Magnetostructural transformation (MST), a coinciding magnetic and crystallographic transition, has attracted great attention for its multimagnetoresponsive effects, such as magnetic shape memory effect,^[1] exchange bias effect,^[2] magnetoresistance^[3,4] magnetic field-induced strain^[5] and large magnetocaloric effect (MCE).^[6,7] Recently, it has been found that MnMX-based alloys, where M is Co or Ni and X is nonmagnetic sp element Si or Ge,^[8] exhibit a thermo-induced structural transformation between high-temperature hexagonal and low-temperature orthorhombic phase. On condition that the transformation temperature (T_t) is tuned into a temperature region between the Curie-temperatures of the two phases ( and , the subscripts H and O correspond to the hexagonal and orthorhombic phases, respectively.), MST between paramagnetic (PM) hexagonal and ferromagnetic (FM) orthorhombic phases can be achieved.^[9] The accompanied large magnetization difference (ΔM) is favorable for the magnetic-field-induced MST, resulting in large magnetic entropy change ().^[10] The obtained is comparable to those of some famous magnetocaloric materials,^[11] indicating the potential applications in magnetic cooling refrigerator.

In MnMX family, MnNiGe alloy is a special member since the hexagonal MnNiGe is a ferromagnet with , while the orthorhombic one is spiral antiferromagnetic (AFM) below its Néel temperature ∼346 K.^[12] On the premise that the magnetic state of orthorhombic phase keeps unchanged, the only way to fulfill an MST with large ΔM is to reduce the T_t to a value lower than and thus the MST between AFM orthorhombic and FM hexagonal phase can be obtained.^[1,13,14] Although this AFM–FM MST can be induced by the magnetic field, there exist two accompanying disadvantages. One is the low-temperature MST as well as the associated physical effect, which is not convenient to the potential utilization at ambient temperature.^[15] The other is the narrow temperature region, in which the MST can be fulfilled, because it is reported that the MST will soon disappear once the T_t is below .^[16]

The promising way to solve the two disadvantages is to change the ground magnetic state of orthorhombic phase from AFM to FM. In that case, the MST can be established between PM hexagonal and FM orthorhombic phase in a temperature range from to .^[16] This temperature region is as large as 141 K and also covers room temperature. As reported before, the magnetic state of orthorhombic phase is sensitive to the atomic environment and the introduction of magnetic element Fe or Co can efficiently change its magnetic state into FM state.^[17,18] In this work, we will use a nonmagnetic element V to activate the FM coupling in orthorhombic phase. Simultaneously, the V-doping also reduces the T_t, and thus leading to PM-FM MST. Based on this improvement, large MCE is achieved in this V-doped MnNiGe alloy.

Polycrystalline samples with compositions of MnNi_{1 − x}V_xGe (x = 0.10, 0.12, 0.14, 0.16, and 0.18) were prepared by arc melting high-purity metals in argon atmosphere. The ingots were then sealed in a vacuum quartz tube and annealed at 1123 K for 72 h followed by rapid quenching in ice water. The structure data were obtained using a room-temperature powder x-ray diffraction (XRD) with Cu–Kα radiations (Bruker D8 Advance). Magnetic measurements were performed by using a physical property measurement system (PPMS, Quantum Design). Density-functional theory (DFT) calculations were performed to investigate the V position by using the general gradient approximation method (GGA) implemented in VASP code.^[19] The plane-wave cut-off energy of 400 eV and high density of k grid of 19×19×19 points were used. In this study, we turned off the spin-orbit interaction and only considered the FM spin arrangements during the calculations.

Figure 1 shows the room-temperature powder XRD patterns of MnNi_{1 − x}V_xGe (x = 0.10, 0.12, 0.14, 0.16 and 0.18) alloys. For the sample with x = 0.10, orthorhombic phase (TiNiSi-type structure, Pnma, space group 62) predominates. With increasing x to a value higher than 0.12, single hexagonal phase (Ni₂In-type structure, , space group 194) can be found. According to the XRD data, it can be concluded that the increase of V-content will reduce the T_t from 470 K, at which stoichiomtric MnNiGe alloy experiences the hexagonal-orthorhombic structural transformation,^[20] to a value lower than room temperature. According to the XRD patterns, the lattice parameters of the samples with x = 0.12–0.18 are calculated. As is well known, the TiNiSi-type orthorhombic structure is an orthorhombic distortion of the Ni₂In-type hexagonal structure and the reduction of ratio tends to stabilize the hexagonal structure. As listed in Table 1, with the increase of x, the values of decrease gradually. It further reveals the decrease of T_t.

Fig. 1.

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	Fig. 1. (color online) Powder XRD patterns of MnNi1 − xVxGe (x = 0.10, 0.12, 0.14, 0.16 and 0.18) alloys at room temperature.

Table 1.

Table 1.

Table 1. Lattice parameters of the samples with x = 0.12–0.18. . samples aH cH cH/aH x = 0.12 4.0958(8) 5.4112(0) 1.3211(6) x = 0.14 4.0998(4) 5.4074(6) 1.3189(4) x = 0.16 4.1003(9) 5.4062(5) 1.3184(9) x = 0.18 4.0971(8) 5.3974(2) 1.3173(7)

Table 1. Lattice parameters of the samples with x = 0.12–0.18. .

In ordered Ni₂In-type hexagonal compounds, the atomic occupancy follows a basic rule that the element with more valence electrons preferentially occupies Ni site, while the element with less covalent electrons intends to occupy Mn site.^[21] According to this, the V atoms should not occupy Ni sites, but intend to occupy Mn sites and simultaneously extrude the same number of Mn atoms to the vacant Ni sites. The corresponding chemical formula is therefore (Mn_{1 − x}V_x)(Mn_xNi_{1 − x})Ge instead of MnNi_{1 − x}V_xGe. The two possible V atomic occupancies are illustrated in Fig 2. To check the V position, DFT calculations are carried out. During the calculation, x is assumed to be 0.25 for simplicity. The obtained results show that the (Mn_{1 − x}V_x)(Mn_xNi_{1 − x})Ge displays a lower free energy (−9437.1082 eV) than MnNi_{1 − x}V_xGe (−9086.0930 eV), indicating that the V atoms obey the atomic occupancy rule.

Fig. 2.

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	Fig. 2. (color online) Schematic diagrams of the two possible V atomic occupancies.

To investigate the magnetostructural coupling, the temperature dependences of magnetization (M–T) curves under a magnetic field of 5 T are shown in Fig. 3(a). The reason for us to choose such a high magnetic field is to overcome the influence of magnetocrystalline anisotropy on the measured magnetization so that the variation of saturated magnetization with V-content can be reflected in these M–T curves. In order to eliminate the potential virgin effect,^[22] each sample is zero-field cooled to a temperature far below T_t prior to the measurement. The heating and cooling rate during measurement are both chosen to be 3 K/min. For the sample with x = 0.10, a continuous and reversible magnetic transition between FM and PM state is observed at ∼210 K. It indicates that this sample is stabilized at orthorhombic state and no structural transformation occurs below 400 K. With increasing x to 0.12, a sharp MST between PM hexagonal and high-magnetization orthorhombic phases is obtained. The obvious hysteresis demonstrates the first-order nature of the structural transformation. Similar phenomena can also be found in the samples with x = 0.14 and 0.16. Further increasing x will eliminate the structure transformation, leading to a smooth magnetic transition of hexagonal phase at 340 K in the sample with x = 0.18.

Fig. 3.

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	Fig. 3. (color online) (a) M–T curves of MnNi1 − xVxGe (x = 0.10, 0.12, 0.14, 0.16, and 0.18) alloys under 5 T. Isothermal magnetization curves of the sample with x = 0.12 (b) and (c) 0.14. (d) Phase diagram of V-doped MnNiGe alloys.

Since the orthorhombic MnNiGe possesses a spiral AFM ground magnetic state, the observed high magnetization is challenged. Figure 3(b) shows the isothermal magnetization curves of the sample with x = 0.12 at 160, 180, 200, and 220 K. Each curve exhibits a slope changing with increasing magnetic field, and the critical field (see inset of Fig. 3(b)), at which the slope changes, increases from 0.217 to 0.283 T with the temperature decreasing from 220 to 160 K. This phenomenon is a typical behavior of the magnetic-field-induced metamagnetic transition from AFM to FM state in MnNiGe-based alloy.^[23–25] It should be noted that the critical magnetic field in stoichiometric MnNiGe is as large as 10 T. Therefore, V-doping greatly reduces the critical field. With further increasing x, the metamagnetic transition disappears and the sample with x = 0.14 exhibits FM behavior (Fig. 3(c)).

In MnMX alloys, since the Mn atoms are relatively isolated, magnetic moment is mainly localized on the Mn atoms.^[21,26] The arrangement of Mn moments in orthorhombic phase is extremely sensitive to the nearest Mn–Mn separation.^[27] Large Mn–Mn separation stabilizes ferromagnetic coupling while a small one leads to a spiral AFM state.^[18] As mentioned above, the V atoms occupy the Mn sites. The large atomic radius of V atom will intend to increase the Mn–Mn separation after the structural transformation, and thus leading to ferromagnetic coupling between two Mn atoms. Therefore, the appearance of ferromagnetic state can be attributed to the increased Mn–Mn separation induced by V doping. Opposite case appears in Mn_{1 − x}Ni_{1 + x}Ge alloys, in which Mn atoms are replaced by smaller Ni atoms so that AFM state is maintained.^[28] On the other hand, as a nonmagnetic element, V atom cannot play the same role as Fe or Co atom, which can activate Fe–Mn or Co–Mn ferromagnetic coupling. However, the introduced V atoms can enlarge Mn–Mn separation, which also leads to local ferromagnetic coupling.

According to the M–T curves, the phase diagram of V-doped MnNiGe is obtained. As shown in Fig. 3(d), MST between FM orthorhombic and PM hexagonal phase can be established in a temperature region of 130 K, which ranges from (210 K) to (340 K). The width of this temperature region in which MST can be obtained is comparable to those of some other MnMX alloys, such as MnNiGe–CoNiGe system,^[14] MnNiGe:Fe system,^[16] and (Mn, Fe)Ni(Ge, Si) alloy.^[29] In our system, the values of and are almost the same as that in orthorhombic MnNiGe alloy, indicating that V-doping cannot influence the strength of magnetic exchange. In Fig. 3(d), , and T_t in heating and cooling process are defined as temperatures, at which is maximum. The values of in heating and cooling process of the samples with x = 0.10–0.18 are further listed in Table 2.

Table 2.

Table 2.

Table 2. Values of Tt in heating and cooling process of the samples with x = 0.10–0.18. . Samples /K /K x = 0.10 388.9 a 420.5 a x = 0.12 280.4 293.6 x = 0.14 262.7 283.6 x = 0.16 241.3 264.1 x = 0.18 — — a measured by differential scanning calorimetry (DSC).

Table 2. Values of Tt in heating and cooling process of the samples with x = 0.10–0.18. .

Due to the large Δ M between FM orthorhombic and PM hexagonal phase, the MST can be triggered by the magnetic field when the temperature is close to T_t. Figure 4(a) shows the M–T curves for the sample with x = 0.12 under magnetic fields of 0.1 (blue) and 5 T (orange). The hysteretic MST obviously shifts to the right with the increase of magnetic field, indicating that the MST can be induced by applying magnetic field. Additionally, it can be found that the low-field magnetization of orthorhombic phase increases with increasing the temperature. This behavior is due to the fact that reducing temperature will enlarge the critical field needed to drive the metamagnetic transition from AFM to FM state.

Fig. 4.

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	Fig. 4. (color online) (a) M–T curves for sample with x = 0.12 under magnetic fields of 0.1 (blue) and 5 T (orange). Dark green and pink curves are for fitted curves. (b) Red curve denotes ΔV vs temperature with magnetic field variation of 0–5 T. Blue dotted curve refers to temperature dependence of with the magnetic field variation of 0–5 T.

The magnetic-field-induced MST is always accompanied by considerable , which is highly related to the orthorhombic phase volume change during applying magnetic field.^[17,30] Here, we use a method, instead of Maxwell relation which is thought to be inapplicable in the calculation of from a first-order transition, to estimate the based on calculating transformation volume fraction.^[31] First, the M–T curve for the sample with x = 0.12 during heating is normalized. Then, the obtained curves are differentiated and fitted by using the following Gaussian equation:

In this work, the ferromagnetism and MST between PM hexagonal and FM orthorhombic phase are achieved in nonmagnetic V-doped MnNiGe alloys. V atoms intend to occupy Mn sites and simultaneously extrude the same number of Mn atoms to the vacant Ni sites. It enlarges the Mn–Mn separation in orthorhombic phases, which further induces the ferromagnetic coupling between Mn moments. The V-doping also reduces the T_t into a temperature region between and so that PM–FM MST is established. The MST can be induced by the magnetic field, leading to considerable MCE.