Alexei V. Sobolev

Ferromagnetic Order, Strong Magnetocrystalline Anisotropy, and Magnetocaloric Effect in the Layered Telluride Fe3−δGeTe2

The ternary transition-metal compound Fe(3-δ)GeTe2 is formed for 0 < δ < 0.3. X-ray diffraction and Mössbauer spectroscopy reveal its layered crystal structure with occasional Fe vacancies in the Fe2 site, whereas no Fe atoms occupy the interlayer space, so that only van der Waals interactions exist between adjacent layers. We explore magnetic behavior and ensuing functional properties of Fe(2.9)GeTe2 via neutron diffraction, thermodynamic and transport measurements, Mössbauer spectroscopy, and electronic structure calculations. Below T(C) = 225 K, Fe(2.9)GeTe2 is ferromagnetically ordered with the magnetic moments of 1.95(5) and 1.56(4) μ(B) at T = 1.5 K, both directed along c, which is the magnetic easy axis. Electronic structure calculations confirm this magnetic structure and reveal a remarkably high easy-axis anisotropy of 4.2 meV/f.u. Mössbauer spectra reveal the magnetic ordering too, although a drastic influence of Fe vacancies on quadrupolar splittings and local magnetic fields has been observed. A moderate magnetocaloric effect with the magnetic entropy change upon the ferromagnetic ordering transition, -ΔS ∼ 1.1 J·kg(-1)·K(-1) at 5 T, is found.

Crystal structure, physical properties, and electronic and magnetic structure of the spin S = 5/2 zigzag chain compound Bi2Fe(SeO3)2OCl3.

We report the synthesis and characterization of the new bismuth iron selenite oxochloride Bi2Fe(SeO3)2OCl3. The main feature of its crystal structure is the presence of a reasonably isolated set of spin S = 5/2 zigzag chains of corner-sharing FeO6 octahedra decorated with BiO4Cl3, BiO3Cl3, and SeO3 groups. When the temperature is lowered, the magnetization passes through a broad maximum at Tmax ≈ 130 K, which indicates the formation of a magnetic short-range correlation regime. The same behavior is demonstrated by the integral electron spin resonance intensity. The absorption is characterized by the isotropic effective factor g ≈ 2 typical for high-spin Fe(3+) ions. The broadening of ESR absorption lines at low temperatures with the critical exponent β = 7/4 is consistent with the divergence of the temperature-dependent correlation length expected for the quasi-one-dimensional antiferromagnetic spin chain upon approaching the long-range ordering transition from above. At TN = 13 K, Bi2Fe(SeO3)2OCl3 exhibits a transition into an antiferromagnetically ordered state, evidenced in the magnetization, specific heat, and Mössbauer spectra. At T < TN, the (57)Fe Mössbauer spectra reveal a low saturated value of the hyperfine field Hhf ≈ 44 T, which indicates a quantum spin reduction of spin-only magnetic moment ΔS/S ≈ 20%. The determination of exchange interaction parameters using first-principles calculations validates the quasi-one-dimensional nature of magnetism in this compound.

Spatially modulated magnetic structure of AgFeO2: Mössbauer study on 57Fe nuclei

The results of the Mössbauer study of ferrite AgFeO2 manifesting multiferroic properties (at T ≤ TN2) have been presented. The hyperfine interaction parameters of 57Fe nuclei have been analyzed in a wide temperature range including the points of two magnetic phase transitions (TN2 ≈ 7–9 K and TN1 ≈ 15–16 K). It has been shown that the Mössbauer spectra of the 57Fe nuclei are sensitive to the variations of the character of the magnetic ordering of Fe3+ ions in the studied ferrite. The results of the model identification of a series of spectra (4.7 K ≤ T ≤ TN2) under the assumption of the cycloid magnetic structure of ferrite AgFeO2 have been presented. The analysis of the results has been performed in comparison with the literature data for other oxide multiferroics.

Local structure, chemical bond parameters and hyperfine magnetic interactions of 57Fe and doped 119Sn atoms in the orthoferrites TlFeO3 and TlFe0.99Sn0.01O3

M?ssbauer spectroscopy has been applied to study the magnetic hyperfine interactions of 57Fe and 119Sn probe atoms within TlFeO3 and TlFe0.99Sn0.01O3 ferrites. According to the magnetic measurements, the magnetic ordering temperature TN (= 560?K) for TlFeO3 is much lower than that for any other orthoferrites RFeO3 (R = rare earth). It is suggested that such difference in TN may be explained by different characteristics of the Tl?O and R?O chemical bonds involved and the induced competition with the Fe?O bonds. The effects of covalency and overlap distortion on the spin and charge densities at 57Fe nuclei are discussed in relation with the hyperfine interactions observed. By using cluster molecular orbital (MO) calculations, it was shown that the low value of the hyperfine magnetic field HFe(= 538?kOe) at 57Fe nuclei in TlFeO3 is strongly associated with a decrease of the covalent mixing parameter (b?)2 = 0.048 and (b?)2 = 0.009 values, which characterize the Fe?O bond covalence. It was elucidated that the value of super-transferred hyperfine field (HSTHF) at 119Sn nuclei in TlFe0.99Sn0.01O3 is sensitive mainly to the angle of the Sn?O?Fe bonds.

Structure of the local environment and hyperfine interactions of 57Fe probe atoms in DyNiO3 nickelate

The local structure of DyNiO3 nickelate at both sides of the insulator (T < Tim) ↔ metal (T > Tim) phase transition was studied by probe 57Fe Mössbauer spectroscopy. The character of change in the hyperfine parameters of probe iron atoms specifically near the phase-transition temperature (T ≈ Tim) was analyzed.

Mössbauer investigations of the CaMn7O12 double manganite with the nuclei of 57Fe probe atoms

AbstractThe CaMn7O12 double perovskite-like manganite is studied using Mössbauer spectroscopy with 57Fe impurity atoms. The hyperfine parameters of the Mössbauer spectra are found to reflect the specific local structure of this oxide. It is found that the phase transition at T ≈ 90 K is associated with the ordering of the magnetic moments of manganese cations in the octahedral sublattice. The structural phase transition R % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfKttLearuqr1ngBPrgarmWu51MyVXgatC% vAUfeBSjuyZL2yd9gzLbvyNv2CaeHbd9wDYLwzYbItLDharyavP1wz% ZbItLDhis9wBH5garqqtubsr4rNCHbGeaGqiVy0df9qqqrpepC0xbb% L8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yqaqpe% pae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabeqaam% aaeaqbaaGcbaWaa0aaaeaaiqaacaWFZaaaaaaa!3AFE!

Electronic state of 57Fe probe atoms in perovskite-type Ni(III) and Cu(III) oxides

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119Sn and 57Fe Mössbauer study of the local structure of perovskite-type ferrites CaFe2 − x N x O5 (N = Sc, Al) and manganite CaMn7O12

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