Windows open for highly tunable magnetostructural phase transitions

An attempt was made to tailor the magnetostructural transitions (MSTs) over a wide temperature range under the principle of isostructural alloying. A series of wide Curie-temperature windows (CTWs) with a maximal width of 377 K between 69 and 446 K were established in the Mn1-yCoyNiGe1-xSix system. Throughout the CTWs, the magnetic-field-induced metamagnetic behavior and giant magnetocaloric effects are obtained. The (Mn,Co)Ni(Ge,Si) system shows great potential as multifunctional phase-transition materials that work in a wide range covering liquid-nitrogen and above water-boiling temperatures. Moreover, general understanding to isostructural alloying and CTWs constructed in (Mn,Co)Ni(Ge,Si) as well as (Mn,Fe)Ni(Ge,Si) are provided.

Magnetic materials with considerable caloric effects in the vicinity of magnetoelastic or magnetostructural transitions (MSTs) show great potential in solid-state refrigeration. [1][2][3] Currently, a large number of caloric materials have been successively found, including the magnetocaloric materials working at different temperatures. 4 Tunable magnetocaloric effects (MCE) over a wide temperature range may provide expended chance for different applications. Especially, high-temperature MCEs, which may be used at elevated temperatures especially above water boiling point (373 K) for magnetic heat pumps, 5 electric power generation, 6,7 or magnetic cooling, 8,9 are seldom reported.
In recent years, hexagonal MM'X (M, M' = transition metals, X = carbon or boron group elements) compounds have been extensively investigated due to the tunable MSTs and associated giant MCEs. [10][11][12][13] In these MM'X compounds, the MSTs, from Ni 2 In-type hexagonal parent phase to TiNiSi-type orthorhombic martensite with different magnetic states, can be obtained by coupling martensitic structural transitions (MTs) with magnetic transitions. The Curie-temperature windows (CTWs), 12,14 constructed by Curie temperatures of parent (T C A ) and that of martensite (T C M ) phases, were revealed and used to maximize the magnetic-energy change in magnetic-field-induced MSTs. Many MSTs have been tuned into CTWs and exhibited remarkable magnetoresponsive effects. [12][13][14][15][16][17][18][19][20] During the practice of MST tuning, we further proposed a principle of isostructural alloying to manipulate the structural transitions and the magnetic couplings at one time. 14 The chemically substituting elements were determined from the viewpoint of alloying the isostructural counterparts before performing the experiments. Till now, CTWs have been established in many alloy systems, [21][22][23][24][25][26][27] in which the tunable MSTs with giant MCEs were obtained.
In our previously reported results, broad CTWs have been constructed by using isostructural alloying in both Mn 1−y Fe y NiGe and Mn 1−y Co y NiGe systems. 14,21 By further applying isostructural alloying to Mn 1−y Fe y NiGe and MnNiSi, a unprecedentedly wide CTW of 400 K has been achieved. 28 According to the principle of isostructural alloying, a proper isostructural counterpart with a MT and a T C M both at high temperatures should be selected to tune MSTs of Mn 1−y Fe y NiGe to high temperatures. 14, 21 This is why MnNiSi was chosen, whose MT temperature (T t ) and T C M are as high as 1200 K and 622 K respectively. 29 were prepared by arc melting high-purity metals four times in argon atmosphere. All ingots were annealed at 1123 K in an evacuated quartz tube for five days and then cooled slowly to room temperature. The phase structures of the samples were characterized by powder x-ray diffraction (XRD) with Cu-K α radiation. The differential scanning calorimetry (DSC) with permanent-magnet assisted thermogravimetric analysis (TGA) was used to detect the structural and magnetic transitions. With the aid of the upward magnetic pull forces provided by the magnets, a change in sample weight can be detected across the magnetic transition by TGA.
The magnetic measurements were performed on superconducting quantum interference device (SQUID) magnetometer, physical property measurement system (PPMS) in the range of 5 ~ 400 K, as well as the vibrating sample magnetometer (VSM, VersaLab, 3 T) for temperatures above 400 K.
Room-temperature XRD analysis of Mn 1−y Co y NiGe 1-x Si x system were performed.   Based on all data from XRD, magnetic and DSC-TGA measurements (Figs. S1 and S2 in supplemental material), 34 we propose a magnetostructural phase diagram (see complete phase diagram in Fig. S3) 34 of Mn 1-y Co y NiGe 1-x Si x (y = 0.2, 0.3, 0.4; 0 ≤ x ≤ 1), as shown in Fig. 2 Fig. 2(a) and (b). The theoretical T t of y = 0.3 and 0.4 alloy series will also increase from B 0 and C 0 , respectively, to higher temperatures. With parent phase being stabilized deeply, nevertheless, it needs more Si contents (x = 0.26 for y = 0.3 and x = 0.48 for y = 0.4) to awaken the "dead" transition above T C A (the lower T cr , denoted by A 1 , B 1 and C 1 in Fig. 2(a)). As mentioned above, for MM'X compounds the magnetic ordering around T C A can suppress the MTs. 12 In order to further analyze the ferromagnetism behavior within the broad CTWs obtained in this study, we measured the magnetization M(H) curves of for Mn 1−y Co y NiGe 1-x Si x ( Fig. 1(a)) and 12.4% for Mn 1−y Fe y NiGe 1-x Si x . 28 The large   At room temperature, the samples crystallize in a Ni 2 In-type hexagonal parent structure when x ≤ 0.25 and a TiNiSi-type orthorhombic martensite structure for x ≥ 0.30. The diffraction peaks of parent and martensite phases are indexed.  All the MSTs and Curie magnetic transitions of Mn 1-y Co y NiGe 1-x Si x samples at high temperatures above 400 K were further measured by DSC-TGA. Figure S2 presents the results of Mn 0.8 Co 0.2 NiGe 1-x Si x sample series. All transition temperatures are determined by the peak maximum in DSC curves. Consistent with the case in magnetic measurements below 400 K (Figure 1(b) in main context), T t continuously increases with increasing Si content (x). For x = 0.35 ( Figure S2(a)), the first-order structural MT occurs with a remarkable enthalpy change. Synchronously, a measured change of sample weight was detected in both heating and cooling processes, as shown in TGA curves in Figure S2(a). This indicates that the sample changes its structure between PM parent and FM martensite phases, which is coherent with the results characterized by M(T) curves in Figure 1 . For x = 0.40 ( Figure S2(b)), in the cooling process a similar weight change was observed across the forward transition. However, upon heating there is no change in TGA curve across the reverse transition. Instead, a remarkable weight change was observed between the forward and reverse transitions. An enlarged image of DSC curve (inset to Figure S2  Based on all data from XRD, magnetic and DSC-TGA measurements, we provide a complete phase diagram of Mn 1-y Co y NiGe 1-x Si x (y = 0.2, 0.3, 0.4; 0 ≤ x ≤ 1), as shown in Figure S3. As we can clearly see, the T t increases with increasing Si content for y = 0.2, 0.3, 0.4. In case of low Si contents, T t < T C A , no MT can be observed due to the suppression of magnetic ordering at T C A .
With increasing Si content, T t happens from paramagnetic parent to ferromagnetic martensite phase between the two red dashed lines, which means that magnetic and structural transitions are coupled together. Further increasing Si content, T t is higher than T C M and the MT happens at high-temperature paramagnetic state.