Structural, magnetic, and magnetocaloric properties of R2NiMnO6 (R = Eu, Gd, Tb)

The crystal structure, cryogenic magnetic properties, and magnetocaloric performance of double perovskite Eu2NiMnO6 (ENMO), Gd2NiMnO6 (GNMO), and Tb2NiMnO6 (TNMO) ceramic powder samples synthesized by solid-state method have been investigated. X-ray diffraction structural investigation reveal that all compounds crystallize in the monoclinic structure with a P21/n space group. A ferromagnetic to paramagnetic (FM-PM) second-order phase transition occurred in ENMO, GNMO, and TNMO at 143, 130, and 112 K, respectively. Maximum magnetic entropy changes and relative cooling power with a 5 T applied magnetic field are determined to be 3.2, 3.8, 3.5 J/kgK and 150, 182, 176 J/kg for the investigated samples, respectively. The change in structural, magnetic, and magnetocaloric effect attributed to the superexchange mechanism of Ni2+–O–Mn3+ and Ni2+–O–Mn4+. The various atomic sizes of Eu, Gd, and Tb affect the ratio of Mn4+/Mn3+, which is responsible for the considerable change in properties of double perovskite.

www.nature.com/scientificreports/ was seen due to spin-phonon coupling caused by the lower ionic radii of rare earth element (Ln 3+ ). The essential factor in determining structural and magnetic or magnetocaloric properties in double perovskites is cation ordering between B and B' elements, as well as distortion in bonding owing to various A-site elements, which is thought to be the cause of complimentary findings. The small distortions caused by defects and disorders may cause redistribution of electron density, resulting in substantial changes in electronic characteristics, magnetic ordering, and magnetocaloric properties. Unfortunately, few research have been conducted to quantitatively demonstrate the relationship between the structure of double perovskite, namely B-O-B′ bonding, bond length, and superexchange mechanism, and subsequent magnetocaloric characteristics in these double perovskite systems.
In this article, we studied the influence of rare earth element ionic size in R 2 NiMnO 6 (R = Eu, Gd, Tb) double perovskite on structural, magnetic, and magnetocaloric properties at cryogenic temperatures. The various sizes of rare earth elements influence the bond angle studied by Rietveld refinement and X-ray photoemission (XPS) revealing the Mn 4+ /Mn 3+ ionic distribution in the double perovskite. At cryogenic temperatures, the magnetocaloric characteristics are revealed to comprehend its usage as a magnetic refrigerant for cooling applications.

Results and discussion
The rietveld refinement of room temperature X-ray diffraction data is used to study the phase purity and crystal structure of the samples. The rietveld refined XRD pattern of the ENMO, GNMO, and TNMO samples are shown in Fig. 1a. According to the diffraction patterns, all the investigated compositions crystallized in the monoclinic  www.nature.com/scientificreports/ crystal system with space group P2 1 /n. The structural parameters are refined using the TOPAS program by Rietveld's profile fitting technique 25 . Table 1 show the refinement parameters of the investigated samples, such as lattice parameters, bond length, goodness of fit (χ 2 ), and bond angle. It is observed that when the atomic number of rare earth elements increases, cell volume and crystal density increase, but cell volume declines. The atomic arrangement in monoclinic structure of ENMO, GNMO, and TNMO are shown in Fig. 1b. When compared to ENMO and TNMO, the crystal structure of GNMO is significantly tilted (β = 124°). The octahedral arrangement of NiO 6 and MnO 6 edges are shared to create a cross chain network in a single unit cell. The tilting of angle of the octahedron and the stability of the perovskite changes with the growth of the radius of the A, B, and B′ metal ions in the double perovskite structure 26 . The structurally stability of double perovskite, is defined by Goldschmidt tolerance factor (t) which is given by relation, where, r A , r B , r B′ , and r O represent ionic radii of A, B, B' ′ and O site element in double perovskite.
The t values of ENMO,GNMO, and TNMO determined by using above Eq. (1) are 1.02, 1.01, and 1.00, respectively. As we know, the double perovskite structure is stable when the value of t is between 0.78 and 1.05. The calculated values of t are within this range, indicating that the investigated samples are stable 3 .
The X-ray photoemission spectroscopy (XPS) technique is used to investigate the chemical oxidation states and the ligand coordination of the samples. The oxidation state analysis of Manganese (Mn) in ENMO, GNMO, and TNMO samples were done, and XPS spectra for Mn2p are shown in Fig. 2a-c. To fit the spectra, Shirley background subtraction was employed. The deconvoluted XPS peak of Mn 2p 3/2 breaks into two peaks at 641.2 eV and 643.2 eV, which correspond to Mn 4+ and Mn 3+ , respectively 27 . The ratio of Mn 4+ /Mn 3+ for ENMO, GNMO, and TNMO were found to be 1.02, 0.76, and 1.64, respectively, showing that Mn 4+ is more dominant in ENMO and TNMO, whereas Mn 3+ is more dominant in GNMO. The Mn 4+ /Mn 3+ ratio demonstrates the change in surface oxidation state induced by distinct A-site rare earth element with varying ionic radii and which indicates that the superexchange mechanism of Ni 2+ -O-Mn 3+ and Ni 2+ -O-Mn 4+ in the investigated compounds are distinct. However, the structural instability caused by the various ionic sizes of the A-site elements in double perovskite causes uncertainty in the Mn 4+ /Mn 3+ ratio, which influences the magnetic properties of the compounds. Figure 3a depicts temperature-dependent magnetization (MT) curves of ENMO, GNMO, and TNMO samples recorded with a 100 Oe applied magnetic field between 2 and 300 K. It is apparent that when the temperature increased, the magnetization in the samples dropped due to the magnetic phase transition temperature from ferromagnetic to paramagnetic. The MT curve verifies the ferromagnetic to paramagnetic phase change caused by the well-known superexchange exchange phenomenon linked with Ni 2+ -O-Mn 4+ /Ni 2+ -O-Mn 3+ . To calculate the Curie temperature (T C ), the temperature dependences of dM/dT for all samples are presented in the inset of figure. It is defined as the minimum of the dM/dT curve, and the Curie temperatures for ENMO, GNMO, and TNMO samples are found to be 143, 130, and 112 K, respectively. The Curie temperature progressively moves to lower temperature when the rare earth size Eu 3+ (0.947 Å) > Gd 3+ (0.938 Å) > Tb 3+ (0.923 Å) declines, which is likely related to reductions in the Ni-O-Mn bond angle [161.84 (Eu), 153.07 (Gd), and 147.30 (Tb)] 28 . However, as the size of rare earth ions decreases in double perovskite R 2 CoMnO 6 (R = La,…,Lu) compounds, the transition temperature changes linearly from 204 K for La 2 CoMnO 6 to 48 K for Lu 2 CoMnO 6 due to long-range magnetic order originating from the dominating Co 2+ and Mn 4+ superexchange interactions 29,30 . The ordered alignment  31 . In the case of TNMO, it is noted that magnetization decreases at low temperatures (2-25 K), similar with Dy 2 NiMnO 6 , and this is caused by anti-parallel alignment of rare earth (Tb) magnetic moments as opposed to transition metal (Ni and Mn) magnetic moments. The temperature dependent inverse susceptibility χ −1 (T) is shown in Fig. 3b,c,d. In paramagnetic region, the linear fitting observed for the experimental χ -1 (T) curve with the Curie-Weiss (C-W) equation,  32 .
To examine the magnetocaloric properties, the isothermal magnetization (MH) curves at various temperatures were measured before and after the T C . The temperature intervals ΔT = 3 K near T C and ΔT = 5 K in the rest of temperature region were kept constant. Figure 4a-c depicts isothermal magnetization of ENMO, GNMO, www.nature.com/scientificreports/ and TNMO samples in a magnetic field range of 0-5 T. The MH curves show that when the magnetic field is low, the MH curves increase quickly, and when the magnetic field is high, the MH curves strive to saturate, and this phenomenon is related with the ferromagnetic behavior of magnetic materials. The MH curves exhibit linear behavior at higher temperatures, confirming the paramagnetic nature of the materials, and this is owing to thermal agitation, which disorients the magnetic moments at higher temperatures. To understand, the order of magnetic phase transition, well-known Arrott plots (M 2 vs H/M) were studied, and which are derived from the magnetic isotherms shown in Fig. 4d-f. According to Banerjee's criteria, the slope of Arrott plots is significant in determining the type of magnetic phase transition. The negative slope represents the first-order phase transition while the positive slope verifies the second-order phase transition 33 . The Arrott plot show a positive slope at all temperatures for the samples examined. As a result, we can confirm that the ferromagnetic-paramagnetic transition is of the second-order type. The "S"-shaped Arrott plot, indicates that GNMO and TNMO samples suffer a weak first-order phase transition, but ENMO totally exhibits second-order phase transition. The Arrott plot analysis indicates that the first order spin re-orientation at lower temperature resulted in an FM/AFM transition in these samples. The existence of disordered B-sites with magnetic ions of Ni and Mn in mixed valence states might explain the complex magnetic structure found in these compounds. In the case of a second order phase transition, the order of degree of magnetic domains, variation in lattice volume, and latent heat of phase transformation are all extremely modest. It is possible that it may be one of the reasons why magnetic entropy changes in second-order phase transition materials are lower than those in first-order phase transition materials. The magnetic entropy change calculated by utilizing the isothermal magnetization data, shown in Fig. 4, and which is initiated by the changing the applied magnetic field from 0 to H is determined by applying the wellknown Maxwell thermodynamic correlation, which is given by the equation 34 , Another essential measure for determining the efficacy of MCE materials is evaluating the cooling efficiency of the materials, which is referred to as relative cooling power (RCP). It is defined as, an amount of heat transferred between temperatures corresponding to the full width at half maximum of magnetic entropy change curve, and it is evaluated by the following equation,  Table 2 summarizes the comparison of transition temperature, magnetic entropy change, and RCP values for the investigated samples and other reported double perovskite compounds. The -ΔS M and RCP values for ENMO, GNMO, and TNMO does not show much change with different rare earth elements in double perovskite, however there is shift in Curie temperature (T C ) observed. Table 2 show that the MCE characteristics for the investigated DPs samples are comparable with other DPs materials, showing that ENMO, GNMO, and TNMO samples are also important for magnetic cooling applications. The comparative research given in the table, the Curie temperature falls from 143 to 84 K with decreasing ionic radii of the rare earth element in Ln 2 NiMnO 6 (Ln = Eu, Gd, Tb, Dy, Ho, and Er) double perovskite, however there are no significant changes in MCE and RCP values. Figure 6 depicts the magnetic entropy change and Curie temperature of ENMO, GNMO, and TNMO samples with respect to increasing ionic radius of rare earth elements (Eu, Gd, and Tb). The magnetic entropy change with different A-site element of the investigated double perovskite varies and does not display a simple monotonic trend as the Curie temperature falls gradually. To determine the order of magnetic phase transition another technique was described in the literature, in which the field dependence of − ΔS M of the sample was determined by utilizing the relation ΔS Max = aH n , where a is constant and n is an exponent linked to the magnetic order 40,41 . Figure 7 depicts − ΔS Max with respect to magnetic field along with power law fitting, and the resulting values of "n" are 0.73, 0.73, and 0.75 for ENMO, GNMO, and TNMO samples, which are somewhat higher than mean field ferromagnets (n = 0.67). However, for single phase ferromagnets, the exponent "n" is considered as function of magnetic field and temperature 2,42 , and is written as,   43 . Expecting a change in MCE properties with different rare earth elements having different ionic radii in A 2 BB'O 6 double perovskite is not much feasible, because of phase transition in double perovskite is associated with M 2+ -O-Mn 4+ superexchange interaction but playing with working temperature this could be one of tool to tune the Curie temperature.

Conclusions
In summary, we used solid-state method to synthesize Eu 2 NiMnO 6 (ENMO), Gd 2 NiMnO 6 (GNMO) and Tb 2 NiMnO 6 (TNMO) double perovskite, and we discovered that all the samples have monoclinic structure with P2 1 /n space group. XRD, XPS, MT, and MCE data have been systematically investigated. All the samples exhibit a second-order magnetic phase transition, with curie temperatures (T C ) of 143, 130, and 112 K for ENMO, GNMO, and TNMO, respectively. The drop in Curie temperature is owing to a decrease in the ionic radii of Eu, Gd, and Tb in the double perovskite structure, which is related with the structural disorder and superexchange interaction. The -ΔS M and RCP at an applied field of 5 T are found to be 3.2, 3.8 J/kg K, 3.5 J/kg K and 150, 182, 176 J/kg K respectively, for the studied samples. The -ΔS M and RCP suggest that the investigated compounds Eu 2 NiMnO 6 , Gd 2 NiMnO 6 , and Tb 2 NiMnO 6 may be considered as magnetic refrigerants with wider temperature range and making them potential magnetic refrigerant materials.

Experimental
Polycrystalline double perovskite compounds of Eu 2 NiMnO 6 (ENMO), Gd 2 NiMnO 6 (GNMO), Tb 2 NiMnO 6 (TNMO) in the present study were synthesized by conventional solid-state method. The stoichiometric amount of the precursors Eu 2 O 3 , Gd 2 O 3 , Tb 2 O 3 , NiO, and MnO 2 were combined and ground in the mortar before being heat-treated at 900 °C for 24 h, after regrinding heat-treated at 1100 °C for 24 h, and finally all samples sintered at 1300 °C for 48 h after regrinding. The phase formation of sintered ceramic compounds was investigated by X-ray diffraction (XRD) by using the X-ray diffractometer (Rigaku) and analyzed by Rietveld method by using TOPAS software. The X-ray photoelectron spectroscopy (XPS, VersaProbe, PHI 5000) experiment was performed using an Al-K α radiation source under 15 kV voltage and 5 mA current. The XPS data was examined by using the XPSpeak41 software. The temperature dependent magnetization and isothermal magnetization of the samples www.nature.com/scientificreports/