Giant and highly anisotropic magnetocaloric effects in single crystals of disordered-perovskite RCr0.5Fe0.5O3 (R = Gd, Er)

Magnetic anisotropy is crucial in examining suitable materials for magnetic functionalities because it affects their magnetic characteristics. In this study, disordered-perovskite RCr0.5Fe0.5O3 (R = Gd, Er) single crystals were synthesized and the influence of magnetic anisotropy and additional ordering of rare-earth moments on cryogenic magnetocaloric properties was investigated. Both GdCr0.5Fe0.5O3 (GCFO) and ErCr0.5Fe0.5O3 (ECFO) crystallize in an orthorhombic Pbnm structure with randomly distributed Cr3+ and Fe3+ ions. In GCFO, the long-range order of Gd3+ moments emerges at a temperature of TGd (the ordering temperature of Gd3+ moments) = 12 K. The relatively isotropic nature of large Gd3+ moment originating from zero orbital angular momentum exhibits giant and virtually isotropic magnetocaloric effect (MCE), with a maximum magnetic entropy change of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {S}_{M}$$\end{document}ΔSM ≈ 50.0 J/kg·K. In ECFO, the highly anisotropic magnetizations result in a large rotating MCE characterized by a rotating magnetic entropy change \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {S}_{\theta }$$\end{document}ΔSθ = 20.8 J/kg·K. These results indicate that a detailed understanding of magnetically anisotropic characteristics is the key for exploring improved functional properties in disordered perovskite oxides.

( S M = 11.3 J/kg·K for ΔH = 0-4.5 T) have also been discovered. Various studies have hypothesized that large magnetic moments of magnetic rare-earth ions with strong anisotropy would significantly affect the cryogenic MCE. However, these studies focused only on polycrystalline specimens containing a large number of grains of all spatial orientations resulting in the average effect for observed physical properties.
To investigate the role of magnetic rare-earth ions and influence of anisotropic characteristics on MCE, single crystals of GdCr 0.5 Fe 0.5 O 3 (GCFO) and ErCr 0.5 Fe 0.5 O 3 (ECFO) were grown. For GCFO, large Gd 3+ moments align below T Gd (the ordering temperature of Gd 3+ moments) = 12 K, which exhibits a relatively isotropic nature. The giant MCE is evidenced by the near-reversible magnetization along and perpendicular to the c-axis induces the maximum magnetic entropy change of S M = 49.8 and 48.8 J/kg·K, respectively. In contrast, Er 3+ moments aligned along the c-axis below T Er (the ordering temperature of Er 3+ moments) = 11 K give rise to a highly anisotropic MCE. This generates a giant rotational MCE, i.e., �S θ = 20.8 J/kg·K. In view of the distinct magnetic aspects of disordered perovskites, these results contribute to the fundamental and applied research on magnetic materials. Figure 1a and 1b show the X-ray diffraction patterns measured at room temperature for the ground GCFO and ECFO, and the simulated patterns analyzed by the Rietveld refinement using the Fullprof software, respectively. The refined results indicate that GCFO and ECFO form an orthorhombic disordered perovskite with the Pbnm space group. The lattice constants were observed to be a = 5.3318 Å, b = 5.5674 Å, and c = 7.6379 Å for GCFO and a = 5.2411 Å, b = 5.5451 Å, and c = 7.5496 Å for ECFO. Additional details of crystallographic data are summarized in Table 1 To examine the magnetic properties of GCFO and ECFO single crystals, the dependence of T on magnetic susceptibility χ = M/H was measured at H = 0.01 T on warming after zero-field cooling (ZFC) and cooling in the same field (FC). The anisotropic χ 's were obtained for the orientations that are parallel (H//c) and perpendicular to the c-axis (H ⊥ c), as shown in Fig. 2a and b for GCFO and Fig. 2c and d for ECFO, respectively. Based on a previous study, the canted antiferromagnetic order of Fe 3+ magnetic moments in GdFeO 3 manifests at T N = 661 K 37 . Unlike other orthoferrites, the canted moments along the c-axis do not rotate on further cooling. In GCFO, half of the Fe 3+ ions are replaced by Cr 3+ ions. However, the same tendency of canted moments aligned along the c-axis is sustained because χ for H//c appears to be larger than that for H ⊥ c in the overall T range, except for the low-T regime ( Fig. 2a and b). The Gd 3+ moments in GCFO are antiferromagnetically ordered along magnetic easy-axis c in the low-T region, evidenced by the smaller magnitude and peaky feature of χ for H//c. Even without an orbital moment (L = 0 of Gd), Gd compounds still possess a very small magnetic anisotropy due to weak dipole-dipole interaction of the large Gd spin. This weak anisotropy is the reason why the Gd spins are pointing towards the c-axis, i.e., a particular easy-axis 38 .

Results and discussion
In ErFeO 3 , the canted antiferromagnetic order of Fe 3+ magnetic moments with a small net moment aligned along the c-axis occurs at T N ≈ 640 K 39,40 with Γ 4 magnetic structure 41 . On further cooling, the net magnetic moment rotates to the a-axis by 90˚ at T SR = 113 K by forming the Γ 2 (F x C y G z ) magnetic structure, followed by the long range antiferromagnetic order of Er 3+ magnetic moments aligned along the c-axis (Γ 1 (C z -type order)) at T = 3.4 K 42,43 . In ECFO, the χ properties of H//c and H ⊥ c directions in the overall T range are strikingly different because of the strong anisotropic nature of the system ( Fig. 2c and d). In contrast to the GCFO, the ECFO exhibits spin reorientation transition at T SR,1 ≈ 180 K, which indicates a considerable decrease in χ along the c-axis and an escalation of χ perpendicular to the c-axis. As shown in Fig. 3, we constructed the Belov-Arrott plot to determine the order of magnetic phase transition at T SR,1 ≈ 180 K. The slope was found to be positive for the overall regime of spin-reorientation, which suggests a second-order phase transition 44,45 . However, EFCO also exhibits signatures of a first-order phase transition such as thermally hysteretic behavior between ZFC and FC data (Fig. 2c) and the absence of a distinct peak in the specific heat (Fig. 2f). Thus, further studies are required to clearly identify the order of this spin-reorientation transition 46,47 . As T was further decreased, sharp anomalies were observed around 10 K indicating the antiferromagnetic order of Er 3+ moments.
The T dependence of the heat capacity divided by T (C/T) measured at H = 0 T for GCFO exhibits a sharp increase below T Gd = 12 K, indicating the ordering of Gd 3+ moments, as shown in Fig. 2e. The influence of the ordering of magnetic Gd 3+ moments on C/T in the low T regime was estimated by subtracting the contributions from Cr 3+ and Fe 3+ ions below T Gd . The subtracted part of C/T was obtained from the following equation: where γ , ρ , andβ are coefficients corresponding to the electron, magnon, and phonon contributions of the Cr 3+ and Fe 3+ moments, respectively. Fitting the data to Eq. (1) resulted in the grey curve of C/T in the inset of Fig. 2e, which indicates the contribution from the interactions of Fe 3+ -Fe 3+ , Cr 3+ -Cr 3+ and Cr 3+ -Fe 3+ pairs and the interaction between the Gd 3+ and Cr 3+ /Fe 3+ sublattices at low T. The estimated entropy change based solely on the order of Gd 3+ moments S Gd in zero H was observed to be 7.5 J/mole•K. S Gd is 21.7% of the expected  www.nature.com/scientificreports/ value of fully-saturated Gd 3+ moments, i.e., 2Rln(2J + 1) = 34.6 J/mole•K, where R is the gas constant and J is the total angular momentum ( J = 7/2 for Gd 3+ ions). Previous experimental studies on neutron diffraction on the polycrystalline ECFO suggest that the spin configuration transforms from representation Γ 4 (G x A y F z ) to Γ 2 (F x C y G z ) on lowering T across T SR, 1 26 . During the ordering of Er 3+ moments at T Er = 11 K, the C z component belonging to Γ 1 was observed on the Er 3+ sublattice. Further decrease in T causes the 2nd spin-reorientation transition at T SR,2 = 7 K on the Cr 3+ /Fe 3+ sublattice where the G y component identified as another Γ 1 phase . Across T SR,2 , larger Er 3+ moments were also observed. Furthermore, the measured C/T value reveals two different transitions, i.e., T Er and T SR,2 at low-T regime, as shown in the inset of Fig. 2f. After subtracting the contribution of the Cr 3+ /Fe 3+ sublattice represented by the gray curve, S Er in zero H was estimated to be 4.11 J/mole•K, which is 8.9% of the expected value of the fully saturated Er 3+ moments, 2Rln(2J + 1) = 46.1 J/mole•K ( J = 15/2 for the Er 3+ ions). T (K) 10 20 30  www.nature.com/scientificreports/ In GCFO, same M values at the maximum H and similar shapes of M curves for different orientations imply the moderately isotropic nature of the Gd 3+ moments associated with the half-filled 4f. electronic configuration (S = 7/2 and L = 0). Therefore, the crystal field effect affected by the symmetry of local environment can be minimum [48][49][50] . Contrarily, the Er 3+ ion exhibits strong anisotropic properties in the ECFO system because its angular momentum (L = 6) breaks the local symmetry and the crystal field effect substantially affects the magnetocrystalline anisotropy 43,50 .
The contrasting magnetic properties of GCFO and ECFO lead to different MCE characteristics measured using the initial M curves with dense T steps for T = 2-30 K, as shown in Fig. 5. In GCFO, the almost isotropic magnetic properties resulted in the typical decreasing trend of the M values similarly for the two different orientations as T is increased (Fig. 5a and 5b). For H//c in ECFO, rapid increase in the initial M curve in the low-H regime at 2 K becomes broaden as T increases; hence, the M value at low H is lower than that at higher T, as plotted in the inset of Fig. 5c. This characteristic of the initial M curves varies above 10 K; thus, the M value exhibits a typical reduction in most of the H regime as T increases. At H ⊥ c, owing to the smaller magnitude and smooth variation of M values, the overall magnitude of M is reduced marginally but continually in the entire regime of H as T increases (Fig. 5d).
The MCE in GCFO and ECFO was quantified by calculating the isothermal magnetic entropy change, S M , at a given T using the Maxwell's relation:   Fig. 5c) considerably cancelled S M ; hence, S M for ΔH = 0-9 T was calculated to be 6.5 J/kg·K at 3 K (Fig. 6c). As T increases further, S M continues to increase and peaks sharply at T SR,2 with a maximum S M value of 39.1 J/kg·K. This feature was derived from the largest drop of isothermal M across T SR,2 , which was verified by measuring initial M curves repeatedly in the low-T regime for various samples of ECFO crystals. Above T SR,2 , S M shows a broad variation and its value is approximately 20 J/kg·K. In contrast with S M for H//c, the overall magnitude of S M for H ⊥ c is largely reduced and its maximum value turns out to be 13.2 J/kg·K for ΔH = 0-9 T (Fig. 6d). Additionally, S M was estimated up to T = 200 K for investigating the influence of χ variation at H = 0.01 T across the spin-reorientation transition of T SR,1 (Figs. 2c and d). As shown in Fig. 7, magnitude and anisotropy of the estimated S M relevant to the spin-reorientation of Cr 3+ /Fe 3+ moments were not pronounced. We also estimated relative cooling power (RCP) for both GCFO and ECFO crystals to show the potential of our single crystals as magnetic cryo-refrigerant. The  Figure 8 displays the resulting �S θ taken at T = 3, 7, 10 and 29 K for H = 9 T. The dissimilar T dependence of S M between H//c and H ⊥ c in the low T regime (Fig. 6c and d) engenders the angle-dependent modulation of �S θ , which changes significantly with T. �S θ at 3 K alters negligibly with θ rotation; however, �S θ at T SR,2 = 7 K shows a sudden increase to 15°. The   Fig. 9 exhibits a clear feature at T SR,2 = 7 K 63 .
As T increases further, the gradual increase of �S θ with θ demonstrates a maximum �S θ of 9.9 and 13.5 J/kg·K at 10 and 29 K, respectively.

Conclusion
This study investigated the anisotropic magnetic and magnetocaloric properties of disordered-perovskite

Methods
Single crystals of GCFO and ECFO were synthesized using conventional flux method with PbO, PbO 2 , and PbF 2 fluxes in a high-T furnace. The stoichiometric ratios of Gd 2 O 3 /Er 2 O 3 , Cr 2 O 3 , and Fe 2 O 3 powders for GCFO and ECFO were mixed and ground using a pestle in a corundum mortar. The mixture was pelletized and calcined at 1000 °C for 12 h. The calcined pellet was finely re-ground, pelletized, and sintered at 1200 °C for 12 h. The same procedure was repeated at 1250 °C for 24 h. The pre-sintered power containing fluxes was heated to 1260 °C in a platinum crucible for 24 h until it was completely dissolved. Thereafter, it was slowly cooled to 850 °C at the rate of 2 °C/h and further cooled to room temperature T at the rate of 100 °C/h. To identify the crystallographic structures of GCFO and ECFO crystals, X-ray diffraction was performed using an X-ray diffractometer (Ultima IV, Rigaku Corp., Japan) with Cu-K α radiation. The T and H dependences of DC magnetization (M) were obtained using a vibrating sample magnetometer at T = 2-300 K and H = -9-9 T in a physical properties measurement system (PPMS, Quantum Design, Inc., USA). The dependence of T on specific heat (C) was measured using the standard relaxation method and PPMS.