Compared with the traditional gas compression/expansion refrigeration technology, the magnetic refrigeration (MR) technology based on the magnetocaloric effect (MCE) possesses advantages, such as high efficiency and energy saving, being green, stable, and reliable. The basic mechanism of the MCE is magneto-thermodynamic phenomenon of a magnetic material, which is evaluated from the isothermal magnetic entropy change ( and the adiabatic temperature change ( in a changing magnetic field.^[1] The researches of magnetic refrigeration materials mainly concentrate in the areas of room temperature and low temperature, such as LaCaMnO₃,^[2] Gd₅Si₂Ge₂,^[3] MnFeX (X = P, As, Si, Ge),^[4] and Ni–Mn–Ga,^[5,6] RCo₂,^[7,8] RNi₂ (R = Gd, Dy, Ho, Er),^[9–11] RCoAl (R = Tb, Dy, Ho),^[12,13] etc.

The low-temperature magnetocaloric materials are very important in the field of hydrogen liquefaction and air science. In the past few years, the rare earth intermetallic compounds have become a main research subject. Recently, in the rare earth oxides, the intermetallic oxide has become a new research hotspot such as DyMnO₃,^[14] Gd₂NiMnO₆,^[15] HoMn₂O₅,^[16] etc. The perovskite oxide RMO₃ (R = rare earth; M = transition metal) system has a strong coupling between spin, lattice, orbital and charge degrees of freedom. It shows unique optical, electrical, magnetic and excellent magnetoresistance effect, catalytic performance, and magnetocaloric effect.

The EuTiO₃ exhibits G-type AFM order, and the modulated magnetic ground state with antiferromagnetic (AFM) and ferromagnetic (FM) phase co-existence has been reported in EuTiO₃, based on the first-principles calculation.^[17,18] There are two corresponding mechanisms here. One is the super-exchange mechanism that leads to antiferromagnetic (AFM) exchange, and the other is the indirect exchange mechanism that leads to ferromagnetic (FM) exchange. What is more, the EuTiO₃ has strong spin lattice coupling. The competition between AFM and FM phases is a delicate balance, so the magnetic state can be easily switched from AFM to FM, such as a increasing with the lattice constant relaxed or c increasing, with a fixed.^[19] The EuTiO₃ has taken the center stage of research due to the observations of the giant magnetocaloric (GMC) and giant magneto-electric (GME) effects around its AFM ordering temperature of Eu spins ( K).^[20–24] It has been shown that a small number of (Ba and Sr) substituteions at the Eu site or (Cr and Nb) substitution at the Ti site drive the system into an FM metallic state. Rubi reported the magnetoelectric effect in Eu_1−xBa_xTiO₃ for .^[20] Mo et al. reported the giant magnetocaloric effects in Eu_1−xSr_xTiO₃ and EuTi_1−xCr_xO₃.^[21,22] Roy et al. reported the in magnetoelectric EuTi_0.85Nb_0.15O₃ compound.^[23]

In the present paper, we show the great enhancements of both and RC upon applying moderate field changes (1 T) to Eu_0.9R_0.1TiO₃ (R = La, Ce) compounds with respect to EuTiO₃. The values of and RC are calculated to be 10.8 J/kgK and 51.8 J/kg for Eu_0.9Ce_0.1TiO₃ and 11 J/kg Kand 39.3 J/kg for Eu_0.9La_0.1TiO₃ at magnetic field change of 10 kOe, respectively The reversible giant low-field MCE and RC make them an excellent candidate for the applications in a cooling device

The Eu_0.9R_0.1TiO₃ (R = La, Ce) samples were prepared by the sol-gel method. Stoichiometric mixtures of europium oxide (Eu₂O₃), butyl phthalate (Ti(OCH and ceria (CeO₂) or lanthanum oxide (La₂O₃), were dissolved in 25 mL of nitric acid (HNO₃ 8 mol/L). Then ethylene glycol (C₂H₆O₂) was added as a dispersant, and stirred for an hour. Next, the solution was heated at 90 °C until a dry gel was obtained. Finally, the samples were pretreated at 900 °C for two hours in air to obtain Eu₂Ti₂O₇ and (R = La, Ce), and these materials were annealed at 1100 °C in 10% H₂ and 90% Ar atmosphere for 3 h.

The structures of the Eu_0.9R_0.1TiO₃ (R = La, Ce) were determined by x-ray diffraction (XRD) with Cu Kα radiation. The magnetic properties of the samples were measured by physical property measurement system (PPMS).

Figure 1(a) shows the XRD patterns of EuTiO₃ and Eu_0.9R_0.1TiO₃(R = La, Ce) single crystal at 295 K. They crystallize into the cubic perovskite structure with Pm3m space group. The lattice parameters are determined respectively to be a = 3.908(2) Å for EuTiO₃, a = 3.910(4) Å for Eu_0.9Ce_0.1TiO₃ and a = 3.911(5) Å for Eu_0.9La_0.1TiO₃ with and by the Rietveld technique using GSAS program shown in Table 1. In oxides, oxygen defects are almost inevitable, which approve to be intrinsic defects, and believed to have a critical influence on their properties.^[25] The oxygen content in Eu_0.9R_0.1TiO₃ (R = La, Ce) samples is determined by the thermogravimetric analysis (TGA). The perovskite EuTiO₃ is obtained by pyrochlore Eu₂Ti₂O₇ under the hydrogen reduction reaction: the reduction of Eu³⁺ to Eu²⁺. Hence, the Eu_0.9R_0.1TiO₃ (R = La, Ce) can oxidize Eu₂Ti₂O₇ and Ti₂O₇(R = La, Ce) through the following reaction:

The oxygen nonstoichiometric parameter δ of perovskite phase can be calculated from the weight gain of the sample during oxidation on heating in air from the following formula:

Fig. 1.

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	Fig. 1. (color online) (a) XRD patterns of the EuTiO3 and Eu0.9R0.1TiO3 (R = La, Ce). (b) Thermogravimetry (TGA) trace of Eu0.9Ce0.1TiO3 and Eu0.9La0.1TiO3.

Table 1.

Table 1.

Table 1. Values of and RC under the field changes of 10 kOe and 20 kOe for EuTiO3, Eu0.9Ce0.1TiO3, and Eu0.9La0.1TiO3. . Samples Lattice parameters T T /Jkg−1K−1 RC/Jkg−1 /Jkg−1K−1 RC/Jkg−1 EuTiO3 3.908 9.8 27 21.1 98 Eu0.9Ce0.1TiO3 3.910 10.8 51.8 18.8 114.6 Eu0.9La0.1TiO3 3.911 11.0 39.3 19.6 103.4

Table 1. Values of and RC under the field changes of 10 kOe and 20 kOe for EuTiO3, Eu0.9Ce0.1TiO3, and Eu0.9La0.1TiO3. .

Figure 1(b) shows the TGA results of the EuTiO₃ and Eu_0.9R_0.1TiO₃ (R = La, Ce) samples, obtained by measuring the weight gain in a temperature range from room temperature to 700 K in air at a rate of 5 K/min. The δ values for EuTiO₃, Eu_0.9La_0.1TiO₃, and Eu_0.9Ce_0.1TiO₃ are calculated to be −0.006, 0.012, and 0.027, from the weight gain of , and , respectively. The negative sign suggests a slight vacancy of oxygen in EuTiO₃ and the positive sign refers to a slight excess of oxygen in the Eu_0.9R_0.1TiO₃ (R = La, Ce), so the compositions can be written as Eu_0.9La_0.1TiO_3.012 and Eu_0.9Ce_0.1TiO_3.027. The small oxygen excess may be caused by cationic deficiency at the Eu sites according to valence state compensation, but they are smaller than the theoretical values (Eu_0.9La_0.1TiO_3.05 and Eu_0.9Ce_0.1TiO_3.05). It indicates that there are oxygen defects in the Eu_0.9R_0.1TiO₃ (R = La, Ce).

Figure 2(a) shows the zero-field cooling (ZFC) and field cooling (FC) of the Eu_0.9R_0.1TiO₃ (R = La, Ce) compounds under an applied magnetic field of 0.1 kOe. The EuTiO₃ exhibits a significant AFM order, and phase transition from AFM to PM at with no thermal hysteresis. However, the M–T curves for Eu_0.9La_0.1TiO₃ and Eu_0.9Ce_0.1TiO₃ exhibit FM order and the ferromagnetic Curie temperature increases from and 5.5 K, respectively. They may be because the increase of the lattice constant leads to the magnetic transition from AFM to FM as Ce(x = 0.1) and La(x = 0.1) substitute for Eu. The microscopic origin of the strong spin-lattice coupling in EuTiO₃ is derived from the superexchange mechanism between Eu²⁺ 4f spins via the 3d states of nonmagnetic Ti⁴⁺ ions along the a, b-plane diagonal, and the AFM phase also comes from a superexchange mechanism.^[26,27] Therefore, the change of lattice constant affects the superexchange mechanism, thereby changing the magnetic state. The ZFC and FC curves for Eu_0.9La_0.1TiO₃ and Eu_0.9Ce_0.1TiO₃ exhibit completely reversible behaviors as observed usually in the second-order phase transition, but an obvious bifurcation appears below the transition temperature. The spin-glass systems, ferromagnetic materials with high anisotropy, and materials with competing magnetic interactions all can lead to thermomagnetic irreversibility.^[28] Figure 2(b) shows the temperature dependence of inverse susceptibility (1/χ vs. T) and the Curie–Weiss fitting of the 1/χ (T). The values of effective magnetic moment ( 6.89 for EuTiO₃ for Eu_0.9Ce_0.1TiO₃, and for Eu_0.9La_0.1TiO₃ can be obtain based on the , which belongs to a reasonable region (.^[29]

Fig. 2.

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	Fig. 2. (color online) (a) Temperature dependence of ZFC and FC magnetizations of the EuTiO3 and Eu0.9R0.1TiO3 (R = La, Ce) under a magnetic field of 0.1 kOe; (b) the temperature dependence of inverse susceptibility fitting to the Curie–Weiss law.

Figure 3 shows the isothermal magnetization curves at temperatures between T = 2 K and T = 28 K under the applied magnetic fields ranging from 0 to 50 kOe. For EuTiO₃, the isothermal magnetization curve increases linearly at low magnetic field and tends to be saturate with increasing magnetic field below Nair temperature ( as shown in Fig. 3(a). In the low fields, the magnetization value at low temperature is smaller than at the higher temperature as shown in Fig. 4(a₁). These are typical AFM natures. With the La³⁺ and Ce³⁺ ions instead of Eu²⁺ ions, the data of magnetization isotherms increase rapidly at low magnetic fields and tend to be saturate under strong magnetic fields below as shown in Figs. 3(b) and 3(c). The magnetization curves do not intersect in Figs. 4(a₂)–4(a₃). These are typical FM natures, which suggest magnetic transition from AFM to FM for the substitution of Eu for Ce and La. Figure 3(d) shows the increase and decrease model of isothermal magnetization curve at 2 K, indicating no magnetic hysteresis. The magnetic moments of per Eu atom, respectively, are for EuTiO₃, for Eu_0.9Ce_0.1TiO₃, and for Eu_0.9La_0.1TiO₃ at 5 T: these values are smaller than the expected values for Eu²⁺ ion, which is likely to be attributed to the crystalline electric field.

Fig. 3.

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	Fig. 3. (color online) (a)–(c) Magnetization isotherms of the EuTiO3 and Eu0.9R0.1TiO3 (R = La, Ce) collected in a temperature range of 2–28 K; (d) field dependence of magnetization data for increasing and decreasing field model at 2 K.

Fig. 4.

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	Fig. 4. (color online) (a) Magnetization isotherms of the EuTiO3 and Eu0.9R0.1TiO3 (R = La, Ce) collected at 2, 3, 4, and 5 K under low field; (b) Arrott plot of the EuTiO3 and Eu0.9R0.1TiO3 (R = La, Ce) at 2, 3, 4, 5, and 6 K under low field.

Based on the Banerjee criterion,^[30] when the slope of the Arrott plot is negative, the magnetic transition is expected to be of the first order; on the contrary, it will be of the second order. Figure 4(b) shows the curves of for . The negative slope of EuTiO₃ implies the first order below . However, the Arrott plots of Eu_0.9La_0.1TiO₃ and Eu_0.9Ce_0.1TiO₃ suggest that the phase transition is of the second order for the positive slope.

The values of can be calculated by the magnetization isotherms data from the Maxwell relation .^[31] Figure 5(a) shows the temperature dependence of the for EuTiO₃ and Eu_0.9R_0.1TiO₃ (R = La, Ce) under magnetic fields. The values of increase with applied magnetic field increasing. The values of are 40.7 J/(kgK) for EuTiO₃ 33.8 and 34.4 J/(kgK) for Eu_0.9R_0.1TiO₃ (R = La, Ce) under magnetic field change 0–50 kOe, respectively. The decrease of is attributed to the decreasing of the Eu²⁺ with doping La (x = 0.1) and Ce (x = 0.1) respectively. In order to further confirm the magnetic transition after Eu has been substituted by La (x = 0.1) and Ce (x = 0.1), the values of are calculated under low magnetic field change as shown in Fig. 5(b). The negative values of for EuTiO₃ compound suggest the AFM order below . However, when Eu is substituted by La (x = 0.1) and Ce (x = 0.1), the positive values of below transition temperature indicate the AFM–FM magnetic transition.

Fig. 5.

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	Fig. 5. (color online) (a) Temperature dependence of for the EuTiO3 and Eu0.9R0.1TiO3 (R = La, Ce) under different magnetic fields; (b) under low magnetic field.

Figure 6 shows the values of for EuTiO₃, Eu_0.9La_0.1TiO₃, and Eu_0.9Ce_0.1TiO₃ changing with temperature under magnetic field changes of 10 kOe and 20 kOe. Under the magnetic field change of 10 kOe, the values of are 9.8 J/(kgK), 10.8 J/(kgK), and 11 J/(kgK) for EuTiO₃, Eu_0.9La_0.1TiO₃ and Eu_0.9Ce_0.1TiO₃, respectively. The values of increase below the transition temperature with doping La (x = 0.1) and Ce (x = 0.1), which may be caused by the magnetic transition AFM–FM. Another key parameter is .^[31] Table 1 shows the RC values of EuTiO₃, Eu_0.9La_0.1TiO₃, and Eu_0.9Ce_0.1TiO₃, which are calculated under 10 kOe and 20 kOe of the magnetic field. The values of RC are evaluated to be 51.8 J/kg and 114.6 J/kg for Eu_0.9Ce_0.1TiO₃ and 39.3 J/kg and 103.4 J/kg for Eu_0.9La_0.1TiO₃, respectively under 10 kOe and 20 kOe of the magnetic field. Compared with the RC value of EuTiO₃ the RC is significantly increased with La (x = 0.1) and Ce (x = 0.1). The MCE and RC values of the Eu_0.9R_0.1TiO₃ (R = La, Ce) compounds are improved, which are an excellent candidate for cryogenic magnetic refrigeration. We hope that our study will further promote the research on magnetocaloric and relevant cooling devices.

Fig. 6.

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	Fig. 6. (color online) Temperature dependence of for EuTiO3 and Eu0.9R0.1TiO3 (R = La, Ce) under magnetic fields of (a) 10 kOe and (b) 20 kOe.

Giant reversible magnetocaloric effects and magnetic properties in Eu_0.9La_0.1TiO₃ and Eu_0.9Ce_0.1TiO₃ are investigated. When part of Eu²⁺ ions are substituted by La³⁺ and Ce³⁺ ions, there is a slight excess of oxygen in the Eu_0.9R_0.1TiO₃ (R = La, Ce), which could compensate for the mismatch of chemical valence and increase the lattice constant. It leads to the magnetic ground state switched from AFM to FM. The large low-field enhancements of and RC are 10.8 J/(kgK) and 51.8 J/kg for Eu_0.9La_0.1TiO₃ and 11 J/(kgK), and 39.3 J/kg for Eu_0.9Ce_0.1TiO₃ for the case of magnetic field change of 10 kOe while pure EuTiO₃ counterpart has J/(kgK) and RC = 27 J/kg. The increases of and RC can be attributed to magnetic phase transition. The giant reversible MCE and large RC suggest that the compounds could be promising materials in low temperature and low magnetic field refrigerants. Our study shows that the MCE and RC are improved, which will promote further research on magnetocaloric and relevant cooling devices.