Influence of fabrication conditions on giant magnetocaloric effect of Ni–Mn–Sn ribbons

Giant magnetocaloric effect (GMCE) of materials is of interest in research by virtue of its application potential in the field of magnetic refrigeration. Magnetic refrigeration is based on the magnetic entropy change principle of materials under variation of magnetic field. The application of magnetocaloric materials in refrigerators has the advantage of avoiding environmental pollution (unlike refrigerators using compression gases), improving the cooling efficiency (saving energy), reducing noise and fitting to some special cases. The main problems to be addressed to improve the practical applications of magnetocaloric materials are: (i) creating GMCE in low field, because it is very difficult to create large magnetic field in civil engineering devices; (ii) performing the magnetic phase transition of materials with GMCE at room temperature; and (iii) extending the working temperature range (range with GMCE) for materials to be cooled in a large temperature range. In addition, some other properties of materials such as heat capacity, electrical conductivity, thermal conductivity, durability and price should be improved for the application of GMCE materials. There are two kinds of GMCE, positive and negative. Both the two effects can be applied in practice and can take place in the same material.

The GMCE in Heusler alloys has been attracting the interest of many scientists [1–9]. Magnetic entropy change (ΔS_m) in this kind of magnetocaloric materials is quite large and has both the positive and negative sign. Besides that, the GMCE in the Heusler alloys can be controlled to occur in the room temperature region. The magnetocaloric materials in amorphous or nanocrystalline structure have also been investigated recently [9–12]. The main advantages of amorphous or nanocrystalline materials are capabilities for GMCE, low coercivity, high resistivity, room temperature magnetic phase transition and low cost, which are necessary requirements for practical application. One of the useful methods to fabricate alloys with amorphous or nanocrystalline structure is the rapid quenching technique. In this work, we investigate the magnetocaloric effect of Ni₅₀Mn_50−xSn_x ribbons (x = 11–15) prepared by means of melt-spinning and subsequently annealing. The goal of this research is to combine the advantages of both the Heusler alloy and rapidly quenched materials for application in magnetic refrigeration.

The alloys with nominal compositions of Ni₅₀Mn_50−xSn_x ribbons (x = 11, 12, 13, 14 and 15) were prepared from pure metals (99.9%) of Fe, Ni and Sn. Arc-melting method was first used to ensure the homogeneity of the alloys. Melt-spinning method was then used to fabricate the ribbon samples. Thickness of the ribbons is about 30 μm. Some of the ribbons were annealed at various annealing temperatures (T_a) and time (t_a) in vacuum. The structure of the samples was examined by powder x-ray diffraction (XRD) method. The magnetic and magnetocaloric properties of the samples were characterized by magnetization measurements.

Figure 1 shows typical XRD patterns of Ni₅₀Mn_50−xSn_x alloy ribbons before and after annealing at 1123 K for 5 h. XRD patterns of the as-quenched ribbons reveal two main diffraction peaks corresponding to Ni₂MnSn phase (austenitic L2₁-cubic structure) and look very similar as the Sn-concentration (x) varies. However, the diffraction peaks of the main crystalline phase (Ni₂MnSn) slightly shift to higher values of the 2θ when Sn-concentration is increased (see the inset of figure 1(a)). That means lattice constants of the crystal are changed by Sn-concentration. The change of the lattice constants probably leads to a variation of magnetic orders in the alloys as presented below. After annealing at 1123 K for 5 h, the structure of all the ribbons is clearly different from that of the as-quenched ones. Other crystalline phases such as Ni₃Sn₂ and Mn_1.77Sn are formed. The number and intensity of the diffraction peaks of these annealed ribbons is dependent on Sn-concentration.

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Magnetic properties of the alloys were investigated by magnetization measurements. Magnetic hysteresis measurement indicates that all the as-quenched and annealed ribbons are soft magnetic. The thermomagnetization measurement reveals the magnetic phase transitions in the ribbons. Figure 2 presents thermomagnetization curves in an applied magnetic field of 12 kOe of Ni₅₀Mn_50−xSn_x alloy ribbons before and after annealing at 1123 K for 5 h. In as-quenched state, the alloy has an antiferromagnetic–ferromagnetic (AFM–FM) transition. The temperature (T_P) and magnitude of the AFM–FM transition strongly depends on Sn-concentration. The T_P of the alloy is fast decreased from 302 to 182 K by an increase of 2 at.% of the Sn-concentration (from 12 to 14 at.%). The AFM–FM transition of the samples with Sn-concentrations of 11 and 15 at.% is not observed in the temperature range of 100–350 K. Thus, the AFM order, which relates to a martensite 10 M-orthorhombic structure, is very sensitive to the Sn-concentration in the alloy. The AFM–FM transition greatly affects on magnetocaloric effect of the alloy as shown later. After annealing at 1123 K for 5 h, no AFM–FM transition is observed in the ribbons with all the Sn-concentrations, while the ferromagnetic–paramagnetic (FM–PM) transition (T_C) occuring near 330 K is almost unchanged by this annealing process (figure 2(b)). Nevertheless, the AFM–FM transition still exists in the samples annealed at 1273 K for 15 and 30 min (figure 3(a)). By increasing the annealing time, the T_P is shifted to higher temperatures, the T_C is nearly unchanged and the magnetization is decreased.

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Figure 3(b) shows thermomagnetization (M–T) curves in various magnetic field of Ni₅₀Mn₃₇Sn₁₃ (x = 13) alloy ribbons annealed at 1273 K for 15 min. From these M–T curves, the magnetization versus magnetic field (M–H) curves (figure 4(a)) can be deduced. Based on M–H curves, magnetic entropy change (ΔS_m) can be calculated by using Maxwell's relation

$$\begin{eqnarray} &&\Delta S_{\rm{m}} = \int_{H_1 }^{H_2 } {\left( {\frac{{\partial M}}{{\partial T}}} \right)_H } {\rm{d}}H. \end{eqnarray} \tag{ 1 }$$

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Temperature dependence of the magnetic entropy change (ΔS_m–T) in a 12 kOe-magnetic field variation of the Ni₅₀Mn₃₇Sn₁₃ sample before and after annealing at 1273 K for 15 min is presented in figure 4(b). The ΔS_m–T curves of both the two samples have two extrema with opposite signs. One extremum corresponds to the maximum of the magnitude of the positive GMCE and the other is of the negative GMCE. The positive GMCE is due to the AFM–FM transition and the negative GMCE is related to the FM–PM transition in the materials. The maximum positive and negative magnetic entropy changes are quite large: |ΔS_m|_max = 5.7 J kg⁻¹ K⁻¹ and | − ΔS_m|_max = 1.4 J kg⁻¹ K⁻¹ for the as-quenched sample, |ΔS_m|_max = 5.2 J kg⁻¹ K⁻¹ and | − ΔS_m|_max = 1.9 J kg⁻¹ K⁻¹ for the annealed sample.

It should be noted that by appropriate annealing processes, both the maximum magnetic entropy change and full-width at half-maximum of the magnetic entropy change of the ribbons can be regulated in room temperature region. One can see in figure 4(b) that by annealing at 1273 K for 15 min, the maximum positive magnetic entropy change of the Ni₅₀Mn₃₇Sn₁₃ alloy ribbon is shifted from ∼265 to ∼285 K and the maximum negative magnetic entropy change of this sample, which occurs at ∼310 K, increases from 1.4 to 1.9 J kg⁻¹ K⁻¹. The full-width at half-maximum of the negative magnetic entropy change of the ribbons is quite large (>20 K). Both the positive and negative magnetocaloric effects of the ribbons take place quite closely in room temperature region. Thus, it is possible for combining all the positive and negative magnetocaloric effects of the material for magnetic refrigeration application at room temperature. The influence of annealing conditions on magnetocaloric effect of the Ni–Mn–Sn ribbons needs to be studied further to achieve an optimum of this effect for practical applications.

The magnetic and magnetocaloric properties of the Ni₅₀Mn_50−xSn_x ribbons strongly depend on Sn-concentration and annealing process. The AFM–FM transition of the alloy is just observed at a narrow range of the Sn-concentration (x = 12–14) and can be controlled by the annealing process. Both the positive and negative magnetic entropy changes are quite large, |ΔS_m|_max > 5.2 J kg⁻¹ K⁻¹ and | − ΔS_m|_max >1.4 J kg⁻¹ K⁻¹ with external magnetic field change ΔH = 12 kOe. The full-width at half-maximum of the magnetic entropy change is also large making Ni₅₀Mn_50−xSn_x ribbons possible for application in magnetic refrigeration.

This work was supported by the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam under grant number of 103.02-2011.23 and the Converging Research Center Program funded by the Ministry of Education, Science and Technology (2012K001431), South Korea. A part of the work was done in the Key Laboratory for Electronic Materials and Devices, and Laboratory of Magnetism and Superconductivity, Institute of Materials Science, VAST.