The structure and magnetic properties of ferromagnetic shape memory alloys containing iron

An experimental investigation of the structural and magnetic properties of Iron based ferromagnetic shape memory alloys Pd57In25Fe18, Ti50Pd40Fe10, Ti50Pd35Fe15, FeMnSi, Fe66.7Mn26.8Si6.5 and Fe57.4Mn35Si7.6 is reported. Magnetisation measurements and high resolution powder neutron diffraction measurements were used to characterise the structural properties of each alloy. The parent phase of the Pd57In25Fe18 specimen has a FCC unit cell, space group Fm3m and lattice parameter a=6.293 ± 0.006Å. The Fe atoms are essentially equally distributed on to the 4a and 4b sites with the Pd occupying the 8c site. On cooling the sample transforms into a tetragonal structure with I 4 / mmm space group and cell parameters a=4.097 ± 0.004Å and c=7.289 ± 0.007Å. Magnetisation measurements performed on the specimen found Tc=140 ± 10K and a moment per formula unit of 1.39 ± 0.03μB. The inverse susceptibility has a Curie-Weiss variation, which yields a paramagnetic Curie temperature of 159 ± 10K, an effective paramagnetic moment of 6.93 ± 0.06μB and a paramagnetic moment μp of 2.86 ± 0.03μB which is close to that generally observed for Fe on an FCC lattice. Neutron diffraction measurements performed on Ti50Pd50-xFex compounds have confirmed the parent phase to be a cubic B2-type structure with the Pm3m space group with lattice parameters a=3.134 ± 0.007Å when x=10 and a=3.118 ± 0.009Å when x=15. On cooling, the martensitic phase transformation occurs and the alloys transform into an orthorhombic B19 structure ( Pmma ) with a 3-fold modulation. The lattice parameters were determined as a=13.853 ± 0.001Å, b=2.872 ± 0.001Å and c=4.513 ± 0.002Å when x=10 and a=13.465 ± 0.002Å, b=2.923 ± 0.002Å and c=4.526± 0.001Å when x=15. The Curie temperature has been estimated using the Brillouin J=1 function as Tc~ 470 ± 10K when x=10 and Tc~523 ± 10K when x=15. For the FeMnSi specimen, the stoichiometry of alloys is consistent with that of C1b compounds. The F 43m space group was employed for the cubic phase with a lattice parameter a=5.684 ± 0.008Å. On cooling the material transforms into a hexagonal martensite phase with 3 P6 /mcm II space group and lattice parameters, a=6.867 ± 0.001Å and c=4.772 ± 0.002Å. In the Fe57.4Mn35Si7.6 compound there is no structural phase transition, the system orders in a FCC unit cell with space group Fm3m and lattice parameters a=3.616 ± 0.007Å. In the Fe66.7Mn26.7Si6.5 case at 500K the alloy orders in a FCC unit cell with space group Fm3m and lattice parameters a=3.611 ± 0.001Å. On cooling, the material transforms into a HCP cell with the space group 3 P6 / mmc and the lattice parameters a=2.540 ± 0.002Å and c=4.108 ± 0.001Å. In the FeMnSi alloy below 69K there is the onset of an incommensurate phase down to 5K, all the Fe-Mn-Si based alloys order with a type-1 FCC antiferromagnetic structure. In the case of FeMnSi, TN=69 ± 3K and for the Fe57.4Mn35Si7.6 and Fe66.7Mn26.8Si6.5 specimens TN has been estimated using the Brillouin J=2 function, to be TN~389 ± 10K and 315 ± 10K respectively. From neutron measurements, at 5K a magnetic moment of 2.69 ± 0.04μB per site and 2.20 ± 0.05μB per site was calculated for the Fe57.4Mn35Si7.6 and Fe66.7Mn26.8Si6.5 specimens respectively.