Since the discovery of magnetostructural transition  and the metamagnetic shape memory effect  in NiMn-based Heusler alloys, numerous research studies have been performed since the alloys are hopeful candidates for application as magnetic actuators  and magnetocaloric refrigerants . Currently developed metamagnetic shape memory alloys include Ni-(Co)-Mn-In [2,5], Ni-Co-Mn-Sn , Ni-Co-Mn-Ga  and Ni-Co-Mn-Al  systems. Among these alloy systems, because of the low cost, several groups have also focused on the Ni-Co-Mn-Al system. To develop magnetocaloric materials, attempts have been made to evaluate isothermal entropy change  as well as adiabatic temperature change . Other groups have also investigated the properties of sputtered film  and melt-spun ribbon samples .
In addition to the application aspect, the Ni-Co-Mn-Al alloys also show interesting fundamental physical phenomena similar to those of other NiMn-based alloys. Like Ni-(Co)-Mn-In alloys [13,14], the thermal transformation arrest (TTA) phenomenon has been observed for Ni45Co5Mn31Al19 (Co5Al19; we adopt the denotation of Ni50−xCoxMn50−yAly as CoxAly in this article)  and Co10Al17  alloys. The temperature dependence of entropy change during martensitic transformation shows an abrupt decrease above the TTA temperature and becomes almost zero below it . On the other hand, the kinetic behaviors, enlargement of magnetic-field hysteresis has also been observed for Co5Al19  and Co10Al17 , as has also been observed for Ni-(Co)-Mn-In [5,14] alloys. For Ni-Co-Mn-Sb and Ni-Co-Mn-In alloys, the temperature hysteresis during martensitic transformation  as well as the magnetic field hysteresis [18,19] during magnetic field-induced transition obviously vary with different sweeping rates of temperature and magnetic field. For Co5Al19, however, an almost equivalent hysteresis has been found by comparison between the results under a pulsed magnetic field and a steady magnetic field .
For both the application and fundamental aspects, the entropy change during martensitic transformation is of great importance. Some systematic work has been performed on Ni-Mn-In-X alloys [5,14,20–23], Ni-Mn-Sn-X alloys [22,24] and Ni-Mn-Ga-X alloys [25,26]. However, for the Ni-Co-Mn-Al system, there are currently only indirect calculations for a single alloy  and estimations deduced from the Maxwell equation . Very recently, we reported the pseudo-binary magnetic phase diagram of the CoxAly alloys . This phase diagram is shown in Figure 1, where the Al content dependence of martensitic transformation starting temperature TMs and the Curie temperature of the parent phase TCP are shown. Based on these alloys reported in the phase diagram, a systematical investigation on the transformation entropy change ΔS for CoxAly alloys by direct measurement was performed.
2. Experimental Methods
Refer to Ref.  for details of the preparation of the CoxAly samples. Specific heat from around 150 up to 663 K was measured by the heat flow method using a commercial Netzsch DSC 204 F1 Phoenix® equipped with a μ sensor, the measurements being calibrated using a standard sapphire sample according to DIN 51007. Some of the samples, which are listed in Table 1 in Ref. , were subjected to composition measurements by EPMA. Since their compositions were very close to the nominal compositions, the nominal compositions were directly used in this research for simplicity.
3. Experimental Results
Figure 2 shows the specific heat measurements for CoxAly alloys without martensitic transformation. A strong bending of the baseline was observed for Co5Al21 while typical lambda-shaped peaks were observed for Co10Al19 and Co15Al17. According to our previous report , these peaks correspond to the Curie temperature of the parent phase. The Curie temperatures, indicated by TCP in the figure, are taken from Ref. . It can be seen that, with increasing Co content, not only the value of TCP, but also the specific heat around the TCP increases. It has been reported that for the CoxAl25 alloys, the magnetic moment increases with increasing Co content . Therefore, a higher Co content results in a larger lambda-shaped peak of the specific heat around TCP, which is contributed to by the strong spin fluctuation  near and above the TCP.
Figure 3(a) shows the specific heat curves for Co5Aly alloys. Only the heating process was measured, in order to avoid the complexity of B2 → L21 diffusion which occurs at relatively high temperatures. For Co5Al18.5 to Co5Al20, first-order transformations, which correspond to the reverse martensitic transformation, were detected. Reverse martensitic transformation temperature is defined as the peak temperature shown as TP since this is the temperature with the largest heat absorption. Transformation enthalpy change ΔH was obtained by calculating the area of the peaks, as shown by the dashed lines in the figure. Generally, straight lines connecting both the baselines were used to determine the area. For Co5Al19, however, since the baseline obviously bends around TP and the transformation interval, i.e., the difference between the starting and finishing temperatures is very large, a cubic polynomial fitting rather than a linear one was used to determine the baseline. Hence the transformation entropy change ΔS can be calculated by
The obtained TP, ΔH and ΔS are listed in Table 1. For Co5Al20, since the TTA phenomenon has been reported even under zero magnetic field , it was impossible to obtain a full martensitic transformation during the current DSC measurement. Thus, its ΔH and ΔS are shown with parentheses in Table 1, indicating an underestimation. It can be seen that the ΔH drastically decreases with decreasing TP. Here, note that for the specific heat data shown in Figure 3, at temperatures away from the martensitic transformation, the values show the specific heat of the sample, whereas for the temperature range near the martensitic transformation, the amount of latent heat is added due to the nature of the heat flow method. According to DIN 51007, the absolute values of the specific heat were calculated based on the assumption that the system backgrounds during each measurement are identical. However, a small change of the system background may exist and this may correspond to a small amount of error of several J · mol−1K−1, shown as Cp,err in Figure 3(a). (Also note that Figure 3(a) is an enlarged view compared to the scales in Figures 3(b) and 3(c)) However, the error Cp,err hardly affects on the determination of ΔS. As shown in Figure 3(a), bending of the baseline instead of the typical lambda-shaped peak was observed for Co5Al20 and Co5Al21 at the TCP. The reported TCPs  are indicated in the figure.
Figure 3(b) shows the specific heat curves for Co10Aly. Results similar to those in Figure 3(a) were obtained. However, for Co10Aly, magnetic transition of the parent phase was clearly observed, which is indicated in the figure. For Co10Al17.5, ΔH and ΔS were likely underestimated due to the TTA phenomenon .
For Co15Aly, Figure 3(c) shows the results of specific heat measurements. For the series of Co15Aly, the samples have very close compositions; therefore, it is quite apparent that the ΔH gradually decreases with decreasing TP.
The data of ΔS shown in Table 1 are plotted against the Al content in Figure 4(a), together with the TCP reported for the CoxAly alloys . The same tendency for ΔS to decrease with increasing Al content was also found in all the series. It is important to note that the transformation type changes at TCP, i.e., a paramagnetic parent to paramagnetic martensite transformation is to the left side of TCP while a ferromagnetic parent to paramagnetic martensite transformation is to the right side of TCP. Hence, on crossing the TCP, it was found that the ΔS begins to abruptly decrease, which shows similar behavior to that of Ni-Co-Mn-In alloys [22,23,30]. It is of great importance that this behavior of ΔS may be one of the vital evidences showing the influence of magnetism on the phase stability as well as the magnetostructural transformations. As shown in Figure 2, the magnetic contribution to entropy changes with the Curie temperature, therefore the ferromagnetic ordering should also influences greatly on the stability of the parent phase. This is consistent with the earlier reported experimental results as well as theoretical predictions [22,31]. Moreover, the magnetic contribution may also affect the modes of lattice vibration, and an stabilization effect of the parent phase may also exist, as the impact on the martensite phase in Ni-Mn-Ga [32,33]. A systematical study and selective comparison between representative metamagnetic and ferromagnetic shape memory alloys are required to further understand this question.
In Figure 4(b), the ΔS is plotted against TP, and the earlier data for Co5Al19  and Co10Al17  are also plotted. It can be seen that ΔS decreases with decreasing TP and approaches zero at an efficiently low temperature. A general consistency was found.
ΔS for Co7Aly film was also estimated using the data reported by S. Rios et al. . Using the Clausius-Clapeyron equation
where dT/dH = 2.1 K/T  is the martensitic transformation shift under different magnetic fields. ΔM ≈ 585 emu/cc is the magnetization difference between parent and martensite phases estimated from Figure 3(a) in Ref. , where the magnetization for the martensite phase is taken to be zero. Taking the lattice parameter to be a = 2.88 Å [11,28], the ΔS is calculated to be 4.0 J/mol·K at 173 K (not plotted in Figure 4(b)). This value is much larger than those of other reports shown in Figure 4(b). The possibility of overestimation here can be considered as follows. First, when calculating the ΔM, the magnetization for the martensite phase was taken to be zero. However, this may not be true since the sample by S. Rios et al. had been subjected to ageing heat treatment . Though systematical research for the magnetic properties of the aged martensite phase in Ni-Co-Mn-Al alloys has not been performed to date, with reference to similar alloy systems such as Ni-Mn-In , Ni-Co-Mn-Ga , and Ni-Mn-Ga , there is a possibility of an increase in magnetization in the martensite phase. Note that as long as the heat treatment is kept as the condition in this research, i.e., no ageing treatment at low temperature being performed, the martensite phase has a paramagnetic-like weak magnetism, as reported for Co5Aly and Co10Aly . Thus, an overestimated ΔM may have resulted in the overestimation of ΔS. Second, the Co7Aly film undergoes a martensitic transformation with large thermal hysteresis where the TTA phenomenon occurs. Therefore, the actual value of dT/dH might be much larger than 2.1 K/T. Actually, the dT/dH approaches infinity at the TTA temperature, and a dT/dH as large as 100 K/T can be estimated from Figure 3(b) in Ref. .
Moreover, for the CoxAly alloys shown in Figure 4(b), the temperature at which ΔS becomes zero should correspond to the TTA temperature TA. The TAs are indicated for Co5Aly, Co10Aly and Co15Aly. The values of TAs for Co5Aly (40 K) and Co10Aly (160 K) show good consistency with earlier reports [15,16]. For Co15Aly, the interval of the martensitic transformation is very large and MFIT measurement for it has not been successfully performed even with pulsed magnetic fields up to 55 T. Therefore, we do not have any information on ΔS at low temperature for this series. The dashed line in Figure 4(b) roughly extrapolates the TA for Co15Aly, assuming that the temperature dependence of ΔS has a shape similar to those of Co5Aly and Co10Aly. This gives a value of about 300 K, which is much higher than 190 K, which is reported from thermomagnetization measurements . The reason is discussed as follows. First, as shown in Figure 3(c), the Al content dependence of TP is very large, and the composition measurements by EPMA also found that the composition difference between different grains is as large as several permillage. This may result in an obvious transformation temperature distribution inside the sample, resulting in an underestimation of TA by thermomagnetization measurements. Second, as shown in Figure 1, the TCP decreases with increasing Al content when the Co content is held constant. Generally the ΔS begins to decrease abruptly only below the TCP [22,37]; therefore, a higher TA can be observed in a sample with a higher TCP. Hence, a TA distribution may exist in the sample if the sample has Co or Al inhomogeneity. Third, the Ni-Co-Mn-Al alloy system shows much better ductility than other NiMn-based alloy systems, where even polycrystalline samples can be subjected to compression testing and show superelasticity behavior . Hence, a strong binding in the grain boundary can be expected, and the constraint by the grain boundary may become nucleation sites for the martensitic transformation. Thus, martensitic transformation may still occur at the grain boundary due to this non-chemical reason at a temperature at which martensitic transformation otherwise would have stopped. This reason can also result in an underestimation of the TA by thermomagnetization measurements. However, in order to measure the TA for Co15Aly precisely, use of a stronger non-destructive pulsed magnet  is necessary. Nevertheless, a rough estimation of the TA as 190–300 K can still be concluded based on the results of the present study.
In this research, specific heat measurements by the heat flow method were performed systematically on Ni50−xCoxMn50−yAly metamagnetic shape memory alloys. Second-order ferromagnetic–paramagnetic transition as well as first-order martensitic transformation were directly observed via specific heat measurements. The transformation entropy change ΔS was estimated from the latent heat by the specific heat measurements. The ΔS decreases with increasing Al content under the series with fixed Co content. The decreasing tendency enlarges below the Curie temperature of the parent phase. The ΔS estimated in this work was found to be consistent with findings of earlier reports.
This work was supported by a Grant-in-Aid for Scientific Research and by a grant from the Japan Society for the Promotion of Science (JSPS).
Conflict of Interest
The authors declare no conflict of interest.
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|Nominal||TP/K||ΔH/kJ · mol−1||ΔS/J · mol−1K−1|
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