Yangkun He

Magnetostructural coupling near room temperature in Ni46-xFexCu4Mn34Ga16 alloys

We report the magnetostructural coupling near room temperature in Ni46-xFexCu4Mn34Ga16 (0 ≤ x ≤ 10) alloys. The martensitic transformation temperature was detected over the whole composition range and was decreased by the substitution of Fe for Ni. The martensitic and austenitic Curie temperatures, TCM and TCA, were observed for 0 ≤ x ≤ 6 and 4 ≤ x ≤ 10, respectively. With the increasing Fe content, TCA was slightly increased and TCM was more rapidly increased. The paramagnetic state of the martensite phase collapsed for x > 6 with the presence of the ferromagnetic austenite phase. The magnetostructural coupling transition from paramagnetic martensite to ferromagnetic austenite was obtained within the temperature range of 300–350 K which was near room temperature.

Investigating non-Joulian magnetostriction

Ferromagnetic materials change their shape under an applied magnetic field—a phenomenon known as magnetostriction. This phenomenon was first described for iron by Joule in 1842, and is generally believed to be volume-conserving. We therefore read with interest the Letter by Chopra and Wuttig1, which reports that samples of Fe100−x–Gax (galfenol) crystals demonstrate “giant” non-volume-conserving (nonJoulian) magnetostriction, and embarked on an extensive study of crystals of similar compositions, dimensions and heat treatments, using strain gauges and capacitive dilatometry to measure magnetostriction for many different combinations of strain direction and applied magnetic field. In every case, we found that the volume was conserved within experimental error, and so we conclude that magnetostriction in galfenol can generally be regarded as Joulian. The initial claim1 was based on strain-gauge measurements in the plane of slow-cooled or quenched, disk-shaped crystals of Fe82.9Ga17.1 and Fe73.9Ga26.1. These samples were 5 mm in diameter and 0.4–0.5 mm thick. A magnetic field of up to 3,000 Oe was applied in-plane. The crystals were found to expand in all, or almost all, of the directions that were tested. No measurement of strain was reported for the [001] direction perpendicular to the plane of the disks because it was assumed that “a negligible magnetization normal to the plane of the disk at comparable fields implies that no volume change occurs along [001].”1 Strain-gauge data for the [001] direction of another slow-cooled Fe82.9Ga17.1 crystal were published subsequently as an Addendum2, in support of the original assertion. In our attempt to verify the idea that the enhanced magnetostriction of galfenol is largely non-Joulian, we grew 12 different rod-, diskor cuboid-shaped crystals with compositions of Fe83Ga17 or Fe74Ga26, heights (or thicknesses) ranging from 70 mm to 0.1 mm and diameters (or widths) ranging from 16 mm to 7 mm. Here we focus on the four of these crystals that were subjected to the same heat treatments as described in ref. 1 (annealed at 1,033 K for 30 min and then either quenched or slow-cooled at 10 K min−1). We measured the magnetostriction (λ, in parts per million (p.p.m.)) using strain gauges or, for the disk-shaped crystals with thicknesses of 0.5 mm, by capacitive dilatometry3 in the [001] direction. The most complete datasets were obtained for a single crystal of Fe83Ga17 with dimensions of 10.6 mm × 10.6 mm × 2.4 mm in the quenched and slow-cooled states. For this crystal we measured the saturation magnetostriction along the [100], [010], [001], [110] and [110] directions in a magnetic field large enough to saturate the magnetization applied along any one of these directions, yielding 17 independent measurements for the crystal in each state. Defining the magnetostriction components (in p.p.m.) for a given applied field (i) and measurement (j) direction as λij q and λij sc for the quenched and slow-cooled states, respectively, and using brackets ‘[...]’ to denote an array of these components, we find

Giant magnetostriction in nanoheterogeneous Fe-Al alloys

As a potential magnetostrictive material, Fe-Al alloys exhibit excellent mechanical properties, low cost, and moderate magnetostriction, but the magnetostriction mechanism is still a mystery. Here, we elucidate the structural origin of magnetostriction in Fe-Al alloys and further improve the magnetostriction five-fold via Tb doping. Nanoinclusions with a size of 3–5 nm were found dispersed in the A2 matrix in Fe82Al18 ribbons. The structure of the nanoinclusions is identified to be tetragonally modified-D03 (L60), which are considered to create the tetragonal distortion of the matrix, leading to the enhanced magnetostriction. Furthermore, a drastic enhancement of the magnetostriction up to 5 times was achieved by trace Tb doping (0.2 at. %). Synchrotron X-ray diffraction directly revealed the increased tetragonal distortion of the matrix caused by these Tb dopants. The results further enrich the heterogeneous magnetostriction and guide the development of magnetostrictive materials.As a potential magnetostrictive material, Fe-Al alloys exhibit excellent mechanical properties, low cost, and moderate magnetostriction, but the magnetostriction mechanism is still a mystery. Here, we elucidate the structural origin of magnetostriction in Fe-Al alloys and further improve the magnetostriction five-fold via Tb doping. Nanoinclusions with a size of 3–5 nm were found dispersed in the A2 matrix in Fe82Al18 ribbons. The structure of the nanoinclusions is identified to be tetragonally modified-D03 (L60), which are considered to create the tetragonal distortion of the matrix, leading to the enhanced magnetostriction. Furthermore, a drastic enhancement of the magnetostriction up to 5 times was achieved by trace Tb doping (0.2 at. %). Synchrotron X-ray diffraction directly revealed the increased tetragonal distortion of the matrix caused by these Tb dopants. The results further enrich the heterogeneous magnetostriction and guide the development of magnetostrictive materials.

When does magnetostriction not conserve volume

Magnetostriction is the strain associated with collective magnetic order.