Kun Xu

Two successive magneto-structural transformations and their relation to enhanced magnetocaloric effect for Ni55.8Mn18.1Ga26.1 Heusler alloy.

In the present work, two successive magneto-structural transformations (MSTs) consisting of martensitic and intermartensitic transitions have been observed in polycrystalline Ni55.8Mn18.1Ga26.1 Heusler alloy. Benefiting from the additional latent heat contributed from intermediate phase, this alloy exhibits a large transition entropy change ΔStr with the value of ~27u2009J/kg K. Moreover, the magnetocaloric effect (MCE) has been also evaluated in terms of Maxwell relation. For a magnetic field change of 30u2009kOe, it was found that the calculated value of refrigeration capacity in Ni55.8Mn18.1Ga26.1 attains to ~72u2009J/kg around room temperature, which significantly surpasses those obtained for many Ni-Mn based Heusler alloys in the same condition. Such an enhanced MCE can be ascribed to the fact that the isothermal entropy change ΔST is spread over a relatively wide temperature interval owing to existence of two successive MSTs for studied sample.

Magnetocaloric effect and negative thermal expansion in hexagonal Fe doped MnNiGe compounds with a magnetoelastic AFM-FM-like transition

We report a detailed study of two successive first-order transitions, including a martensitic transition (MT) and an antiferromagnetic (AFM)-ferromagnetic (FM)-like transition, in Mn1-xFexNiGe (xu2009=u20090, 0.06, 0.11) alloys by X-ray diffraction, differential scanning calorimetry, magnetization and linear thermal expansion measurements. Such an AFM-FM-like transition occurring in the martensitic state has seldom been observed in the M(T) curves. The results of Arrott plot and linear relationship of the critical temperature with M2 provide explicit evidence of its first-order magnetoelastic nature. On the other hand, their performances as magnetocaloric and negative thermal expansion materials were characterized. The isothermal entropy change for a field change of 30u2009kOe reaches an impressive value of −25.8u2009J/kg K at 203u2009K for xu2009=u20090.11 compared to the other two samples. It demonstrates that the magneto-responsive ability has been significantly promoted since an appropriate amount of Fe doping can break the local Ni-6Mn AFM configuration. Moreover, the Fe-doped samples reveal both the giant negative thermal expansion and near-zero thermal expansion for different temperature ranges. For instance, the average thermal expansion coefficient ā of xu2009=u20090.06 reaches −60.7u2009×u200910−6/K over Tu2009=u2009231–338u2009K and 0.6u2009×u200910−6/K over Tu2009=u2009175–231u2009K during cooling.

Structural and magnetocaloric properties in hexagonal MnNiGa alloys with Co doping

Hexagonal MnNiGe-based alloys are a series of novel functional materials with potential magnetostructural transitions (MSTs). Accordingly, it was investigated the magnetic features of bulk hexagonal MnNiGa alloy and attempted to partially substitute Mn by Co atoms to tailor its structural and magnetic properties. Nonetheless, the introduction of magnetic Co atom fails to bring about the first-order phase transition and gives rise to the emergence of second phase with cubic structure instead. For ternary MnNiGa parent alloy, the second-order nature of transition is confirmed by both the absence of thermal hysteresis and the standard Arrott plot. To the end, the values of isothermal entropy change are determined by Maxwell relation, and the maximal values follow the trend predicted by the mean-field theory. Its broad transition region (~53xa0K) leads to only a very small value of entropy change (~2.4xa0J·kg−1·K−1 at a field change of 3xa0T). In turn, the wide transition ensures a relative large refrigerant capacity (~89.4xa0J·kg−1), which is comparable to that of MnNiGe-based systems. Although the substitution of Co for Mn site is unsuccessful, the chemically modified MnNiGa is still a promising candidate for the application of magnetocaloric effect (MCE) with merits of higher magnetization and better mechanical performance than MnNiGe-based systems.

Large Room Temperature Magnetocaloric Effect in Ni50Mn34(Co,Cu)In15

Effect of Co or Cu slightly introduced in Ni50Mn35In15 on martensitic transformation and magnetocaloric effect was investigated. The small doping of Co can modify exchange interaction between Mn atoms, resulting in the ferromagnetic ordering of the parent phase and a large magnetization difference across the martensitic transformation. For Cu-doped sample, a large was obtained, and gives rise to a large magnetic entropy change of 58.4 J/kg K for 5 T near room temperature accompanying with smaller hysteresis losses. The study on the doping system may have significant impact on realization of room-temperature magnetic refrigeration.

Electrical transport properties and giant baroresistance effect at martensitic transformation of Ni43.7Fe5.3Mn35.4In15.6 Heusler alloy

The electrical transport properties at martensitic transformation (MT) in polycrystalline Ni43.7Fe5.3Mn35.4In15.6 have been intensively investigated under different hydrostatic pressures. For this alloy, the experimental results show that applying a higher hydrostatic pressure can convert its MT from the metamagnetic type into the paramagnetic type. It provides a unique opportunity to separate the relative contributions of electron-spin and electron-lattice scatterings across the metamagnetic MT based on the dynamical Clausius-Clapeyron equation, which delivers a deeper insight into the resistivity change of metamagnetic MT for the Mn-rich Ni-Mn based Heusler alloys. In addition, the studied alloy also reveals a giant positive baroresistance (BR) effect with a saturated value of 115% at 242u2009K. This performance originates from the combined effect of electron-spin and electron-lattice scatterings associated with a prominent hydrostatic pressure-induced MT, which contribute 46% and 69% to the overall BR rati...

Realization of metamagnetic martensitic transformation with multifunctional properties in Co50V34Ga16 Heusler alloy

In this work, we have developed a ferromagnetic shape memory alloy Co50V34Ga16 with a metamagnetic martensitic transformation (MT) from the high-magnetization austenitic phase to the low-magnetization martensitic phase. As a consequence of a strong coupling between structure and magnetic degrees of freedom, the metamagnetic MT of this alloy is relatively sensitive to the external magnetic field, thus giving rise to a field-induced reverse MT. Associated with such a unique behavior, both considerable inverse magnetocaloric effect (9.6u2009J/kgu2009K) and magnetostrain (0.07%) have also been obtained under the magnetic field change of 3u2009T. Our experimental results indicate that this kind of Co-V based alloy probably becomes an alternatively promising candidate for applications in magnetic sensors and magnetic refrigeration.In this work, we have developed a ferromagnetic shape memory alloy Co50V34Ga16 with a metamagnetic martensitic transformation (MT) from the high-magnetization austenitic phase to the low-magnetization martensitic phase. As a consequence of a strong coupling between structure and magnetic degrees of freedom, the metamagnetic MT of this alloy is relatively sensitive to the external magnetic field, thus giving rise to a field-induced reverse MT. Associated with such a unique behavior, both considerable inverse magnetocaloric effect (9.6u2009J/kgu2009K) and magnetostrain (0.07%) have also been obtained under the magnetic field change of 3u2009T. Our experimental results indicate that this kind of Co-V based alloy probably becomes an alternatively promising candidate for applications in magnetic sensors and magnetic refrigeration.

Tuning Exchange Bias Effect in Ni50Mn36Sn14 Heusler Alloy

The tuning exchange bias HE at martensitic state of Ni50Mn36Sn14 alloy has been investigated by means of hysteresis loop measurement. It was found that the whole loop can be tuned by HFC from a double-shifted to a single-shifted hysteresis loop, leading to an appearance of maximum HEat HFC = 0.5 kOe. This behavior could be ascribed to the competition between two types of AFM clusters and HFC,which exhibits predominantly atlow HFC range, while the competition between FM clusters and HFC, which becomes predominant at high HFC range.