Liquid helium temperature zones are widely used in low temperature physics, superconducting technology, aerospace, and so on.^[1] Magnetic refrigeration technology is one of the high efficiency refrigeration techniques and can be applied to extreme research environment.^[2] Magnetocaloric effect materials are the core of magnetic refrigeration technology. Currently, the rare earth intermetallic compounds are a larger family in a low temperature region, such as DyCo₂Si₂,^[3] ReFe₂Si₂ (Re = Pr and Nd),^[4] ErCoSn,^[5] Er₄CoCd,^[6] Tm₄CoCd,^[7] Re₂CuSi₃ (Re = Dy and Ho),^[8] HoCoAl,^[9] the quaternary compound ReT₂B₂C (where Re represents a rare earth element and T a transition metal),^[10] etc.

Recently, rare earth metal oxides are gradually becoming another important research direction of the low-temperature magnetocaloric materials, such as ReCrO₄ (Re = Ho, Gd, Lu),^[11] Pr₂CoMnO₆,^[12] Re₂BaCuO₅ (Re = Dy and Er).^[13] Li et al. reported that when the magnetic field changed from 0 T to 5 T, the maximum values of magnetic entropy change () could reach 7.5 J/kg · K and 9.2 J/kg · K for Dy₂Cu₂O₅ and Ho₂Cu₂O₅ respectively.^[14] Balli et al. reported the anisotropic magnetocaloric effect in single-crystal HoMn₂O₅, the value of was 13.1 J/kg · K in the easy axis direction as the change of magnetic field (ΔH) is 0 T–7 T.^[15] In 2016, Koushik Dey et al. reported that ReVO₄ compounds also exhibited huge value of ΔS_M at low temperature. The values of were 41.1 J/kg · K, 7.94 J/kg · K, and 19.7 J/kg · K for GdVO₄, HoVO₄, and ErVO₄ when ΔH = 5 T.^[16]

Around the temperature region of liquid helium, the magnetic order arrangement of the Cubic perovskite material EuTiO₃ is G-type antiferromagnetic (AFM).^[17] In EuTiO₃, Ti is tetravalent (3d⁰) and Eu is divalent with large spin moment (S = 7/2) caused by the stable 4f⁷ electronic configuration. The single crystal EuTiO₃ shows a large reversible magnetocaloric effect, the value of is 42.4 J/kg · K as ΔH = 5 T.^[18] The polycrystalline EuTiO₃ also shows a large ^[19] Previous reports indicate that the improvement of ferromagnetism (FM) exchange could effectively increase the −ΔS_M around the transition temperature when Δ H = 1 T, such as Eu_{1 − x}Ba_xTiO₃ (11.6 J/kg · K),^[20] Eu_{1 − x}Sr_xTiO₃ (10 J/kg · K),^[21] Eu_{1 − x}La_xTiO₃ (10.8 J/kg · K),^[22] and EuTi_0.9V_0.1O₃ (11 J/kg · K).^[23] When parts of Ti ions are substituted by Nb ions, the magnetic state changes into FM metallic state.^[24,25] In 2016, Roy et al. reported that a large magnetocaloric effect of single crystal EuTi_0.85Nb_0.15O₃ (14.7 J/kg · K) could be obtained when Δ_H = 1 T. Roy speculated the Nb⁴⁺ (4d¹) in the EuTi_0.85Nb_0.15O₃ based on metallic behavior.^[26] The introduction of an itinerant electron mediates the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between the localized moments of Eu²⁺ ions and the system and exhibits ferromagnetic effect above a critical value of Nb concentration.^[24,25] But, if the Nb ions exhibit Nb⁵⁺ (4d⁰), it is also possible to introduce an itinerant electron occupying an empty orbit of the Ti 3d state, resulting in a decrease in Eu 4f–Ti 3d–Eu 4f hybridization and the suppression of AFM coupling, because the AFM exchange comes from the superexchange of Eu²⁺ 4f spins via the Ti⁴⁺ 3d states.^[27] For EuTi_{1 − x}Nb_xO₃ compounds, the contribution of increasing FM exchange needs to be further confirmed and the magnetocaloric effect needs to be studied systematiclly.

In this study EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds are obtained by introducing Nb⁵⁺ ions into EuTiO₃. The value of is 13.1 J/kg · K for EuTi_0.85Nb_0.15O₃ when Δ_H = 1 T, which is more greatly improved than that for EuTiO₃. The improvement of ΔS_M is attributed to the increase of FM exchange due to the electron doping and lattice expansion in the EuTiO₃.

A series of EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds are prepared by sol–gel method in this work. Firstly, europium oxide (Eu₂O₃) and five niobium chlorides (NbCl₅) are added to an aqueous solution of nitric acid according to a stoichiometric ratio and stirred until they are completely dissolved. Next, tetrabutyl titanate (Ti(OC₄H₉)₄) is added and stirred for 0.5 h to clarify. Secondly, ethylene glycol (C₂H₆O₂) is added as dispersant and stirred for 0.5 h. Then the configured homogeneous solution is dried at 353 K and thoroughly ground. Finally, the power sample is sintered in a tube furnace at 1373 K for 3 h to obtain the precursor, next the precursor is sintered in mixed gas (8% H₂ and 92% Ar) at 1573 K for 5 h to obtain EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) series of samples.

The structures of EuTi_{1 − x}Nb_xO₃ compounds are determined by a Rigaku 2500/PC x-ray diffractometer with a Cu Kα radiation source, and the wavelength was 1.54184 Å. The element valence is confirmed by x-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The isothermal magnetization curves are measured by using a commercial superconducting quantum interference device (SQUID) magnetometer and model MPMS-7 from Quantum Design Inc.

Figure 1 shows XRD diffraction peak for each of EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds, indicating that the EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) are single-phase crystals and belong to a cubic perovskite structure (space group 221). The refined lattice parameter increases from 3.9055 Å to 3.9273 Å with doping Nb ions increasing as shown in the inset of Fig. 1. It is attributed to the larger size of Nb ions (∼ 0.69 Å), after Ti ions have been replaced by Nb ions.

Fig. 1.

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	Fig. 1. XRD patterns of EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds. Observed data are denoted by crosses, and calculated profile is continuous line overlying them. Short vertical lines indicate angular positions of Bragg positions, and the green curve is the difference between the experimental data and the calculated data. Illustration shows variation of cell volume.

In order to determine the valence of Nb ion in the EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds, figures 2(a) and 2(b) show the XPS spectra of Nb 3d and Ti 2p for EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds. There are dual positions for Nb 3d (Nb 3d^3/2 and Nb 3d^5/2), and Nb 3d^5/2 is 206.5–206.9 corresponding to the chemical valence of Nb⁵⁺, which is different from the previously reported value of Nb⁴⁺.^[24] The Ti³⁺ was observed in the XPS due to the balance of the valence state.

Fig. 2.

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	Fig. 2. XPS spectra of Nb 3d and Ti 2p for EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds.

Figure 3 shows the zero-field-cooling (ZFC) and the field-cooling (FC) temperature dependence of magnetization for EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds under an applied magnetic field of 0.01 T. With the decrease of temperature, EuTi_{1 − x}Nb_xO₃ compounds experience a change from paramagnetic (PM) state to FM state, which are different from what EuTiO₃ compound does. The transition temperature of EuTi_{1 − x}Nb_xO₃ compound is determined according to the dM/dT as shown in the inset of Fig. 3. When the temperature is higher than the phase transition temperature, the thermal hysteresis is observed in none of the samples. And a bifurcation is observed below transition temperature, which is probably caused by a domain wall pinning effect. But the bifurcation decreases with the number of Nb ions substituting for Ti ions, which may be attributed to the enhancement of FM coupling. Previous reports explained that Nb doping induced ferromagnetism, which is most likely to result from the ferromagnetic interaction between localized Eu-4f spins, mediated by itinerant electrons introduced by chemical doping.^[28] The FM ordering of the localized Eu²⁺ spin is mediated by the itinerant electrons of the dopant Nb⁴⁺ (4d¹).^[26] However, the Nb ions exhibit Nb⁵⁺ as shown in XPS, which maybe introduces electrons into the empty Ti-3d band such as Eu_{1 − x}La_xTiO₃ compounds.^[22] In EuTiO₃ the antiferromagnetic coupling comes from the superexchange interactions with Ti-3d (t_2g) empty states.^[27] Therefore, the 3d¹ of Ti ions maybe suppresses antiferromagnetic coupling and promotes ferromagnetic interaction between 4f spins neighboring Eu²⁺ ions through RKKY. What is more, Akamatsu et al. reported that the increase of lattice parameters in Eu_{1 − x}Ba_xTiO₃^[20] and Eu_{1 − x}Sr_xTiO₃^[21] could improve the FM coupling.^[27] The ionic radius of Nb⁵⁺ is larger than that of Ti⁴⁺ which obviously increases the lattice constant as shown by XRD in Fig. 1. In short, there are two factors that could improve FM exchange: electronic doping and lattice expansion.

Fig. 3.

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	Fig. 3. Temperature-dependent magnetization M curves of ZFC and FC for EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) under magnetic field of 0.01 T, with inserts showing dM/dT versus T curves at low temperature under magnetic field of 0.01 T.

Figure 4 shows the isothermal magnetization curves in a temperature range from 3 K to 28 K under magnetic fields ranging from 0 kOe to 50 kOe (1 Oe = 79.5775 A · m⁻¹). The isothermal magnetization curves of EuTi_{1 − x}Nb_xO₃ compounds are nonlinear when T_C < T < 16 K, indicating the existence of short-range FM state in the paramagnetic state. None of the isothermal magnetization curves crosses with other one in a low magnetic field range of 0 kOe–10 kOe at 3 K, 4 K, 5 K, and 6 K as shown in Fig. 5, which also implies that the EuTi_{1 − x}Nb_xO₃ compounds all present the FM state.

Fig. 4.

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	Fig. 4. Magnetization isotherms of EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds measured in temperature range of 3 K–28 K under applied magnetic fields ranging from 0 kOe to 50 kOe.

Fig. 5.

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	Fig. 5. Magnetization isotherms of EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds measured in temperature range of 3 K, 4 K, 5 K, 6 K under applied magnetic fields ranging from 0 kOe to 10 kOe.

According to the Maxwell relation , the value of −ΔS_M is calculated by the isothermal magnetization curve.^[29] Figure 6 shows the relationship between −ΔS_M and temperature for each of the EuTi_{1 − x}Nb_xO₃ compounds when Δ_H is in a range of 0 T–5 T. The values of are 38.0 J/kg · K, 37.5 J/kg · K, 39.6 J/kg · K, and 34.6 J/kg · K, respectively, for EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds at transition temperature when ΔH = 5 T. Although the values of −ΔS_M for EuTi_{1 − x}Nb_xO₃ compounds decrease when ΔH = 5 T, the values of −ΔS_M are 13.1 J/kg · K and 11.9 J/kg · K, respectively, for EuTi_0.85Nb_0.15O₃ and EuTi_0.8Nb_0.2O₃ when Δ_H = 1 T, which are larger than that for EuTiO₃ (9.8 J/kg · K). The values of −ΔS_M for these compounds are all greater than those for other rare earth metal oxide compounds near the liquid helium temperature, such as R₂NiMnO₆ (R = Dy, Ho, and Er),^[30] GdMnO₃,^[31] and DyScO₃,^[32] and so on. The increase of −ΔS_M may be attributed to the enhancement of FM exchange due to electronic doping and crystal lattice expansion.

Fig. 6.

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	Fig. 6. Temperature-dependent magnetic entropy change under magnetic fields 1 T–5 T for EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds.

In practical application, magnetic refrigeration materials need not only a large −ΔS_M, but also a wide range of phase change temperature, so that they can ensure better cooling effect in a sufficiently wide operating temperature range. The refrigeration capacity (RC) is another important parameter. It represents the exchange of energy between heat and cold in the process of heat exchange, which is directly related to energy efficiency. The RC is calculated from , where T₁ and T₂ are the temperatures corresponding to half of the peak for the integration limit. The values of RC and −ΔS_M for the EuTi_{1 − x}Nb_xO₃ compounds and EuTiO₃ are shown in Table 1 when ΔH = 1 T, 2 T, and 5 T. It could be observed that the RC values of the EuTi_{1 − x}Nb_xO₃ compounds are larger than that of the EuTiO₃ compound. Especially under the magnetic field changing from 0 T to 1 T the values of RC are 86 J/kg and 80 J/kg for EuTi_0.85Nb_0.15O₃ and EuTi_0.8Nb_0.2O₃ respectively, which are three times larger than that for the EuTiO₃ (27 J/kg). It could be clearly observed that the EuTi_0.85Nb_0.15O₃ and EuTi_0.8Nb_0.2O₃ present FM and larger −ΔS_M under a low change of magnetic field. At the same time, the temperature span (the full width at half maximum of the −ΔS_M) is expanded, so greatly improving the RC, which is more conducive to practical applications. In summary, the EuTi_{1 − x}Nb_xO₃ compounds are considered as a good candidate for magnetic refrigeration materials in liquid helium temperature regions.

Table 1.

Table 1.

Table 1. Values of −ΔSM and RC under field change of 1 T, 2 T, and 5 T for EuTiO3, EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds. . Samples −ΔSM/(J/kg · K) RC/(J/kg) 1 T 2 T 1 T 2 T 5 T EuTiO3 9.8 22.3 27 98 292 EuTi0.95Nb0.05O3 10.3 21 36 104 307 EuTi0.9Nb0.1O3 9.6 20.3 33 100 304 EuTi0.85Nb0.15O3 13.1 22.6 86 174 397 EuTi0.8Nb0.2O3 11.9 20 80 158 365

Table 1. Values of −ΔSM and RC under field change of 1 T, 2 T, and 5 T for EuTiO3, EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds. .

The second-order phase transition is more suitable for magnetic refrigeration cycles. Based on Banerjee’s^[33] criterion, if the Arrott plot has a positive slope, the magnetic transition is a second-order phase transition, while the negative slope or S-type belongs to the first-order phase transition. The Arrott plots of the EuTi_{1 − x}Nb_xO₃ compounds show positive slopes as shown in Fig. 7, indicating the second-order phase transition. The phenomenological universal curve of the −ΔS_M has been proposed as another method to distinguish between the second order phase transition and the first order phase transition.^[34] The normalize ΔS′(θ) can be folded into a curve under different magnetic fields. The reference temperature T_r1 and T_r2 correspond to the peak of . The temperature axis is rescaled in a different way below and above T_C as shown below:

Fig. 7.

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	Fig. 7. Arrott plots of EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds.

Figure 8 shows a ΔS′(θ) plot of different magnetic fields for the EuTi_{1 − x}Nb_xO₃ compounds. It can be observed that the ΔS′(θ) curves almost overlap, which further prove the second-order phase transitions for EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds.

Fig. 8.

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	Fig. 8. θ-dependent ΔS′(T) for typical field changes in EuTi1 − xNbxO3 (x = 0.05, 0.1, 0.15, and 0.2) compounds.

A series of EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds is prepared by the sol–gel method. The magnetic and magnetocaloric effects of compounds are studied. The compounds are of the cubic perovskite structure and the Nb element is confirmed to be pentavalent by XPS. The substitution of Nb⁵⁺ ions for Ti⁴⁺ ions increases the lattice constant and achieves electronic doping, which leads FM exchange to increase. The values of for the EuTi_{1 − x}Nb_xO₃ (x = 0.05, 0.1, 0.15, and 0.2) compounds are 10.3 J/kg · K, 9.6 J/kg · K, 13.1 J/kg · K, and 11.9 J/kg · K, values of RC are 36 J/kg, 33 J/kg, 86 J/kg, and 80 J/kg when ΔH = 1 T, respectively. In addition, the EuTi_{1 − x}Nb_xO₃ compounds all experience the second-order phase transitions without hysteresis loss. In summary, EuTi_{1 − x}Nb_xO₃ compounds are considered as one of the most promising magnetic refrigeration materials in the liquid helium temperature zone.