Tuning the giant Magnetocaloric Effect and refrigerant capacity in Gd_1–xY_xCrO₃ (0.0 ≤ x ≤ 0.9) perovskites nanoparticles

1.   Introduction

Magnetic refrigeration based on the magnetocaloric effect (MCE) has attracted increasing interest in recent years due to the clean energy feature and it can be an alternative to conventional vapor compression refrigeration ^[1]. "The MCE, which is an intrinsic property of magnetic materials, is defined as the thermal response (heating or cooling) of magnetic solids during the application or removal of an external magnetic field" ^[2]. For temperatures below the critical temperature of the magnetic phase transition, the magnetic moments can be aligned by applying external magnetic field on the material. During this process the magnetic entropy (S_M) of the material (under the adiabatic conditions) decreases because of the moments ordering and to compensate for the decrease in the magnetic entropy the lattice entropy (under the adiabatic conditions) increases, which causes an increase in the temperature ^[3].

Rare-earth orthochromites RCrO₃ (where R = Y or trivalent rare-earth ion) are a class of interesting materials which showed various magnetic properties for example magnetization Reversal (MR), spin reorientation (SR), change in the magnetic entropy (ΔS_M), and exchange bias (EB) ^{[4,5,6,7,8,9]}. These different magnetic properties are temperature and magnetic-field-controlled which makes them potential materials for many applications such as spintronics, magnetic switches, thermally assisted random access memory devices, magnetic refrigeration based on the magnetocaloric effect ^{[10,11,12,13]}.

In the recent years, many researchers focused on the search for magnetic materials with large change in the magnetic entropy (ΔS_M) and large values of the relative cooling power (RCP). The RCrO₃ perovskite compounds have canted antiferromagnetic structure where Cr³⁺ ions occupy the octahedral sites and they are coupled antiparallel with slight canting of Cr³⁺ ions, resulting in week ferromagnetic below the critical Néel temperature. The interaction between the internal magnetic field produced by Cr³⁺ ions and the magnetic moments of the trivalent rare-earth ions can be weak or strong depending on the trivalent rare-earth ion and the temperature, and at certain range of temperatures spin reorientation occurs. Mahana et al. ^[14] studied the magnetic properties of polycrystalline GdCrO₃ prepared by solid-state technique, followed by annealing at 1400 ℃. They found that this compound "exhibits a giant magnetocaloric effect (MCE) with a maximum entropy change of 36.97 J/kg·K, a diabatic temperature change of 19.12 K and a refrigeration capacity of 542 J/kg for a field change of 7 T at low temperatures". The GdCrO₃ single crystal was grown with the laser-diode-heated floating-zone technique have been studied by Zhu et al. ^[15] and they found that (-ΔS_M) is about 57.5 J/kg·K at 6 K and at a change in the applied magnetic field of 14 T. Oliveira et al. ^[16,17] studied the "effect of chemical pressure on the magnetocaloric effect of perovskite-like, RCrO₃ (R-Yb, Er, Sm and Y)" and the MCE in YCrO₃. They found that for YCrO₃ the magnitude of the change in the magnetic entropy is equal to 3.5 at a temperature about 141 K and a change in the external magnetic field of 5 T.

In most of the published work, the researchers use solid-state reaction techniques, hydrothermal, or ball-milling to prepare their RCrO₃ samples. These methods usually take long time for the preparation and it needs high temperatures (≈1400 ℃) and long times (≈24–48 h) for sintering, which results in polycrystalline samples with particle sizes in the micrometer range. Although the findings of our recent research have not yet been formally published, the results are encouraging. Where we recently succeeded in preparing Gd_1–xY_xCrO₃ nanoparticles using the very simple auto-combustion method followed by annealing at lower temperatures and shorter times (700 ℃ and for 4 h). First, we have in detail structural, magnetic, and morphological measurements, analysis and discussion about the effects of Y substitution for Gd on the magnetic and structural properties of Gd_1–xY_xCrO₃ nanoparticles. And all the samples are canted antiferromagnetic (below the Néel transition temperature, T_N), and exhibit large exchange bias, and negative magnetization at low temperatures (≈110–10 K), which is responsible about the magnetic switching behavior in that temperature range. Changes in some of the magnetic properties is found due to the Y substitution for Gd, where the coercive field increases as we increased x reaching the maximum value (of 4.8 kOe) for the compound with x = 1.0 at a temperature of 125 K. For low temperatures (2–10 K) the coercive field is small (0.3 kOe, for x = 0) and it increases linearly by a small amount reaching 1.5 kOe for x = 0.9. In addition to that the magnitude of the exchange bias field increases as we increased the yttrium concentration, x, reaching a maximum at x = 0.7 then decreases. The increase in the magnitude of the exchange bias field was found to depend on the temperature for each compound. And we found that T_N = 171 ±2 K for x = 0 which is close to T_N = 171–177 K for polycrystalline GdCrO₃ compound ^{[18,19,20,21]}, we also found that T_N decreases linearly with increasing the yttrium concentration reaching 146 ±2 K for x = 0.9. In addition, we found that T_SF = 14 ±2 K for x = 0 which is close to T_SF = 12 ±2 for polycrystalline compound ^{[22,23,24,25]}, we also found that T_SF decreases linearly (by small amount) with increasing the yttrium concentration reaching 5 ±2 K for x = 0.9, indicating that the magnetocaloric effect in GdCrO₃ can be manipulated via Y-substitution.

In this paper, we present the results of a detailed temperature and field dependent magnetization and systematic study of the effect of yttrium substitution in Gd_1–xY_xCrO₃ on the magnetic and magnetocaloric properties of the prepared nanoparticle compounds.

2.   Materials and methods

Different nanoparticle powder samples of Gd_1–xY_xCrO₃ (where x = 0.0, 0.3, 0.5, 0.7, and 0.9) were synthesized by the auto-combustion method, using glycine as fuel and nitrates as oxidants, followed by annealing at 700 ℃. X-ray diffraction measurements (XRD) were performed by using (PANalytical X' Pert, UK). The average particle size was determined from the images of the high-resolution transmission electron microscope. The magnetic measurements were done using a vibrating sample magnetometer (VSM), attached to a Dynacool Physical Property Measuring System (PPMS, Quantum Design, USA) in magnetic fields up to 9 T and in the temperature range of (2–300) K.

3.   Results and discussion

Figure 1 shows the x-ray diffraction pattern with the Rietveld-fitting for Gd_0.5Y_0.5CrO₃. As we see from the figure that the sample has a single-phase and crystallized into a distorted orthorhombic structure and all the peaks in the diffraction pattern are indexed based on the space group (Pbnm). The crystallite size was calculated from the width of the different peaks and the average value is found to be (54 ±5) nm. All the other samples in this study showed similar diffraction patterns with very small shifts in the peak's positions due to the very small change in the unit cell volume, and different values of the average crystallite sizes.

The morphology and the microstructure of the samples were investigated from the micrographs of the TEM images. Figure 2 shows the image for a representative sample, Gd_0.5Y_0.5CrO₃. The image shows that most the nanoparticles have spherical shape with an average diameter of about (55 ±6) nm. The other samples under investigation showed similar images with an average diameter between 53 and 110 nm.

Figure 3 shows the isofield magnetization versus temperature curves during the zero field cooled (ZFC) and the field cooled (FC) modes at an applied magnetic field of 200 Oe for a representative sample with x = 0.5. It is clear from Figure 3 that the sample is paramagnetic above the Néel transition temperature, T_N, and it is canted antiferromagnetic (or weak ferromagnetic) below T_N with different magnetic features. Between the two compensation temperatures (T_comp-1 and T_comp-2) the sample undergo a magnetization reversal, where the magnetization changes from positive to negative. Another interesting feature is observed at a temperature called "spin reorientation or flipping temperature (T_SF)", where the Gd³⁺–Cr³⁺ coupling is rotated to align the Gd³⁺ moments parallel to the applied magnetic field.

In order to investigate the magnetocaloric effect (MCE) and the relative cooling power, or refrigerant capacity, in Gd_1–xY_xCrO₃ compounds we measured the dependence of the isothermal magnetization (M) on the applied magnetic field (μ_oH) at different temperatures. The applied magnetic field was increased in an isothermal manner from 0 to 90000 Oe in steps of 200 Oe. As we can see from Figure 4, at low temperature there is a visible curvature close to saturation (at the lowest temperatures) and as we increase the temperature the curvature decreases and also the magnetization decreases. In this study, the measured samples have a thin cylindrical shape and the applied magnetic field was along the length of the cylinder. We assumed that the internal magnetic field (H_int) is equal to the applied magnetic field (H_app or simply H) because the contribution to the internal field from the demagnetizing field (D) is negligible (H_int = H_app-D ≈ H_app, where the maximum order of D is ≈ 60 Oe compared with 90000 Oe for the maximum H_app).

The change in the magnetic entropy at a certain temperature and for a change in the applied magnetic field between 0 and H is found using Maxwell relation giving by Eq 1 ^[26,27]. ΔS_M were found at different temperatures between 4.5 and 77.5 K, and at different values of the change in the applied magnetic field between 1 and 9 T.

Figure 5 shows the magnitude of the change in the magnetic entropy as a function of temperature and for different values of the change in the magnetic field (1 T ≤ Δμ_oH ≤ 9 T). The magnitudes of ΔS_M for all the samples above 50 K are relatively small and they are not shown in the figures, to make the figures clear for the readers. It is clear from the figure that for the compound with x = 0 the magnitude of the change in the magnetic entropy has a maximum at 8.5 K, while the other samples did not show any maximum in the temperature range from 4.5 to 77.5 K.

The dependence of negative the change in magnetic entropy (-ΔS_M) on the yttrium composition (x) and on the different values of the change in the applied magnetic field (Δμ_oH = 1 to 9 T), at different temperatures T = 4.5, 8.5, and 77.5 K, for the different Gd_1–xY_xCrO₃ (0.0 ≤ x ≤ 0.9) nanoparticles is illustrated in Figure 6. It is clearly seen from this figure that as we increase the change in the applied field the magnitude of the change in magnetic entropy increases. At T = 4.5 K, negative the change in magnetic entropy is found to increase with increasing the yttrium concentration in the compound reaching a maximum at x = 0.3 then decreases as we reduce the composition. The magnitude of ΔS_M showed the maximum value of 45.6 J/kg·K for x = 0.0 and μ_oH = 9 T at 8.5 K, which is the highest value for all the compounds under investigation. The wide temperature range of the magnitude of ΔS_M from 3.5 to 77.5 K, as shown in Figures 4 and 5, indicates an advantageous feature in the MCE field due to the nano-structuring of Gd-Y based oxide materials. This range in the temperature covers the cryogenic range from helium-liquefaction to nitrogen-liquefaction temperatures.

The magnitude of the change in the magnetic entropy is one of the measures to determine the applicability of the material for MCE applications. Another important measure is the relative cooling power (RCP) which is defined as "the amount of heat transferred between two temperatures", or refrigerant capacity (RC) which is defined as "amount of heat transfer between the hot and cold sinks in an ideal refrigeration cycle". RCP or RC depends on the magnitude of the change in the magnetic entropy between the two temperatures T₁ and T₂ and can be calculated according to the formula in Eq 2 ^[26,27]. Based on this equation we evaluated RCP between T₁ = 3.5 K (for x = 0.0) or 4.5 K (for the rest of the samples) and T₂ = 77.5 K, using the magnitude of the change in magnetic entropy curves (-ΔS_M versus T).

The evaluated RCP values for the perovskite compounds as a function of yttrium composition (x) and as a function of the change in the applied magnetic field are shown in Figure 7. We can see from this figure that the compound with x = 0.0 has the largest value for the relative cooling power and x = 0.9 has the lowest value. What is interesting in this system of nanoparticle compounds that we have relatively large values (≈100 J/kg) for the RCP even at low values of the change in the applied magnetic field 1 to 2 T).

To better understand and establish the relation between the magnitude of the change in the magnetic entropy and the change in the applied magnetic field we can assume |ΔS_M| depends on (ΔH)ⁿ, based on the analysis by Franco et al. ^[28,29] using the mean field theory yield n ≈1 for T < T_C to n ≈2 for T > T_C.

Figure 8 shows the relation between ln|ΔS_M| and lnH for the different compositions of yttrium, x. The solid lines represent the linear fitting to the experimental data. The slopes of the straight lines, gives the value of n, at T = 4.5 K are 1.5 ±0.2 (for x = 0.0) and 0.9 ±0.2 (for x = 0.3 to 0.9), while at T = 8.5 K they are 1.2 ±0.2 (for x = 0.0) and 0.9 ±0.2 (for x = 0.3 to 0.9). The values of n for all the nanoparticle compounds agree with the expected value and agree with the previously published values of 1.3 to 1.6 at temperatures below 20 K for GdCrO₃ by Shi et al. ^[27].

To shed light and to get deeper insight of the order of magnetic phase transition one can use Arrott plots. Using the data of the isotherm's magnetization curves (magnetization (M) as a function of the external field (H), we determined M² and H/M, then we plotted Arrott plots (M² versus H/M) as shown in Figure 9 for the different nanoparticle compounds. From the slope of the curves of Arrott plots ^[18], the order parameter of the magnetic transition can be estimated. At low temperature, all the curves show a downward concave curvature and as we increase the temperature the plots are displaced to the right and then at higher temperatures the curves become straight upwards. All the curves in an applied magnetic field up to 9 T show a positive slope reveals that the canted-antiferromagnetic to paramagnetic phase transition is second-order in nature. A negative slope usually represents a first order magnetic transition, which was not found in our system of nanoparticles.

The magnetic behavior of the studied samples changes from canted antiferromagnetic below the ordering temperature of Cr³⁺ (T_N) to paramagnetic above T_N (which is between 145 and 170 K for the different compounds investigated in this study). The Gd³⁺ ordering temperature in GdCrO₃ single crystal is 2.3 K (which is below our measured range). The orbital angular momentum (L) for Gd³⁺ ions is zero, so the contribution to the total angular momentum (J) comes from the spin angular momentum (S = 7/2).

Due to this, the contribution to the magneto crystalline anisotropy from the crystal field effect can be neglected which makes the magnetization almost isotropic and large in our compounds ^[19,20,21]. The giant magnitude of the change in the magnetic entropy may be attributed mainly to the Gd³⁺–Gd³⁺ interaction and partially due to magnetic entropy induced by the spin-reorientation of Cr³⁺ and Gd³⁺ ions due to the Cr³⁺–Gd³⁺ magnetic interaction in the non-collinear canted-antiferromagnetic systems. In this study we found a maximum value of -ΔS_M 45.6 J/kg·K for x = 0.0 and μ_oH = 9 T at 8.5 K for -ΔS_M, which is comparable with the value of 52.5 J/kg·K for GdCrO₃ single crystal reported by Das et al. ^[21]. For the MCE applications, it is important to have large values of the magnitude of the change in the magnetic entropy and a large value for the relative cooling power at an achievable low magnetic field, which can be produced by permanent magnets (in the range of 1–2 T). In our system of nanoparticles, we can see that a large values for |ΔS_M| (3–11 J/kg·K) and RCP (30 to 85 J/Kg) can be reached in a change in the applied magnetic field of 1 to 2 T.

5.   Conclusion

In summary, we used the auto-combustion method followed by annealing at 700 ℃ to prepare the Gd_1–xY_xCrO₃ perovskite nanoparticles compounds. We found that all the synthesized samples form nano-particles with diameters in the range (53–110 ±6) nm and crystallized into a distorted orthorhombic structure with the space group (Pbnm). We have systematically investigated the magnetic and MCE properties of the nanoparticle (Gd, Y)-chromites including the temperature-dependent, the magnetic field-dependent, and the composition-dependent. The giant values of -ΔS_M and RCP and the wider temperature range of -ΔS_M suggest that these compounds may be considered as magnetic refrigerants materials in the cryogenic range from helium-liquefaction up to hydrogen-liquefaction temperatures.

Acknowledgments

Al-Omari would like to thank Sultan Qaboos University for the support provided during this study under the grant number IG /SCI/PHYS/20/05. We would like to thank Mr. Abdul Rahman Al-Nabhani for the help in TEM measurements, and the CAARU team for the XRD measurements.

Conflict of interest

The authors declare no competing interests.