RESUMO
The magnetization distribution in Co2FeSi which has the largest moment per formula unit â¼6 µB of all Heusler alloys, has been determined using polarized neutron diffraction. The experimentally determined magnetization has been integrated over spheres centred on the three sites of the L12 structure giving µ Fe = 3.10(3) µB and µ Co = 1.43(2) µB, results which are slightly lower than the moments in atomic spheres of similar radii obtained in recent LDA + U band structure calculations (Li et al 2010 Chin. Phys. B 19 097102). Approximately 50% of the magnetic carriers at the Fe sites were found to be in orbitals with eg symmetry. This was higher, ≃65%, at the Co sites. Both Fe and Co were found to have orbital moments that are larger than those predicted. Comparison with similar results obtained for related alloys suggests that there must be a finite density of states in both spin bands at the Fermi energy indicating that Co2FeSi is not a perfect half-metallic ferromagnet.
RESUMO
Magnetization and high resolution neutron powder diffraction measurements have been made on the magnetic shape memory alloy Ni(1.84)Mn(1.64)In(0.52). The compound undergoes a broad structural phase transition, which on heating starts at â¼150 K and finishes at â¼215 K. On cooling there is a â¼20 K hysteresis. The high temperature parent phase is cubic (a = 5.988 Å) with the L2(1) structure in which the excess Mn atoms occupy the vacancies on the Ni and In sites. The magnetic moment is located mainly on the Mn atoms with the same magnitude on both the 4a (Mn) and 4b (In) sites. The low temperature martensite is monoclinic with parameters a = 4.405(2), b = 5.553(2), c = 12.950(2) Å, ß = 86.47(10)° and space group P2/m. The magnetic properties of the martensitic phase are complex and indicate metamagnetic behaviour.
RESUMO
Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory alloy Mn(2)NiGa have confirmed that it is ferromagnetic with a Curie temperature above 500 K. The compound undergoes a broad structural phase transformation ΔT â¼ 90 K with a mean transition temperature T(M) â¼ 270 K. The high temperature parent phase is cubic (a = 5.937 Å) and has a modified L 2(1) structure. At 500 K the ordered magnetic moment essentially all on the 4a site is 1.35 µ(B)/Mn. The low temperature martensite has space group I4/mmm and is related to the cubic phase through a Bain transformation a(tet) = (a(cub) + b(cub))/2, b(tet) = (a(cub) - b(cub)) and c(tet) = c(cub) in which the change in cell volume is < 2.6%. In this structure at 5 K the ordered moment of ≈2.3 µ(B) is again found to be confined to the sites with full Mn occupation and is aligned parallel to c. Neutron diffraction patterns obtained at 5 K suggested the presence of a weak incommensurate antiferromagnetic phase characterized by either a ((1/3)0(1/3)) or (00(1/3)) propagation vector.
RESUMO
Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni(2)Mn(1.48)Sb(0.52) have confirmed that it is ferromagnetic below 350 K and undergoes a structural phase transition at T(M)≈310 K. The high temperature phase has the cubic L2(1) structure with a = 5.958 Å, with the excess manganese atoms occupying the 4(b) Sb sites. In the cubic phase above ≈310 K the manganese moments are ferromagnetically aligned. The magnetic moment at the 4(a) site is 1.57(12) µ(B) and it is almost zero (0.15(9) µ(B)) at the 4(b) site. The low temperature orthorhombic phase which is only fully established below 50 K has the space group Pmma with a cell related to the cubic one by a Bain transformation a(orth) = (a(cub) + b(cub))/2; b(orth) = c(cub) and c(orth) = (a(cub) - b(cub)). The change in cell volume is ≈2.5%. The spontaneous magnetization of samples cooled in fields less than 0.5 T decreases at temperatures below T(M) and at 2 K the magnetic moment per formula unit in fields up to 5.5 T is 2.01(5) µ(B). Neutron diffraction patterns obtained below ≈132 K gave evidence for a weak incommensurate magnetic modulation with propagation vector (2/3, 1/3, 0).
RESUMO
Polarized neutron diffraction has been used to study the magnetization distribution in two isostructural inter-metallic compounds NiMnSb and PdMnSb. Band structure calculations have predicted that whereas the former should be a half metallic ferromagnet the latter should not. Measurements made at 5 K on different crystals show that disorder can occur between the A (Mn) and B (Sb) sites in both alloys and in the case of NiMnSb, by partial occupation of the void D sites by Ni. In all the crystals most of the moment was found on the Mn atoms in the A sites; in NiMnSb it is due to spin only but in PdMnSb there is evidence for a significant orbital contribution (g = 2.22). The magnitudes of the moments associated with each atom are in fair agreement with the theoretical values; however, the distribution of magnetization around the Mn atoms is found to have nearly spherical symmetry (40% e(g)) rather than the 50% e(g) character expected from the band structure.