Room-temperature magnetic refrigeration is an innovative cooling technology with the advantages of high efficiency, low cost, and environmental friendliness. In this technology, magnetic materials are used as the refrigerants. To maximize the cooling capacity of magnetic refrigeration at room temperature (RT), the magnetic refrigerants should exhibit a large magnetic entropy or temperature change under a magnetic field.^[1–4] During the last decade, the large magnetocaloric effect (MCE) has been successively observed in magnetic alloys that experience a magnetic-field-induced first-order phase transition, including Gd–Si–Ge-based alloys,^[5] MnAs_1−xSb_x,^[6] Mn–Fe–P–As,^[7] La (Fe_1−xSi_x)₁₃,^[8] and especially, Ni–Mn-based magnetic shape memory alloys.^[9,10]

Rare-earth-free Ni–Mn-based magnetic shape memory alloys (Ni–Mn–X, X = Al, Ga, Sn, In and Sb) display a structural transformation between magnetic martensite and austenite. Various investigations on the structural, magnetic, and electrical properties of these alloys have been carried out.^[11–17] It is found that the first-order magnetostructural transformation (MST) from the high-temperature austenite to the low-temperature martensite is accompanied by an obvious magnetization difference (ΔM) in some alloys with specific compositions. Due to the existence of ΔM, the MST can be driven by a magnetic field, and thus a large MCE can be achieved.^[18–22]

Among Ni–Mn–X alloys, Ni–Mn–Sn attracts considerable attention due to its tunable MST, large MCE, and relatively low cost. From the application point of view, the designed material as a magnetic refrigerant should display the MCE and large effective refrigeration capacity (RC_eff) at RT. In this work, we select Ni₄₈Co₂Mn₃₈Sn₁₂ as the starting composition, which experiences a structural transformation above RT. By changing the Ni/Mn ratio, the structural transformation temperature (T_t) is decreased to RT. Meanwhile, the structural transformation is tuned into the Curie temperature window, which leads to a large ΔM and be in favor of the magnetic-field-induced structural transformation (MFIST). Based on this improvement, large MCE, reduced hysteresis loss (HL), and improved RC_eff are obtained.

The polycrystalline Ni_48−xCo₂Mn_38+xSn₁₂ (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys were prepared by arc-melting appropriate amounts of high purity elements Ni (99.995 wt.% purity), Co (99.95 wt.% purity), Mn (99.998 wt.% purity), and Sn (99.99 wt.% purity) in argon atmosphere. The samples were re-melted three times for homogeneity. The obtained ingots were annealed at 900 °C in vacuum for 24 h, and then quenched into cold water. The martensitic and reverse transformation behaviors were identified by differential scanning calorimetry (DSC) with the heating/cooling rate of 10 K/min. The crystal structures were investigated by powder x-ray diffraction (XRD) using Cu-K_α radiation at RT. The compositions of the elaborated alloys were confirmed by x-ray energy dispersive spectrometry (EDS). Magnetic measurements were performed on a physical property measurement system (PPMS). Isothermal magnetization (M–B) curves were measured using a so-called loop process to avoid the irreversibility caused by the magnetic-field-induced first-order MST.^[23] Before each M–B measurement, the samples were cooled down to full martensitic state, and then gradually heated to the measurement temperature with a ramp rate of 3 K/min. To ensure the temperature stability of the measurement, a waiting time of 3 min was hold at the target temperatures.

The XRD patterns of all the samples are shown in Fig. 1. For x = 0, the XRD pattern demonstrates a 4O modulated martensitic structure at RT. It suggests that the structural transformation in Ni₄₈Co₂Mn₃₈Sn₁₂ occurs above RT. While for x = 2.5, the presence of (111), (220), (400), and (422) peaks indicates a L2₁ austenitic structure with the lattice constant of a = 5.978 Å. The coexistence of austenitic and martenitic phases is observed in the samples with 0 < x < 2.5 at RT. The XRD data indicates that T_t decreases with x increasing.^[24]

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. (color online) Powder XRD patterns of Ni48−xCo2Mn38+xSn12 (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys at RT.

The thermal-induced structural transformation was investigated by DSC measurements. As shown in Fig. 2, the large endothermic and exothermic peaks in the heating/cooling processes and the large thermal hysteresis indicate the occurrence of first-order structural transformation between martensite and austenite in these Heusler-type Ni_48−xCo₂Mn_38+xSn₁₂ (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys. As x increases from 0 to 2.5, the T_t shifts to the lower temperatures by around 85 K and 110 K in the heating and cooling processes, respectively. It is known that the T_t of Ni–Mn-based shape memory alloys is sensitive to the electron concentration (e/a), and it decreases with the e/a decreasing.^[25–28] The e/a calculated from the EDS results for our samples are listed in Table 1. It can be found that with the increase of x, the e/a decreases from 8.115 to 8.025, resulting in the observed decrease of T_t.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (color online) DSC curves for Ni48−xCo2Mn38+xSn12 (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys in heating and cooling processes.

Table 1.

Table 1.

Table 1. The measured compositions and the characteristic phase-transition temperatures of Ni48−xCo2Mn38+xSn12 (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys. . x Ni/at.% Co/at.% Mn/at.% Sn/at.% As/K Af/K Ms/K Mf/K e/a 0 48.13 1.69 38.08 12.10 343 358 343 315 8.115 1.0 46.50 2.17 39.32 12.01 325 342 315 302 8.079 1.5 45.40 2.34 40.95 11.31 315 334 302 294 8.069 2.0 45.57 2.14 40.26 12.03 304 325 297 287 8.049 2.5 45.15 2.05 40.46 12.34 251 278 225 213 8.025

Table 1. The measured compositions and the characteristic phase-transition temperatures of Ni48−xCo2Mn38+xSn12 (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys. .

Figure 3 shows the temperature dependence of magnetization (M–T) of Ni_48−xCo₂Mn_38+xSn₁₂ (x = 1.0, 1.5, 2.0, and 2.5) alloys under an applied field of 0.01 T during the heating and cooling processes. For x = 1.0, due to the fact of that the T_t in the heating process is close to the Curie-temperature of austenite (T_C,A ≈ 340 K), the ferromagnetic austenite is not obviously observed in the heating process, but can be found in the cooling process. The existence of thermal hysteresis proves the first-order nature of the structural transformation between martensite and austenite. With x increasing from 1.5 to 2.5, the T_t shifts to the lower temperatures. As a result, the ferromagnetic austenite appears both in the heating and cooling processes. It is found that T_C,A is nearly stable with the alteration of composition. The Curie temperature of martensite (T_C,M) is not observed in the samples with x = 1.0, 1.5, and 2.0 in the temperature range of 250–400 K, but is found in the x = 2.5 sample. In the heating process, the sample with x = 2.5 experiences the magnetic ordering-disordering transition of martensite, and then the first-order MST from the low-magnetization martensite to ferromagnetic austenite. The T_C,M of x = 2.5 is around 208 K. A phase diagram including austenitic start temperature A_s, austenitic finish temperature A_f, martensitic start temperature M_s, martensitic finish temperature M_f, T_C,M, and T_C,A for Ni_48−xCo₂Mn_38+xSn₁₂ (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys is shown in Fig. 4(a). According to Fig. 4(a), it can be found that with the increase of x, the T_t is decreased into the temperature window between T_C,A and T_C,M. Thus, the samples with x = 1.5 and 2.0 exhibit the room-temperature MST between the weak-magnetization martensite and ferromagnetic austenite. This transition is accompanied by a large ΔM, which is favor of MFIST. Due to the existence of ΔM between martensite and austenite, the MST can be induced by a magnetic field. The M–B curves for Ni_48−xCo₂Mn_38+xSn₁₂ (x = 1.0, 1.5, and 2.0) alloys with a temperature interval of 3 K in the heating process are shown in Figs. 4(b)–4(d). With the increase of the magnetic field, an obvious slope change is observed when the temperature is close to A_s. This behavior suggests the existence of MFIST from the martensite to austenite.^[29–31]

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. (color online) M–T curves for Ni48−xCo2Mn38+xSn12 (x = 1.0, 1.5, 2.0, and 2.5) alloys under an applied magnetic field of 0.01 T during the heating and cooling processes.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. (color online) (a) Phase diagram for Ni48−xCo2Mn38+xSn12 (x = 0, 1.0, 1.5, 2.0, and 2.5) alloys as a function of x. (b)-(d) M–B curves for Ni48−xCo2Mn38+xSn12 (x = 1.0, 1.5, and 2.0) alloys with a maximum magnetic field of 5 T.

According to the M–B curves, the MCE of Ni_48−xCo₂Mn_38+xSn₁₂ (x = 1.0, 1.5 and 2.0) alloys is estimated by the Maxwell equation^[32]

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. (color online) Temperature dependence of magnetic entropy change for Ni48−xCo2Mn38+xSn12 (x = 1.0, 1.5, and 2.0) alloys under the magnetic field variations of 0–2 and 0–5 T. The inset shows the x dependence of average HL for Ni48−xCo2Mn38+xSn12 (x = 1.0, 1.5 and 2.0) alloys under a magnetic field variation of 0–5 T.

Table 2.

Table 2.

Table 2. The transformation width W(=Af − As), Tcold, Thot, magnetic entropy change ΔSM, hysteresis loss (HL), and effective refrigeration capacity (RCeff) of Ni48−xCo2Mn38+xSn12 (x = 1.0, 1.5, and 2.0) alloys. . x W/K Tcold/K Thot/K ΔSM(ΔB = 5 T)/J⋅kg−1⋅K−1 HL/J⋅kg−1 RCeff/J⋅kg−1 1.0 17 332 338 19.22 63.13 23.36 1.5 19 319 329 13.64 41.51 60.79 2.0 21 308 225 7.71 30.70 67.60

Table 2. The transformation width W(=Af − As), Tcold, Thot, magnetic entropy change ΔSM, hysteresis loss (HL), and effective refrigeration capacity (RCeff) of Ni48−xCo2Mn38+xSn12 (x = 1.0, 1.5, and 2.0) alloys. .

In summary, we investigate the structural transformation and MCE in Ni_48−xCo₂Mn_38+xSn₁₂ alloys with x = 0, 1.0, 1.5, 2.0, and 2.5. The partial substitution of Ni by Mn stabilizes the austenite phase and leads to the decrease of T_t. For the samples with x = 1.5 and 2, the MST between the weak-magnetization martensite and ferromagnetic austenite is achieved at RT. This transition is accompanied by a large ΔM, which is favor of MFIST. Accompanied by the MFIST, a considerable MCE is obtained. Maximum values of ΔS_M are 19.22 J⋅kg⁻¹⋅K⁻¹, 13.64 J⋅kg⁻¹⋅K⁻¹, and 7.71 J⋅kg⁻¹⋅K^{− 1} for x = 1.0, 1.5, and 2.0 under the magnetic field variation of 0–5 T, respectively. Although the maximum ΔS_M is reduced with x increasing, the RC_eff is remarkably improved.