Structural and magnetic properties of DyCrO₃

The magnetocaloric effect (MCE) is a phenomenon in which the temperature of material changes when it is exposed to an adiabatically changing magnetic field.¹ This phenomenon has attracted much research due to its potential application in magnetic refrigeration (MR), a technology providing highly efficient and environment-friendly cooling compared to the traditional gas compression/expansion techniques.^1–6 A suitable material for MR should have a large magnetic entropy change (ΔS_M) and large relative cooling power (RCP).^1–6 It should be noted that the efficiency of the MCE is related to the magnetic moment of the ions in the material. Rare-earths Gd, Ho, Tb and Dy have a large magnetic moment; therefore, their oxides exhibit large magnetocaloric effects and may be useful in MR.¹ Rare-earth chromites, represented by chemical formula RCrO₃ (where R denotes a rare earth element), not only exhibit MCE,^1–6 but they also possess unusual physical properties, such as field and temperature-dependent magnetization reversal, exchange bias, magnetoresistance effect, magnetodielectric effect, spin flipping and multiferroicity.^7–22 They have the potential to be utilized in multiferroic applications, solid oxide fuel cells, catalysis, sensors, oxygen ion conductors.²² Most rare-earth chromites are isostructural and crystallize in orthorhombically distorted perovskite structures belonging to the centrosymmetric space group Pbnm.¹² The Cr³⁺ ions are located at the center of the polyhedron of oxygen atoms that have octahedral tilting distortions.¹² These compounds show canted antiferromagnetic (AFM) behaviour and a weak ferromagnetic (FM) moment in the temperature range of 113 K to 300 K, because of the antisymmetric Dzyaloshinskii-Moriya (DM) interaction.^8–23 It has been established that the magnetic behaviour of RCrO₃ compounds is strongly influenced by the Cr³⁺–Cr³⁺ super-exchange interaction rather than the R³⁺–Cr³⁺ and R³⁺–R³⁺ super-exchange interactions.^23–25 The Néel temperature due to ordering of Cr³⁺–Cr³⁺ (T_N^Cr) decreases with the reduction in the cationic radius of R³⁺ from as in the case of LaCrO₃ to LuCrO₃, which is attributed to the decrease in Cr–O bond length and dilution of Cr³⁺–Cr³⁺ interactions.^23–25 At low temperatures, the R³⁺ sublattice orders. There is no critical temperature associated with this ordering and it is independent of the ionic radius of the rare-earth.^3,26 AFM ordering occurs for the Cr³⁺ ions at a relatively higher temperature when compared to the T_N of rare-earth sublattice.^3,26

In this present work, the DyCrO₃ compound is considered. DyCrO₃ has an orthorhombically distorted perovskite structure (Pbnm) at room temperature, and it becomes G-type AFM and shows weak ferromagnetism below T_N^Cr ≈ 146 K due to the ordering of Cr moments. The Dy moments orders antiferromagnetically below T_N^Dy ≈ 2.2 K.^1,3,11,22 Considering the background from the literature, DyCrO₃ can be considered as an important compound in view of its structure as well as the observed magnetic behaviour. Thus, it should prove interesting to study their structural, magnetic, and MCE properties adopting a simple, cost-effective sol-gel technique.

Dysprosium (III) nitrate pentahydrate, 99.9% (Dy(NO₃)₃·5H₂O), chromium (III) nitrate nonahydrate, 99% (Cr(NO₃)·9H₂O) and ethylene glycol (C₂H₆O₂) were used for the synthesis of both DyCrO₄ and DyCrO₃ powder by means of sol-gel techniques,²⁷ from stock solutions of 0.23 M of Dy(NO₃)₃.5H₂O and 0.5 M Cr(NO₃).9H₂O. Ethylene glycol was added to the solution mixture for gelation. The gel residue was then left overnight for aging, dried, powdered and calcined for 2 h in a furnace at 1000 °C to obtain powders of DyCrO₃, respectively. An x-ray diffraction (XRD) study to identify the crystalline nature and phase of the samples was done on an Empyrean PAN analytical powder x-ray diffractometer using monochromatic Cu-K_α radiation (⁠$λ=1.54Å)$⁠. The morphology and microstructure of DyCrO₃ powder were investigated using a multipurpose JEM-2100 transmission electron microscope (TEM) operated at an accelerating voltage of 200 kV. The average particle sizes of the samples were measured based on the TEM imaging of particles. The magnetic properties were carried out using a 14 T Cryogenic Measurement System with a vibrating sample magnetometer (VSM) insert.²⁸ 

The structure and phase purity of the DyCrO₃ sample was confirmed through analysis of XRD spectra, as shown in Fig. 1(a). The as-synthesized sample was found to be in the amorphous phase. Calcining the amorphous powder at 1000 °C resulted in the formation of DyCrO₃, having orthorhombically distorted perovskite structure belonging to centrosymmetric space group Pbnm.²² This calcination temperature was chosen based on the chemical phase diagram of RCrO_3+y (with y = 0 or 1)²⁵ compounds to form the desired amorphous material. Rietveld refinement of the powder XRD patterns was performed using general structure analysis system (GSAS) software (Fig. 1(a)). The difference between calculated and experimental data (as seen in Fig. 1(a)) is minimal. The reliability of fitting has been confirmed by a variety of factors, including weighted profile factor (wR), the goodness of fit (GOF) and χ². Crystallographic parameters of this sample were obtained from the refinement and are presented in Table I. The obtained lattice parameters are consistent with those reported in the literature.^1,18,29

In order to study the morphological features of the DyCrO₃ samples, TEM measurements were carried out. Fig. 1 (b) shows the TEM image of DyCrO₃. The DyCrO₃ particles are well-formed with irregular shapes, as shown in Fig. 1 (b). The average particle size was estimated to be 215 ± 3 nm from the log-normal fitting of the particle size distribution histogram (inset of Fig. 1 (b)). The selected area electron diffraction (SAED) patterns indicating spot patterns corroborate the crystalline nature of the samples (inset of Fig. 1 (b)).

The susceptibility as a function of temperature $χT$ curves were obtained using zero-field-cooling (ZFC) and field-cooling (FC) measurement protocols,²⁸ with an applied magnetic field of 0.05 T for DyCrO₃ are shown in Fig. 2. The ZFC curve approximately traces the path of the FC curve. The transition $TNCr$ observed in the inset of Fig. 2(a) is associated with the canted AFM ordering (G–type) of Cr moment due to an antisymmetric Dzyaloshinskii–Moriya (DM) interaction, whereas the Dy moment retains its paramagnetic (PM) ordering.²⁹ The susceptibility increases for T < $TNCr$ because of the PM response of Dy³⁺ ions, as these continuously rotate to align with the Cr³⁺ before it antiferromagnetically orders at T_SR.^24,29 The transition observed at T_SR is attributed to the second-order spin reorientation of Cr³⁺ moment from G_xF_z–type to the G_zF_x–type canted AFM order, as observed in other orthochromite perovskites, La_0.5Pr_0.5CrO₃²⁴ and GdCrO₃.²⁷ The ordering of Dy³⁺ moments cause a sudden decrease in susceptibility below T_SR.²⁹ In this study, $TNCr$ and the T_SR was determined from the peaks in the $dMdTT$ curves and found to be 147.1 ± 0.1 K and 4.81 ± 0.04 K, respectively. The value of $TNCr$ determined in this study is higher compared to those previously reported in the range 145 K to 146 K.^2,11,29–31

At temperatures above $TNCr$⁠, the DyCrO₃ displays Curie-Weiss behaviour and the χ⁻¹(T) curve is shown in Fig. 2(b). The PM region of the ZFC of the sample was fitted by the Curie-Weiss (CW) law given by³⁰ 

where k_B = 1.38062 × 10⁻²³ J.K⁻¹ is the Boltzmann constant, θ_CW is the PM CW temperature, N_A = 6.022 × 10²³ mol⁻¹ is Avogadro’s constant and $μeff=gμ0j(j+1)$ is the net effective moment. In this case θ_CW and μ_eff were determined to be −129.5 ± 0.2 and 12.66μ_B, respectively. The negative θ_CW indicates that the dominant exchange interaction is AFM. It is also noted that these values are very large when compared to those in previous literature where θ_CW varies from −19.5 K to −61.7 K and μ_eff from 8.8 to 11.7 µ_B.^2,11,29,31 The obtained higher θ_CW and μ_eff values in the present case could be indicative of the existence of vacancies that most probably exist in the Dy and/or Cr atomic sites, due to the dissimilar morphologies of the various DyCrO₃ samples resulting from the different synthesis method followed in the referenced studies.¹¹ The distinct features in magnetic data can be ascribed to the complex interplay of magnetic cations when the particle size is in micrometre range.²⁷ 

Magnetization as function of the applied magnetic field, M(μ₀H), studies on DyCrO₃ were carried out for $T<TNCr$ and are shown in Fig. 2(c). The M(μ₀H) behaviour is in line with previously reported results.²⁹ The material is observed to have multiple magnetic states, AFM with weak FM order below the Néel temperature and then becomes a pure antiferromagnet below the spin reorientation temperature. The behaviour seen in the Arrott³² plots is shown in Fig. 2 (d). The change in the curvature or gradient of the curves across the transition temperature is clearly noticeable (Fig. 2 (d)). The trend in curvatures between $TNCr$ and T_SR appears complex and requires detailed study. However, this proved difficult for the current results because of the multiple magnetic states present in the sample as outlined above and will be addressed in a later report.

The dependence of the magnetization on the applied field M(μ₀H) measurements was used to investigate the MCE properties of the DyCrO₃. The MCE properties can be characterized mainly by entropy change, ΔS_M, that is dependent on temperature (T) and magnetic field (H), given by¹ 

where M_i+1 and M_i represent measured values of magnetization at temperatures of T_i+1 and T_i, respectively, and ΔH is the differential element of the applied magnetic fields. The relative cooling power (RCP) is given by¹ 

where ΔT_FWHM is the full width at half maximum (FWHM) of the temperature dependent ΔS_{M, Max} curve. The M_i+1 and M_i used to calculate the ΔS_M values for the sample was obtained from fitting the M(μ₀H) curves with the law of approach to saturation magnetization equation:³³ 

where M_S is the spontaneous magnetization, H is the applied magnetic field, a and b are fitting constants and K is the high field magnetization resulting from an increase in spontaneous magnetization by the application of a field. Based on Eq. (2), the calculated values of ΔS_M of the DyCrO₃ are shown in Fig. 3(a). The maximum of ΔS_M is observed at about 10 K, showing that the MCE of this sample is dependent on the Dy³⁺ moment. The maximum ΔS_M increases for the sample with an increase in the applied field. The maximum ΔS_M and RCP (Fig. 3 (b)) for this sample is large compared to the observed values of 10.5 J.kg⁻¹.K⁻¹ at 4 T, 15 J.Kg⁻¹.K⁻¹ at 4.5 T and 8.4 J.kg⁻¹.K⁻¹ at 4 T as reported in the literature.^1,11,29

According to Oesterreicher et al.,³⁴ the field dependence of the maximum ΔS_M can be expressed as following equation:³¹ 

where a is a proportionality constant and the exponent n is related to magnetic state which depends on temperature and magnetic field. The exponent n takes the value of 1 or 2 for either FM or PM state, respectively. Provided that the materials follow the Curie-Weiss law. On the other hand, the field dependence of the relative cooling power (RCP) has the following relation:³⁵ 

where C is a proportionality constant and δ is a critical constant. The critical exponents n = 1 and δ → ∞ were obtained from the linearization of Eqs. (4) and (5). As the complex magnetic behaviour of the DyCrO₃ compound is influenced by the Cr³⁺–Cr³⁺, R³⁺–Cr³⁺ and R³⁺–R³⁺ super-exchange interactions, which are dominant at different temperatures.^23–25 The obtained value of n shows the (weak) ferromagnetic state resulting from the canted spin structure of the Cr³⁺ sublattice. This value is further confirmed by the value of δ → ∞ following the relation:³⁶ 

where β is a critical constant.

Pure phase DyCrO₃ was successfully synthesized using the sol-gel technique. The phase formation from the as-synthesized amorphous sample was done by choosing the calcination temperature of 1000 °C, resulting in the formation of pure phase DyCrO₃. The phase formation of the compound was confirmed through XRD measurements. TEM studies revealed the DyCrO₃ particle morphology with an average particle size 215 ± 3 nm, while the SAED pattern confirms the crystallinity, with the EDS verifying the pure elemental composition. The AFM with weak FM transition temperature, $TNCr$⁠, and the spin reorientation temperature, T_SR, were determined to be 147.1 ± 0.1 K and 4.81 ± 0.04 K, respectively, using Lorentzian fits over the $dMdT$ as a function of the T curve. Arrott plots reflect the various magnetic states; however, the critical exponents proved difficult to obtain from these plots because of the multiple magnetic states present in the sample and will be addressed in a later report. M(μ₀H) measurements were used to investigate the MCE properties of the DyCrO₃. The maximum values of ΔS_M and RCP for this sample are large when compared to values previously reported. The critical exponents δ and n further confirmed the weak FM behaviour in the sample. The large MCE observed in the temperature range of 10 – 80 K makes DyCrO₃ a potential candidate for MR applications.

Financial aid from the SANRF (Grant No. 120856), as well as the University of Johannesburg (UJ) URC and FRC (Grant Nos. 282810, 083135) is acknowledged. The use of the NEP Cryogenic Physical Properties Measurement System at UJ, obtained with the financial support from the SANRF (Grant No. 88080) and UJ, RSA, is acknowledged. The use of the Spectrum facility in the FoS at UJ is acknowledged.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding authors upon reasonable request.