Magnetic properties and magnetocaloric effects of Gd₆₅(Cu,Co,Mn)₃₅ amorphous ribbons

1. Introduction

The magnetocaloric effect (MCE) refers to conversion of magnetic energy of a magnetic substance into thermal energy by changing applied external magnetic field. Magnetic refrigeration based on MCE has been proposed to be the most promising alternative for conventional gas compression refrigeration, because the former is more energy-efficient and environmentally friendly, lower noise and cost effective [[1], [2], [3]]. Up to now, many intermetallic compound materials with giant MCE have been discovered and intensively studied, such as Gd₅(Si_xGe_1-x)₄ [4,5], La(Fe_1-xSi_x)₁₃ [[6], [7], [8]], Dy₂In [9], MnFeP_1-xAs_x [10], MnAs_1-xSb_x [11], Mn₂Sb [12], and Ni-Mn-based alloys [[13], [14], [15]]. However, in these systems, the materials undergo the field-induced first-order magneto-structural phase transitions show an abrupt variation at magnetic transition temperature, leading into a narrow transition temperature range and a large hysteresis loss. These shortcomings hinder their refrigeration efficiency. Recently, amorphous alloys with the second order magneto-structural phase transition and large magnetic-order-transition region have been discovered, such as Gd-based amorphous alloys [[16], [17], [18]]. These amorphous alloys also possess many unique properties that are superior to those of crystalline alloys, such as negligible magnetic and thermal hysteresis, high electrical resistivity, high corrosion resistance and tailorable magnetic transition temperature [17]. Therefore, they are considered to be ideal candidates for magnetic refrigerants.

Gd-based amorphous alloys are generally fabricated in a composition range near the eutectic point. However, the melt-spun of Gd₆₅Mn₃₅ binary alloy [19] contains Gd-rich nano-precipitates in the amorphous matrix, which show the similar structure as the high Gd content amorphous/Nano crystalline alloy [20]. Many efforts have been devoted to improve the glass forming ability (GFA) of Gd-based alloys. From Gd₆₅Mn₃₅ binary alloy, the Ge and Si [19,21] substitutions for Mn contents have been investigated. For the Gd₆₅Mn_35-x(Ge,Si)_x (0≤x≤10) alloys, the excellent refrigeration capacity (RC) values are 625 and 660 J kg^-1 for Gd₆₅Mn₃₀Ge₅ and Gd₆₅Mn₂₅Si₁₀ alloys, respectively, in favor of magnetic Ericsson cycle. Compared with Gd-Mn binary alloys, the binary Gd-Co alloys show higher GFA [22]. Due to the magnetic Co addition, stronger exchange coupling leads into the higher Curie temperature (T_c) but reduced saturation magnetization due to antiparallel alignment of Gd moment with Co moment. Ni and Al are also used to improve the GFA of Gd-based alloy at the cost of either reduced saturation magnetization or lowered T_c [23]. As a later-transitional element, Cu addition can drastically reduce the liquidus temperature of Gd-based alloy to form Gd-Cu eutectic. Mn as an unique element is found to form many Mn-based crystallized magnetocaloric systems, because Mn atoms contribute large atomic moment. And the Mn-Mn exchange coupling determines the fundamental magnetic properties. However, its roles in the Gd-based amorphous systems have not been studied intensively. Therefore, in present work, starting from Gd₆₅Mn₃₅ binary alloys, Co and Cu have been gradually replaced by Mn element, to investigate its influences on the amorphous structrue and magnetic properties.

2. Experimental

The Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) and Gd₆₅Cu_25-xCo₁₀Mn_x (x = 10, 5) ingots were firstly prepared by arc-melting pure Gd, Mn, Co and Cu in argon atmosphere. In order to ensure composition homogeneity, the ingots were turned over and remelted four times. The weight losses during the arc-melting were less than 1.0 wt%. The ribbons were obtained by single-roller melt-spinning with a copper-wheel speed of 30 m/s. The structure of melt-spun ribbons were checked by X-ray diffraction (XRD) using a Panalytical X’Pert PRO diffractometer with Cu-K_α radiation. Thermal analysis was carried out at a heating rate of 15 K/min by using a NETZSCH DSC (STA449 C) differential scanning calorimeter (DSC) under a protective atmosphere. The magnetization as a function of temperature and magnetic field was measured by Cryogenic vibrating sample magnetometer (VSM) with the maximum field of 7 T.

3. Results and discussion

Fig. 1 compares the DSC curves of the Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons during the heating process. It is seen that all the ribbons exhibit mainly three obvious exothermic peaks related to crystallization, which means the formation of glassy alloys. With increased temperature, the first sharp exothermic peak is observed at $\widetilde{5}$32 K for Gd₆₅Cu₁₀Co₂₀Mn₅ ribbon followed by the second exothermic peak at $\widetilde{5}$78 K. With increased Mn substitution for Co, the first exothermic peak shifts to lower temperature and the exothermic enthalpy corresponding to the first and second peaks is decreasing. It means that Mn substitution destabilizes the disordered structure, lowering the glass stability of the Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) serious alloys. For Gd₆₅Cu_25-xCo₁₀Mn_x (x = 10, 5) series alloys, with increasing the Cu element, the first exothermic peark is further lowered to 507 K with gradually disappeared second one at higher temperature.

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Fig. 1.   DSC curves of the melt-spinning ribbons for the Gd₆₅(Cu,Co,Mn)₃₅ series alloys with the heating rate 15 K/min.

Fig. 2 compares the XRD patterns of the Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons along with those of Gd₆₅Cu_25-xCo₁₀Mn_x (x = 10, 5) ribbons. The broad hump between 2θ = 30° to 35° is observed in all ribbons and no obvious crystalline peak is found, indicating the typical amorphous structures. With increased substitution amount of Mn for Co, the diffraction angle of the hump in Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons is gradually shift from 2θ = 32.7° to 2θ = 31.5°. While, with more Mn substitution for Cu, the diffraction angle of Gd₆₅Cu_25-xCo₁₀Mn_x (x = 10, 5) ribbons shifts again to the lower angle.

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Fig. 2.   XRD patterns of the melt-spinning ribbons for Gd₆₅(Cu,Co,Mn)₃₅ series alloys.

Due to the Gd, Co and Mn inter/intra-atomic exchange coupling, the temperature (T) dependences of the magnetization (M) are measured under 0.05 T in the cooling process to detect the phase transformation from paramagnetism to ferromagnetism as shown in Fig. 3. The Curie temperature (T_c) defined by the maximum of the “absolute value” of dM/dT. In Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons, the magnetization is gradually increased with cooling down. It is seen that the magnetization starts to increase for Gd₆₅Cu₁₀Co₂₀Mn₅ at 191 K, indicating the T_c. By gradually increasing Mn substitution amount for Co, the T_c is only slightly increased to be 208 K. While compared with that of Gd₆₅Cu₂₀Co₁₀Mn₅, more Mn substitution for Cu leads into increased T_c, which may be because that more magnetic moments from Mn atoms participate the exchange coupling to enhance the T_c. The T_c values of all the present alloys are much higher than that of Gd₅₅Ni_xAl_45-x (x = 15, 20, 25, 30) [23,24] and Gd₅₅Co_20+xAl_25-x (15≤x≤30) [24,25] amorphous ribbons which are in the vicinity of 100 K as seen in Table 1.

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Fig. 3.   Temperature dependence of magnetization under a magnetic field of 0.05 T for Gd₆₅(Cu,Co,Mn)₃₅ series alloys.

To further detect how the saturation magnetization changes due to Mn substitution, the variations of the magnetization as a function of the applied magnetic field (H) measured at 10 K are presented in Fig. 4. The magnetization can be saturated at 5 T in all Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons. Meanwhile, with more Mn substitution for Co, the slope of M-H curve is increased but the saturation magnetization is reduced. Moreover, the M-H curves in Gd₆₅Cu₁₀Co₂₀Mn₅ and Gd₆₅Cu₁₀Co₁₅Mn₁₀ ribbons appear to be slightly discontinuous compared with that of Gd₆₅Cu₁₀Co₁₀Mn₁₅ ribbon. While increasing the Cu content, the magnetization can be easily saturated with increased external field. The saturation magnetization is almost equal in Gd₆₅Cu_25-xCo₁₀Mn_x (x = 5 and 10) ribbon, and obviously higher than those in Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons. It is known that the Gd moment is usually antiparallel coupled with Co moment. The reduced magnetization in Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons compared with those of Gd₆₅Cu_25-xCo₁₀Mn_x (x = 5, 10) is mainly due to high Co composition.

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Fig. 4.   Isothermal magnetization curves of Gd₆₅(Cu,Co,Mn)₃₅ series alloys measured at 10 K.

The isothermal magnetization curves of these ribbons have been measured near their respective T_c as shown in Fig. 5. In the M-H curves of Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) amorphous ribbons, it is seen that at low temperature, the magnetization shows the tendency to be difficultly saturated, which is also observed in M-H curves at 10 K in Fig. 4. It may be ascribed to the field-induced change on moment configuration since the Co moment is supposed to be antiparallel with Gd moment. With increasing temperature, such feature disappears and the typical S-shaped M-H curves observed. Further increased temperature, the magnetization curve shows the linear dependence with respect to external field, indicating the end of ferromagnetic phase transition. The similar features are observed in the isothermal M-H curves in Gd₆₅Cu_25-xCo₁₀Mn_x (x = 10, 5) ribbons. However, at low temperature, magnetization is comparably easier to be saturated than those in Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons.

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Fig. 5.   TIsothermal magnetization curves of Gd₆₅(Cu,Co,Mn)₃₅ series alloys measured at various temperatures.

The magnetic-entropy changes (ΔS_m) of the melt-spun ribbons were calculated based on the magnetic isotherms in the vicinity of T_c by using the Maxwell relation. Fig. 6 shows the temperature dependence of ΔS_m under magnetic-field changes 0-7 T (Fig. 6(a)) and 0-5 T (Fig. 6(b)) for all amorphous ribbons. For the field change of 0-7 T, the maximum of entropy change (ΔS_m^max) of 6 J kg^-1 K^-1 has been obtained in Gd₆₅Cu₁₀Co₂₀Mn₅ amorphous ribbon. By increasing the Mn substitution for Co, the ΔS_m^max is decreased to $\widetilde{5}$ J kg^-1 K^-1 in Gd₆₅Cu₁₀Co₁₅Mn₁₀ and Gd₆₅Cu₁₀Co₁₀Mn₁₅ ribbons, due to the decreased saturation magnetization. Noting that the corresponding temperature to obtain the ΔS_m^max is only 140 K, which is much lower than the corresponding T_c of $\widetilde{2}$00 K in Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) amorphous ribbons. As comparison, by decreasing the Mn amount from x = 10 to x = 5 in Gd₆₅Cu_25-xCo₁₀Mn_x ribbon, the ΔS_m^max is recovered to 5.8 J kg^-1 K^-1 in Gd₆₅Cu₂₀Co₁₀Mn₅ ribbon and 5.2 J kg^-1 K^-1 in Gd₆₅Cu₁₅Co₁₀Mn₁₀ ribbon. And the corresponding temperature to obtain the ΔS_m^max is about 200 K, which is close to the corresponding T_c of these two alloys. Most of the magnetocaloric materials have been reported to show sharp or broad triangle peak-like ΔS_m-T curves, meaning that the largest ΔS_m values are only achieved at the transition points while drastically decreasing with temperature far away from the transition points [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. Comparing the five amorphous alloys in the present work, the Gd₆₅Cu₁₀Co₁₅Mn₁₀ and Gd₆₅Cu₁₀Co₁₀Mn₁₅ ribbons show flater ΔS_m-T curves. It is reported that amorphous composite showed flattened ΔS_m profile within a tailorable temperature range by selecting amorphous alloys with appropriate T_c and ΔS_m peaks, because the nanocomposite and amorphous matrix show several successive second-order magnetic phase transitions, thus giving rise to a remarkable broadening of the full-width at the half-maximum of the magnetic entropy change curve (δT_FWHM) [26]. It is supposed that some nano-crystals or inhomogeneous structure may exist in the amorphous matrix of both Gd₆₅Cu₁₀Co₁₅Mn₁₀ and Gd₆₅Cu₁₀Co₁₀Mn₁₅ ribbons, which is consistent with the corresponding DSC curves with smaller exothermic enthalpy corresponding to the first and second peaks. Under the magnetic field change of 0-5 T, the ΔS_m^max in the range of 4-5 J kg^-1 K^-1 have also been obtained for the investigated Gd₆₅(Cu,Co,Mn)₃₅ series alloys. These values are comparable to Gd₆₅Mn_35-xGe_x amorphous ribbons (4.1-4.5 J kg^-1 K^-1, field change of 0-5 T) [19] and Gd₆₅Mn_35-xSi_x amorphous ribbons (4.6-4.7 J kg^-1 K^-1, field change of 0-5 T) [21], but lower than that of Gd₅₅Ni_xAl_45-x (x = 15, 20, 25, 30) [23,24] and Gd₅₅Co_20+xAl_25-x (15≤x≦30) amorphous ribbons [24,25].

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Fig. 6.   Magnetic entropy changes of Gd₆₅(Cu,Co,Mn)₃₅ series alloys versus temperature under the field change of 0-7 T and 0-5 T.

Another relevant parameter characterizing the refrigerant efficiency of the material is the refrigeration capacity, RC, which is measured by different methods in literature [23,24]. One method, RC_AREA, calculated by integrating numerically the area below the ΔS_m-T curve using the temperatures at half maximum of ΔS_m^max as the integration limits [24]. The other method defines RC_FWHM=δT_FWHM*ΔS_m^max [23]. It is noted that a larger δT_FWHM is advantageous for practical application in order to obtain a large RC. In the present work, under the field change of 0-7 T, the RC_FWHM of 708 J kg^-1 has been achieved in Gd₆₅Cu₁₀Co₂₀Mn₅ ribbons and greatly increased to $\widetilde{8}$50 J kg^-1 in Gd₆₅Cu₁₀Co₁₅Mn₁₀ and Gd₆₅Cu₁₀Co₁₀Mn₁₅ ribbons, due to the broader ΔS-T curves. However, significantly widened δT_FWHM over 180 K has been achieved in Gd₆₅Cu₁₅Co₁₀Mn₁₀ and Gd₆₅Cu₂₀Co₁₀Mn₅ ribbons, leading into greatly enhanced RC_FWHM to 1000 J kg^-1. The values of RC_AREA, calculated by integrating numerically the area below the ΔS_m-T curves are also given in Table 1. The largest RC_AREA of 813 J kg^-1 is achieved for Gd₆₅Cu₂₀Co₁₀Mn₅ ribbon.

From the above results we can see the increased Mn substitution for Co or Mn substitution for Cu led into increased T_c for Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) and Gd₆₅Cu_25-xCo₁₀Mn_x (x = 10, 5) amorphous ribbons. Gd₆₅Cu₁₀Co_25-xMn_x (x = 5, 10, 15) ribbons. Meanwhile, with more Mn substitution for Co, the slope of M-H curve is increased but the saturation magnetization is reduced for Gd₆₅Cu₁₀Co_25-xMn_x series alloys. The increasing Cu element and lowering Mn addition also lead to higher ΔS_m^max for the both series alloys. The Gd₆₅Cu₂₀Co₁₀Mn₅ ribbon shows the highest RC values but lowest T_c temperature because less magnetic moment from Mn atoms participate the exchange coupling to enhance the T_c. By comparing with other rare-earth-based amorphous systems, it is seen that Gd-based amorphous ribbons in present work possess higher T_c, which is elevated with higher Gd composition, and wider δT_FWHM. More significantly, the higher RC values have also been achieved. These excellent properties make the Gd₆₅(Cu,Co,Mn)₃₅ system a promising magnetocaloric candidates used in the temperature range of liquid nitrogen.

4. Conclusion

The Gd₆₅(Co,Cu,Mn)₃₅ amorphous ribbons have been prepared by melt spinning. The effects of Mn substitution for Co and Cu have been studied. Mn is found to thermodynamically destabilize the amorphous ribbons. The Curie temperature is found to be around 200 K for the series alloys. Saturation magnetization is decreased with increased Mn substitution for Cu. For a field change of 0-7 T, the ΔS_m^max ranged between 5 J kg^-1 K^-1 to 6 J kg^-1 K^-1 has been obtained in all amorpohous ribbons. Significantly, high refrigeration capacity of 708 J kg^-1 has been achieved in Gd₆₅Cu₁₀Co₂₀Mn₅ ribbons, but significantly enhanced to 1000 J kg^-1 in Gd₆₅Cu₁₅Co₁₀Mn₁₀ and Gd₆₅Cu₂₀Co₁₀Mn₅ ribbons. Such excellent refrigeration capacity with considerable magnetocaloric effect indicates that Gd₆₅(Co,Cu,Mn)₃₅ system is a suitable magnetocaloric candidate.

Acknowledgement

This work was supported financailly by the National Natural Science Foundation of China (Nos. 51674082, 51771049 and 51790484).