Study On Thermaldynamical Effect Of Phase Transformation And Magnetic Properties In A New Type Of Ni-Mn Based Quaternary Heusler Alloys

The Mn rich Ni-Mn based Heusler alloys which is as a new type of functional materials, have been attracted much attention in condensed matter physics and material science due to their potential application. The novel physical mechanisms and properties exhibited by these alloys are mainly included as follows. First, such material belongs to ferromagnetic intermetallic compound. They undergo martensitic transformation from a high-magnetization austenite phase with conventional Heusler type L21 structure to a low-magnetization martensitic phase with L10 (or orthogonal) modulated structure, causing a large difference in magnetization (ΔM) between two phases. Second, as same as conventional Ni-Mn-Ga ferromagnetic shape memory alloy (FSMA), the Mn rich Ni-Mn based Heusler alloys show a thermalelastic martensitic transformation, leading a temperature-controlled two way shape memory effect. Third, different from Ni-Mn-Ga alloys, the martensitic transformation in these alloys can be induced by external magnetic field. Associated with field-induced martensitic transformation (FIMT), these alloys reveal metamagnetic shape memory effect (MSME), large output stress and large inverse magnetocaloric effect (MCE). In this paper, we prepared a series of Mn rich Ni-Mn based quaternary Heusler alloys through introducing appropriate transition elements including Co or Cu. Based on study of thermodynamical effect of martensitic transition, a detailed discussion of abundant physical properties during martensitic transformation are carried out in these quaternary Heusler alloys.Both the structure and martensitic transition in Ni_50-xCo_xMn₃₈Sn₁₂ (x=2, 4, 6) polycrystalline have been investigated by means of magnetic measurements. It is found that the martensitic transformation shifts to lower temperature with the increase of Co concentration. At the same time, it is also found that appropriation of Co can enhance ferromagnetic exchange for their parent phases, while the magnetic exchange of martensitic phases is nearly unchanged, thus giving rise to evident increase ofΔM between both phases. On the basis of above analysis and combing with thermodynamical mechanisms, we deduced a general equation of calculating the critical magnetic field, which can drive a complete martensitic transformation. Based on this expression, the thermodynamical effect upon FIMT for Ni_50-xCo_xMn₃₈Sn₁₂(x=4, 6) have been compared, it is indicates that the behavior of FIMT not only depends onΔM between with their both phases, but also relies on the span of temperature as well as thermal hysteresis during their martensitic transformation. We have substituted Cu for Ni into Ni-Mn-Sn in the form of Ni₄₆Cu₄Mn₃₈Sn₁₂alloy. Both the spontaneous transition strains and MSME at free condition in this alloy have been firstly studied. A large spontaneous strain with the value of about 0.12% upon martensitic transformation has been observed in this alloy, which is significantly larger than that in ternary Ni-Mn-Sn alloy. In addition, such a value of strain can be obtained through a fully reverse martensitic transformation induced by a magnetic field at 284 K, exhibiting a considerable metamagnetic shape memory effect without any prestrain. However, the demagnetizing energy can not overcome the elastic energy on account of the lowΔMbetween both phases of this sample, thus giving rise to one way MSME.The studies of strain, output stress and inverse MCE upon martensitic transformation have been firstly reported in Ni₄₅Co₅Mn₃₇In₁₃ polycrystalline. A spontaneous phase transition strain with the value of about 0.4% in this alloy can be acquired by applying and removing magnetic field. Owing to the"magnetic training"effect in this sample is so weak that a large and reproducible two-way MSME with nonprestrain can be observed in this sample. Associated with MSME, the output stress with hydrostatic form in this sample is about 4.2 kbar. Furthermore, the sample also shows a large inverse MCE upon FIMT. According to our calculate, the saturated isothermal entropy change is about 27 J/kg K and remains unchanged in a broad temperature interval, causing that the refrigeration capacity in this sample achieves about 340 J/kg.We present a case study of calculative method of inverse MCE corresponding to first-order structure transition for Heusler-type ferromagnetic martensitic transformation materials. Our results indicated that the"giant inverse MCE"upon martensitic transition evaluated by Maxwell relation in this alloy are unphysical results. This is due to coexistence of both martensitic and austenitic phases as well as thermal hysteresis during martensitic transition. However, a careful study demonstrated that the spurious results during martensitic transition can be well removed by using Clausius-Clapeyron equation based on magnetization measurements. Thus, the inverse MCE related to martensitic transformation in Heusler alloys system is evaluated by using Clausius-Clapeyron equation based on magnetization measurements is a scientific method.