[en] Well-crystallized, natural goethite has been investigated as a function of temperature. The saturation magnetic hyperfine field was found to be 507 kOe and the Neel temperature Tsub(N) = (400 +- 2) K. The antiferromagnetic structure is stable in external fields of up to 60 kOe and the spin direction is at right angles with respect to the EFG's principal axis. At T/Tsub(N) < 0.5, the reduced hyperfine field varies as T² and in the range 0.8 < T/Tsub(N) < 0.97 as Tsup(1/3). From the temperature dependence of the isomer shift, the Debye temperature was found to be 440 K from which a relative Moessbauer fraction of 0.96 against hematite was evaluated. (Auth.)

[en] Fe₂TiO₄ has been investigated using ⁵⁷Fe Moessbauer spectroscopy with the aim of determining the shape and the temperature dependence of the separate A- and B-site quadrupole splitting distributions. This has been impossible hereto because both contributions are strongly overlapping and the normal doublet spectra do not contain enough information to differentiate between both. It is found that, through the use of external magnetic fields, the necessary information to perform the separation is unveiled. Furthermore, the change in shape as a function of temperature of the distributions thus obtained is well understood from the known behaviour of Fe²⁺. (Auth.)

[en] Several samples of hematite, one a natural specimen, are considered in order to study the specific effect of the morphology on the magnetic structure and especially on the Morin transition. It seemed that mainly the weak-ferromagnetic contribution is affected by the morphology, as well as the transition temperature and region. (Auth.)

[en] Short communication

[en] For badly resolved Moessbauer spectra of ₆₁Ni and ₁₂₁Sb it is demonstrated that even for moderate thickness the analysis with a sum of Lorentzians instead of the complete transmission integral may introduce significant deviations in determining hyperfine interaction constants. Graphs with the relative deviations for purely magnetic and quadrupolar interactions are shown. From these plots it is indicated when the use of the complete transmission integral is recommended. (orig.)

No abstract available

[en] Some aspects of a comprehensive Moessbauer spectroscopic study at variable temperatures and in applied magnetic fields of the spinel system Co_xFe_3-xO₄ with 0≤x≤0.6 are presented. Emphasis is given to the dipolar contribution B_dip to the magnetic hyperfine field at the octahedral sites. Its dependence on the temperature and on the Co concentration is determined and discussed. (orig.)

[en] Moessbauer spectra for three Al-hematite powders were recorded at 4.2 K in applied field of 140 kOe. Particle sizes, as determined from X-ray diffraction line broadenings, are of order 100 nm. In zero field the powders are in a pure antiferromagnetic state. It is found that the applied-field is too low to produce a spin flop into the basal plane in all individual grains constituting the powders. Only portions undergo this transition, while remaining portions undergo a screw-rotation transition whereby the antiferromagnetic spin axis is canted away from the c-axis and rotates perpendicular to the external field. It is argued that the angle of the external field with respect to the c-axis determines which of the two distinct spin reorientation mechanisms takes place. A critical angle can be calculated from the numerical results of the spectral analyses. Its value depends on the field strength and on the Al substitution

[en] Published in summary form only

[en] ⁵⁷Fe Moessbauer spectroscopy is performed on synthetic Mn-substituted goethites and hematites. The hyperfine parameters and in particular the magnetic hyperfine field are found to be less influenced by Mn substitution than in the case of Al. On the other hand, Mn substitution suppresses drastically the Morin transition in hematite. (Auth.)