Effect of Al Substitution on Structural, Magnetic, and Magnetocaloric Properties of Er₆Fe_23−xAl_x (x = 0 and 3) Intermetallic Compounds

Abstract

The structural, magnetic, and magnetocaloric properties of Er₆Fe_23−xAl_x (x = 0 and 3) intermetallic compounds have been studied systematically. Samples were prepared using the arc furnace by annealing at 1073 K for one week. Rietveld analysis of XRD shows the formation of pure crystalline phase with cubic Fm-3m structure. Refinement results show that the unit cell volume decreases with increasing Al content. The Curie temperature Tc of the prepared samples was found to be strongly dependent on the aluminum content. This reduces magnetization and the ferrimagnetic phase transition temperature (Tc) from 481 K (for x = 0) to 380 K (for x = 3), is due to the substitution of magnetic element (Fe) by non-magnetic atoms (Al). With the increase of the Al content, a decrease in the values of magnetic entropy is observed. The magnitude of the isothermal magnetic entropy (|∆SM|) at the Tc decreases from 1.8 J/kg·K for x = 0 to 0.58 J/kg·K for x = 3 for a field change 14 kOe. Respectively, the relative cooling power (RCP) decreases with increasing Al content reaching 42 Jkg⁻¹ for x = 0 to 28 Jkg⁻¹ for x = 3.

1. Introduction

Intermetallic compounds that contain rare-earth elements are particularly interesting because of their temperature- and pressure-dependent structural and physical transitions that make them potential candidates for magnetic applications [1,2,3,4,5,6,7,8,9].

Investigations on the structure and the magnetic properties of R₆M₂₃ (M = Mn, Fe) have been reported in the literature [10]. It was found that R₆Fe₂₃ compounds crystallize in Th₆Mn₂₃ type structure and with a space group of Fm-3m.

Rare earth 4f-Transition metals 3d intermetallic compounds of 6:23 stoichiometry show very interesting magnetic properties [11]. The magnetic characterization of the rare earth 5d states is of special significance for the magnetic properties of the R–M materials as they mediate the magnetic interaction between the R and M sublattices [12].

Er₆Fe₂₃ is of great interest among the compounds in the Er–Fe system, because it contains a large amount of iron in the unit cell. There are 116 atoms per unit cell occupying five non-equivalent crystal sites (24e, 4b, 24d, 32f₁, and 32f₂). The magnetic properties of Er₆Fe₂₃ were studied by Boltich et al. [10]. This compound presents compensation temperature at 95 K and a ferrimagnetic behavior at 493 K. However, there have been few studies on Er₆Fe₂₃ since then. This seems to be due to the difficulty entailed in preparing the Er₆Fe₂₃ intermetallic compound.

Neutronic study on Dy₆Mn₂₃ and Er₆Mn₂₃ [13] shows that this collinear ferromagnetic arrangement of the Mn sublattice persists at low temperature when the R sublattice is ordered. Both neutron diffraction study and magnetization measurements on single crystals [13,14,15] provide that the R sublattice then adopts a complex non-collinear magnetic order. More importantly, the R–Mn exchange coupling is stronger with the 24d and 4b sites than with the 32f₁ and 32f₂ sites. This yields a behavior in apparent contradiction with the usual sign of the R–Mn coupling in intermetallic: in R₆Mn₂₃ the low temperature magnetization is higher with the heavy lanthanides than with the light ones [16,17,18,19].

In the present work, we focused on the preparation, structure, magnetic properties, and magnetocaloric of the Er₆Fe_23−xAl_x (x = 0 and 3) compounds and the influence of Al on the magnetic properties.

2. Experimental

Polycrystalline samples of Er₆Fe_23−xAl_x (x = 0 and 3) were synthesized by arc-melting stoichiometric amounts of the constituent elements: Fe, Er, and Al (with purity 99.99%) under protective argon atmosphere. The pellets were turned over and remelted several times, in order to ensure a good homogeneity. The samples were wrapped in a tantalum sheet and annealed at 1073 K for one week to improve the atomic diffusion kinetics and to ensure a good crystallinity. After arc-melting, these alloys were finished by direct water quenching of the pellets from the furnace. The intermetallics were ground to powder by means of a mortar to prepare them for measurements. The powders were sieved using a 40 μm pore size metallic sieve.

The final product was characterized using powder X-ray diffraction (XRD) on a Brucker diffractometer (Brucker, Thiais, France) using copper radiation (λKα1 = 1.540600 Å, λKα2 = 1.544390 Å) to determine the crystallographic structure and identify the present phase. Forty milligrams of powder was loaded on a sample holder as a randomly oriented powder. Scans of 2θ between 20° and 80° with steps 0.015° and counting times of 13.5 s per point were measured at 300 K. In this latter procedure, the Bragg peak breadth is a combination of both instrument and sample dependent effects. To remove these aberrations, it is necessary to assemble a diffraction pattern from the line broadening of a standard material with a good crystallinity such as silicon, to determine the instrumental broadening. Silicon is added as an internal standard, the pattern is established by the Rietveld process [20] using Fullprof code as a single-phase pattern. The Si unit-cell parameter fixed at a = 12.4312 Å and U = 0.007754, V = −0.010764, and W = 0.005756. The data of Er₆Fe₂₀Al₃ were analyzed and refined with the same technique of refinement using for silicon. U, V, W-half-width parameters normally characterizethe instrumental resolution function.

The magnetic and magnetocaloric measurements of the sample were performed using the Physical Properties Measurement System PPMS9 (Quantum Design), at low temperature, under an applied field up to 5 T in a temperature range from 10 K to 300 K and by the magnetometer SQUID: (Superconducting QUantum Interference Device) at high temperature, between 300 K and 500 K (for magnetic and magnetocaloric measurement).

3. Results and Discussion

3.1. X-ray Diffraction Results

Erbium ternary compounds of the Er–Fe–Al system contain a large number of intermetallic compounds were constructed at 1073 K in our laboratory [21]. We extended our investigation in a systematic search of new interesting intermetallic compounds in this ternary diagram, and confirmed a significant extension by the X-ray analysis. This result showed that the samples contained single phase of Er₆Fe_23−xAl_x (x = 0 and 3). A comparison of the X-ray diffraction patterns of the Er₆Fe_23−xAl_x (x = 0 and 3) compounds revealed that there is no change in the patterns, except the shift in position of Bragg reflections, towards higher angle sides, indicating variation in the lattice parameters. The index results of X-ray diffraction patterns indicated that all the alloys crystallizes in the cubic Th₆Mn₂₃-type crystal structure, Th₆Mn₂₃-type structure with space group Fm-3m and lattice parameters a = 11.991–12.068 Å. The variation of the unit cell parameters shows that the unit-cell parameters decrease with the increase of Al contents. There are 116 atoms per unit cell occupying five non equivalent crystal sites (24e, 4a, 24d, 32f₁, and 32f₂). The 24e positions are accessible only to the rare earth atoms, whereas iron atoms occupy the other positions.

The analysis of the XRD patterns of the Er₆Fe₂₀Al₃ compound shows that this system crystallizes in the cubic Th₆Mn₂₃-type structure and Fm-3m space group. The lattice parameter is: a = 12.0082(2) Å. This value is slightly smaller than that of Er₆Fe₂₃ (a = 12.01(3) Å) [22]; in agreement with a simple steric effect of Al (r_Fe = 1.26 Å) substitution by a bigger atom (r_Al = 1.43 Å). The representative Rietveld refinement results for ErFe₂₀Al₃ are shown in Figure 1 and listed in

Table 1. Details of structure refinement for Er₆Fe₂₀Al₃.

Compound	Er6Fe20Al3
Type structure	Th6Mn23
Space group	Fm-3m
Cell parameters:
A (Å)	12.0082(2)
Volume (Å3)	1748.91(3)
Dcal (g·cm−3)	7.199
RBragg	3.21
Rfactor	2.37
Rp	6.68
Rwp	8.58
χ2 = [$\frac{{\mathrm{R}}_{\mathrm{wp}}}{{\mathrm{R}}_{\exp }}$]2 (%)	2.26

. The observed and the calculated diffraction patterns for Er₆Fe₂₀Al₃ at room temperature match well and show that the sample contains the single Th₆Mn₂₃ phase.

We have determined the distribution of Al atoms over the three possible Fe crystallographic sites. According to the Rietveld refinement and especially to the occupation refinement, we demonstrated and determined that the Al atoms are distributed in three crystallographic sites: 32f₁, 32f₂, and 24d. The best agreement factors R_F and R_B of the Er₆Fe₂₀Al₃ compound have been obtained with Al located in the 32f₁, 32f₂, and 24d (

Table 2. Atomic coordinates, occupations, and equivalent isotropic displacement parameters for Er₆Fe₂₀Al₃.

Atoms	Sites	x	y	z	Occ	Ueq
Fe1	32f1	0.1215(2)	0.1215(3)	0.1215(3)	0.890(2)	0.037(6)
Al1	32f1	0.1215(2)	0.1215(3)	0.1215(3)	0.091(2)	0.037(6)
Fe2	32f2	0.3234(2)	0.3234(2)	0.3234(2)	0.880(2)	0.021(6)
Al2	32f2	0.3234(2)	0.3234(2)	0.3234(2)	0.110(2)	0.021(6)
Fe3	24d	0.00000	0.25000	0.25000	0.740(2)	0.023(6)
Al3	24d	0.00000	0.25000	0.25000	0.259(1)	0.023(6)
Fe4	4a	0.00000	0.00000	0.00000	1.00	0.011(5)
Er	24e	0.2978(2)	0.00000	0.00000	1.00	0.001(4)

Ueq (Ų) = 1/3Σ_iΣ_j U_ij a_i* a_j* a_i a_j.

). In fact, in our groups, we have proven in the Er₆Fe_17.66Al_5.34C_0.65 compound [23] that only one of the two 32f sites exhibited a significant deviation from the full occupancy with Fe atoms, indicating a substitution with Al atoms. Carbon atoms in this compound localized a new octahedral site. However, with the same structure type, Lemoine et al. [24] did not demonstrate the distribution of Co in the Gd₆(Mn_1−xCo_x)₂₃ system.

In the structure of Er₆Fe₂₀Al₃ Figure 2, all metal atoms have high coordination numbers, as is usually the case for intermetallic phases. The Er atoms are coordinated by 12 Fe/Al, one Fe, and four Er atoms. The Er–Er distances of 3.445(2) Å are rather large, considering the metallic radius of erbium for the coordination number (CN) 17. Nevertheless, if one discounts these erbium neighbors, the coordination polyhedron of the erbium atom would look rather incomplete. The Er–Fe/Al or Er–Fe distances cover the range between 2.963(3) Å and 3.588(2) Å.

3.2. Magnetic Properties

The Curie temperature is a direct measure of the exchange interaction, which is the origin of ferromagnetism. This interaction depends on the interatomic distance. In fact, T_C is determined by three kinds of exchange interactions: 3d–3d exchange interaction between magnetic Fe moment sublattice, 4f–4f exchange interaction between the magnetic moments of rare earth sublattice and the inter-sublattice 3d–4f and 3d–5d exchange interaction, which probably favor a ferrimagnetic collinear structure and the anisotropies of the Er, Al, and Fe siteshad to be considered.

Figure 3 shows the thermomagnetic curve of Er₆Fe_23−xAl_x with x = 0 and 3. The value of T_C (which are flagged with arrows) has been estimated from the minimum of the temperature derivative of the magnetization, dM = dT, vs. temperature which has presented in the set of Figure 3.

For x = 0, the T_C is equal to 481 K. The magnetic result is in good agreement with those already reported [10,25]. In the compound Er₆Fe₂₃, moments of erbium and iron couples antiparallel and the Curie temperature depends on the interactions Fe–Fe, Er–Fe and Er–Er [25]. The large exchange interaction depends on the Fe–Fe distance. It is obvious that the Fe–Fe interactions play the most significant role in the magnetic order of Er₆Fe₂₃ compound.

The magnetic transition T_C decreases from 481 K, for x = 0, to 380 K, for x = 3. We can conclude in the case of ternary Er₆Fe₂₀Al₃ that the interatomic-distance Fe–Fe decreases with increasing the content of aluminum. When Fe is substituted by non-magnetic Al, Fe–Fe interactions become weaker, leading to a Curie temperature decrease and a tendency magnetic disorder. According to the dilution law [26], when iron atoms are substituted by aluminum atoms, the magnetization of the sublattice decreases and, consequently, the molecular magnetic moment of the compound increases because the magnetization of the erbium sublattice in Er₆Fe₂₃ is larger than the iron sublattice. This decrease of Curie temperature is comparable with a system having a same type of structure (Th₆Mn₂₃), in the magnetic properties of Gd₆(Mn_1−xFe_x)₂₃ alloys (x < 0.2) [27], the substitution of Mn by Fe leads to a decrease of the T_C down to 175 K for x = 0.2 [27,28,29,30,31].

Figure 4 shows that the Er₆Fe₂₀Al₃ compound has a minimum magnetization at 75 K. Indeed, the net moment at low temperature points in the direction of the Er moment, while in others, it points in the direction of the Fe moment, as evidenced by the presence of the compensation temperature at T_comp = 75 K. Before this temperature, the Er free ion moment coupled antiparallel to the Fe moments. The magnetic properties of the Er₆Fe₂₀Al₃ sample can readily be understood in reference to the Er₆Fe₂₃ compound from a model in which the Er moments are coupled antiparallel to the iron moments. This behavior is similar to the Er₆Fe₂₃ magnetic behavior (T_comp = 95 K). This decrease is due to substitution by the Al atoms, and is comparable with the Er_6−xY_xFe₂₃ [32]. The magnetic saturations at 300 K of Er₆Fe_23−xAl_x (x = 0 and 3) are about 14.74 and 10.24 μ_B, respectively.

3.3. Magnetocaloric Effect

In order to understand the influence of Al substitution in the magnetocaloric properties, we plotted the magnetization vs. the applied magnetic field (from 0 to 14 KOe) obtained at various temperatures for the Er₆Fe_23−xAl_x (x = 0 and 3) samples in Figure 5. These curves provide evidence of a close relationship between the magnetization M and the applied magnetic field H. Below T_C, the magnetization M increases sharply with magnetic applied field for (H < 0.5 kOe) and then saturates above 1 kOe. The saturation magnetization shifts to higher values with the decreasing temperature. When the temperature approaches T_C, the magnetization change becomes large.

To explain with precision the Curie temperature and the transition nature, the Arrott plots obtained from the magnetic field dependence of isothermal magnetization, are shown in Figure 6. The Banerjee criterion [33] has been used to determine the nature of the magnetic phase transition in intermetallics. The positive or negative slope of M² vsμ₀H/M (Arrott plot) curves indicates whether the magnetic phase transition is second or first order, respectively. In this compound, the positive slope of the Arrott plots confirms a characteristic of the second-order magnetic transition.

The magnetocaloric properties of Er₆Fe_23−xAl_x have been calculated from magnetization vs. applied magnetic field (M(H)) curves, measured for temperatures within a 50 K interval centered around the T_C of each sample. The desired entropy variation ΔS(T) is then investigated, following the Maxwell relation [34,35] (Equation (1))

$$\Delta S_M\left(T,H\right) =S_M\left(T,H\right)-S_M\left(T,\mathbf{0}\right)= {{\displaystyle \int }}_{\mathbf{0}}^{{\mathsf{\mu }}_{\mathbf{0}}{H}_{\max }}{(\frac{\mathbf{\partial }S\left(T,H\right)}{\mathbf{\partial }H})}_{\mathrm{T}}dH$$

(1)

From Maxwell’s thermodynamic relation (Equation (2))

$${\left(\frac{\mathbf{\partial }{S}_M\left(T,H\right)}{\mathbf{\partial }H}\right)}_{\mathbf{T}}={\left(\frac{\mathbf{\partial }M\left(T,H\right)}{\mathbf{\partial }T}\right)}_{\mathbf{H}}$$

(2)

One can obtain the following expression (Equation (3))

$$\Delta S_M{\left(T,\Delta H\right)}_{\Delta H}= {{\displaystyle \int }}_{\mathbf{0}}^{{\mathsf{\mu }}_{\mathbf{0}}{H}_{\max }}{(\frac{\mathbf{\partial }M\left(H,T\right)}{\mathbf{\partial }T})}_{\mathbf{H}}dH$$

(3)

where M is the magnetization, T is the temperature,and μ₀H_max is the maximum external field, with μ₀ = 1 (c.g.s).

The Figure 7 presents the magnetic entropy change ΔS_M for both samples as a function of temperature under different magnetic fields. A remarkable feature is the exact correspondence between the maximal ΔS_M and the recorded ferrimagnetic transition at T_C, while ΔS_M distributes over a relatively wide T range. The maximum entropy change of only 0.58 J/kg K and 1.8 J/kg K upon a filed variation of 14 kOe was obtained for Er₆Fe₂₀Al₃ and Er₆Fe₂₃, respectively, indicating relatively small magnetic order fluctuations around the ferrimagnetic transitions.

For the materials actually used in magnetic refrigeration, the most meaningful parameter is the relative cooling power (RCP). The RCP based on the magnetic entropy change ΔS_M is defined as [3,36,37]

$$RCP=\left|-\Delta S_M\left(T,H\right)\right|\times \mathsf{\delta }{T}_{FWHM}$$

(4)

where δT_FWHM is the full-width at half the maximum of the magnetic entropy change curve. The calculated RCP values for ΔH = 14 kOe are 28 and 42 J/kg K for Er₆Fe₂₀Al₃ and Er₆Fe₂₃, respectively. The RCP values show a difference between the two samples, due to the Fe/Al substitution.

We list in

Table 3. Curie temperature T_C, magnetic entropy change (ΔS_M), and relative cooling power (RCP) for R₆M₂₃ compounds

Compound	TC (K)	H (kOe)	ΔSM (max) (J/kg·K)	RCP (J/kg)	Reference
Y6Mn23	505	50	2.1	-	[38]
Nd6Mn23	441	50	2.9	-	[38]
Sm6Mn23	456	50	2.6	-	[38]
Er6Mn23	430	50	34.3	-	[38]
Dy6Mn23	447	50	24.6	-	[38]
Gd6Mn23	489	10	1.6	-	[38]
Gd6(Mn0.875Co0.125)23	265	10	2.4	-	[38]
Gd6(Mn0.85Co0.15)23	219	10	2.9	-	[38]
Er6Fe20Al3	481	14	0.58	28	This work
Er6Fe23	380	14	1.8	42	This work

the T_C, the ΔS_M (max), and the RCP values for our materials with 14 kOe magnetic applied field in comparison with other results reported in the literature. The lower values of (ΔS_M (max)) compared with those of the Er₆Fe_23−xAl_x compounds are explained by the small value of the iron magnetic moment which is associated with a non-collinear Er magnetic structure. The magnetic entropy depends on the magnetic applied field and the nature of rare-earth.

4. Conclusions

In conclusion, we determined the effect of the Al substitution on the magnetic behavior, magnetic phase transition and magnetocaloric properties of Er₆Fe_23−xAl_x (x = 0 and 3) compounds. The structural study revealed that the samples with x = 0 and 3 were crystallized in the cubic Th₆Mn₂₃ type structure. In addition, the Curie temperature decreased with increasing Al concentration. We have found that the samples under went a second-order magnetic phase transition. The decrease of T_C was accompanied by a reduction of the isothermal entropy change. For a magnetic field change of 0–14 kOe, a maximum magnetic entropy values of 0.58 J/kg·K and 1.8 J/kg·K for Er₆Fe_23−xAl_x (x = 0 and 3) compounds are determined around T_C. The values of this parameter are lower than those reported in Th₆Mn₂₃ alloys, which can be attributed to the presence of the contribution of Er free moment antiparallel to the Fe moment in this system.