Realization of magnetostructural coupling by modifying structural transitions in MnNiSi-CoNiGe system with a wide Curie-temperature window

The magnetostructural coupling between structural and magnetic transitions leads to magneto-multifunctionalities of phase-transition alloys. Due to the increasing demands of multifunctional applications, to search for the new materials with tunable magnetostructural transformations in a large operating temperature range is important. In this work, we demonstrate that by chemically alloying MnNiSi with CoNiGe, the structural transformation temperature of MnNiSi (1200 K) is remarkably decreased by almost 1000 K. A tunable magnetostructural transformation between the paramagnetic hexagonal and ferromagnetic orthorhombic phase over a wide temperature window from 425 to 125 K is realized in (MnNiSi)1−x(CoNiGe)x system. The magnetic-field-induced magnetostructural transformation is accompanied by the high-performance magnetocaloric effect, proving that MnNiSi-CoNiGe system is a promising candidate for magnetic cooling refrigerant.

Magnetostructural transformation (MST), a coupling between the structural and magnetic transition, attracts considerable attention due to various associated interesting magnetoresponsive effects, such as magnetic shape memory effect 1 , magnetic field-induced strain 2,3 , magnetoresistance 4,5 and magnetocaloric effect [6][7][8][9] . These effects show potential applications in sensors 10 , magneto-mechanical devices 11,12 , energy-harvesting devices 13 , magnetic cooling refrigeration 14,15 , and so on. In order to realize MST in a phase-transition material, a large magnetization difference (ΔM) between the two structural phases is essential and is always pursued to increase the magnetic field-driving capacity 1 . If MST in a given system is tuned to occur between a paramagnetic (PM)/antiferromagnetic (AFM) state and a ferromagnetic (FM) state 1,16 , rather than between two FM states 17 , a large ΔM will be gained. As an example, MST is widely observed in Heusler-type magnetic shape memory alloys, which show ferromagnetic martensitic transitions 18,19 .
Based on this viewpoint, another type of magnetic shape memory material, the hexagonal MnM′ X compound (M′ = Co or Ni, X = Si or Ge) is designed to obtain MST [20][21][22][23] . In stoichiometric MnM′ X compounds, the structural transformation takes place from a martensitic-like hexagonal Ni 2 In to an orthorhombic TiNiSi structure during the cooling process 24 . The main challenge in MnM′ X compounds is that the structural transformation temperature (T t ) is usually much higher than the magnetic-ordering temperatures of both hexagonal and orthorhombic phases [25][26][27] , and the transformation occurs between two PM states [25][26][27] , resulting in a low ΔM and low magnetic field-driving capacity 17 .
It is known that, in MnM′ X compounds, the stability of Ni 2 In and TiNiSi structures is highly dependent on the covalent bonds between M′ and X atoms 28,29 , and that between the neighbouring Mn and Mn atoms 16 . The stoichiometric tuning 20,23 , foreign-atoms-substituting 16,21,22,30,31 and applied stress 32  atoms, T t of MnCoGe can be reduced from 420 K to below Curie temperature of the orthorhombic phase (T C ) and MST from PM hexagonal to FM orthorhombic phase is realized during cooling 22,31 . In the case of MnNiGe system, the stoichiometric tuning can reduce T t from 470 K to below than 200 K and obtain MST from FM hexagonal to AFM orthorhombic phase with decreasing temperature 20 . Moreover, it is reported that if T t lies in the Curie-temperature window (CTW), which is the range between Curie temperatures of the hexagonal and orthorhombic phases 16,33,34 , the structural transition will couple with magnetic state changes, bringing a large ΔM. The CTW is expected to be broad enough, so that MST and the coupled magnetoresponsive effects can be freely tailored in a large temperature range. However, in MnM′ Ge-based compounds, the relatively low magnetic ordering temperatures restrict the further enlargement of CTW.
Aimed at the potential applications with magnetoresponsive effects, a good candidate of MnM′ X compound with MST should have both a large ΔM and a wide CTW. As a member of MnM′ X family, MnNiSi has a high T C of 622 K 35 , indicating a potential large CTW. However, T t in stoichiometric MnNiSi alloy is as high as 1200 K 24 , which is almost two times higher than those in MnCoGe and MnNiGe [24][25][26][27] . Compared to the previously mentioned alloy systems, to effectively tune down T t to below T C is a big obstacle in MnNiSi system before the realization of MST 36,37 . Recently, the isostructural alloying opens up a new feasibility to realize the magnetostructural coupling in MnNiSi-based compounds 16,33,34,[38][39][40][41] . By applying this method, the wide CTW can be further obtained. In this work, by Co and Ge co-substitution, we perform the isostructural alloying of MnNiSi and CoNiGe. In this (MnNiSi) 1−x (CoNiGe) x system, T t is successfully tuned down to below than T C , and the tunable MST between PM hexagonal and FM orthorhombic phase can be obtained in a large CTW by altering the CoNiGe-content. Due to the large ΔM between two phases, the observed MST can be induced by the external magnetic field at room temperature (RT). The effect is accompanied by a large magnetocaloric effect, indicating the potential applications in magnetic cooling refrigerator.

Results
Structural characterization. Figure 1(a) shows X-ray diffraction (XRD) patterns of (MnNiSi) 1 42 . With the increase of CoNiGe-content, the structure of (MnNiSi) 1−x (CoNiGe) x samples at RT changes from the orthorhombic TiNiSi to hexagonal Ni 2 In phase and no other impurity phase is found. For the samples with x ≤ 0.36, TiNiSi structure dominates at RT; while for x = 0.37, the XRD pattern indicates the coexistence of TiNiSi and Ni 2 In structures; when x ≥ 0.38, the single Ni 2 In structure is observed. XRD results suggest that the introduction of CoNiGe will stabilize Ni 2 In structure and lower down T t from 1200 K to below RT 24 . From the crystallographic point of view, TiNiSi structure can be understood as the distortion of Ni 2 In structure and the axes of the two structures are related as a ortho = c hex , b ortho = a hex and c ortho = 3 a hex 25 . It is known that the stability of Ni 2 In structure is associated with the low c hex /a hex (a ortho /b ortho ) 39,43 . In (MnNiSi) 1−x (CoNiGe) x system, the value of a ortho /b ortho (c hex /a hex ) decreases with increasing CoNiGe-content, especially at the range of x ≤ 0.36, shown in the inset of Fig. 1(a). It further supports that Co and Ge co-substitution can reduce T t .
To further investigate the thermo-induced structural transformation in (MnNiSi) 1−x (CoNiGe) x system, the temperature-dependent XRD measurement during heating is performed on (MnNiSi) 0.66 (CoNiGe) 0.34 . As shown in Fig. 1(b), (MnNiSi) 0.66 (CoNiGe) 0.34 exhibits TiNiSi structure at below 390 K, while becomes pure Ni 2 In structure at above 433 K. When the temperature is 413 K, two structures coexist, suggesting that TiNiSi structure transits to Ni 2 In structure with increasing temperature. According to XRD analysis, the temperature-dependent unit-cell volume is calculated, shown in the inset of Fig. 1(b). A large decrease of 2.5% in unit-cell volume is observed during the structural transformation on heating, indicating a remarkable atomic displacement during the structural reconstruction.
MST and magnetic phase diagram. In order to confirm the CoNiGe-content dependent T t , DSC measurements of (MnNiSi) 1−x (CoNiGe) x (x = 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42) were carried out upon cooling and heating with a ramp rate of 10 K/min, which are shown in Fig. 2. The observed large endothermic/exothermic peaks during heating/cooling cycles are associated with the latent heat of the first-order structural transitions between TiNiSi and Ni 2 In structures. The thermal hysteresis between heating and cooling cycles signifies the first-order nature of structural transformation. For the sample with x = 0.34, the endothermic peak appears at 410 K, which agrees well with the temperature-dependent XRD analysis. It is also found that the endothermic and exothermic peaks shift towards lower temperatures with the increase of CoNiGe-content. This phenomenon verifies that the introduction of CoNiGe can reduce T t of MnNiSi from 1200 K to be below RT.
Since the magnetic properties of MnM′ X alloys are sensitive to the Mn-Mn distances 44 , the large distortion of unit-cell during the structure transformation may bring about considerable changes of magnetic states. The temperature dependences of magnetization (M-T) for (MnNiSi) 1−x (CoNiGe) x (x = 0.36, 0.38 and 0.41) measured during heating and cooling with a ramp rate of 2 K/min at a magnetic field of 0.1 T are shown in Fig. 3(a) (some other representative M-T curves are shown in the supporting information). A sharp magnetic transition from the high-temperature PM to low-temperature FM state is observed during cooling. The transition shifts to the lower temperatures with the increase of CoNiGe-content. The obvious thermal hysteresis, reflecting the irreversibility between cooling and heating cycles, suggests the first-order nature of the transition. T t , here defined as the temperature where |dM/dT| is the maximum, agrees well with the DSC measurement. These results prove that the studied samples experience MST between FM orthorhombic and PM hexagonal phase as the temperature changes. While it is worth noting that Mn atoms carry the majority of magnetic moments in MnNiSi alloys 35 . To investigate the saturated magnetization, M s , of MnNiSi-CoNiGe system, M-B curves of some samples are measured at 4.2 K in Fig. 3(b). As shown in the inset, M s decreases with increasing CoNiGe-content, and it is lower  (x = 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41  and 0.42) samples. The red and blue arrows denote the heating and cooling processes, respectively. than that of stoichiometric MnNiSi (2.62 μ B /f.u.) 35 . It indicates that the substitution of large-moment Mn atoms by small-moment Co atoms modifies the exchange interactions between Mn-Mn atoms 37 .
According to DSC and magnetic measurements, the structural and magnetic phase diagram of (MnNiSi) 1−x (CoNiGe) x system is obtained, as shown in Fig. 4. The sample with x = 0.32 transits from PM hexagonal to PM orthorhombic phase at 450 K, then to FM orthorhombic phase at 425 K during cooling. Upon the further increase of CoNiGe-content to 0.43, T t continuously decreases to 125 K. When x is higher than 0.43, MST disappears and a weak magnetic spin-glass-like state is found, similar as Mn 1−x Fe x NiGe 16 . Thus, a CTW ranged from 425 to 125 K is established in (MnNiSi) 1−x (CoNiGe) x system, where alloys undergo a tunable MST coupled with a magnetic transition from PM to FM state.

Magnetic-field-inducing MST and magnetocaloric effect. In the case of Ni-Mn based ferromagnetic
shape memory alloys, due to the large ΔM between FM austenite and weak magnetic martensite, the austenite is energetically more favorable in the magnetic field, giving rise to the decrease of T t and the magnetic-field-inducing MST from the martensite to austenite 1 . Similarly, in (MnNiSi) 1−x (CoNiGe) x system with a first-order PM-FM transition, the magnetic field will stabilize FM TiNiSi structure and lead to a MST from the hexagonal to orthorhombic phase. For the sample with x = 0.38, T t increases 4.8 K under a magnetic field of 5 T, indicating that MST can be induced by the magnetic field ( Fig. 5(a)). Due to the discontinuities of spin and lattice, MST is associated with a large magnetic entropy change (ΔS). It is known that the maximum value of ΔS can be estimated from M-T curves measured at different constant fields (here, B = 0.1 and 5 T, respectively) using the Clausius-Clapeyron equation: (1) For (MnNiSi) 0.62 (CoNiGe) 0.38 , the calculated maximum value of ΔS is − 41.8 J/(kg·K), where ΔM = 41 A·m 2 / kg, ΔB = 4.9 T and ΔT = 4.8 K (Fig. 5(a)). To further confirm the magnetocaloric effect during the transition, ΔS in the heating process is also calculated from the isothermal magnetization curves (M-B curves in Fig. 5(b)) using the Maxwell relation. As shown in Fig. 5(c), the maximum value of ΔS is − 40.3 J/(kg·K) at 295 K, which agrees with the value obtained by Clausius-Clapeyron equation. In (MnNiSi) 1−x (CoNiGe) x system, the large CTW offers the possibility to obtain large ΔS values in the temperature range of nearly 300 K. As an example, the substitution levels of x = 0.40 and 0.39 give rise to ΔS of − 31.7 and − 30.5 J/(kg·K) for ΔB = 5 T at 245 and 270 K, respectively (Fig. 5(c)). The observed maximum ΔS is larger than some promising magnetocaloric systems, such as some MnM′ Ge-based systems 16,30,33 , Ni-Mn based alloys 7,8 and rare earth-transition metal intermetallic compounds 45 . The effective refrigeration capacity (RC eff ), which is calculated by subtracting the average hysteresis loss (HL) from the refrigeration capacity (RC) value, is commonly adopted to evaluate magnetocaloric effect 46 . For sample with x = 0.38, RC value is 170.1 J/kg around room temperature, numerically calculated by integrating the area under ΔS-T curves, using the temperatures at half-maximum of the peak as the integration limits. And HL, calculated from the area surrounded by hysteresis loops (M-B curves), is negligible, as shown in the inset of Fig. 5(c). Therefore, RC eff is about 169.8 J/kg for this sample under the field change of 0-5 T. These indicate the potential applications for the RT magnetic cooling refrigerator. Besides, as mentioned above, the MnNiSi-CoNiGe system undergoes large changes of lattice parameters and unit volume during the transition ( Fig. 1(a)), which can be induced by the applied magnetic field. This may be utilized for the strain-based applications 16 .

Discussion
The observed MST between FM orthorhombic and PM hexagonal phase is achieved by adjusting T t into the CTW. However, as mentioned above, T t of MnNiSi is as high as 1200 K and cannot be efficiently decreased by the conventional methods 36,37 . Here, the question is why Co and Ge co-substitution can sharply reduce T t to below T C , leading to the observed MST. In MnM′ X alloys, covalent bonds form between M′ and X atoms and between the neighbouring Mn and Mn atoms both in TiNiSi and Ni 2 In structures 16,28,29 . These covalent bonds act as skeletons, stabilizing the crystalline structure 16,29,47 . The structure transformation can be understood as a competition between the strengths of covalent bonds in TiNiSi and Ni 2 In structures 16,34,47 . Thus, altering the strength of covalent bonds can influence T t . For the system with M′ = Ni, T t of MnNiGe lies around 470 K 24,25 , which is 730 K lower than that of MnNiSi 24 . This suggests that altering the main-group elements can influence the strength of covalent bond, leading to the change of T t . Similar phenomenon is found in Al substituted MnNiGe alloy 48 . Except for altering elements which form M′ -X covalent bond, the decrease of T t is also observed when Mn is replaced by other 3d-atoms, such as Mn 1−x Fe x CoGe alloy 49 . Based on the site-preference rule 50 , Fe atoms occupy Mn sites and induce lattice change in c axis. This change will influence the separation of Mn atoms, leading to the enhancement of the strength of Mn-Mn covalent bonds, which is helpful to stabilizing the Ni 2 In structure. For MnNiSi, T t cannot be efficiently reduced by the single-element substitution 36,37 . However, when CoNiGe is introduced, Co atoms occupy 3d-atom Mn sites, Ge atoms occupy main-group Si sites, and T t decreases by almost 1000 K with increasing CoNiGe-content.
From the view of applications, a large CTW enables tunable magnetoresponsive effects in a wide temperature range. In this (MnNiSi) 1−x (CoNiGe) x system, the CTW is as large as 300 K, which is comparable to the previously reported systems, such as MnCoGe-based system 23 , Mn 1−x Co x NiGe 33 , MnNiGe:Fe system 16 , (Mn, Fe)Ni(Ge, Si) 34 and Gd-Si-Ge alloys 51 . It is known that the realization of PM-FM-type MST should meet the condition that T t can be tuned into the temperature window between the magnetic-ordering temperatures of orthorhombic and hexagonal phases. In (MnNiSi) 1−x (CoNiGe) x system, T C of orthorhombic phase is around 425 K (shown in Fig. S1). However, with the increase of CoNiGe-content, a spin-glass-like state appears and the magnetic order-disorder transition of hexagonal phase is still not observed (Fig. S1). This phenomenon indicates that the magnetic ordering temperature of hexagonal phase is lower than 125 K. Therefore, a large CTW between 425 and 125 K is established in (MnNiSi) 1−x (CoNiGe) x system.

Conclusions
To summarize, we have successfully realized a PM-FM magnetostructural coupling in (MnNiSi) 1−x (CoNiGe) x system. By introducing CoNiGe which possesses a stable Ni 2 In structure, T t decreases sharply, resulting in the first-order MST between FM orthorhombic and PM hexagonal phase. This is due to the enhancement of covalent bonds which modifies the relative stability of two structures. Besides, a large CTW of 300 K is established, which will benefit the multifunctional applications of the materials over a wide temperature range. Due to the large ΔM during the transition, MST can not only be induced by temperature, but also by magnetic field. Large, tunable magnetocaloric effect generated from MST is obtained. The magnetic entropy change of (MnNiSi) 0.62 (CoNiGe) 0.38 reaches − 40.3 J/(kg·K) under the field change of 0-5 T around RT, suggesting the potential applications in magnetic cooling refrigerator.

Methods
The samples with nominal compositions of (MnNiSi) 1 44) were prepared by arc-melting the appropriate amounts of raw materials in a water-cooled copper crucible under a high purity argon atmosphere for three times. As-cast ingots were annealed in vacuumed quartz tubes at 1073 K for four days before quenched into the cold water.
The crystal structures of the specimens were identified by X-ray diffraction (XRD, Bruker, D8 Advance) analysis with Cu-Kα radiation. The structural transitions were investigated by differential scanning calorimetry (DSC, Mettler Toledo, DSC 3).
Magnetic measurements were performed with vibrating sample magnetometer (VSM, LakeShore, 7407) and Physical Property Measurement System (PPMS, Quantum Design, Dynacool). Isothermal magnetic entropy change (ΔS) was calculated from the isothermal magnetization curves using Maxwell equation (2): To avoid the irreversibility in the magnetic-field-induced first-order MST, isothermal magnetization curves were measured using a so-called loop process 52 : in the heating process, the isothermal magnetization were measured with a temperature interval of 2 K in the magnetic field variation from 0 to 5 T. For each M-B curve, the samples were initially cooled down to complete orthorhombic region (for our samples, it's around 150 K) at 5 K/ min. Then the samples were heated slowly to the measuring temperature at 3 K/min. To guarantee the temperature stability of measurement, a waiting time of 300 s was hold at the initial and targeted temperatures. Besides, for the samples with x = 0.38, 0.39 and 0.40, the highest loop temperatures were 301, 288 and 261 K, respectively.