1 Introduction

In recent years, rare-earth (R)–transition metal (T) intermetallic compounds of the RxTy type have been extensively studied due to the R–T exchange coupling interaction, which is always ferromagnetic when R is a light rare earth and antiferromagnetic with the heavy rare earths [1, 2]. Depending on the constituent element or composition, various properties such as magnetovolume effect [3], magnetocrystalline anisotropy [4, 5], magnetic-field-induced martensitic transformation, strain effect [6], magnetoresistance effect [7, 8] and magnetocaloric effect [9, 10] were observed. Among them, the binary R6Mn23 compounds crystallize in the Th6Mn23-type structure (space group Fm-3m), which comprises one crystallographic site for the R atoms (24e) and four sites for the Mn atoms (4b, 24d, 32f1 and 32f2) [11,12,13]. And, the magnetic transition temperature of R6Mn23 compounds is at high Curie temperature (TC = 400–500 K) [14,15,16,17]. In addition, among the binary Gd–Mn systems, Gd6Mn23 compound (TC ~ 500 K) is believed to violate this antiferromagnetic coupling for a long time, which is because the magnetization of Gd6Mn23 is larger than that of Y6Mn23 [1, 18,19,20,21], and the magnetization of R6Mn23 involves a heavy rare-earth element [20]. Neutron diffraction experiments [21] had also confirmed that the Gd–Mn coupling was antiferromagnetic in fact, as always observed. More recently, the magnetocaloric properties of the Th6Mn23-type R6Mn23 compounds were systematically studied [22], including their derivatives Gd6−xRxMn23 [23], Gd6(Mn1−xCox)23 (x ≤ 0.3) [24] and Gd6(Mn1−xFex)23 (x ≤ 0.2) [25]. In these ferromagnetic materials, the R and T (T = Mn or Fe) sublattices order magnetically at two different temperatures which can be altered by chemical replacement. For example, Fe substitution can effectively reduce the ordering temperature from TC = 489 K for x = 0.0 to TC = 176 K for x = 0.2 in Gd6(Mn1−xFex)23 [25]. Similar results were found in Co-substituted Gd6(Mn1−xCox)23 system [24].

In this work, based on the research results of references [22,23,24,25] and our related research on Gd–Fe–Mn system [26], we have to further focus on the influences of the Mn/Fe ratio on magnetic and magnetocaloric properties of the Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) compounds, which expand the Fe composition range. This work helps to further understand the physical properties of Gd6Mn23 system and to explore their potential value for future cooling applications.

2 Experiment

Polycrystalline Gd6(Mn1−xFex)23 (x = 0.0–0.5) samples were synthesized by arc melting technique using stoichiometric amounts of Gd (99.99%), Mn (99.99%) and Fe (99.99%) under a purified argon atmosphere. To ensure homogeneity, the ingots were re-melted three times. Then, the samples are wrapped in tantalum foils, sealed in a high-vacuum quartz tube, annealed at 800 °C for 5 days and finally quenched in ice water. The crystal structure was studied using powder X-ray diffraction (XRD) patterns with a Cu Kα1 radiation experiment at room temperature. Magnetization measurements were taken in the temperature range from 10 to 380 K with an external applied magnetic field up to 50 kOe using a physical property measurement system (PPMS-9, Quantum Design.)

3 Results and Discussion

Figure 1 shows the XRD patterns measured at room temperature and Rietveld refinement results of Gd6(Mn1−xFex)23 (x = 0.0–0.5) compounds. In order to reveal the crystal structure, the XRD patterns were analyzed using FullProf package. The original record data are indicated by red circles, and the calculated profile is the continuous line overlying them. The lower curve shows the difference between the observed and calculated intensity. The detailed analysis of the results confirms that all samples exist as a single cubic phase with the Th6Mn23-type structure (space group of Fm-3m) and form a continuous solid solution [18,19,20,21,22,23,24,25]. The values of the lattice constants derived through the Rietveld refinement are presented as a function of Fe concentration x in Fig. 2, and the values are listed in Table 1. The R-values and GOF of these refinements consequences are also given in Table 1, suggesting that these results agree well with those reported for Gd6Mn23 alloys [12, 13, 18,19,20,21,22,23,24,25]. As is well known, the displacement of foreign atoms with distinct radii can lead to changes in lattice parameters. In this compound, the lattice dimensions decrease almost linearly with the Fe content, owing to the smaller metallic radius of Fe compared to that of Mn.

Fig. 1
figure 1

Rietveld refined powder XRD patterns of Gd6(Mn1−xFex)23 (x = 0.0–0.5) compounds (Color figure online)

Full size image
Fig. 2
figure 2

Lattice constants a and the unit-cell V values of Gd6(Mn1−xFex)23 as a function of Fe content x (Color figure online)

Full size image
Table 1 The lattice constants a, unit-cell volume V, GOF and TC of Gd6(Mn1−xFex)23 compounds
Full size table

The magnetization as a function of temperature (M(T)) for Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) compounds is measured in the 200 Oe external field from 10 to 380 K on heating process, as shown in Fig. 3a. The results show that the magnetization decreases rapidly with the increase in temperature, suggesting the occurrence of a magnetic phase transition from ferromagnetic to paramagnetic state. Here, TC is defined as the temperature where dM/dT is the minimum. It has been previously reported that the TC of Gd6Mn23 compounds is 489 K [18,19,20,21,22,23,24,25]. As shown in the inset of Fig. 3a, the TC of the compound changes with the Fe content. From Ref. [25], the TC is 369 K at x = 0.1 and 176 K at x = 0.2. In this work, the TC at x = 0.3, 0.4 and 0.5 is 99 K, 89 K and 293 K, respectively. As is well known, the structural and magnetic properties of these compounds are strongly dependent on their composition [15, 16, 18,19,20,21,22,23,24,25,26,27,28]. Therefore, three reasons can account for the drastic change in TC when the x increases from 0.0 to 0.4: (i) It is recognized that the substitution of foreign atoms with different radii, which can be treated as ‘chemical pressure,’ may possess an effect similar to applying external mechanical pressure [29]. Since the radius of Fe ions is smaller than that of Mn, the substitution of Fe for Mn can induce the contraction of lattice in Gd6(Mn1−xFex)23 compounds and consequently modify the relative stability between the ferromagnetic and paramagnetic state, leading to the decrease in TC. (ii) The substitution of Fe for Mn would enhance the antiferromagnetic coupling interactions between Mn and Fe in Gd6(Mn1−xFex)23 compounds and take the responsibility of decreasing TC to some extent [1, 2, 15, 16, 19, 22,23,24,25]. (iii) In R6Mn23 compounds involving a heavy rare-earth element, the R magnetic moments couple antiferromagnetically with the Mn sublattices to form complex non-collinear magnetic structures [21, 30]. The (usual) negative sign of the Gd–Mn magnetic exchange has been confirmed by neutron inelastic scattering on Gd6Mn23 [31]. The decrease in both the magnetization and the ordering temperature upon Fe substitution in Gd6(Mn1−xFex)23 is due to the Fe atoms carrying a lower magnetic moment than that of Mn atoms in these phases [25]. Interestingly, the TC suddenly increases to 293 K at x = 0.5. The ratio of Fe/Mn = 1:1 is probably a critical value in the system of Gd6(Mn1−xFex)23. When the iron concentration reaches the critical value, the exchange coupling between Fe and Gd is stronger than that between Gd and Mn. Furthermore, the ferromagnetic coupling between Gd and Fe needs a higher temperature to break the Gd and Fe interaction. For this reason, the magnetic transition temperature increases to 293 K at x = 0.5. In order to know the effect of Fe content on magnetic moment of Gd6(Mn1−xFex)23 compounds, magnetic hysteresis loops were investigated at 10 K, as shown in Fig. 3c. It can be seen that the magnetization decreases from 119 emu/g (x = 0.0) to 82 emu/g (x = 0.4), which owns to antiparallel coupling of the magnetic moment between Mn and Fe atoms. However, the magnetization increases to 105 emu/g at x = 0.5. As mentioned above, the strong coupling between Gd and Mn at the critical value occurs in the system, so the overall moment increases to a higher value. And, more remarkable, the Gd6(Mn1−xFex)23 compounds possess very minor magnetic hystereses loss at room temperature, which is beneficial to the improvement in magnetic refrigeration efficiency.

Fig. 3
figure 3

Temperature dependence of magnetization for Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) compounds on heating in the field of 200 Oe (a); the inset shows the TC of Gd6(Mn1−xFex)23 as a function of Fe content x. Magnetization curves for the Gd6(Mn1−xFex)23 compounds measured at 300 K (b) and magnetization curves for the Gd6(Mn1−xFex)23 compounds measured at 10 K (c) (Color figure online)

Full size image

The isothermal magnetization (M–H) curves were measured in a magnetic field up to 50 kOe in the vicinity of TC. When one curve is finished, the magnetic field is reduced to zero isothermally and slowly, and then, the temperature is increased for the next M(H) curve. The temperature was changed in steps of 2 K in the vicinity of TC and in steps of 5 or 10 K far from TC. The field sweeping rate is slow enough to ensure that data are recorded in an isothermal process. Figure 4a, b shows the magnetization isotherms of Gd6(Mn1−xFex)23 compounds for x = 0.3, 0.4 and 0.5, respectively. Obviously, a typical ferromagnetic transition is evident near TC. Based on M–H curves, the magnetic entropy change (ΔSM) for a magnetic field change from 0 to 50 kOe was calculated by using the Maxwell relation [32]:

$$ \Delta S_{M} (T,\Delta H) = \int_{0}^{H} {\left( {\frac{\partial M(H,T)}{\partial T}} \right)_{H} {\text{d}}H} $$

Figure 4d–f shows ΔSM–T curves in the field of 0–5 T for Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) compounds. As expected, the maximum ΔSM was observed at the transition temperatures. ΔSM increases obviously with the increase in applied magnetic field. The maximum of ׀ΔSM׀ for a field change of 0–5 T increases slightly from 3.56 to 3.77 J/kg K with x increasing from 0.3 to 0.4, but reduces to 1.79 J/kg K at x = 0.5. The refrigerant capacity or relative cooling power (RCP) is another important quality factor of the refrigerant materials which is a measurement criterion of heat transfer between the cold and hot reservoirs in an ideal refrigeration cycle. In this work, on the basis of Gschneidner and Pecharsky [33], the RCP value is defined as RCP = − ΔSM × δTFWHM based on the ΔSM–T curve, where δTFWHM is the full width at half maximum. The RCP values of Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) are 44.04, 51.56 J/kg and 108 J/kg for an applied field change of 0–5 T.

Fig. 4
figure 4

The isothermal magnetization curves for Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) compounds, measured in the vicinity of TC (a–c). Temperature dependence of -ΔSM for Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) compounds at TC (d–f) (Color figure online)

Full size image

A conventional and most straightforward method to identify the nature of a phase transition is the Banerjee criterion based on an Arrott plot [34]. It is generally agreed that a negative slope or presence of an inflection point in the Arrott plot is related to a first-order magnetic phase transition (FOMT), while a positive slope or linear behavior near TC indicates a second-order magnetic phase transition (SOMT). Figure 5a–c shows Arrott plots of three samples, which show curves of M2 versus H/M near the TC. For the Gd6(Mn1−xFex)23 (x = 0.3, 0.4, 0.5) compounds, the absence of a negative slope and an inflection point indicates a SOMT behavior. Generally, most magnetic transitions are of second order and the materials with second-order magnetic phase transitions exhibit very small magnetocaloric effect than the materials with first-order magnetic phase transitions [35,36,37].

Fig. 5
figure 5

Arrott plots for the Gd6(Mn1−xFex)23 compounds with ax = 0.3; bx = 0.4 and cx = 0.5 (Color figure online)

Full size image

4 Conclusions

The Gd6(Mn1−xFex)23 compounds (x = 0.0–0.5) have been synthesized and studied. The results show that the samples are a single phase, crystallizing in the cubic Th6Mn23-type structure. With a decrease in the Mn/Fe ratio, the lattice parameter a and unit-cell volume V decrease. From x = 0.0 to x = 0.4, the TC moves to lower temperatures, but increases to 293 K at x = 0.5. The magnetization of all compounds at 300 K decreases to the Fe substitution up to x = 0.5, which is lower than that at 10 K. The type of magnetic phase transition from ferromagnetic to paramagnetic is second order, the maximum values of ׀− ΔSM׀ and RCP for a magnetic field change of 0–5 T for x = 0.3, x = 0.4 and x = 0.5 are 3.56, 3.77 J/kg K and 1.79 J/kg K and 44.04, 51.56 J/kg and 108 J/kg, respectively. The present results may give some clue for searching new magnetocaloric materials with large MCE.