The Magnetic-field-driven Martensitic Transformation And Magnetocaloric Effect In MnCoGe-based Alloys

Magnetic refrigeration technology is a new solid state refrigeration technology with high energy efficiency and environment friendliness based on magnetocaloric effect.It mainly depends on the endothermic and exothermic ability of magnetic refrigerants in the process of magnetization and demagnetization.Therefore,it is particularly essential to find the magnetic refrigerants with excellent magnetocaloric effect.In recent years,focused have been on magnetic phase transition alloys.This kind of alloy has been widely investigated owing to the abundant physical effects such as magnetocaloric effect,magnetic-drive shape memory,large magnetoresistance.Mn-based magnetic phase transition alloys?e.g,MnCoGe-based alloys?have been especially paid more attentions because of their low cost,simple preparation process and relatively high magnetic moment values of Mn ions.Up to now,the magnetic and structural phase transitions can be tuned into coupling by element substitution,interstitial atoms doping,elemental vacancy/excess,and exerting the external pressure.However,the magnetic field has not yet been considered to be an effective means to induce the martensitic transformation in MnCoGe based alloys.In this paper,magnetic-field-driven martensitic transformation is studied by optimizing the alloy composition,which would improve the sensitivity of martensitic transformation to applied field,and the magnetocaloric effect and its mechanism are also studied.Firstly,investigated are the magnetic-field-induced martensitic transformation and magnetocaloric effect in Mn_1-xCo_1+xGe alloys.The bulk samples of Mn_1-xCo_1+xGe alloys were successfully fabricated by high vacuum arc-melting technology.By tuning the ratio of Co/Mn,increasing the valence electron concentration?e/a?,the instability of Mn-moment is promoted,for which the higher degree of magnetic field sensitivity of martensitic transformation is driven to realize the magnetic-field-induced martensitic transformation.More importantly,the largest magnetic entropy change(51.7 Jkg^-1K^-1)and refrigerating capacity(368.7 Jkg^-1)are obtained under the field range of 0-7 T.Secondly,the magnetic-field-induced martensitic transformation and magnetocaloric effect are studied in Mn_1-xCo_1+xGe ribbons.Mn_1-xCo_1+xGe alloy ribbons were fabricated by arc-melting and melt-spinning techniques.The substitution of Co for Mn reduces the structural transformation temperature and realizes a magnetic-field-induced martensitic transformation in these alloy ribbons.The efficient refrigerating capacities of 252.7 Jkg^-1 is obtained under the field change of 0-7 T.Meanwhile,the hysteresis losses are very small compared with the counterpart bulks.Finally,the magnetic-field-induced martensitic transformation and magnetocaloric effect are explored in Mn_1-xNi_xCoGe alloys.The Mn_1-xNi_xCoGe alloys were prepared by arc-melting technique.The transformation temperature of alloy decreases significantly with the increase of Ni-content.The abrupt change of magnetization becomes greater when the transformation approaches to room temperature.In addition,this system alloys have large refrigerating capacites from 205Jkg^-1 to 288 Jkg^-1 and very small hysteresis loss under the field change of 0-7 T.In conclusion,the magnetic field driven martensitic transformation and magnetocaloric effect of MnCoGe based alloys have been studied systematically in this work.This study may provide theoretical basis with important academic value for other potential magnetofunctional properties in MnCoGe based alloys based on the magnetic-field-transition,including magnetostriction and magnetoresistance.Furthermore,these results may give the experimental evidences for and shed light on the further development of new magnetic refrigerant materials.