Mössbauer measurements of GaFeO3 single crystal multiferroic

Mössbauer measurements on single crystal absorbers at room and at low temperatures were performed. The results are fully consistent with previously published reports by other groups. Spectra of single crystals were simultaneously analyzed including magnetic dipole and electric quadrupole interactions. The analysis shows that there is a small component of magnetic moments perpendicular to the magnetic easy axis. Mössbauer data seem not agree with commonly accepted ferrimagnetic structure of GaFeO3.

Magnetic Structures of Some Multiferroics

<p>We studied crystal and magnetic structures of some composite and single-phase multiferroics: (<em>x</em>)MFe₂O₄ + (1-<em>x</em>)BaTiO₃, Ni_3-<em>y</em>Co<em>_y</em>V₂O₈, and Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃. Composite multiferroics (<em>x</em>)MFe₂O₄ + (1-<em>x</em>)BaTiO₃ with <em>x</em> = (0.2; 0.3; 0.4) and M = (Ni, Co) have ferrimagnetic structure, which is described by the propagation vector <strong><em>k</em></strong> = 0. Oxides Ni_3-<em>y</em>Co<em>_y</em>V₂O₈ with <em>y</em> = (0.1; 0.3; 0.5) possess a modulated magnetic structure, described by the vector <strong><em>k</em></strong> = (δ, 0, 0), where δ = 0.283 and 0.348 at 7.4 K for <em>y</em> = 0.1 and 0.5, respectively. In the Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃ multiferroic a magnetic order is destroyed at 600 K and the Fe-ion magnetic moment decreases from µ = 3.46(5) μ_B at 300 K to zero at 600 K.</p>

Computational Design of Mn4 Molecules with Strong Intramolecular Exchange Coupling

The geometric and electronic structures of [Mn44+Mn3+3(µ3-L2 -)3(µ3-X -(OAc) - 3(dbm) -3] (L = O, X = F, dbmH = dibenzoyl-methane) molecule has been studied by first-principles calculations. It was shown in our previous paper that the ferrimagnetic structure of Mn$^{4 + }$Mn$^{3 + }_{3}$ molecules is determined by the $\pi $ type hybridization between the $d_{z^2}$ orbitals at the three high-spin Mn$^{3 + }$ ions and the $t_{2g}$ orbitals at the Mn$^{4 + }$ ion by the $p$ orbitals at the $\mu _{3}$-L$^{2 - }$ ions. To design new Mn$^{4 + }$Mn$^{3 + }_{3}$ molecules having much more stable ferrimagnetic state, one approach is suggested. That is controlling the Mn$^{4 + }$-($\mu _{3}$-L$^{2 - })$-Mn$^{3 + }$ exchange pathways by rational variation in $\mu _{3}$-L ligands to strengthen the hybridization between Mn ions. By this ligand variation, $J_{AB}$ can be enhanced by a factor of 3. Our results should facilitate the rational synthesis of new single-molecule magnets.

The Magnetic Phase Transitions and Magnetocaloric Effect in the Ho(Co1-XAlx)2 and Tb(Co1-XAlx)2 Compounds

Direct measurements of the magnetocaloric effect (MCE) and magnetization have been conducted for the R(Co1-xAlx)2 (R = Tb, Ho) compounds. It was found that the small substitutions of Co by nonmagnetic Al lead to an increase both of the magnetic ordering temperature and the value of MCE. The following increase in Al content decreases the Curie temperature and the value of MCE. The analyses of the MCE effect is conducted considering the ferrimagnetic structure and the itinerant nature of these compounds.

Magnetic State of Y3Fe5-XScxO12 Garnets

Ferrogarnets of the system {Y3}[Fe2-xScx](Fe3)O12 were investigated by neutron diffraction and magnetic methods. Parameters of crystal structure were refined. It was found that deviations from a collinear ferrimagnetic structure emerged at x ~ 0.7. At high concentrations of Sc a growth of the high-field magnetic susceptibility and an abnormal temperature dependences of dynamic susceptibility typical of local angular spin structures were revealed. The fracture temperatures for the local angular structures were determined.

Moderate-pressure Synthesis and Neutron Diffraction Study of New Metastable Oxides

We have synthesized two new series of metastable oxides, namely REMn2O5 and RECu3Mn4O12 (RE = rare earths) under moderate pressure conditions. A novel series of ferrimagnetic oxides has been obtained by replacing Mn3+ by Fe3+ in the parent REMn2O5 compounds (RE = Y, Dy, Ho, Er, Tm, Yb). The crystal structure has been studied by neutron powder diffraction (NPD); it contains chains of edge-linked Mn4+O6 octahedra connected via dimeric groups of Fe3+O5 square pyramids. The magnetic susceptibility and the thermal evolution of the NPD patterns reveal the onset of a ferrimagnetic structure below TC ≈ 165 K, characterized by the propagation vector k = 0. Immediately below TC, the Fe3+ and Mn4+ moments lie in an antiparallel arrangement along the c-axis direction. At lower temperatures, the magnetic moment of the rare-earth cations also participates in the magnetic structure, adopting a parallel arrangement with the Fe3+ spins. Some new derivatives of CaCu3Mn4O12 have been prepared at moderate pressures of 2 GPa by replacing Ca2+ by RE3+ cations in the series RECu3Mn4O12 (RE = Pr, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb); the concomitant electronic injection leads to a substantial contribution to TC. The crystal structures of the new materials were refined in the space group Im3̅ from NPD data for the non-absorbing RE cations. The unit cell parameters are considerably expanded with respect to CaCu3Mn4O12, as a result of the electronic injection. The r. t. magnetic structure displays a ferrimagnetic coupling between Mn3+/4+ and Cu2+ spins; at low temperatures there is an antiferromagnetic coupling of the RE3+ moments with the Mn substructure, which substantially reduces the susceptibility and the saturation magnetization.