Magnetic phase transitions and magnetocaloric effect in the Fe-doped MnNiGe alloys

The magnetic phase transition and magnetocaloric effects in Fe-doped MnNiGe alloys are investigated. The substitution of Fe for Ni decreases the structural transition temperature remarkably, resulting in the magnetostructural transition occurring between antiferromagnetic and ferromagnetic states in MnNil_χFexGe alloy. Owing to the enhanced ferromagnetic coupling induced by the substitution of Fe, metamagnetic behaviour is also observed in TiNiSi-type phase of MnNil-xFezGe alloys at temperature below the structural transition temperature.

Magnetostructural transformation and magnetocaloric effect in Mn_(48-x)V_(x)Ni_(42)Sn_(10) ferromagnetic shape memory alloys

In this work,we tuned the magnetostructural transformation and the coupled magnetocaloric properties of Mn_(48-x)V_(x)Ni_(42)Sn_(10)(x=0,1,2,and 3)ferromagnetic shape memory alloys prepared by means of partial replacement of Mn by V.It is observed that the martensitic transformation temperatures decrease with the increase of V content.The shift of the transition temperatures to lower temperatures driven by the applied field,the metamagnetic behavior,and the thermal hysteresis indicates the first-order nature for the magnetostructural transformation.The entropy changes with a magnetic field variation of 0-5 T are 15.2,18.8,and 24.3 J.kg^(-1).K^(-1)for the x=0,1,and 2 samples,respectively.The tunable martensitic transformation temperature,enhanced field driving capacity,and large entropy change suggest that Mn_(48-x)V_(x)Ni_(42)Sn_(10)alloys have a potential for applications in magnetic cooling refrigeration.

Magnetostructural transition in NiCuCoMnGa alloys

The effects of the addition of Co on the martensitic transformation and Curie transition temperatures of polycrystalline Ni46-xCu4CoxMn33.sGa16.5 （x = 0, 1, 3, 5） alloys are investigated. An abrupt decrease in the martensitic transformation temperature and an obvious increase in the Curie transition temperature of austenite （TA） are observed when Co is doped in the NiCuMnGa alloy. As a result, the composition range for obtaining the magnetostructural transition is extended. Furthermore, the effect of a strong magnetic field on the magnetostructural transition is analyzed. This study offers a possible method to extend the composition range for obtaining magnetostructural transition in Heusler alloys.

Ferromagnetism and magnetostructural coupling in V-doped MnNiGe alloys

The magnetostructural coupling between magnetic and structure transitions plays an important role in the multifunctional applications of magentocaloric materials. In this work, ferromagnetism and magnetostructural transformation are achieved in nonmagnetic V-doped MnNiGe alloys. With simultaneously reducing the transformation temperature and converting antiferromagnetic martensite to ferromagnetic state, the magnetostructural transformation between ferromagnetic orthorhombic phase and paramagnetic hexagonal phase is established in a temperature region as large as 130 K. The magnetic-field-induced magnetostructural transformation is accompanied by considerable magnetocaloric effect.

Magnetic phase transition and magnetocaloric effect in Mn_(1-x)Zn_xCoGe alloys

The magnetic phase transition and magnetocaloric effect are studied in a series of Mn1-xZnxCoGe （x = 0.0l, 0.02, 0.04, and 0.08） alloys. By introducing a small quantity of Zn element, the structural transformation temperature of the MnCoGe alloy is greatly reduced and a first-order magnetostructural transition is observed. Further increasing the Zn concentration results in a second-order ferromagnetic transition. Large room-temperature magnetocaloric effects with small magnetic hysteresis are obtained in alloys with x = 0.01 and 0.02, which suggests their potential application in magnetic refrigeration.

Influence of Ni/Mn ratio on magnetostructural transformation and magnetocaloric effect in Ni48-xCo2Mn38+xSn12(x=0,1.0,1.5,2.0,and 2.5)ferromagnetic shape memory alloys

An investigation on the magnetostructural transformation and magnetocaloric properties of Ni48-xCo2Mn38＋xSn12（x = 0, 1.0, 1.5, 2.0, and 2.5） ferromagnetic shape memory alloys is carried out. With the partial replacement of Ni by Mn in the Ni_（48）Co2Mn38Sn12 alloy, the electron concentration decreases. As a result, the martensitic transformation temperature is decreased into the temperature window between the Curie-temperatures of austenite and martensite. Thus, the samples with x = 1.5 and 2.0 exhibit the magnetostructural transformation between the weak-magnetization martensite and ferromagnetic austenite at room temperature. The structural transformation can be induced not only by the temperature,but also by the magnetic field. Accompanied by the magnetic-field-induced magnetostructural transformation, a considerable magnetocaloric effect is observed. With the increase of x, the maximum entropy change decreases, but the effective magnetic cooling capacity increases.

Revealing essence of magnetostructural coupling of Ni-Co-Mn-Ti alloys by first-principles calculations and experimental verification

In this work,the effects of Co doping on the magnetostructural coupling transformation of Ni_(50-x)Co_(x)Mn_(50-y)Ti_(y)(x=0-15,y=12.5-15)Heusler alloys were systematically investigated through the first-princi-ples calculations and experimental verification.The cal-culation result indicates that the doped Co atoms prefer to occupy the Ni sublattice.The Co atoms tend to flock together in terms of the lowest energy principle.Since the formation energy of the austenite is higher than that of the martensite,the alloys will undergo martensitic transfor-mation for the Ni_(50-x)Co_(x)Mn_(37.5)Ti_(12.5)alloys(x=0-12.5).The magnetostructural coupling point of Ni_(50-x)Co_(x)Mn_(37.5)Ti_(12.5)alloys is predicted in the vicinity of x=11-12.Based on the computational composition Ni_(37.5)Co_(12.5)Mn_(37.5)Ti_(12.5),the Ni_(36)Co_(14)Mn_(36)Ti_(14)alloy with magnetostructural coupling near room temperature was experimentally developed by simultaneously increasing the Ti and Co contents.The largest magnetization change(ΔM)and magnetic entropy changes(ΔS_(m))obtained under magnetic field of 5 T for the martensitic transformation in the Ni_(36)Co_(14)Mn_(36)Ti_(14) alloy are about 87.6 A·m^(2)·kg^(-1)and 21 J·kg^(-1)·K^(-1),respectively.The fracture strength and strain for non-textured polycrystalline Ni_(36)Co_(14)Mn_(36)Ti_(14)alloy reach 953 MPa and 12.3%,respectively.The results show that the alloy not only possesses a large magne-tocaloric effect but also has excellent mechanical proper-ties.In addition,the 6 M modulated martensite is evidenced in the Ni-Co-Mn-Ti alloys via transmission electron microscopy technique.

Large enhancement of magnetocaloric effect induced by dual regulation effects of hydrostatic pressure in Mn_(0.94)Fe_(0.06)NiGe compound

MM'X(M,M'=transition metals,X=carbon or boron group elements)compounds could exhibit large magnetocaloric effect due to the magnetostructural transition,and the composition regulation has been widely studied to realize the magnetostructural transition.Moreover,the magnetostructural transition is also sensitive to the pressure.Herein,the effect of hydrostatic pressure on magnetostructural transformation and magnetocaloric effect has been investigated in Mn_(0.94)Fe_(0.06)NiGe compound.Dual regulation effect of lowering structural transition temperature and strengthening ferromagnetic(FM)state of martensite is realized by applying hydrostatic pressure,which would greatly improve the magnetocaloric effect of Mn_(0.94)Fe_(0.06)NiGe compound.Moreover,the first-principles calculations have also been performed to discuss the origin of the regulation effect under hydrostatic pressure,and it indicates that the hydrostatic pressure can stabilize the hexagonal structure and decrease the structural transition temperature.The maximum isothermal entropy change increases by 109%from 4.3 J/(kg K)under 0 GPa to 9.0 J/(kg K)under 0.402 GPa for a magnetic field change of 0-3 T.This work proves that the hydrostatic pressure is an effective method to regulate the magnetostructural transition and enhance magnetocaloric effect in MM'X compounds.

Phase transition and mechanical properties of Ni_(30)Cu_(20)Mn_(37+x)Ga_(13-x)(x=0–4.5) alloys

Ni30Cu20Mn37＋xGa13-x（x = 0–4.5） alloys were studied with the phase transformation and mechanical properties. With the increase of Mn content, the martensitic transformation temperatures increase and the Curie temperature decreases. Simultaneously, the room temperature microstructure evolves from single phase of austenite to dual phases containing martensite and precipitation. Both the ductility and the strength of the polycrystalline alloys are significantly improved by the precipitation. Coupled magnetostructural transition from weak magnetic martensite to ferromagnetic austenite is obtained in both single-phase and ductile dual-phase alloys.