Coexisting antiferromagnetism and ferromagnetism in mechanically alloyed Fe-rich Fe–Ni alloys: implications regarding the Fe–Ni phase diagram below 400°C

Fe–Ni alloys below the Invar region with compositions Fe_100−xNi_x (x=21, 24, and 27 at%) were prepared by high-energy ball milling technique (mechanical alloying). The as-milled samples, characterized by X-ray diffraction and Mössbauer spectroscopy, contain a mixture of (BCC) and γ (FCC) phases, whereas the samples annealed at 650°C for 0.5 h show a single γ (FCC) phase displaying a single line Mössbauer spectrum at room temperature (RT). At low temperature, the Mössbauer spectra of annealed Fe₇₆Ni₂₄ and Fe₇₃Ni₂₇ alloys show the existence of a magnetically split pattern together with a broad singlet, which are ascribed to a high-moment ferromagnetic Ni-rich phase and a low-moment Fe-rich phase, respectively. The Fe-rich phase in annealed Fe₇₆Ni₂₄ alloy, which is paramagnetic at RT, undergoes antiferromagnetic ordering at 40 K, estimated from the dramatic line broadening of its spectrum, giving rise to a small hyperfine field (e.g. 2 T at 6 K). The coexistence of these phases is attributed to phase segregation occurring in these alloys as a result of enhanced atomic diffusion. The stability of these alloys towards martensitic (FCC→BCC) transformation at low temperatures is discussed in connection with the Fe–Ni phase diagram below 400°C.     

Mössbauer study of the invar Fe–Ni and Fe–Ni–C alloys in magnetic field

F.C.C. Fe–30.3%Ni and Fe–30.5%Ni–1.5%C (wt.%) alloys were studied by means of Mössbauer spectroscopy in external magnetic field B _ext?=?2.5, 5, 7 T parallel to the gamma-beam. It is shown that distribution of effective magnetic field in the alloys is broad and that carbon expands the range of B _eff. The external magnetic field increases B _eff in the Fe–Ni alloy and decreases it more evidently in the Fe–Ni–C alloy. Antiferromagnetic spin coupling along the ferromagnetic component is proposed to explain data.     

Anti-Invar properties and magnetic order in fcc Fe-Ni-C alloy

Anti-Invar effect was revealed in the fcc Fe-25.3%Ni-0.73%C (wt%) alloy, which demonstrates high values of thermal expansion coefficient (TEC) (15-21)×10⁻⁶ K⁻¹ accompanied by almost temperature-insensitive behavior in temperature range of 122-525 K. Alloying with carbon considerably expanded the low temperature range of anti-Invar behavior in fcc Fe-Ni-based alloy. The Curie temperature of the alloy T_C=195 K was determined on measurements of temperature dependences of magnetic susceptibility and saturation magnetization. The Mössbauer and small-angle neutron scattering (SANS) experiments on the fcc Fe-25.3%Ni-(0.73-0.78)%C alloys with the varying temperatures below and above the Curie point and in external magnetic field of 1.5-5 T were conducted. Low value of the Debye temperature Θ_D=180 K was estimated using the temperature dependence of the integral intensity of Mössbauer spectra for specified temperature range. The inequality B_eff=(0.7-0.9)B_ext was obtained in external field Mössbauer measurement that points to antiferromagnetically coupled Fe atoms, which have a tendency to align their spins perpendicular to B_ext. Nano length scale magnetic inhomogeneities nearby and far above T_C were revealed, which assumed that it is caused by mixed antiferromagnetically and ferromagnetically coupled Fe atom spins. The anti-Invar behavior of Fe-Ni-C alloy is explained in terms of evolution of magnetic order with changing temperature resulting from thermally varied interspin interaction and decreasing stiffness of interatomic bond.     

The cation ordering in iron-containing zinc and magnesium orthophosphates determined from Mössbauer spectroscopy

The cation distribution between five- and six-coordinated sites in the isotypic solid solutions γ (Zn, Fe)₃(PO₄)₂ and (Mg, Fe)₃(PO₄)₂ has been determined by means of Mössbauer spectroscopy. Iron is found to be strongly ordered into the six-coordinated sites of γ-(Zn, Fe)₃(PO₄)₂ thus stabilizing this phase with respect to α-Zn₃(PO₄)₂, while (Mg, Fe)₃(PO₄)₂; shows a roughly random cation distribution. Distortion of the coordination polyhedra around iron is suggested to limit the solid solution range of iron in the γ-Zn₃(PO₄)₂ structure type. Isomer shifts for Fe²⁺ in the five-coordinated site are observed to be $\text{1.12±0.01}\text{mm}\text{s}$ at room temperature and $\text{1.24±0.02}\text{mm}\text{s}$ at 77 K.     

Investigations of Al-Dalang and Al-Hawashat meteorites

Mössbauer spectroscopy, X-ray diffraction (XRD) measurements, and electron microprobe analysis (EMPA) have been performed on two meteorites named Al-Dalang and Al-Hawashat after identifying their falling sites in the Western region of Sudan. These two meteorites are ordinary chondrites with similar mineralogy. XRD and EMPA show that the two specimens consist of primary olivine, ortho-pyroxene and later crystallising clino-pyroxene as reaction rims against plagioclase. Fe-metal phases are dominated by kamacite (≈6 wt.% Ni) and minor amounts of tetrataenite (≈52 wt.% Ni). Troilite (FeS) and alabandite (MnS) are optically observed as sulphide phases. The Mössbauer measurements at 295 and 78 K are in agreement with the above characterizations, showing at least two paramagnetic doublets which are assigned to olivine and pyroxene and magnetic sextets assigned to kamacite (hyperfine field ≈33.5 T) and troilite FeS (hyperfine field ≈31 T).     

Magnetic dipolar and electric quadrupolar effects on the Mössbauer spectra of magnetite above the Verwey transition

Mössbauer spectroscopic studies (⁵⁷Fe) of powdered magnetite have been undertaken between 120 K and 880 K. Below the magnetic transition temperature (T _C=839.5 K) three six-line patterns have been fitted to our experimental spectra. The broadening of the B-pattern is explained by two magnetically non-equivalent B-site irons, suggesting broadening due to electron hopping to be negligible. In the paramagnetic state the electric quadrupole splittings of iron at A-and B-sites are found to be constant, independent of temperature, having the values zero and 0.16 mm/s, respectively. The centroid shifts, on the other hand, show above 700 K large deviations from the calculated second order Doppler shift. It is proposed that the deviations arise from a variation in band overlap. The temperature variation of the magnetic fields is found to be proportional to the sublattice magnetization. The difference in the magnetic fields at the two non-equivalent B-sites is measured to be 1.1 T at 310 K.     

High Pressure Mössbauer Studies on FCC Fe53Ni47 Alloy

Perturbed angular correlation measurements of the hyperfine interaction of ¹¹¹In in sapphire show, that after implantation and annealing at 1000°C, the fraction of undisturbed probe atoms exhibiting a unique quadrupole interaction with ν _Q = 219(1) MHz (η = 0) varies between 50% at 4 K, 5% at 100 K and 80% at 973 K in a reversible manner. A possible explanation for this surprising behaviour is the influence of so-called ‘after effects’ following the EC-decay of ¹¹¹In to ¹¹¹Cd. Immediately after the decay the ¹¹¹Cd is in an ionized state, then collects electrons from its surroundings and reaches the ground state. The different electronic configurations that arise during this relaxation process affect the amplitude (f _u) and the damping (δ _u) of the unique quadrupole interaction.     

Field-induced local magnetic moments in γ (FCC) Fe–Ni anti-Invar alloys

Mössbauer spectroscopy in longitudinal external fields (up to 7 T) and SQUID magnetometry (up to 5 T) measurements have been carried out on mechanically alloyed (MA) γ (FCC) Fe_100−xNi_x (x=21, 24, and 27 at%) alloys at room temperature. The zero-field Mössbauer spectra of these alloys show only singlets. The high field Mössbauer results indicate that large amounts of the material is in the paramagnetic state, giving rise to two spectral components with their effective fields almost linearly depend on the external field, but with slopes that are smaller than unity. The in-field Mössbauer spectra of the x=27 at% alloy show an additional component with a hyperfine field of ≈21 T, which is attributed to Ni-rich (>30 at% Ni) clusters (domains) of ferromagnetically ordered HM phase that behaves superparamagnetically at room temperature and shows a non-linear character in the magnetization (M–H) curves at low fields. This HM phase is also present in the x=21 and 24 at% samples but with smaller amounts. The results suggest induced hyperfine fields and hence induced moments in the paramagnetic components, which increases with increasing Ni contents. Taenite-enriched samples from the metal particles of two stony meteorites, Al Kidirate (H6) and New Halfa (L4), are also studied by high field Mössbauer spectroscopy and the results are compared to that of MA samples.     

High-temperature cation ordering in olivine: an in situ Mössbauer study of synthetic (Mg0.55Fe0.45)2 SiO4

High-temperature in situ Mössbauer spectroscopy measurements (300–950°C) were done on synthetic olivine of composition (Mg_0.55Fe_0.45)₂ SiO₄ (=Fa45) in order to study the distribution of Fe^2?+? over the M1 and M2 octahedral sites. The spectra are fit with two doublets, which are assigned to Fe^2?+? at the M1 (smaller splitting) and M2 sites. The Fe^2?+? site-occupancies at M1 and M2, obtained from the Mössbauer relative areas, suggest that Fe^2?+? has a slight preference for the M1 site at temperatures below ~500°C, with a tendency of disordering around this temperature. At higher temperatures, Fe^2?+? again prefers to occupy the M1 site, where it shows a considerable order at this site up to 750°C. At still higher temperatures, the spectra indicated partial reduction of the Fa-component into metallic iron and the resolution of the doublets was severely deteriorated.     

Non-Magnetic Stainless Steels Reinvestigated – a Small Effective Field Component in External Magnetic Fields

Hyperfine Interactions - Three standard non-magnetic stainless steels of composition (wt%) Fe70Cr19Ni11, Fe70Cr17Ni13 and Fe69Cr18.5Ni10.3Mn1.8Ti0.4 have been investigated by Mössbauer...