K.-U. Neumann

Magnetic and Crystallographic Properties of Shape Memory Alloys Ni2Mn1+xSn1-x

X-ray powder diffraction and magnetization measurements were done on the magnetic shape memory alloys Ni2Mn1+xSn1-x. The alloys with 0≤x≾0.4 crystallize in the cubic L21 structure and exhibit the ferromagnetic behavior. X-ray diffraction patterns indicate that the excess Mn atoms occupy the Sn sites. Furthermore, magnetization measurements make clear that the Mn atoms, which substitute for Sn sites, are coupled antiferromagnetically to the ferromagnetic manganese sublattices. The alloys with 0.4≾x≤0.6 undergo a martensitic transition from the high temperature L21 structure to the orthorhombic 4O one. These alloys show a variety of magnetic transitions. A magnetic phase diagram of Ni2Mn1+xSn1-x system is discussed qualitatively on the basis of the interatomic dependence of the exchange interactions.

The magnetic and structural properties of the magnetic shape memory compound Ni2Mn1.44Sn0.56

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).

Atomic and magnetic order in the shape memory alloy Mn2NiGa.

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 ∼ 90 K with a mean transition temperature T(M) ∼ 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.

Magnetic order in Pd2TiIn: A new itinerant antiferromagnet?

Abstract The bulk magnetic properties of the intermetallic compound Pd 2 TiIn which has the Heusler L2 1 structure are reported. From these investigations it appears that the material orders antiferromagnetically below approximately 110 K. The effective paramagnetic moment obtained from the Curie-Weiss behaviour of the susceptibility is 4.9μ B a value much higher than can be accounted for on the basis of a localised moment associated with Ti atoms. From these results it is suggested that the material is probably an itinerant antiferromagnet with finite-temperature properties characterised by spin fluctuations.

Neutron scattering from invar alloys

Abstract Neutron scattering experiments from invar alloys are reviewed. The discussion is restricted to results obtained on Fe 65 Ni 35 and Fe 3 Pt alloys. Particular attention is given to polarised neutron techniques which enable a detailed test of relevant theories to be made. These measurements suggest that many of the theories put forward to explain the invar effect are inappropriate. Finally, the results of some recent polarised-neutron measurements are presented.

Superweal ferrimagnetism arising from strong coupling in paramagnetic systems

Abstract Alloys are considered which can be thought of as being composed of two strongly coupled paramagnetic subsystems. Using Landau theory it is shown that when a magnetic coupling of sufficient strength is switched on between the paramagnetic subsystems the whole system becomes unstable towards a magnetically ordered state. It is shown that for a positive coupling constant a ferrimagnetic state results with a small ferromagnetic moment. For small external magnetic fields the ferromagnetic component is only weakly field and temperature dependent. The relevance of this model for the interpretation of superweak ferrimagnetism recently observed in some Heusler alloys is briefly discussed.

Stability of martensitic domains in the ferromagnetic alloy Ni2MnGa: a mechanism for shape memory behaviour

The martensitic phase transition in Ni2MnGa, fundamental to its shape memory behaviour, can be described by two successive 110 type shears leading to 36 possible different orientations for the axes of the pseudo-tetragonal martensitic phase. The distribution and orientation of the domains formed on cooling Ni2MnGa into the martensitic phase has been studied using single-crystal neutron diffraction with a multi-detector. The number of domains actually occurring was rather low and was reduced by residual strain. Thermal cycling through the phase transition was found to further reduce the number of domains occurring, which may stabilize after several cycles. Uniaxial stress or a magnetic field applied parallel to [001] is able to switch domains whose pseudo-tetragonal axis is [100] or [010] to ones of type [001]. The results suggest that plastic deformation in the martensitic phase takes place by twinning (change of domain), rather than by slip and that the shape memory property arises from the fixed orientation relationship between the martensitic twins and the high temperature cubic axes.

Atomic order and magnetization distribution in the half metallic and nearly half metallic C1b compounds NiMnSb and PdMnSb

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.

Ferromagnetism in an intermetallic compound containing palladium, titanium and aluminium

Abstract Ferromagnetism has been observed in a ternary intermetallic compound whose constituent elements themselves do not order magnetically. The compound formed at the stoichiometric composition Pd2TiAl has the cubic Heusler L21 structure and a lattice parameter of 6.322 A. Magnetisation measurements made between 2 and 300 K and in applied fields of up to 5 T indicated a ferromagnetically ordered state. On the basis of these measurements, the saturation magnetic moment per Pd2TiAl formula unit was determined to be (0.21 ± 0.005)μB. Furthermore, the spontaneous magnetisation only decreases by approximately 3% between 2 and 300 K. If a Brillouin variation of the magnetisation is assumed, then a Curie temperature must be at least 900 K. The observed decrease in the magnetisation follows very closely a T2 dependence expected for Stoner behaviour. However, since the spontaneous moment is roughly half that of nickel and the Curie temperature is nearly that of iron, the compound does not fall into the category of a weak itinerant ferromagnet. A model based on a band structure description is put forward to account for the observations.

Magnetic and atomic order in the potential half metallic ferromagnets Ru2−xFexCrGe

Neutron powder diffraction and magnetization measurements have been used to study the crystallographic and magnetic properties of Ru2−xFexCrGe with x = 0, 1.0 and 1.7. Ru2CrGe was found to have the Heusler L21 structure and to order antiferromagnetically below TN = 16.5 K to a face centred cubic type 2 structure, propagation vector . Only the Cr atoms carry a magnetic moment, which is oriented perpendicular to the propagation vector and has magnitude 1.45 μB at 5 K. Replacement of Ru by Fe causes the alloys to become ferromagnetic with accompanying disorder amongst the Fe, Cr and Ru atoms with only the Ge atoms remaining completely ordered. Both the neutron diffraction data and the magnetization results for the ferromagnetic alloys lead to non-integral values of the magnetic moment per formula unit differing significantly from the 2 μB predicted by band structure calculations. The materials can therefore not be expected to display ideal half metallic ferromagnetic properties.