Phase Transition And Magnetic Properties Of New Transition Metal-based First-order Phase Transition Materials

1.MnFe_0.5Ni_0.4M_0.1Si?M=Ni?Co?Fe?Cr?Ti?Cu?Al?was prepared by arc melting under high-purity argon atmosphere.Magnetism and magnetostructural transitions of MnFe_0.5Ni_0.4M_0.1Si?M=Ni?Co?Fe?Cr?Ti?Cu?Al?have been investigated on the basis of crystal structure and magnetic measurements.The results show that the crystal structure changes from orthorhombic to hexagonal for M_0.1=?Ni?Fe?Co?Cr?Ti?Cu?Al?_0.1.Magnetic measurements show that the Curie temperature increases in the order of M_0.1=?Cr?Ti?Fe?Cu?Co?Ni?Al?_0.1.When M_0.1=?Fe?Co?Ni?_0.1,the ferromagnetic to paramagnetic transition is sharp,while when M_0.1=?Cr?Ti?Cu?Al?_0.1,the ferromagnetic to paramagnetic transition is slow and the magnetic moment is small.The study found that the Curie temperature of MnFe_0.6Ni_0.4Si material can be near room temperature.The maximum magnetic entropy of MnFe_0.6Ni_0.4Si is 2.35J/kg K for a field change of 3T near room temperature.2.The structural and magnetic phase diagrams of MnFe_0.6Ni_0.4Si_1-xGe_x alloys?0?x?1?are explored using powder X-ray diffraction,magnetic and calorimetric measurements.At room temperature,the crystal structure evolves from the orthorhombic TiNiSi-type to the hexagonal Ni₂In-type with the increase in Ge.A magneto-structural coupling leading to a first-order ferromagnetic transition is found at 204 K for x⁰.5.Differential scanning calorimetry reveals a recalescence-like event at the first cooling across the structural transition.In-field specific heat measurements are used to quantify the giant magnetocaloric effect and show that the latent heat of the magneto-structural transition is less than in closely related Mn MX alloys,which leads to sizable transition sensitivity to the magnetic field and finite adiabatic temperature change.3.The crystal structure of MnFe_0.6Ni_0.4Si_1-xAl_x?0?x?0.15?series compounds is changed from orthogonal TiNiSi to hexagonal Ni₂In structure around Al⁰.05.When 0.06?Al?0.07,the M-T curve results show that a first-order phase transition occurs,and its thermal lag is about20 K.When the temperature was measured for the first time,the sample was found to have an virgin effect.The analysis of variable temperature XRD results showed that the sample coexisted orthogonally and hexagonally at low temperature,and could not gradually become a hexagonal structure until T_C.4.In this experiment uses the MnFe₄Si₃ single crystals were successfully grown by solvent method where tin was used as solvent.The polycrystalline sample used for single crystal growth was prepared by high energy ball milling and solid-phase sintering process.The mixture of the polycrystalline sample and tin?ratio of 1:15?was sintered at 1150°C,followed by spontaneous cooling nucleation from the saturated solution and precipitation growth by slow cooling.It can be seen from the SEM images that the size of MnFe₄Si₃ single crystals are about several hundred microns and the crystals have a sharp angular appearance on the surface.The c axis is confirmed to be the hard magnetic axis,both in single crystal and polycrystalline MnFe₄Si₃,and a large,dominant,K₁ anisotropy constant(^-2.5 MJ m^-3)is found at low temperatures.