We report new results of a 57 Fe Mössbauer study of multiferroic 3R-AgFeO 2 powder samples performed in a wide temperature range, including two points, T N1 » 14 K and T N2 » 9 K, of magnetic phase transitions. At the intermediate temperature range, T N2 < T < T N1 , the 57 Fe Mössbauer spectra can be described in terms of collinear spin-density-waves (SDW) with the inclusion of many high-order harmonics, indicating that the real magnetic structure of this ferrite appears to be more complicated than a pure sinusoidally modulated SDW. The spectra at low temperatures, T < T N2 , consist of a Zeeman pattern with line broadenings and sizeable spectral asymmetry. It has been shown that the observed spectral shape is consistent with a transition to the elliptical cycloidal magnetic structure. An analysis of the experimental spectra was carried out under the assumption that the electric hyperfine interactions are modulated when the Fe 3+ magnetic moment rotates with respect to the principal axis of the EFG tensor and emergence of the strong anisotropy of the magnetic hyperfine field H hf at the 57 Fe nuclei. The large and temperatureindependent anharmonicity parameter, m » 0.78, of the cycloidal spin structure obtained from the experimental spectra results from easy-axis anisotropy in the plane of rotation of the iron spin.Analysis of different mechanisms of spin and hyperfine interactions in 3R-AgFeO 2 and its structural analogue CuFeO 2 points to a specific role played by the topology of the exchange coupling and the oxygen polarization in the delafossite structures.
The relationship
between hyperfine interactions, local structure,
and magnetism in ordered double perovskites A2NiMnO6 (A = Sc, In, and Tl) is investigated by nuclear forward scattering
with the 61Ni nuclear transition. Special attention is
given to a quantitative determination of on-site and transferred hyperfine
fields on the 61Ni nuclei. The anomalous small value of
the saturated hyperfine magnetic field H
hf(0) in antiferromagnetic Sc2NiMnO6 is explained
by spin-orbital coupling and strong Ni-3d and O-2p hybridization.
The temperature evolution of H
hf(T) is reproduced with Néel temperature T
N1 ≈ 37 K, which is incompatible with the earlier
assumption that magnetic ordering in the Ni sublattice takes place
below T
N2 ≈ 17 K. Significantly
reduced fields H
hf(0) in In2NiMnO6 (∼21 kOe) with a cycloidal magnetic structure
and Tl2NiMnO6 (∼18 kOe) with collinear
ferromagnetic ordering are related to the supertransferred hyperfine
field (H
STHF) induced by the nearest Mn4+ neighbors. Taking into account the angular dependence H
STHF(ϑ) on the Ni–O–Mn bond
angle ϑ, we have shown that H
STHF in the A = In and Tl perovskites has the negative sign, thus drastically
reducing the resulting H
hf value.
Hyperfine parameters of Fe 7 (PO 4 ) 6 phosphate were examined using 57 Fe Mossbauer spectroscopy in a wide temperature range 4.2 ≤ T ≤ 300 K. Four different iron sites were successfully detected: two for the high-spin Fe 2+ and two for Fe 3+ ions. Mossbauer spectra below T N1 ≈ 47 K were analyzed by diagonalization of the full nuclear-interaction Hamiltonian. The noticeable difference between the saturated hyperfine fields (B hf ) for the 5-foldcoordinated site (∼5.0 T) and the octahedral site (∼10.1 T) was related to the symmetry of the local crystal field. Using crystal field calculations of the energy levels based on the point symmetry of the (Fe 2+ O 5 ) and (Fe 2+ O 6 ) sites, we estimated the intra-atomic magnetic dipolar and electron orbital current contributions to the B hf field. The observed line broadening below the second magnetic phase transition at T N2 ≈ 16 K is approximated by the bimodal distribution of the hyperfine fields p(B hf ) that is characteristic of spin-modulated magnetic systems.
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