In recent years, the development of magnetic refrigeration technology based on the magnetocaloric effect (MCE) has provided a new choice for traditional gas refrigeration with great significance in saving energy and environment protection.^[1] In varying magnetic fields, the MCE is the magneto–thermodynamic phenomenon of a magnetic material evaluated from the isothermal magnetic entropy change (−ΔS_M) and the adiabatic temperature change (ΔT). A number of materials exhibiting giant −ΔS_M have been found near transition temperatures, such as LaCaMnO₃,^[2] MnAs_{1 - x}Sb_x,^[3] MnFeX (X = P, As, Si, Ge),^[4] La (Fe, M)₁₃ (M = Si, Al), Ni–Mn–Ga alloy new materials,^[5,6] and so on. In addition, it is also valuable to study at low temperatures in the fields of hydrogen liquefaction and air science.

In the past few years, the research of large MCE materials at low temperatures mainly focused on the rare earth intermetallic compounds and the classical materials: RAl₂,^[7–10] RNi (R = Gd, Ho, Er),^[11] RMAl (M = Ni, Co, Cu),^[12,13] etc. Currently, oxide has become a new research hotspot. The spin degree of freedom, the orbital degree of freedom, and the coupling and interaction between lattice degrees of freedom in the perovskite RTiO₃ (R = rare earth) system, make the RTiO₃ system show many peculiar physical phenomena. In EuTiO₃, Ti is tetravalent (3d⁰) and Eu is divalent with a large spin moment (S = 7/2) due to the stable 4f⁷ electronic configuration. The cubic perovskite EuTiO₃ has aroused widespread concern with the G-type AFM order, large magneto–electric (ME) effect and the quantum paraelectric (PE) behavior.^[14,15] In EuTiO₃, there may be two exchange mechanisms based on the first-principles calculation of the nearest neighbor Eu interaction. One is the super-exchange mechanism that leads to antiferromagnetic (AFM) exchange, which is between Eu²⁺ 4f spins via the 3d states of nonmagnetic Ti⁴⁺ ions. The other is the indirect exchange mechanism, which leads to ferromagnetic (FM) exchange via the Eu 5d states.^[16,17] The competition of AFM and FM phases is a delicate balance. The magnetic ground state can be switched from AFM to FM due to its strong spin lattice coupling in the EuTiO₃, when the lattice constants were changed such as a increased with the lattice constant relaxed or c increased with a fixed.^[18] The delicate change may cause an important effect on the magnetic entropy. It has been shown that a small amount of (Ba and Sr) substitution at the Eusite or (Cr and Nb) substitution at the Ti site drives the system into a FM metallic state. For example, Rubi reported the magnetic entropy change in magnetoelectric Eu_1−xBa_xTiO₃ for 0.1 < x < 0.9.^[19] Mo et al. reported the giant magnetocaloric effect in Eu_{1 - x}Sr_xTiO₃ and EuTi_1−xCr_xO₃.^[20,21] Roy et al. reported the −ΔS_M in magnetoelectric EuTi_0.85Nb_0.15O₃ compound.^[22]

In this paper, with Ti to be substituted by Co, EuTi_1−xCo_xO₃ compounds exhibit a giant reversible MCE. At the same time, it is possible that Co with a certain magnetic moment will impact the nature of the sample. The results show that EuTi_1−xCo_xO₃ processes an effective application prospect at low temperature magnetic refrigeration.

The samples of EuTi_{1 - x}Co_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) were synthesized by the sol–gel method. In the first step, a stoichiometric amount of europium oxide (Eu₂O₃), butyl phthalate (Ti(OC₄H₉)₄) and cobalt nitrate (Co(NO₃)₂), were dissolved into 25 mL of nitric acid (HNO₃ 8 mol/L). Then, the ethylene glycol was added as a dispersant and then continually stirred for about 1 h to make it completely dissolved. In the second step, the solution was heated at 90 °C until a dry gel was obtained. In the third step, the samples were pretreated at 400 °C for 30 min, then cooled down and ground into powder. In the fourth step, the samples were heated 900 °C for 2 h in air to remove carbon. Finally, the samples were annealed at 1100 °C in 10% H₂ and 90% Ar atmosphere for 3 h to obtain EuTi_{1 - x}Co_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) materials.

The structures of the EuTi_{1 - x}Co_xO₃ were determined by x-ray diffraction (XRD) with Cu Kα radiation at room temperature, and then the magnetic properties of the samples were analyzed by a physical property measurement system (PPMS).

Figure 1 shows the XRD patterns of the EuTi_1−xCo_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) samples. No other impurities can be found such as Eu₂Ti₂O₇, Eu₂O₃, and TiO₂ and crystallizing in the cubic perovskite structure (space group 221). With the increase of Co content, the diffraction peaks slightly move to the left due to the size of Co²⁺ (∼ 0.745 Å) which is bigger than Ti⁴⁺ (∼ 0.605 Å) as shown in the inset of Fig. 1. Eu³⁺ or oxygen vacancy is easliy formed due to mismatch of valence (Co²⁺ ion and Ti⁴⁺ ion). The oxygen vacancy is favorable for the magnetic transition.^[23]

Fig. 1.

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	Fig. 1. (color online) The XRD patterns of the EuTi1 - xCoxO3 (x = 0.1 (a), 0.075 (b), 0.05 (c), 0.025 (d), 0 (e)). Inset: the corresponding local amplification figure.

Figure 2 shows the function of magnetization versus temperature in the cases of zero-field cooling (ZFC) and field cooling (FC) of the EuTi_{1 - x}Co_xO₃ compounds under an applied magnetic field of 0.1 kOe. The EuTiO₃ is clearly AFM type and has phase transition from AFM to PM at T_N = 5.7 K with no thermal hysteresis. The T_N = 5.7 K (AFM transition temperature) is similar to the previous results.^[24,25] With the increase of Co doping content (x = 0.025, x = 0.05, x = 0.075, and x = 0.1), the ZFC and FC curves of the samples have an obvious bifurcation below the transition temperature, which may be the nature of spin-glass-like behaviors.

Fig. 2.

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	Fig. 2. (color online) Temperature dependences of ZFC and FC magnetizations of EuTi1−xCoxO3 (x = 0.1 (a), 0.075 (b), 0.05 (c), 0.025 (d), 0 (e)) under the magnetic field of 0.1 kOe.

In Fig. 3, the magnetization data of the magnetic field increases coincide with the decreases at 2 K, indicating no magnetic hysteresis; there is a beneficial effect on the magnetic entropy and cooling capacity of the material without magnetic hysteresis. The inset of Fig. 3 shows the magnetic moment calculated per Eu²⁺ atom in EuTi_{1 - x}Co_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) compounds for 50 kOe at 2 K. With doping Co²⁺ ions, the magnetic moment decreases from 6.1μ_B (x = 0) to 5.5μ_B (x = 0.025) and from 6.0μ_B (x = 0.05) to 5.6μ_B (x = 0.075, 0.1). It is possible that some Eu²⁺ ions were converted to Eu³⁺ ions (without magnetic moments), leading to the decrease of total magnetic moments as the Co²⁺ ion content increases. On the other hand, the Eu²⁺ ions did not convert to Eu³⁺ ions, but produced the oxygen vacancies. However, Co²⁺ ions have a certain magnetic moment, the exchange may decrease the total magnetic moment which comes from Eu²⁺4f spins and the Co²⁺3d spins (further research is needed to confirm the exchange mechanism between Eu²⁺4f spins and the Co²⁺3d spins).

Fig. 3.

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	Fig. 3. (color online) The field dependences of magnetization data both increasing and decreasing field model at 2 K. Inset: the μB–x curve.

Figure 4 shows the isothermal magnetization curves from 2 K to 5 K with an increment of 1 K at 0–50 kOe applied magnetic field. In Fig. 4 (x = 0), the high temperature magnetization is greater than the value at low temperatures under low fields. On the contrary, the situation is the opposite in higher fields. It is consistent with the AFM ordering of the compound below T_N.^[12] However, there are no intersections among the curves as shown in Fig. 4 (x = 0.025, 0.05, 0.075, 0.1), which can be seen in the FM magnetic ground state. The FM coupling gradually increases, and slightly dominates in the delicate balance between AFM and FM phases, when the Ti is substituted by Co. Figure 5 shows the Arrott curve (the curve of M² for H/M in the case of isothermal) of EuTi_{1 − x}Co_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1). When the slope of the curve is negative, the magnetic transition is the first order, whereas it will be of second order based on the Banerjee criterion.^[26] The EuTiO₃ is the first order with the negative slope of the Arrott plots below T_N. The results of EuTi_1−xCo_xO₃ (x = 0.025, 0.05, 0.075, 0.1) suggest that the phase transition is second order for the positive slope of the Arrott plot.

Fig. 4.

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	Fig. 4. (color online) Magnetization isotherms of EuTi1 - xCoxO3 compounds collected at 2, 3, 4 and 5 K under low fields (x = 0 (a); x = 0.025 (b); x = 0.05 (c); x = 0.075 (d); x = 0.1(e)).

Fig. 5.

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	Fig. 5. (color online) The Arrott plot of the EuTi1 − xCoxO3 compounds at 2, 3, 4, 5, and 6 K under low fields (x = 0 (a); x = 0.025 (b); x = 0.05 (c); x = 0.075 (d); x =0.1(e)).

The values of ΔS_M can be calculated by the magnetization isotherms using the Maxwell relation .^[27] Figure 6 shows the values of −ΔS_M in the samples of EuTi_{1 − x}Co_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) under different magnetic fields and different temperatures. The for EuTi_1−xCo_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) reach 40.5 J⋅kg⁻¹·K⁻¹, 33.9 J⋅kg⁻¹·K⁻¹, 38.9 J⋅kg⁻¹·K⁻¹, 35.6 J⋅kg⁻¹·K⁻¹, and 34.9 J⋅kg⁻¹·K⁻¹, respectively. The values of −ΔS_M monotonically increase as the applied magnetic field increases and the maximum values of are obtained near the magnetic transition temperature. Figure 7 shows the values of −ΔS_M of EuTi_1−xCo_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) as a function of the temperature with the magnetic field changed from 10 kOe, 20 kOe to 50 kOe. Since the value of −ΔS_M below the transition temperature is increased with the increase of the doping Co ion content, it can be obviously observed that the value of −ΔS_M below the transition temperature is significantly increased, under a magnetic field change 0–10 kOe, the value of reaches 10 J⋅kg⁻¹·K⁻¹ for EuTi_0.95Co_0.05O₃. Meanwhile, the values of −ΔS_M are almost the same under a magnetic field change 0–20 kOe and 0–50 kOe. Therefore, the decrease of magentic moment caused by Co and Eu interactions in the large magnetic field is disadvantageous. It is larger and more comparable than those of most potential magnetic refrigerant materials in a similar magnetic transition temperature under the same field change (20 kOe), such as TmCuAl (17.2 J⋅kg⁻¹·K⁻¹),^[28] ErMn₂Si₂ (20 J⋅kg⁻¹·K⁻¹),^[29] ErRu₂Si₂ (11 J⋅kg⁻¹·K⁻¹).^[30]

Fig. 6.

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	Fig. 6. (color online) Temperature dependences of magnetic entropy change for EuTi1−xCoxO3 under different magnetic fields (x = 0 (a); x = 0.025 (b); x = 0.05 (c); x = 0.075 (d); x = 0.1 (e)).

Fig. 7.

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	Fig. 7. (color online) Temperature dependences of magnetic entropy change for EuTi1−xCoxO3 (x = 0, 0.025, 0.05, 0.075, 0.1) under magnetic field 10 kOe (a), 20 kOe (b), and 50 kOe (c).

RC is a measure of how much heat can be transferred between the cold and the hot sinks in an ideal refrigeration cycle, which is another important parameter. The RC, defined as a cooling capacity of , where T₁ and T₂ are the temperatures at half maximum of the peak taken as the integration limits.^[31] The RC values of EuTi_{1 − x}Co_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) were calculated under 10 kOe, 20 kOe, and 50 kOe of the magnetic field, as shown in Table 1. The maximum values of RC reach 38.5 J⋅kg⁻¹ and 296.1 J⋅kg⁻¹ for EuTi_0.95Co_0.05O₃ under the magnetic field changes of 10 kOe and 50 kOe, respectively. Therefore, Eu_1−xCo_xTiO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) compounds appear to be promising materials for low temperature refrigeration.

Table 1.

Table 1.

Table 1. The content of Co3+ dependences of RC for EuTi1 − xCoxO3 (x = 0, 0.025, 0.05, 0.075, and 0.1) under magnetic fields of 10 kOe, 20 kOe, and 50 kOe. . Samples ΔH = 1 T ΔH = 2 T ΔH = 5 T /J⋅kg−1·K−1 RC/J⋅kg−1 /J⋅kg−1·K−1 RC/J⋅kg−1 /J⋅kg−1·K−1 RC/J⋅kg−1 EuTiO3 9.8 27 21.1 98 40.5 291.5 EuTi0.975Co0.025O3 8.4 30.6 18.2 96.0 33.9 274.7 EuTi0.95Co0.05O3 10.0 38.5 20.3 99.2 38.9 296.1 EuTi0.925Co0.075O3 9.8 36.7 19.7 100.1 35.6 278.5 EuTi0.9Co0.1O3 9.1 34.8 18.8 98.7 34.9 275.2

Table 1. The content of Co3+ dependences of RC for EuTi1 − xCoxO3 (x = 0, 0.025, 0.05, 0.075, and 0.1) under magnetic fields of 10 kOe, 20 kOe, and 50 kOe. .

A giant reversible MCE in EuTi_{1 − x}Co_xO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) compounds was observed under low fields. Due to the larger size of Co²⁺ ion (∼ 0.745Å) than that of Ti⁴⁺ ion (∼ 0.605 Å), the diffraction peaks showed a left shift after Co²⁺ ion doping. The EuTiO₃ exhibited the G-type AFM, while the delicate balance between AFM and FM phases in the EuTiO₃ compound was changed, when the Ti⁴⁺ ions were substituted by Co²⁺ ions in the compound. Under a low magnetic field change (0–10) kOe, the values of achieved 10 J⋅kg⁻¹·K⁻¹ and the RC reached 38.5 J⋅kg⁻¹ for EuTi_0.95Co_0.05O₃. In conclusion, the experimental results show that EuTi_1−xCo_xO₃ compounds can be considered as promising materials for low-temperature and low-field magnetic refrigerant.