Magnetic And Electrical Properties Of Oriented NiMn-based Alloys And VS_xSe_2-x Lamellar Materials

As a ferromagnetic shape memory alloy,NiMn-based Heusler alloy has the coupling properties of magnetic,electrical,thermal and phase transformation.In this paper,the precursor ingot with the nominal composition of Mn_44.7Ni_43.5Sn_11.8 and Ni₄₅Mn_36.5In_13.5Co₅ are prepared by arc-melting.Subsequently,the Mn_44.7Ni_43.5Sn_11.8 ribbons are prepared by melt-spun.Then,textured Ni₄₅Mn_36.5In_13.5Co₅ alloy was obtained by the liquid-metal-cooling directional solidification method.The effect of hydrostatic pressure on the martensitic transformation,exchange bias behavior and magnetocaloric effect of Mn_44.7Ni_43.5Sn_11.8 ribbons have been systematically studied.By applying hydrostatic pressure of 0.75GPa,the martensitic transformation temperature(T_M)of the sample increases from266.5 K to 281.0 K.The value of dT_MdP is about 19.3 K GPa^-1 under the magnetic field of 30 kOe.The results show that the working temperature span of room temperature magnetic refrigeration can be broaden by hydrostatic pressure.At ambient pressure,the variation of 30 kOe external magnetic field makes the(35)S_{ma x} value of the sample reaches 35.9 J kg^-1 K^-1,which indicate that the highly textured Mn_44.7Ni_43.5Sn_11.8 alloy will be a potential magnetic refrigeration materials.In additon,the exchange bias(EB)of the ribbons was investigated at 50 K.Through heat treatment,the coercivity(H_C)of the sample decreased significantly,while the EB effect(H_E)increased.After applying hydrostatic pressure,the value of H_C remained unchanged,but the value of H_E slightly increased.Giant magnetoresistance(MR),martensitic transformation,and magnetocaloric effect(MCE)have been investigated in highly textured Ni₄₅Mn_36.5In_13.5Co₅ alloys.The unique bamboo-like transverse stripes perpendicular to the long axis of grains were observed in the scanning electron microscope images.The result shows that the direction of crystal growth is easy to magnetized.When the direction of the external magnetic field is along the crystal growth direction,the magnetocaloric effect is more obvious in the low external magnetic field.Compared with randomly oriented Ni₄₅Mn_36.5In_13.5Co₅ alloys,at a higher temperature(268 K),a larger value(64%)of the MR under a smaller magnetic fields(10 kOe)was obtained.The enhancement of the MR should be attributed to the special crystallographic texture prepared by the directional solidification method.VX₂(X=S,Se)is an intrinsic magnetic material with a layered structure.Compared with the bulk,the nanosheets has more novel properties,and it will also have a strong application potential in the optical field.In this paper,we focus on the fabrication of VX₂ powder and nanosheets.By vacuum sealing tube heating,we can simply prepare VS_xSe_2-x powder sample with room temperature magnetism.The nanoscale VSe₂ materials were prepared by CVD method,mechanical stripping and liquid phase ultrasonic exfoliation.In the process of liquid phase ultrasonic treatment,it is found that the side reaction can be used as a new way to synthesize crystalline selenium nanowires and V₂O₅ thin films,and the reaction mechanism is explored.