J.P. Qu

Negative thermal expansion and magnetic properties of Y2Al3Fe14−xMnx compounds

The structure and magnetic properties of Y2Al3Fe14−xMnx (x=0–12) compounds have been investigated by means of x-ray diffraction and magnetization measurements. The Y2Al3Fe14−xMnx compounds have a hexagonal Th2Ni17-type structure. Their unit-cell volumes increase slowly with increasing x first, then increases rapidly with the further increase of x. This implies that there exists a positive spontaneous volume magnetostriction in the magnetic state of Y2Al3Fe14−xMnx compounds. X-ray diffraction of the Y2Al3Fe11Mn3 compound from 150 to 300 K shows that there appears a negative coefficient of thermal expansion, ᾱ≈−7.5×10−5/K, from 185 to 200 K. The Curie temperature and the saturation magnetization of these compounds show a rapid drop with increasing x.

Giant magnetostriction on Fe85Ga15 stacked ribbon samples

Large magnetostrictions of −1300 and +1100 ppm related in the different directions have been obtained in our stacked Fe85Ga15 ribbon samples. In the case of non-180° domain magnetization in the high anisotropic samples, the magnetostrictions are mainly attributed to the existence of Ga clusters which preferentially orient with the ribbon normal due to the ribbon grain texturing. Forming the modified DO3 structure, the Ga–Ga atom pairs distribute in the matrix and cause the x-ray diffraction peak split in melt-spun ribbons. As a special micromorphology, Ga clusters highly condensed in some nanoscale dots have also been experimentally observed.

Large magnetostriction in Fe100−xAlx(15⩽x⩽30) melt-spun ribbons

Magnetostriction of Fe100−xAlx(15⩽x⩽30) al1oys has been largely improved by using melt-spun method. The large magnetostriction up to −700ppm obtained in Fe81Al19 sample is about 5 times as large as that in conventional bulk samples of Fe–Al composition. It has been ascribed to the high concentration of Al–Al atom pairs created by melting-spinning method and their strongly preferential orientation in [100] textured ribbon plane. The remarkable anisotropic magnetostriction reflects the magnetoelastic competition occurring in those strong textured and thin ribbon samples. The composition dependence of the magnetostriction in ribbon samples has been found to be consistent with bulk materials.

Superelasticity of CoNiGa : Fe single crystals

We have fabricated CoNiFeGa single crystals with excellent superelasticity. The superelastic strains of 4% and 6.7% in compression have been obtained along the [001] and [110] directions, respectively. These single crystals show strong anisotropy in strains, superelastic parameters, and even transformation path related to the different crystalline directions. A large superelastic strain up to 11% has been obtained in tension test. The perfect superelasticities have also displayed in bending and torsion tests.

Magnetostriction in (CexTb1−x)0.5Pr0.5Fe2 compounds

The structural and magnetostrictive properties of (CexTb1−x)0.5Pr0.5Fe2 were investigated. The Ce concentration must exceed x=0.6 in order to obtain the pure Laves phase. The magnetostriction and anisotropy constant increase with the Tb content in this system. The anisotropy value of PrFe2 is lower than that of isostructural DyFe2. From the x-ray step-scanned data, we find that λ111 of PrFe2 is 1310 ppm. A large increase can be observed under a small prestress of 6 MPa for Ce0.5Pr0.5Fe2.

Characterization of martensitic transformation in ferromagnetic shape memory alloys Co-Ni-Ga melt-spun ribbons

The martensitic transformation in Co50Ni20Ga30 ribbon has been characterized in detail by means of X-ray diffraction and magnetic measurements. The sample showed a structural transition from the body-centered cubic with indexed lattice parameters of a=5.743 Å to martensite with a tetragonal structure with lattice parameters of a=b=5.422 Å and c=6.401 Å. (c/a>1). By comparing the results of the diffraction intensity and the magnetic susceptibility measurements, we found that the proceeding of the martensitic transformation can be divided into several different steps during cooling from 283 K to 213 K. During heating from 673 K to 833 K, the peak width became very wide and the intensity became very low. It reveals an order-disorder transformation.