Hydrostatic pressure-tuned magnetostructural transition and magnetocaloric effect in Mn-Co-GeIn compounds

Polycrystalline MnCoGe0.99In0.01 with magnetostructural transition temperature (Tmstr) around 330 K has been prepared by arc-melting technique, and the pressure-tuned magnetostructural transition as well as the magnetocaloric effect (MCE) has been investigated. The experimental results indicate that a pressure (P) smaller than 0.53 GPa can shift Tmstr to lower temperature at a considerable rate of 119 K/GPa with the coupled nature of magnetostructural transition unchanged. However, as P reaches 0.53 GPa, the martensitic structural transition temperature (TM) further shifts to 254 K while the magnetic transition temperature of austenitic phase (TCA) occurs at around 282 K, denoting the decoupling of magnetostructural transition. Further increasing P to 0.87 GPa leads the further shift of TM to a lower temperature while the TCA keeps nearly unchanged. Therefore, the entropy change (ΔS) of the MnCoGe0.99In0.01 under different magnetic fields can be tailored by adjusting the hydrostatic pressure.

Polycrystalline MnCoGe 0.99 In 0.01 with magnetostructural transition temperature (T mstr ) around 330 K has been prepared by arc-melting technique, and the pressuretuned magnetostructural transition as well as the magnetocaloric effect (MCE) has been investigated.The experimental results indicate that a pressure (P) smaller than 0.53 GPa can shift T mstr to lower temperature at a considerable rate of 119 K/GPa with the coupled nature of magnetostructural transition unchanged.However, as P reaches 0.53 GPa, the martensitic structural transition temperature (T M ) further shifts to 254 K while the magnetic transition temperature of austenitic phase (T C A ) occurs at around 282 K, denoting the decoupling of magnetostructural transition.Further increasing P to 0.87 GPa leads the further shift of T M to a lower temperature while the T C A keeps nearly unchanged.Therefore, the entropy change (∆S) of the MnCoGe 0.99 In 0.01 under different magnetic fields can be tailored by adjusting the hydrostatic pressure.© 2017 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/1.5006688

INTRODUCTION
MnCoGe-based materials with Ni 2 In-type hexagonal structure [1][2][3][4][5][6][7] show large magnetocaloric effect (MCE), 1 barocaloric effect (BCE) 2 and giant negative thermal expansion (NTE) 3 behavior.These multifunctional properties warrant people's interest for this kind of materials due to the potential application in solid state refrigeration technique, as well as high-precision pressure-sensitive sensors and devices.The martensitic phase transition (T M ) from the Ni 2 In-type hexagonal structure (space group P6 3 /mmc) to the TiNiSi-type orthorhombic structure (space group Pnma) is supposed to be responsible for these fascinating properties. 4toichiometric MnCoGe alloy shows ferromagnetic properties with Curie temperature at about 345 K. 5 In the paramagnetic region, a martensitic structural transition from high temperature Ni 2 Intype hexagonal to low temperature TiNiSi-type orthorhombic structure takes place at about 420 K with a -3.9% negative volume change. 5,8It has been also revealed that its intrinsic Curie temperature of hexagonal phase locates at about 283 K. 9,10 More attractively, the magnetic interactions and the structural transition can be adjusted by either chemical pressure, i.e. substitutions, 6 dopings, 7 and interstitial elements 7 or physical pressure. 1,11][14] Early studies by Niziol et al 11 showed that the separated magnetic and structural transitions can be made to be coincident during a range of pressure.Here, we report the influence of hydrostatic pressure on the T mstr and MCE for this alloy.A series of pressure (P) including ambient pressure (namely 0 GPa), 0.24 GPa, 0.53 GPa, and 0.87 GPa were selected to study the pressure effect.Under 0 GPa, the MnCoGe 0.99 In 0.01 alloy shows a considerable large entropy change ∆S ∼ -17.9 Jkg -1 K -1 for a field change of 5 T around its T mstr ∼330 K.We found that the application of a pressure smaller than 0.53 GPa can significantly shifts T mstr to lower temperature at a considerable rate of 119 K/GPa.As the pressure reaches 0.53 GPa, T M is further pushed to 254 K while T C A is still at a much higher temperature about 282 K, demonstrating the decoupling of magnetic and structural transition.

EXPERIMENTAL DETAILS
The alloy with nominal composition MnCoGe 0.99 In 0.01 was prepared by arc-melting technique under Ar atmosphere atmosphere (99.996%) with a base vacuum of 10 -4 Pa.To make the ingots homogeneous, each one was re-melted for three times.The commercial purities of Mn, Co, Ge, In are 99.9 wt%, 99.9 wt%, 99.999 wt%, and 99.99 wt%, respectively.1wt% extra Mn was added during sample synthesis to compensate Mn loss.The obtained ingots were wrapped with Mo foil and subsequently homogenized in a sealed quartz tube under vacuum of 10 -4 Pa at 875 • C for 6 days, then cooled down to room temperature in the furnace.Magnetic properties were measured in superconducting quantum interferometer device (SQUID, MPMS-7 T).Hydrostatic pressure was applied by using a Be-Cu pressure cell, where Daphne 7373 was used as the pressure transferring medium.The pressure inside the cell was calibrated by the shifts of the superconductive transition temperature of Pb.Magnetic entropy change ∆S was calculated based on the magnetization data using Maxwell relation.

RESULTS AND DISCUSSION
Temperature dependence of magnetization (M-T curve) under a magnetic field of 100 Oe for zerofield-cooling (ZFC) and field-cooling (FC) is shown in figure 1(a).The thermal hysteresis between heating and cooling cycles indicates the first-order nature of the transition.MnCoGe 0.99 In 0.01 shows the T mstr at ∼330 K, defined as the peak position of dM/dT on heating, under 0 GPa and a thermal hysteresis about ∼12 K. Upon application of hydrostatic pressure, T mstr shifts to lower temperatures at a rate of ∼119 K/GPa with the coupled nature of magnetostructural transition unchanged as the pressure is below 0.53 GPa.From figure 1(a), one can notice that the gap of thermal hysteresis (∼12 K) keeps nearly unchanged under a pressure of 0.24 GPa compared to 0 GPa while the T mstr locates at around 285 K.
When the pressure reaches 0.53 GPa, the structural and magnetic transitions become separated.The martensitic transition occurs at round T M ∼ 254 K while magnetic transition of austenitic phase at round T c A ∼ 282 K, as shown in figure 1(a).Further increasing the pressure to 0.87 GPa leads to the further lower T M at ∼226 K while the T c A keeps nearly unchanged.For the MM`X (where M, M`are 3d transition metals and X is Si or Ge) compounds, the austenitic phase has a smaller unit cell volume than the martensitic phase, indicating that the substitution with smaller atoms or vacancies may shift T M to a lower temperature by stabilizing hexagonal phases.Indeed, chemical pressure induced magnetostructural transition has been reported in MnCo 1-x Ge, 15 Mn 1-x CoGe, 16 and Mn 1-x Cr x CoGe. 6esides, valence electron concentration (e/a) also plays an important role in lowering T M and creating magnetostructural transition. 2 Recently, doping larger atoms with fewer valance electrons has been studied in Mn 1-x Al x CoGe 17 and MnCo 1-x Zr x Ge. 18 It was found that the replacement of Mn by Al makes the martensitic transformation temperature decrease, as a result, a first-order magnetostructural transition occurs at 0 ≤ x ≤ 0.01 (For x = 0, the Mn loss during the melting is supposed to be responsible for the coupling of the structural and magnetic transitions.).For 0.01 < x ≤ 0.02, T M is below T C and the magnetic and structural transitions become separated.For MnCo 1-x Zr x Ge, the coincidence of structural and magnetic transitions has been observed at x = 0.02.Our previous studies have demonstrated that the substitution of Ge for In can also shift the T M to lower temperatures in MnCoGe 1-x In x , resulting in the magnetostructural coupling at 0 < x < 0.03. 2 At x = 0.01, the T mstr appears at 330 K for MnCoGe 0.99 In 0.01 .Hydrostatic pressure has a similar effect with chemical pressure in reducing volume and stabilizing the hexagonal phase, thus bringing the T M down to low temperature. 12Neutron diffraction investigation revealed that the Mn-Mn interlayer distance can be rapidly shortened and the covalent banding between Mn-Mn atoms is strengthened upon an application of hydrostatic pressure, hence leading to the stabilization of the austenitic phase and the shift of T mstr to low temperature. 2igure 1(b) displays the temperature dependent magnetization (M-T curve) measured on heating and cooling at a high magnetic field of 5 T under different pressures.One can notice the impact of magnetic field on the magnetization and magnetostructural transition under pressure.The phase transition becomes less sharp, while the gap of thermal hysteresis remains nearly the same, ∼12 K, for the all cases of 0 GPa, 0.24 GPa, and 0.53 GPa.The change of magnetization (∆M) across the transition also shows a difference, which is 53.4,44.7, 36.1 emu/g for 0 GPa, 0.24 GPa and 0.53 GPa respectively, predicting possible decrease of entropy change with pressure.For P = 0.53 GPa, two distinct transitions T M and T C A can be seen from the M-T curves at 0.01 T (figure.1(a)), which still can be identified at 5T when the logarithmic coordinate is applied for the vertical axis (not shown).It means that the application of 5 T magnetic field does not lead to the magnetostructural coupling.For P=0.87 GPa, the transition is significantly broadened due to the decoupling of magnetic and structural transition.An interesting feature is that the saturated magnetization at 5 K keeps the nearly same for the cases of 0 GPa, 0.24 GPa and 0.53 GPa.However, the saturated magnetization at 5 K (M s (5 K)) under 0.87 GPa largely decreases (see the inset of figure 1(b)).The reason can be ascribed to the change of ferromagnetic coupling between Mn atoms under pressure noting the magnetic moment of the alloy is totally dominated by Mn atoms. 9Early studies about the effect of hydrostatic pressure on the saturation magnetization of Mn-riched Ni 50-x Mn 25+x+y Ga 25-y showed that physical pressure can lead to a decrease of saturation magnetization. 19Detailed studies concluded that the magnetic coupling between Mn atoms is not affected by atomic distance simply, the atomic arrangement and ordering also play an important role.Although no enough structural information can be available for the effect of a high pressure on the atomic occupation in the martensitic orthorhombic phase for present MnCoGe 0.99 In 0.01 , a critical change of local environments particularly for the Mn-Mn pairs and their surroundings induced by a high pressure should play a dominated role for the observed large decrease of saturation magnetization under 0.87 GPa.
Figure 2 shows the field dependence of magnetization (M-H curves) measured up to 5 T at different temperatures under different pressures.Large magnetic hysteresis at temperatures around the T mstr can be observed at 0 GPa, which indicates a field-induced metamagnetic transition from the paramagnetic austenstic phase to the ferromagnetic martensitic phase.However, as the pressure increases, the hysteresis gradually disappeared.For P=0.87 GPa, no clear magnetic hysteresis can be identified.The intrinsic hysteresis is expected to be connected with the electronic band structure and the nucleation during the transition. 20The process of metamagnetic transition in LaFe 11.7 Si 1.7 has been interpreted in terms of activation model. 20For radii smaller than the critical size of the nucleation, the large surface energy forces new phase bubble to disappear.The application of pressure may influence the atomic distance and hence alters the covalent banding and electronic band structure.As a result, the nucleation during the phase transition may become harder, leading to the gradual disappearance of magnetic hysteresis with pressure.Besides, the frictions from domain rearrangements during the transition also contribute to the hysteresis loss.An application of pressure may significantly influence the microstructure, and the distribution of residual stress and domains have been changed under pressure, which should also play a role to the change of magnetic hysteresis with pressure.
Magnetic hysteresis, which is a sign of field-induced metamagnetic transition, is further studied for P=0 GPa.We measured the M-T curves using SQUID-VSM in the absence of pressure under different magnetic fields of 0.5 T, 1 T, 2 T, and 5 T, as shown in figure 3. Similar magnetic hysteresis appears at temperatures around the T mstr .From figure 3(a), one may notice that the T mstr shifts to higher temperatures with increasing magnetic field.The normalized M-T curves are shown in figure 3(b).T mstr is defined as the temperature corresponds to the peak position of dM/dT on heating.One can notice that the T mstr shifts to higher temperature at a rate of 1.34 K/T, similar to the driving rate by  magnetic field in the reported MnCrCoGe. 1 However, when comparing figure 1(a) and (b), we found that the 5 T magnetic field shifts the T mstr from 329 K to 333 K at a rate about 0.8 K/T at 0 pressure.This deviation of the shift rate might be relative to the possible difference from sample to sample though all of them were from the same ingot.One knows that the magnetostructural transition is critically dependent on the composition, and the microstructure also plays a key role on the process of magnetostructural transition.The as-prepared MnCoGeIn sample is quite brittle and even naturally cracked into powders.It is quite possible that the microstructure has some difference from powder to powder.Finally, we turn our attention to the magnetocaloric effect of MnCoGe 0.99 In 0.01 .Magnetic entropy change ∆S was calculated using Maxwell relation. 21As shown in figure 4, the maximum of |∆S| is 17.9, 15.9, and 10.4 Jkg -1 K -1 for a magnetic field change (∆H) of 0-5 T under pressure of 0 GPa, 0.24 GPa and 0.53 GPa respectively.Although the |∆S| value decreases with increasing pressure, the width of ∆S peak is broadened covering room temperature.For P=0.53, 0.87 GPa, a clear split appears in the ∆S peak due to the decoupling of structural and magnetic transitions, and the ∆S sharply decreases for the case of P=0.87 GPa due to the notable reduction of saturated magnetization (figure 1(b) and its inset).These results suggest that an application of hydrostatic pressure smaller than 0.87 GPa is an effective way in tuning the magnetocaloric effect for present MnCoGe 0.99 In 0.01 alloy.Additionally, refrigeration capacity (RC) is another important parameter that can estimate the usefulness of a material as a magnetic refrigerant.It is defined as T 1 ∆S (T ) dT , where T 1 and T 2 correspond the temperatures at half width of ∆S peak.The calculated RC is about 203, 200 and 157 Jkg -1 under ∆H= 0-5 T for the samples under 0 GPa, 0.24 GPa and 0.53 GPa respectively.However, hysteresis loss injures the performance of RC.People usually use the effective refrigeration capacity (RC eff ) with the deduction of maximal hysteresis loss to effectively evaluate the materials.The evaluated RC eff is about 185, 178 and 169 J/kg at 5 T for 0 GPa, 0.24 GPa and 0.53 GPa, respectively.Moreover, one can also notice that the peak position of ∆S can be continuously tunable from 330 K down to 250 K covering room temperature with pressure, which is particularly meaningful for the hybrid field driven refrigeration applications around room temperature.

CONCLUSION
In summary, the pressure effect on magnetostructural transition and magnetocaloric effect has been studied for the polycrystalline MnCoGe 0.99 In 0.01 alloys.Our results indicate that the magnetostructural transition shifts to lower temperatures with increasing pressure.Although the ∆S decreases as the pressure is smaller than 0.87 GPa, the effective refrigeration capacity RC eff stays almost unchanged due to the reduction of hysteresis loss with pressure.As the pressure is increased to 0.53, 0.87 GPa, the ∆S peak splits due to the decoupling of the structural and magnetic transition and the ∆S value sharply decreases because of the notable reduction of saturated magnetization.The tunable ∆S and RC eff by pressure is of particular significance for developing the hybrid field driven refrigeration applications.

FIG. 1 .
FIG. 1. Temperature dependence of magnetization (M-T curve) for MnCoGe 0.99 In 0.01 measured using ZFC-FC modes under a magnetic field of 100 Oe (a), and the M-T curve at a 5T (b) magnetic field on warming and cooling.Inset: saturation magnetization at 5 K M s (5 K) vs Pressure.

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FIG. 3. (a) Temperature dependence of magnetization (M-T curve) on heating for 0 GPa under magnetic fields of 0.5 T, 1 T, 2 T and 5 T, (b) Normalized M-T curves on heating, (c) dM/dT curves at round the T mstr , (d) The dependence T mstr on magnetic field.

FIG. 4 . 6 Liang
FIG. 4. Temperature dependence of entropy change of MnCoGe 0.99 In 0.01 for the magnetic field changes of 0-2 T and 0-5 T under different pressures.