional theory calculations, with increasing the Sn doping content, the lattice constant and exchange integral between the Fe-Fe bonds are increased, resulting in the increase in Curie temperature. The simulation results are in good agreement with the experimental ones. Through the traditional Banerjee criterion and the exponent n, it is concluded that the second-order phase transition (SOPT) occurs when the Sn doping content is 0.05wt%. The phase transition behavior of the La0.75Ce0.25Fe11.5-xAl0.2Si1.3Snx alloys is very sensitive to the composition change caused by Sn addition, and the required Sn doping content for SOPT occurrence is significantly lower than the doping content of other elements.

2) Sn addition can increase the Curie temperature, widen the magnetic refrigeration temperature range, and reduce the thermal hysteresis. By adjusting the doping content of Sn element, the phase transformation and magnetocaloric properties of the alloy can be adjusted to achieve multi-stage series refrigeration.

3) For the La0.75Ce0.25Fe11.5Al0.2Si1.3 alloy, the Curie temperature is 177 K, the magnetic entropy change is

13.6 J/(kg·K), the thermal hysteresis is 1 K, and the relative cooling capacity is 154 J/kg under magnetic field of 2 T. Thus, the La0.75Ce0.25Fe11.5Al0.2Si1.3 alloy shows great potential as magnetic refrigeration materials at specific temperatures.

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