Zhenzhuang Li

Microstructure and magnetocaloric effect of melt-spun Ni52Mn26Ga22 ribbon

Microstructural features and magnetocaloric properties of Ni52Mn26Ga22 melt-spun ribbons were studied. Results show that there are four types of differently oriented variants of seven-layered modulated (7M) martensite at room temperature, being twin-related one another and clustered in colonies. Due to the coupled magnetic and structural transformations between parent austenite and 7M martensite, the melt-spun ribbons exhibit a significant magnetocaloric effect. At an applied magnetic field of 5 T, an absolute maximum value of the isothermal magnetic entropy change of 11.4 J kg−1 K−1 is achieved with negligible hysteresis losses.

Phase transition and magnetocaloric properties of Mn50Ni42−xCoxSn8 (0 ≤ x ≤ 10) melt-spun ribbons

Mn50Ni42−xCoxSn8 melt-spun ribbons exhibit strong magnetostructural coupling over a wide temperature range from 222 to 355 K. The ΔS M peak and RC eff in Mn50Ni42−xCoxSn8 melt-spun ribbons are comparable with or even superior to those of Ni-rich Ni–Mn-based polycrystalline bulk alloys.

Crystallographic correlation between 5M and 7M martensite in an Ni–Mn–Ga alloy

In Ni–Mn–Ga ferromagnetic shape memory alloys, a structural transformation from one type of martensite to another is frequently observed upon cooling or heating. In this work, the microstructural features associated with the transformation from 5M to 7M martensite in an Ni50Mn26Ga22Cu2 alloy were studied. On the basis of the crystallographic orientation determination and an examination of the microstructure by means of the electron backscatter diffraction technique, the 5M to 7M transformation was found to be accompanied by the thickening of martensite plates. The two kinds of martensite (5M and 7M) possess a specific orientation relationship with (001)5M//(001)7M and [100]5M//[100]7M. Through further lattice distortion, four types of 5M variant can evolve into four 7M martensite variants in one variant colony. The present study is expected to provide a deep insight into the crystallographic correlation between 5M and 7M martensite in Ni–Mn–Ga alloys.

Influence of modulation periods on the magnetic and structural properties of L1 0 -FePt/Fe multi-layer films

L10-FePt/Fe nanocomposite thin films are promising candidates for excellent performance permanent magnets, owing to the large magnetocrystalline anisotropy of FePt (Ku=7×107 erg/cc [1]) and high saturation magnetization of Fe. The theoretical energy product of FePt/Fe nanocomposite thin films could be up to 90 MGOe [2]. However, the maximum energy product has been obtained in FePt/Fe nanocomposite thin films is 52.8 MGOe [3]. It is well known that a uniform and nanoscale soft magnetic phase which completely exchange-coupled with the hard magnetic phase is the key issue to get high energy product. In addition, it is widely believed that grain size, especially of soft magnetic phase, is a critical parameter for the exchange coupling effect. Therefore, considerable attentions have been focused on the design of FePt/Fe nanocomposite thin films. It has been proved that the energy product of multilayer FePt/Fe thin films is higher than that one of bilayer FePt/Fe thin films and could be further improved by optimization of microstructure and modulation periods [4-5]. In the present work, the effects of modulation periods on magnetic and structural properties of FePt/Fe multilayer thin films were investigated.

Microstructural evolution associated with martensitic transformation in Ni-Mn-Ga alloy

Based on the spatially resolved electron backscatter diffraction technique, the microstructural evolution accompanying the martensitic transformation (austenite to 7M martensite) and the intermartensitic transformation (7M martensite to NM martensite) was studied on a polycrystalline Ni53Mn22Ga25 alloy. Results show that the 7M martensite plate groups transformed from initial austenite have a diamond-shape with four twin-related variants. The 7M to NM intermartensitic transformation was accompanied by the thickening of martensite plates. With the experimental results, the characteristics of microstructural evolution during the phase transformations were further analyzed.

Phases Stability of Ni-Mn-Ga Alloys Studied by Ab Initio Calculations

The phase stabilities and magnetic properties in Ni-Mn-Ga alloys are systematically investigated by means of the first-principles calculations within the framework of density functional theory using the vienna ab initio simulation package. The calculated formation energies show that the tetragonal NM martensite is the most stable phase compared with the cubic austenite and the modulated monoclinic martensite for stoichiometric Ni2MnGa. The atomic magnetic moment keeps constant in austenite and NM martensite, whereas those of Ni and Mn in the modulated martensite oscillate according to the atomic position. Furthermore, The formation energies of the various compositions have been systematically calculated.