Martensitic Transition And Its Precursor Effect In Ni-Mn-In-Al Alloys

As a new type of ferromagnetic shape memory alloys(FSMAs),off-stoichiometric Ni50Mn25+xZ25-x(Z = In,Sn,Sb)Heusler alloys have received much attention recently.Compared with conventional Ni-Mn-Ga alloy,such alloys exhibit large magnetization difference between the ferromagnetic high-temperature austenitic phase and weak magnetic low-temperature martensitic phase.The strong magnetoelastic coupling will result in a magnetic-field-induced inverse martensitic transition(MT)from martensite to austenite.Associated with the field-induced inverse MT,a variety of interesting properties,such as magnetic shape memory effect,magnetocaloric effect,and magnetoresistance have been observed in the vicinity of the MT.Change the Mn-Mn distance through external pressure has been proved as an effective method of tuning the MT temperature.Actually,chemical pressure can also be generated by relatively smaller atom substitution conveniently.In this paper,Ni50Mn34In16-xAlk(x=0.5,1)were prepared through isoelectronic substitution of In with smaller radius Al.Measurement results indicate that A1 substitution shift the MT temperatures to high temperature,even near room temperature.Additionally,premartensitic transition can be observed in Ni50Mn34In15.5Al0.5 alloy due to the strong magnetoelastic effect.The main researches are shown following:1).Premartensitic transition in Ni50Mn34In15.5Al0.5 alloypmartensitic state,referring to the intermediate state existing between the symmetric high temperature austenitic phase and a low-symmetry martensitic structure at low temperature,can be observed in the Ni-Mn-Ga ferromagnetic shape memory alloys.The premartensitic phase shows approximately cubic symmetry with a micromodulated domain structure of parent phase.The premartensitic transition is a consequence of magnetoelastic coupling between the magnetic and structural degrees of freedom.However,large magnetic effects associated with the premartensitic transition in Ni-Mn-Ga alloys have seldom been reported up to now due to the quite narrow temperature range and small magnetization change across the premartensitic transition.An intermediate phase transition prior to the martensitic transition has been observed in high-pressure annealing Ni43Mn41Co5Sn11 alloy recently.In Ni50Mn34In16Al0.5 alloy,Al doping would induce internal pressure,which would enhance the magnetoelastic coupling.Resistance,thermal and magnetic measurements indicate the existence of two successive magneto-structural transitions,including MT and premartensitic transition.Thermal/magnetic hysteresis clearly indicate the first-order nature of the two structural transitions.The microstructure evolution observation using in situ optical microscope further confirm the existence of premartensite prior to the typical martensitic image.Due to the large magnetization change across premartensitic transition,applying magnetic field would decrease the phase transition temperatures,indicate the magnetoelastic coupling.The inverse premartensitic transition induced by magnetic field results in large magnetoresistance,and contributes to the enhanced inverse magnetocaloric effect though enlarging the value and temperature interval of magnetic entropy change Sm.2).Near room magentoresistance and magnetocaloric effect in Ni50Mn34Ini5Al alloyFew Ni-Mn based alloys exhibit both large magnetoresistance and magnetocaloric effects near room temperature.Ni50Mn34In15Al alloy exhibits first-order MT and second order magnetic transition near room temperature.Additionally,large magnetization change and small thermal hysteresis can be observed across MT.The results indicate the alloy shows large magnetoresistance(peak value-58%upon heating)and large successive magnetic entropy change,corresponding to first-order and second-order transition(with the peak value of 21 J/Kg K and-8.2 J/Kg K).Due to the small magnetic hysteresis loss,the refrigerant capacity corresponding to first-order and second-order transitions are considerable,reaching to 150J/Kg.