Enhanced magnetic refrigeration properties in Mn-rich Ni-Mn-Sn ribbons by optimal annealing.

The influence of annealing time on temperature range of martensitic phase transition (ΔT(A-M)), thermal hysteresis (ΔThys), magnetic hysteresis loss (ΔMhys), magnetic entropy change (ΔS(M)) and relative refrigeration capacity (RC) of the Mn-rich Ni43Mn46Sn11 melt spun ribbons have been systematically studied. By optimal annealing, an extremely large ΔS(M) of 43.2 J.kg(-1)K(-1) and a maximum RC of 221.0 J.kg(-1) could be obtained respectively in a field change of 5 T. Both ΔT(A-M) and ΔThys decreases after annealing, while ΔMhys and ΔS(M) first dramatically increase to a maximum then degenerates as increase of annealing time. A large effective cooling capacity (RC(eff)) of 115.4 J.kg(-1) was achieved in 60 min annealed ribbons, which increased 75% compared with that unannealed ribbons. The evolution of magnetic properties and magnetocaloric effect has been discussed and proved by atomic ordering degree, microstructure and composition analysis.

Magnetic refrigeration based on magnetocaloric effect (MCE) is a potential refrigeration technology with high efficiency, low cost, and environmentally friendly, which is drawing more attention as an alternative to the existing vapor compression refrigeration. Recently, magnetic refrigerants, such as Gd-Si-Ge1, La-Fe-Si23, Mn-Fe-P-As4 and Ni-Mn-based Heusler alloys5 come to be research focus due to their giant MCE caused by first-order phase transition. But from the point of application, it is important for a magnetic refrigeration material not only has a large magnetic entropy change (ΔSM) but also a large refrigeration capacity (RC)678. Amorphous structure is in favor of a large RC while in a compensation for a relative low ΔSM91011. Therefore, how to increase RC but keep a large ΔSM at same time is a key issue for a magnetic refrigeration material.

Among them, rare-earth-free Ni-Mn-X (X =Sn, In, Sb) Heusler alloy systems have aroused people’s great interest due to their large inverse MCE121314. Additionally, for Ni-Mn-Sn system, its low cost of raw materials, easy fabrication, low energy cost during fabrication etc. make it having more advantages for applications. While, except for MCE, martensitic transition related other magnetic properties such as magnetic hysteresis loss (ΔMhys), cooling capacity (RC) are important factors, which need to be considered before its application1516. Even these parameters are intrinsic, but they are more sensitive to microstructure, microcomposition, phase/magnetic ordering degree, which are strongly influenced by fabrication conditions.

Current research mostly focuses on bulk alloys, synthesized by conventional arc melting followed by prolonged high-temperature annealing. More recently reported melt-spinning technique effectively promotes more homogeneous single-phase materials, substantially shortened annealing stage and improved MCE properties1718. But both samples prepared by these two methods need to be annealed for a period of time in order to obtain a uniform phase. Because of the longer atom diffusion distance in bulk sample, it needs longer annealing time to obtain homogeneous composition. Ghosh and Aksoy et al.1920 have also studied the influence of annealing time on magnetocaloric effect of Ni-Mn-Sn bulk samples at 1173 K from 0 h to 24 h and a maximum magnetic entropy change could be found when annealing time is close to 24 h. In our recent research, an extremely large MCE more than 40 J.kg−1K−1 in a field change of 5 T was achieved in annealed melt-spun ribbon sample compared with bulk sample of 29.5 J.kg−1K−121. For melt-spun ribbons, short time17 about several minutes or longer time18 around several hours has been reported separately. It is obvious that the annealing time has great influence on magnetocaloric effect of Ni-Mn-Sn melt spun ribbons. But it is still not very clear which annealing time is optimal. And there are few reports on systematically studying what’s the important factors influence on refrigeration related parameters for this MCE system.

To our knowledge, a maximum magnetic entropy change ΔSM of 33 J.kg−1K−1 was reported for a Ni-rich alloy of Ni51.6Mn32.9Sn15.5 due to a field change of 5 T, which is almost a record MCE for Ni-Mn-Sn family alloy5. In this article, an extremely large ΔSM of 43.2 J.kg−1K−1 and a high effective RCeff more than 100 J.kg−1 were obtained in Mn-rich Ni43Mn46Sn11 ribbons by optimizing annealing time. The influence of annealing time on martensitic transition related behaviors and magnetocaloric effect of Mn-rich Ni43Mn46Sn11 melt spun ribbons was systematically investigated and discussed. The evolution of microstructure, composition, atomic ordering degree has been introduced to explain the trend of magnetic refrigeration related properties on different annealing time.

Results and discussions

As discussed in the introduction part, it is rare to find an optimal annealing time for melt spun ribbons. In the literatures, either extremely short or long annealing time would miss an optimal annealing condition, which prevented achieving optimal magnetic refrigeration properties. Here, different annealing time from 0 min to 180 mins has been carried out for Mn-rich Ni43Mn46Sn11 melt spun ribbons, and martensitic transition (MT) related magnetic behaviors and MCE has been systematically studied.

Thermal Magnetization, Temperature Range of Martensitic-Austenitic Transition and Thermal Hysteresis

The temperature dependence of magnetization during heating and cooling at a rate of 10 K/min for Ni43Mn46Sn11 ribbons annealed at different time in a low magnetic field of 100 Oe was shown in Fig.1. In the thermal magnetization curve, an abrupt change in magnetization around 200 K belongs to martensitic transformation (MT) proved by many previous researches2223. The MT in Ni-Mn-X (X = Ga, In, Sn, Sb) alloys takes place from a cubic austenitic phase, showing L21 Heusler crystal structure (space group Fm3-m) and next-nearest neighbor atomic order, to a low-symmetry martensitic structure1324. Accordingly, the phase changes from high temperature ferromagnetic (FM) austenitic phase having larger susceptibility, to low temperature martensitic phase, which exhibits small magnetic susceptibility. The temperature range for this transformation ΔTA-M can be calculated using the following formula:

Figure 1

Temperature dependence of magnetization (M-T curves) for ribbons annealed for different annealing time,

(a) as spun ribbon, (b) 10 min, (c) 60 min21, (d) 180 min in the presence of 100Oe field.

Where As, Af, and Ms, Mf are the starting and finishing temperatures of the austenite and martensitic phase, respectively. With the increasing of annealing time, the value of A slightly increases as listed in Table 1. Moreover, the temperature range of martensitic-austenitic phase transition (ΔTA-M) decreased dramatically from 11 K for unannealed sample to 3 K, 3.5 K and 4 K for 10 min, 60 min and 180 min annealed samples. It’s obvious that the unannealed one has the widest ΔTA-M and correspondingly the thermal hysteresis ΔThys during this phase transition is largest for the as spun ribbons (10 K), and then reduces to 7 K after annealing as listed in Table 1. The annealing time has no much different effect on ΔThys.

Table 1

Physical properties of ribbons annealed for different time.

	0 min	10 min	60 min	180 min
As	211	219 K	220 K	221 K
ΔTA-M	11K	3 K	3.5 K	4 K
ΔThys	10 K	7 K	7 K	7 K
MA	65 emu/g	75 emu/g	75 emu/g	67 emu/g
MM	35 emu/g	20 emu/g	20 emu/g	20 emu/g
ΔMA-M	30 emu/g	55 emu/g	55 emu/g	47 emu/g
ΔMhys	57.1 J.kg−1	185.5 J.kg−1	168.6 J.kg−1	123.3 J.kg−1
ΔSM	18.4 J kg−1K−1	43.2 J.kg−1K−1	41.4 J.kg−1K−1	32.3 J.kg−1K−1
RC	107.0 J.kg−1	201.0 J.kg−1	221.0 J.kg−1	185.0 J.kg−1
RCeff	65.9 J.kg−1	102.2 J.kg−1	115.4 J.kg−1	100.9 J.kg−1
AOD	0.06	0.09	0.11	0.07

As: The starting temperature of austenitic phase;

ΔTA-M: Temperature range of martensitic phase transition;

ΔThys: Thermal hysteresis;

MA: Magnetization of austenitic phase;

MM: Magnetization of martensitic phase;

ΔMA-M: Magnetization difference between austenitic phase and martensitic phase;

ΔMhys: Magnetic hysteresis loss;

ΔSM: Magnetic entropy change;

RC: refrigeration capacity, RCeff: effective cooling capacity;

AOD: Atomic ordering degree.

For the as spun ribbon, the composition is inhomogeneous even it has single L21 phase tested by XRD after rapid solidification from the melt, as a result it has larger ΔTA-M, which can be ascribed to the following two reasons. One is that the atoms in the unannealed sample (as spun ribbon) are inhomogeneous and disordered. The other reason is that due to the fast cooling rate, the defects and stress are reserved in the as spun ribbons. The decrease of ΔTA-M for the annealed ribbons is due to the homogeneity of the composition and reducing of the defects and stresses. And along with the more homogeneous composition after different time annealing, atomic order degree (AOD) would also increase, which was the reason why A has a little bit increase, and which may also contribute to the narrower ΔTA-M. At the same time, the magnetic refrigeration performance would also be strongly influenced by annealing, which will be shown and discussed later. However, the composition may be less homogeneous when the annealing time is more than enough, which would have a negative influence on AOD, thus mildly widen the temperature change of phase transformation, and the increased ΔTA-M from 3 K for 10 min sample to 4 K for 180 min annealed sample can further prove it. The change of compositional homogeneity will be proved in the following part by EDS in SEM (see Table 2). The change of atomic ordering degree AOD will be confirmed by the XRD analysis in the following part (see Table 1).

Table 2

Average composition deviation and three different areas’ composition diviation for 10 min and 180 min annealed ribbons.

	ζ	ζa	ζb	ζc
10 min	0.086	0.113	0.062	0.084
180 min	0.224	0.235	0.249	0.189

Isothermal Magnetization and Magnetic Entropy Change ΔSM

In order to investigate magnetic refrigeration performance, isothermal magnetization (M-H) curves up to 50 kOe near TM for melt-spun ribbons annealed for different time were measured as shown in Fig. 2. From the M-H curves, it is obvious that all the samples show a typical ferromagnetic behavior above 217 K, which presents austenitic phase. And a metamagnetic behavior between 205 K and 217 K was due to a field-induced martensitic transition. Apparently, the increasing manner of magnetization around 211 K for 10 min annealed ribbons is much sharper than all the other most obvious metamagnetic transition curves for the other three samples. Another important difference needs to be mentioned is that the magnetization of austenitic phase MA increased from 65 emu/g for the as melt-spun ribbons to about 75 emu/g for the annealed 10 min and 60 min ribbons, but decreased to 67 emu/g as further increase annealing time to 180 min. Meanwhile, the magnetization of martensitic phase MM decreased from 35 emu/g for unannealed ribbons to around 20 emu/g for 10 min, 60 min and 180 min annealed ribbons.

Figure 2

Isothermal magnetization (M–H) for

(a) as spun ribbon, (b) 10 min annealed ribbon, (c) 60 min annealed ribbon21, (d) 180 min annealed ribbon, near martensitic phase transition temperature.

The change of magnetization both for austenitic phase and martensitic phase is due to different annealing time, which affects the magnetic correlations in the alloy. In the austenite phase, there are two positions for Mn atoms, one is the regular Mn site and another is the Sn site, and both of their magnetic moments order ferromagnetically19. After annealing, an increase in the ordering of Mn atomic moments to their respective proper sites happened and as a result, the ferromagnetic correlation increases, which directly leads to an increase in MA. While, the antiferromagnetic correlations between Mn-Mn at different positions strengthened after annealing as the structure of these materials transforms from austenitic phase to martensitic phase on cooling, which results in a decrease of MM20. The deep reason for the change of magnetization can be ascribed to atomic ordering degree, which will be discussed later. Since there is an increase of MA and decrease of MM upon annealing as listed in Table 1, the difference in magnetization between austenitic phase and martensitic phase (ΔMA-M listed in Table 1) was is enlarged, which is favorable to obtain a large magnetic entropy change.

Martensitic transition is a phase transition associated with structure change, which belongs to a first order transition. As mentioned in the first part, a thermal hysteresis ΔThys is obvious as shown in Fig. 1 and listed in Table 1. Usually, the trend of magnetic hysteresis loss ΔMhys upon annealing is similar with that of ΔThys for the first order transition. But in our research, it has a different trend in Ni-Mn-Sn alloy. The dashed area in Fig. 2 represents the maximum ΔMhys during phase transition for each sample and it means that the larger the dashed area, the larger the hysteresis loss. The values of the largest ΔMhys for four samples are listed in Table 1, respectively. It first dramatically increases from 57.1 J.kg−1 for the as spun ribbons to 185.5 J.kg−1 for the 10 min annealed ribbons, and then it gradually decreases to 168.6 J.kg−1 for the 60 min annealed sample and 123.3 J.kg−1 for 180 min annealed sample. But the annealing time has no much difference on ΔThys. It is obvious that the longer the annealing time, the smaller the ΔMhys. The change of ΔMhys upon different annealing time will be discussed in the following part.

The ΔSM of all the samples has been estimated using Maxwell’s thermodynamic relation:

where, T, M, H, and μ0 are the temperature, magnetization, magnetic field intensity, and permeability of free space, respectively. The M-H curves used for the calculation of ΔSM are the curves measured during field increasing. The temperature dependences of magnetic entropy change ΔSM for the samples of as spun and annealed ribbons are plotted in Fig. 3. It can be seen that the ΔSM for the annealed samples increase dramatically compare with that of the as spun ribbons as listed in Table 1, which can be explained by the increase of atom order degree AOD by annealing in the following part. A maximum ΔSM of 43.2 J.kg−1K−1 in a field change of 5 T is obtained in the sample annealed for 10 min. As compared to other samples, the largest value of ΔSM for this sample originates from the sharpest change in magnetization during martensite/austenite structural phase transition, which could be well explained by the increase of atomic ordering degree in the following part. While for the other two annealed samples, the metamagnetic transition is not directly change from martensitic phase to austenitic phase (which can be proved by Fig. 2), but to an intermediate state, therefore the ΔSM is not as large as that of 10 min annealed sample even the difference in magnetization between two phases is similar. On the other hand, ΔSM will decrease with the increasing of annealing time. In our research, it is postulated that the decrease of atomic ordering degree and decrease of compositional homogeneity would result in the depression of ΔSM. And, this will be proved in the following section.

Figure 3

ΔSM as a function of temperature for

(a) as spun ribbon, (b) 10 min, (c) 60 min21 and (d) 180 min annealed ribbon due to 1.0−5.0 T field change.

Refrigeration Capacity (RC), Magnetic hysteresis, and Microstructure

For a magnetocaloric effect material, refrigeration capacity (RC) is an important parameter to be evaluated for its potential application, which estimates the total amount of thermal energy that can be transferred to a hot sink from a cold source in one ideal thermodynamic cycle. RC has been calculated using ΔSM as a function of temperature and integrated it within the full width at half maximum (FWHM) of ΔSM of the materials7. Upon annealing, RC of Ni-Mn-Sn alloy first increases from 140 J.kg−1 for the as spun ribbons to 259.5 J.kg−1 and 269.4 J.kg−1 for 10 min and 60 min annealed samples, respectively, but decreases to 225.8 J.kg−1 for longer time 180 min annealed sample, which are listed in Table 1. Since the value of RC is calculated by the integration ΔSM as a function of T within FWHM, the larger the ΔSM and the wider FWHM are, the higher RC will be. 60 min annealed sample has a relative larger ΔSM, which is only a little bit lower than that of 10 min annealed sample, plus a relative wider FWHM. Therefore, it has the highest RC.

Additionally, magnetic hysteresis loss ΔMhys must be considered for evaluating the usefulness of a magnetic refrigerant during a thermodynamic cycle since a high effective RC is favorable for its application. The value of the maximum magnetic hysteresis loss ΔMhys for each sample, which is the dashed area shown in Fig. 2, is 57.1 J.kg−1, 185.5 J.kg−1, 168.6 J.kg−1 and 123.3 J.kg−1 for as spun, 10 min, 60 min, 180 min annealed samples, respectively, which is listed in Table 1. Magnetic hysteresis loss ΔMhys is an intrinsic behavior for a first order transition. Generally, the more obvious and violent the first order transition is, the larger the magnetic hysteresis is. For the as spun ribbons, due to its inhomogeneous composition and low atomic order degree, the martensitic transition is mild since it undergoes a longer temperature range ΔTA-M, therefore, it has the lowest magnetic hysteresis loss ΔMhys. After a short time (10 min) annealing on this as spun ribbons, L21 phase rapidly becomes homogeneous and uniform, leading to an obvious and violent martensitic transition. Additionally, the stress formed during rapid solidification has no time to release in a short time, resulting in a very large magnetic hysteresis ΔMhys of 185.5 J.kg−1. As the increase of annealing time, the stress can gradually release and at the same time, the grain growth can also reduce frictions2526, which will further lead to a lower hysteresis. Therefore, magnetic hysteresis for 60 min sample reduces to 168.6 J.Kg−1 and further reduces to 123.3 J.Kg−1 for 180 min annealed sample with the prolonging of annealing time.

For rapid quenched sample like melt-spun ribbons without annealing, the stress and defects formed during rapid quench will be reduced after annealing for a period of time, and further annealing will have no much influence. 60 min annealing should be enough to stress release and defect elimination, further increase of annealing time will have no much improvement. At the same time, overtime annealing will deteriorate the homogeneity of the composition as shown in Table 2, which will increase ΔMhys. While, as listed in Table 1, ΔMhys for ribbons annealed for 180 min, still dramatically reduced about 45 J.kg−1 compared with that of annealed for 60 min. There should have other reasons for this decrease of ΔMhys for overtime annealing. Except for the stress formed during solidification process, other kinds of stress developed during the reversible martensitic transformation, such as those caused by a large degree of grain constraints, affect the transformation range, which also can cause a large magnetic hysteresis27. In conventional shape memory alloys, elastic energy storage has been linked to grain size to thickness ratio2829 during superelastic loading and, thus, thermally or magnetically induced martensitic transformation ranges should also be affected by this microstructural parameter. Samples with large grain to thickness ratios require less thermal or mechanical (or magnetic) driving force to achieve the same response than samples with small grain to thickness ratios.

In order to find out how the grain size affects magnetic hysteresis, SEM observation on fracture surface of the ribbons was carried out for all samples. As shown in Fig. 4, all the ribbons have a similar thickness of about 50 μm. It can be seen that the grain size also increases with the increasing of annealing time. Figure 4a shows that the as-spun ribbon consists of thin columnar crystals with diameters of about 4 μm. Columnar crystals proceed with a lateral growth to about 13 μm after 10 min annealing. Meanwhile, the grains have three or four layers across the section as shown in Fig. 4b. Then the columnar crystals further grow to an average of 20 μm after annealed 60 min, at the same time, crystals continue its longitudinal growth, thus the grains have only two layers which can be seen in Fig. 4c. Then they finally grow into an average diameter of 26 μm after annealing for 180 min, when the whole ribbon is made up with one grain/one layer running through the cross section as shown in Fig. 4d. According to SEM results, for the annealed ribbons, the size of grains has a great enhancement along with the increase of annealing time. Grain constraints and elastic energy storage is related to gain sizes, which can explain the decrease of hysteresis upon annealing very well. Therefore, minimizing stored elastic energy and strain/stress is beneficial to reduce magnetic hysteresis. And one possible method to reduce magnetic hysteresis is to have gain size as large as possible, for example single crystal materials may have the lowest hysteresis25.

Figure 4

SEM image of fracture surface for

(a) as spun ribbon, (b) 10 min, (c) 60 min and (d) 180 min annealed ribbon.

Returning to RC, the effective cooling capacity RCeff is defined as a more reasonable criterion for assessing the cooling efficiency. RCeff can be obtained by subtracting the average magnetic hysteresis loss during martensitic transition from RC value. So the values of RCeff for all the studied samples are 65.9 J.kg−1, 102.2 J.kg−1, 115.4 J.kg−1 and 100.9 J.kg−1, respectively as listed in Table.1. It’s obvious that short time (10 min) heat treatment dramatically increase the ribbons’ RCeff from 65.9 to 102.2 J.kg−1, which is mostly due to the large increase of ΔSM. And after a longer annealing (60 min) time, RCeff still increase to 115.4 J.kg−1, which may due to the wider temperature range of ΔSM since ΔSM has a little bit decrease. However, with further increasing of annealing time (180 min), its RCeff decreases to 100.9 J.kg−1. For RCeff, the longer the annealing time will lead to the lower the magnetic hysteresis, but ΔSM also decrease substantially; therefore, the highest RCeff appears when the decrease of magnetic hysteresis can’t compensate for the deterioration of the entropy change. Only after a proper time of annealing, highest RCeff will be obtained; in our case, 60 min annealing is the optimal condition. The RCeff of ribbons annealed for 60 min increased almost twice compared with the unannealed (as spun) ribbons.

Atomic Ordering Degree and Magnetic Refrigeration Performance

As discussed above, atomic ordering degree AOD has an important effect on thermal hysteresis, ΔThys, magnetic hysteresis ΔMhys, magnetization difference between austenitic phase and martensitic phase ΔMA-M and magnetic entropy change ΔSM. In this section, atomic ordering degree will be investigated.

It has been mentioned above that in Ni-Mn-Sn alloys, there exists an ordering transition from relatively disordered cubic B2 phase to long-range ordered L21 phase, as solidified from the high temperature melt3031. As for L21 phase, Mn and Sn atoms occupy their corresponding sites, respectively. Whereas in the high-temperature B2 phase, they are being evenly distributed. The B2-L21 transition occurs at a certain temperature above 800 K depending on its composition. The atomic ordering of L21 phase is also strongly influenced by annealing time. As discussed above, the magnetization change between austenitic phase and martensitic phase, and the enhanced magnetic entropy change ΔSM upon annealing was attributed to the increase of atomic ordering degree. Generally, the higher the atom ordering degree is, the higher magnetization difference across martensitic transition has, and the better the magnetocaloric effect has.

In order to confirm the influence of annealing on the atomic ordering degree, and then prove its effect on magnetic properties, XRD was carried out on all the samples as shown in Fig. 5, in which all the peaks is corresponding to Heusler cubic L21 structure, which indicates that all of the samples are austenitic phase. No martensitic phase was observed since martensite transition temperatures TM are below room temperature. B2 phase is a high-temperature phase compare to L21 phase. This B2-L21 transition takes place promptly even less than 0.5 s in the process of cooling32. So even quenching from B2 phase range, all samples are indexed to be L21 phase no matter annealed or unannealed. That is the reason why the as spun ribbons present L21 phase. But because of the very high cooling rate (more than 105 K/s), the as spun ribbon still reserve some disordered state. Post-heat treatment can realize the structural relaxation and the grain growth, thus gradually eliminating the disordered state through atomic diffusion.

Figure 5

XRD patterns for ribbons annealed for

(a) 0 min, (b) 10 min, (c) 60 min and (d) 180 min annealed ribbon.

Annealing would produce a great influence on the atomic order degree. The L21 order degree can be qualitatively evaluated from the evolution of the intensities of superlattice reflection peaks, such as (111) and (311), whereas the evolution of the (111) peak appears most strikingly3334. In the literatures333435, the evolution of the order parameter associated with the L21 structure was monitored based on . Figure 5 shows the XRD patterns of the four samples. In order to rule out the measuring error, here AOD (atomic ordering degree) was defined as I(111)/I(220), where (220) is the strongest diffraction peak of L21 phase. AOD is 0.06, 0.09, 0.11 and 0.07 for as-spun ribbons, 10 min, 60 min and 180 min annealing ribbons respectively, which is listed in Table 1. One can find that quenching directly from the melt gives a low AOD of 0.06, which gradually increases to 0.09 and 0.11, then decreases to 0.07 when annealing from 0 min to 180 min. With proper annealing time, AOD is improved; therefore ΔMA-M, ΔSM, and ΔThys ΔTA-M have been improved greatly compared with unannealed ribbons as shown in Table 1. Even for the overtime annealed (180 min) samples, it still has higher ΔSM and RCeff than that of unannealed sample.

It is well known, atomic ordering degree AOD depends on microstructure and degree of crystallinity. Generally, longer annealing time is beneficial to obtain a higher AOD. In fact, the AOD for 60 min annealed sample is highest, but its ΔSM is not the largest. While, 10 min annealed sample with a mild AOD but has the highest ΔSM. For excessive longer annealing time for example 180 mins, its AOD decreases. Therefore, except for annealing time, other factors, such as composition may become a more important issue to influence AOD and then affects magnetic properties.

Composition evolution and magnetic entropy change

From the above results and discussions, annealing time favors to achieve a higher AOD and then to have higher magnetic refrigeration performance, such as higher ΔMA-M, ΔSM, and RCeff, lower ΔMhys and ΔTA-M. However, over time annealing, for example 180 min, AOD will decrease from 0.11 to 0.07, therefore a decreasing of ΔSM from ~40 Jkg−1K−1 to ~30 Jkg−1K−1, and increasing of ΔTA-M from 3 K to 4 K. What is the under reason for the decrease of atomic ordering degree AOD becomes an important question. Furthermore, AOD value of 60 min is higher than that of 10 min, but the latter has the highest ΔSM while it has similar ΔMA-M. There are some previous researches363738 which ascribed the decreasing of atom ordering degree for ribbons with over time annealing to the variations of vacancy concentration, but no evidence was provided so far.

In our research, it is assumed except for AOD, that composition evolution may also play an important role. Here, a composition deviation ζ is introduced to investigate composition evolution upon annealing. Figure 6 is SEM image for one ribbon annealed for 10 min, which shows the points of EDS has been done. The average composition for each component in Ni-Mn-Sn as spun sample is defined as NiS0, MnS0 and SnS0; the composition of the five points in the rectangle area is defined as NiS1, MnS1 and SnS1 to NiS5, MnS5 and SnS5. So the composition deviation for Ni can be got from:

Figure 6

SEM image for 10 min annealed ribbon to give an example of how composition deviation was measured and calculated.

Then ζMn and ζSn can be got in the same way. Thus the composition deviation ζ for all the components in a sample is calculated by:

In order to minimize measurement errors, three different areas a, b and c were selected randomly in 10 min and 180 min annealed ribbons. All rectangles are in the same size and keep a same distance from the roll-touched surface. According to the method described above, an average composition deviation ζ is obtained after measured at least three different areas a, b and c by EDS, which is shown in Table 2. It is obvious that ζ for 180 min annealed ribbons is all larger than that of 10 min annealed ribbons, which indicates that the former is more inhomogeneous than the latter. This result proves that the composition also change along with atomic ordering degree upon annealing. Atomic ordering degree AOD is enhanced by increase of annealing time, but the composition deviation ζ is also enlarged. Therefore, there is a equilibrium point, when the increase of AOD can not be compensated by the enlarge of ζ. Thus an optimal annealing time is needed to achieve high magnetic refrigeration properties.

Conclusions

The magnetic refrigeration properties, such as ΔTA-M, ΔMhys, ΔSM, and RCeff, of melt-spun Ni43Mn46Sn11 ribbons after different annealing time have been systematically investigated. An extremely large magnetic entropy change ΔSM of 43.2 J kg−1K−1 in a field change of 5 T was achieved only after short time annealing (10 min) of the as-spun ribbons, but it gradually decreases to 32.3 J kg−1K−1 as annealing time increases to 180 min. At the same time, 10 min annealed ribbons have the largest ΔMhys, while ΔMhys decreases with the increasing of annealing time but with a compensation of decrease of ΔSM. As a result, the largest RCeff of 115.4 J.kg−1 was appeared in the 60 min annealed ribbons. Annealing also favors to have a lower thermal hysteresis, but over time annealing is also unfavorable. For samples with different annealing time, a maximum ΔSM of 43.2 J.kg−1K−1, a lowest ΔMhys of 123.3 J.kg−1, a maximum RCeff of 115.4 J.kg−1 could be obtained in samples with different annealing time. While annealing time has no much influence on ΔTA-M and ΔThys. An optimal annealing time is 60 min that is it has not only a large magnetic entropy change of 41.4 J.kg−1K−1, but also a large effective cooling capacity of 115.4 J.kg−1. The ΔSM increases for 125% and the RCeff increases for 75% comparing the optimal annealing with unannealed ribbons.

Atomic ordering degree, microstructure, and composition have been applied to explain the change of magnetic refrigeration properties on different annealing time. Atomic ordering degree (AOD) enhancement by annealing attributed to a higer ΔMA-M, RCeff, ΔSM. Magnetic hysteresis loss ΔMhys is also influenced by microstructure and composition. The larger crystal size and the uniform of the composition, the lower ΔMhys. Over time annealing leads to a large composition deviation, which is unfavorable to get an optimal magnetic refrigeration performance.

Methods

The as-cast ingot of Ni43Mn46Sn11 was prepared by arc-melting under a high purity argon atmosphere. The purities of all raw elements are better than 99.9%. An additional 1wt. % Mn was added to compensate for its evaporation. The arc-melted ingot was cut into small pieces for use in the following melt-spun experiment. The small pieces were induction melted in a quartz tube with a rectangular nozzle (6 mm*0.5 mm) and then ejected onto a copper wheel rotating at a surface velocity of 15 m/s.

The melt-spun ribbons were sealed in quartz tubes, which were first evacuated, then filled with argon gas, and after that they were annealed at 1173 K for different time. After annealing, the quartz tube was quenched by water. The annealing time varied from 0 to 180 min.

The crystal structure analyses were carried out by x-ray diffraction (XRD) using Cu-K radiation at room temperature. The microstructure and composition were determined by scanning electron microscopy (SEM) using a JSM–6700 F microscope equipped with energy dispersive spectrometry (EDS). The magnetic measurements were carried out using a super conducting quantum interference device (SQUID).

Additional Information

How to cite this article: Zhang, Y. et al. Enhanced magnetic refrigeration properties in Mn-rich Ni-Mn-Sn ribbons by optimal annealing. Sci. Rep. 5, 11010; doi: 10.1038/srep11010 (2015).