Magnetostructural transformation and magnetocaloric response in Mn(Fe)NiSi(Al) alloys

Magnetostructural transformation and magnetocaloric response in Mn(Fe) NiSi(Al) alloys Aun N. Khan, Luis M. Moreno-Ramírez * , Jia Yan Law , Victorino Franco * Dpto. Física de la Materia Condensada. ICMS-CSIC. Universidad de Sevilla, P.O. Box 1065, Sevilla 41080, Spain ARTICLE INFO Keywords: MnNiSi alloys Magnetostructural transition Magnetocaloric effect ABSTRACT An efficient magnetocaloric refrigerant must have certain characteristics such as a sharp transition near the desired working temperature, a large cyclic magnetocaloric response, and the use of raw materials with reduced costs and low environmental consequences. In this sense, this work focuses on a series of rare-earth- and Co-free Mn 0.5 Fe 0.5 NiSi 1-x Al x alloys (x =0.0525, 0.060, 0.0685) with promising magnetocaloric properties. The alloys were synthesized using combined arc melting and induction melting techniques, as this synthesis route provides improved control on the sample composition and homogeneity. We investigated how the heat treatment temperature and Al content affect the magnetostructural and magnetocaloric properties of the alloys. On the one hand, it is found that annealing at 1173 K for 7 days leads to a sharp magnetostructural transformation with no traces of impurities for the alloy with x =0.0525. Under these conditions, a large isothermal entropy change of –11.5 J kg −1 K −1 for 1 T is obtained near room temperature, significantly improving the value of the as-cast sample. On the other hand, following this optimal heat treatment, the influence of Al content is studied: upon increasing the Al concentration the magnetostructural transformation shifts to lower temperatures, ranging from 320 K for x =0.0525 to 220 K for x =0.0685 (measured upon heating). 1. Introduction Magnetic materials undergoing first-order magnetic phase transition (FOMT) are gaining considerable attention due to their large magnetocaloric (MC) responses that can be translated into potential applications in energy-efficient solid-state cooling devices [1,2]. The MC response is quantified either as the entropy change in isothermal conditions (ΔSiso) or as the temperature change under adiabatic conditions (ΔTad) under varying applied magnetic fields [3]. This effect is maximal near magnetic phase transitions, with the first-order type generally producing a higher MC response than that arising from the second-order type [4]. Therefore, showing large MC responses in the desired temperature range, MC materials undergoing FOMT are pivotal for potential cooling applications [5,6]. Since the discovery of the so-called giant MCE in Gd 5 (Si,Ge) 4 alloys [7], much of the research nowadays focuses on FOMT materials. Notable high-performance families of MC materials showing FOPT include the La(Fe,Si) 13 alloys [8], (Mn,Fe) 2 (P,Si) alloys [9], NiMn-based Heusler alloys [10,11] and the MnTX alloys (T =Ni, Co and X =Ge, Si) [12–14]. Among the abovementioned families, MnTX is gaining attention due to its use of abundant elements, as with MnNiSi-based alloys. MnNiSibased alloys undergo a structural transition from a high-temperature Ni 2 In-type structure (space group P6 3 /mmc) to a low-temperature TiNiSi-type structure (space group Pnma) at around 1200 K [12]. A viable approach to tune the transition temperatures is by finding suitable partial substitutions at Mn, Ni and/or Si sites [12,14–16]. Among the possible substitutions at the Mn site, Fe is the most desirable due to its non-criticality, high abundance and low cost [12]. Although the structural transition decreases to ~ 845 K for the Mn-Fe-Ni-Si alloys, further optimization is essential for the practical applications of such materials. Previous research indicates that simultaneous substitution of Fe across Mn and Ni sites or Co along Ni sites for the Mn-Fe-Ni-Si alloys leads to a near room temperature transformation, indicating a rather moderate magnetocaloric response for the alloys (~ – 13 J kg −1 K −1 for 2 T) [16,17]. The addition of Fe 2 Ge to MnNiSi alloys also leads to a magnetostructural transition near room temperature, with isothermal entropy change values as large as 29 J kg −1 K −1 for 2 T [18]. However, for moderate magnetic fields, a low value of adiabatic temperature change (0.6 K for 1.75 T) is reported [18]. Just recently, Z. Zheng et al., reported a high value of adiabatic temperature change of 8.6 K for the * Corresponding authors. E-mail addresses: [email protected] (L.M. Moreno-Ramírez), [email protected] (V. Franco). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2024.176724 Received 13 August 2024; Received in revised form 23 September 2024; Accepted 26 September 2024 Journal of Alloys and Compounds 1008 (2024) 176724 Available online 27 September 2024 0925-8388/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/bync-nd/4.0/ ). MnNiSi-based high-entropy alloys doped with Fe, Co and Ge [19]. However, the need for large magnetic fields imposes a disadvantage for MC applications. A. Biswas et al. performed the Density Functional Theory (DFT) calculations to select the most appropriate elemental substitutions (avoiding critical elements such as Co and Ge) for the Mn-Fe-Ni-Si alloys, partially substituting Al at Si site [12]. For these alloys synthesized by the arc melting technique, they reported a large isothermal entropy change for magnetic field changes of 2 T, from –16 J kg −1 K −1 to –22 J kg −1 K −1 , as the Al content was increased in Mn 0.5 Fe 0.5 NiSi 1-x Al x alloys (x =0.05–0.07), also tuning the transformation from near room temperature to 200 K [12]. Since that work, other composition variations directed to achieving a large magnetocaloric response in these alloys include the variation of the Fe/Ni ratio [20], double substitutions of Fe at both Mn and Ni sites [14], or the doping of an interstitial element such as B in the Mn 0.5 Fe 0.5 NiSi 1-x Al x alloys. In particular, Mn 0.5 Fe 0.5 NiSi 0.94 Al 0.06 B 0.005 , shows a large isothermal entropy change of –22 J kg −1 K −1 for an applied magnetic field change of 2 T [21]. In addition, by looking into the literature, large variations in the MC response of similar compositions are widely found. In addition, investigations directed toward the influence of heat treatment on their MC response are scarce. B. Nuendute et al. investigated the influence of heat treatment conditions for the MnFe 0.53 Ni 0.47 Si 0.94 Al 0.06 alloy, comparing as-cast samples to those annealed for 80 hours at 1173 K and 1273 K, reporting a low isothermal entropy change of – 2.7 J kg −1 K −1 for 2 T at 184 K [22]. In another work, the heat treatment procedure was optimized for melt-spun ribbons of composition Mn 0.5 Fe 0.5 NiSi 0.95 Al 0.055 , reporting a peak isothermal entropy change of ~ – 8 J kg −1 K −1 for 2 T [23]. In this work, we synthesized Mn 0.5 Fe 0.5 NiSi 1-x Al x alloys (x =0.0525, 0.060, 0.0685) using arc melting and induction melting techniques due to the improved magnetocaloric performance previously observed in the all-d-metal Ni(Co)-Mn-Ti Heusler alloys [24]. We investigate the influence of annealing and Al content on the magnetostructural and magnetocaloric properties of this alloy system. Annealing at 1173 K for 7 days is found to be the optimal annealing treatment, for which a sharp magnetostructural transformation is obtained for the x =0.0525 sample. Upon increasing the Al content from 0.0525 to 0.0685 (for optimally annealed samples), the transformation shifts to lower temperatures (100 K decrease upon heating). A large magnetocaloric response of −11.45 J kg −1 K −1 near room temperature for 1 T is achieved (140 % improvement with respect to the as-cast value). This response obtained for relatively low magnetic field changes of 1 T exceeds those previously reported for similar alloy compositions, indicating the strong influence of thermal treatments [14,21,25]. However, increasing the Al content leads to a deterioration of sample homogeneity, resulting in a more gradual magnetostructural transformation and a slight decrease in the associated isothermal entropy change. These results signify the importance of the annealing parameters and sample homogeneity in the MC performance of the novel MnNiSi-based alloys for near room temperature applications. 2. Materials and methods Bulk ingots of nominal composition Mn 0.5 Fe 0.5 NiSi 1-x Al x (x = 0.0525, 0.060, 0.0685), hereby referred to as Al 0.0525 , Al 0.060 and Al 0.0685 , were first arc-melted twice (flipped each time) starting from raw materials (with at least 99.9 % purity) in an argon-controlled atmosphere. The resulting ingots were further melted another 2 times (also flipped each time) using induction melting. To investigate the influence of heat treatment on the MC properties of the alloys, the Al 0.0525 as-cast ingot was annealed at various temperatures (1073 K, 1173 K and 1273 K) for 7 days followed by water quenching. The microstructure and composition were characterized using X-ray diffraction (XRD, Bruker D8I diffractometer with Cu-K α radiation) and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX; FEI™ Teneo). Thermal analysis was carried out using a TA Instruments Q20 differential scanning calorimeter (DSC) at different temperature sweeping rates. The thermal hysteresis (ΔThys)and entropy change of the transformation (ΔStrans)were calculated from DSC scans [26]. Temperature-dependent magnetization measurements were performed using the Vibrating Sample Magnetometry (VSM) option of the Physical Property Measurement System at a sweeping rate of 10 K min −1 . The isothermal entropy change (ΔSiso) was determined from the isothermal magnetization measurements using a discrete Fig. 1. Temperature dependence of magnetization for 1 T (left axis) and its temperature derivative (right axis) for the as-cast and annealed Al 0.0525 samples. A.N. Khan et al. Journal of Alloys and Compounds 1008 (2024) 176724 2 approximation to the following equation [1,27]: ΔSiso = μ 0∫ H 0( ∂ M ∂ T)Hʹ dHʹ,(1) where μ 0 is the magnetic permeability of vacuum, M the magnetization, T the temperature and H the magnetic field. Discontinuous protocols were performed to avoid spurious results, erasing the history of the samples prior to each isothermal magnetization measurement [28–30]. In such a protocol, for heating measurements, the erasing temperature was established well below the transition. Subsequently, the sample was heated under the maximum magnetic field until the desired temperature. When the desired temperature value was reached, the field was decreased under isothermal conditions. To evaluate the magnetocaloric performance of the studied series, the temperature averaged entropy change over a span (ΔT) of 5 K (TEC(5)) was calculated using a discrete approximation to the following equation [31]: TEC(ΔT) = 1 ΔTmax ⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩ ∫ T+(ΔT 2) T−(ΔT 2) ΔSiso(Tʹ)dTʹ ⎫ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎬ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎭ .(2) 3. Results and discussion First, we analyze the influence of the thermal treatments on the magnetostructural transition of the selected Al 0.0525 sample. Fig. 1a- d shows the temperature dependence of magnetization curves (right axis) for the as-cast and annealed Al 0.0525 samples for 1 T along with the dM/dT (left axis) plots. All samples undergo a near room temperature transformation from a low temperature phase with notable magnetization to a high temperature phase with lower magnetization, however, the abruptness of the transformation is significantly modified by the annealing. For the as-cast sample, a gradual magnetization change is observed for both cooling and heating branches, which is evidenced from the derivative plots. Annealing the sample at 1073 K causes an increase in the abruptness of the magnetization change upon heating. A further increase in the annealing temperature to 1173 K leads to a more abrupt change in magnetization upon both heating and cooling. A higher temperature annealing at 1273 K deteriorates the magnetization change compared to the previous 1173 K annealing. A significant hysteresis is observed for all samples (≈40 K); however, this value is affected by thermal inertia due to the sweeping temperature used. Alternatively, thermal hysteresis is obtained for the Al 0.0525 sample optimally annealed at 1173 K from the DSC heating and cooling scans, resulting in a thermal hysteresis of ~ 25 K when extrapolating to static conditions (Fig. A of the supplementary information). It is interesting to note that the magnetization and transition temperatures do not substantially change with annealing temperatures and remain similar for the studied alloys, which indicates only small compositional changes might happen through the series of annealings. Fig. 2a-d shows the backscattered electron (BSE) micrographs taken on polished sample surfaces at room temperature for the different samples. For the as-cast sample, minor contrast throughout the surface is observed. The existing contrast becomes more significant as we anneal the samples, with the micrographs showing a main phase corresponding to a homogeneous matrix with minor lighter regions. Interestingly, this double contrast is more pronounced in the case of the sample annealed at 1173 K (Fig. 2c), the one with more abrupt transformations, where the minor regions are present mainly at the grain boundaries [14]. The elemental compositions for the samples obtained from the EDX analysis are also presented as radar graphs in Fig. 2 (below each corresponding micrograph). The results reveal a relatively similar composition for all the investigated samples when compared with the nominal one, confirming that the contrast observed in the BSE micrographs (main and minor regions) is due to minor compositional variations. Once the optimal annealing temperature of 1173 K was established, the investigations were extended to varying the Al concentration. Fig. 3 (left-panel) shows the room temperature XRD patterns together with the Rietveld analysis using the TOPAS software. As shown, all Bragg peaks are correctly identified using Ni 2 In-type and TiNiSi-type structures without evidence of impurity phases compared to the unidentified impurity phases reported in [12] or the additional Mn 3 Al 2 minor phase reported in [32]. The lines corresponding to the TiNiSi-type become less intense as Al concentration increases, indicating that the transition temperature is shifted to temperatures below room temperature. Table 1 shows the lattice parameters of the alloy series, the phase proportion, and the weighted profile R-factor (Rwp) obtained using the Rietveld analysis. The lattice parameters obtained for the Ni 2 In-type phase remain rather similar. For the TiNiSi-type phase, a decrease in lattice parameter a is associated with an increase in lattice parameter c together with a slight increase in the unit cell volume. Nevertheless, for both phases, the lattice parameters are close to those previously determined in the literature [10,12,16]. It is worth mentioning that the ideal Fig. 2. BSE micrographs of the Al 0.0525 samples: (a) as-cast, (b) 1073 K, (c) 1173 K, (d) 1273 K annealing temperatures and the corresponding elemental compositions obtained by EDX analysis below each micrograph. A.N. Khan et al. Journal of Alloys and Compounds 1008 (2024) 176724 3 Fig. 3. (Left) Room temperature powder XRD analysis and corresponding Rietveld refinement for the studied series: (a) Al 0.0525 , (b) Al 0.060 , and (c) Al 0.0685 . Black bars indicate the Ni 2 In-type phase while grey bars indicate the TiNiSi-type phase. The insets show the corresponding micrographs obtained from the room temperature BSE analysis. (Right) Pie chart showing the elemental compositions of main and minor regions as obtained by the EDX analysis for the investigated series. The outer circle shows the nominal composition whereas the inner two circles show the composition of the main and minor regions, respectively. Table 1 Structural parameters of the studied samples obtained by Rietveld analysis. Alloy TiNiSi-type Ni 2 In-type Rwp a (˚ A) b (˚ A) c (˚ A) Phase proportion (wt%) a (˚ A) c (˚ A) Phase proportion (wt%) Al 0.0525 5.5814 (16) 3.6751 (5) 6.9311 (13) 45.4 (6) 3.9679 (3) 5.1002 (5) 54.6 (6) 6.05 Al 0.060 5.5670 (5) 3.6885 (17) 6.9160 (3) 23.4 (6) 3.9708 (18) 5.1055 (3) 76.6 (6) 4.51 Al 0.0685 5.6170 (4) 3.7210 (2) 6.8390 (3) 18.5 (7) 3.9698 (4) 5.1032 (6) 81.5 (7) 6.33 A.N. Khan et al. Journal of Alloys and Compounds 1008 (2024) 176724 4 relations between the Ni 2 In-type and TiNiSi-type phases parameters, i. e., aortho =chexa, bortho =ahexa and cortho = 3 √ahexa [33], are not completely fulfilled, explaining the large thermal hysteresis observed for the series (further analyzed below). It is evident that upon increase in the Al content, the phase corresponding to the Ni 2 In-type becomes more dominant, increasing from 51.6 wt% for Al 0.0525 to 81.5 wt% for Al 0.0685 . The inset of Fig. 3 shows the room temperature BSE micrographs, whereas their elemental analysis is presented as pie chart at the right-hand side of Fig. 3. For the Al 0.0525 and Al 0.060 samples, the main and minor regions, observed from the BSE micrographs, show a minute change in composition with a similar microstructure. Upon an increase in Al concentration to 0.0685, the minor region indicates a significant depletion in Fe and a small decrease in Si and Al content. Additionally, slightly different contrast can be observed for different grains. This indicates that the homogeneity of the sample is compromised as Al concentration increases. The M(T)plots obtained for an applied magnetic field of 1 T for the investigated alloy series show a clear shift to lower temperatures of the magnetostructural transformation with increasing the Al content (Fig. 4a). The Ttrans decreases from 325 K for Al 0.0525 to 220 K for Al 0.0685 (upon heating). This agrees with the previously shown XRD results at room temperature. As already discussed, an abrupt decrease in magnetization upon increasing temperatures is observed for the Al 0.0525 sample. However, that decrease is more gradual for Al 0.060 and Al 0.0685 samples. This is evidenced in the dM/dT plot upon heating as depicted in Fig. 4b. It can be observed that the width of the transition increases with increasing Al content, obtaining a more gradual transformation. This can be directly related to the inhomogeneities observed from the EDS analysis. Regarding the thermal hysteresis shown in Fig. 4c, the Al content does not affect this magnitude though the observed value is overestimated due to the temperature ramp rate used during measurements (all samples employed for the measurements have similar mass and shape so all should be similarly affected). As shown previously from the DSC analysis for the Al 0.0525 sample (the only sample among the series whose transition temperatures falls within the DSC measured temperature range), the thermal hysteresis values are found to be close to 25 K. This hysteresis must be significantly reduced for improving the applicability of these alloys. The temperature dependence of the isothermal entropy change is plotted upon heating in Fig. 5 for various magnetic field changes. A large magnetocaloric response is obtained near room temperature and at moderate fields for Al 0.0525 sample, where ΔSiso reaches –11.5 J kg −1 K −1 for 1 T. This obtained value is the largest reported so far for Mn-Fe- Ni-Si-Al alloys showing a transition near room temperature (further compared below) [12,14,23]. It is worth mentioning that the sample annealed at 1173 K shows the best performance of the annealed series, being 140 % larger than that of the as-cast sample (Fig. B of the supplementary information), showing the importance of adequate thermal treatments. Upon application of larger magnetic field changes of up to 5 T, the ΔSiso response increases to –42.8 J kg −1 K −1 . This is ~ 93 % of the ΔStrans value (– 46 J kg −1 K −1 ) obtained from DSC by integrating the heat flow divided the temperature along the DSC peak (Fig. A of the supplementary information), indicating the proximity to the completion of the transformation induced by the magnetic field. As the Al concentration increases from 0.0525 to 0.0685, the ΔSiso peak shifts to lower temperatures, accompanied by a decrease in the isothermal entropy Fig. 4. (a) Temperature dependence of magnetization (b) dM/dT as a function of temperature upon heating and (c) transformation temperatures upon heating and cooling for 1 T for Al 0.0525, Al 0.060 and Al 0.0685 samples annealed at 1173 K for 7 days. Fig. 5. Temperature dependence of the isothermal entropy change upon heating for different magnetic fields and temperature averaged entropy change (TEC)over a span of 5 K (inset). A.N. Khan et al. Journal of Alloys and Compounds 1008 (2024) 176724 5 change. This reduction is attributed to a rather gradual change in magnetization for the Al 0.060 and Al 0.0685 samples, as evidenced in Fig. 4. At the same time, as discussed above, this was caused by the deterioration in sample homogeneity for Al 0.0685 , as confirmed from the EDX analysis (Fig. 3). To evaluate the MC performance, the temperature averaged entropy change (TEC)is calculated for the alloy series over a span of 5 K and depicted in the inset of Fig. 5. A rather similar TEC(5) response is observed for the whole investigated alloy series, ranging from ≈5 → 15 → 25 J kg −1 K −1 for applied magnetic field changes of 1 → 3 → 5 T, respectively. This is an interesting feature, as increasing the Al content does not deteriorate the performance independently of the magnetic field change. To further evaluate the magnetocaloric response, Fig. 6 draws a comparison between the investigated series and other Mn-Fe-Ni-Si-Al alloys already reported in the literature for an applied magnetic field change of 1 T, which is comparable to the one achieved in real devices (generated by permanent magnet arrays [34]). The MC response for the investigated alloy series is shown to be comparable with those reported, with the Al 0.0525 sample showing the largest response near room temperature. As discussed, a decreasing trend for the peak of the isothermal entropy change upon increasing Al content is observed. Compared to the literature, this trend is opposite to the data reported in [12] but agrees with the previous trend reported in [25], where the increase in Al content compromises compositional homogeneity (which is also evidenced in our study). 4. Conclusions The influence of heat treatment and Al content has been systematically investigated in Mn 0.5 Fe 0.5 NiSi 1-x Al x alloys (x =0.0525, 0.060, 0.0685), synthesized by a combination of arc melting and induction melting techniques. Heat treating the Al 0.0525 alloy at 1173 K for 7 days indicates single-phase material with composition close to the nominal one. This leads to a sharp magnetostructural transition together with a large isothermal entropy change, which represents the highest value reported so far for the Mn-Fe-Ni-Si-Al alloys near room temperature. Upon increasing Al content from 0.0525 to 0.0685, the transformation shifts to lower temperatures (100 K decrease). Additionally, the isothermal entropy change is slightly reduced by increasing Al content due to minor compositional inhomogeneities. However, the values of the temperature average entropy change, TEC(5),show a rather similar performance for the series. CRediT authorship contribution statement Victorino Franco: Writing – review & editing, Supervision, Methodology, Investigation, Funding acquisition, Conceptualization. Jia Yan Law: Writing – review & editing, Methodology, Investigation, Conceptualization. Luis M. Moreno-Ramirez: Writing – review & editing, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Aun N. Khan: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgment Work supported by Grants PID2019-105720RB-I00 and PID2023- 146047OBI00 funded by MCIN/AEI/ 10.13039/501100011033, US Air Force Office of Scientific Research (FA8655-21-1-7044), the Clean Hydrogen Partnership and its members within the project HyLICAL (Grant No. 101101461), the European Innovation Council, funded by the European Union, via project CoCoMag (Grant no. 101099736), and the Research Council of Norway within project LIQUID-H. J.Y.L acknowledges EMERGIA 2021 fellowship from Junta de Andalucía (Ref. EMC21_00418). ANK acknowledges a FPI fellowship from the Spanish MCIN/AEI. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jallcom.2024.176724. References [1] V.K. Pecharsky, K.A. Gschneidner Jr, Magnetocaloric effect and magnetic refrigeration, J. Magn. Magn. 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