Iron Subsystem Research Articles

Magnetic ordering of Fe and Tb in theab initiodeterminedFeRGe2O7structure (R=Y, Tb)

The crystal structure of $\mathrm{Fe}R{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ ($R=\mathrm{Y}$, Tb) has been solved ab initio from x-ray powder diffraction data. It is monoclinic, space group ${P2}_{1}/m$ (No. 11), $Z=4,$ $a$ (\AA{})=9.6552(4) and 9.6388(8); $b$ (\AA{})=8.5197(3) and 8.4789(7), $c$ (\AA{})=6.6746(3) and 6.7383(5), \ensuremath{\beta} (\ifmmode^\circ\else\textdegree\fi{})=100.761(2) and 100.377(4), and $V({\AA{}}^{3})=539.39$ and 541.69, for $R$=Y and Tb, respectively. Precise oxygen positions were determined for the Tb compound from a room temperature neutron diffraction profile, refined by the Rietveld method to an ${R}_{f}=3.99%$ using 58 parameters. The ${\mathrm{FeYGe}}_{2}{\mathrm{O}}_{7}$ crystal structure contains three kinds of coordination polyhedra: ${R}^{3+}$ coordinated to seven oxygens at slightly different lengths forming a capped octahedron, ${\mathrm{FeO}}_{6}$ distorted octahedra, and four types of ${\mathrm{GeO}}_{4}$ tetrahedra. Its most interesting feature is the existence of flattened chains of $R{\mathrm{O}}_{7}$ polyhedra linked in the $c$ direction through pairs of ${\mathrm{FeO}}_{6}$ octahedra with which they share edges, forming layers running parallel to the $\mathrm{bc}$ crystal plane. Magnetization measurements between 350 and 1.7 K show one peak at 38 K for $R$=Y and two maxima at 42 and 20 K for the Tb compound, which could indicate transitions to antiferromagnetically ordered states. From low-temperature neutron diffraction data on ${\mathrm{FeTbGe}}_{2}{\mathrm{O}}_{7},$ three-dimensional antiferromagnetic ordering is established, both Fe and Tb sublattices getting simultaneously ordered at ${T}_{N}=42\mathrm{K}.$ The propagation vector of the magnetic structure is $k=[0,0,0].$ At 1.7 K the magnetic moments $3.91(7){\ensuremath{\mu}}_{B}$ $({\mathrm{Fe}}^{3+})$ and $7.98(6){\ensuremath{\mu}}_{B}$ $({\mathrm{Tb}}^{3+})$ lie ferromagnetically coupled in the $\mathrm{ac}$ planes, which contain ${\mathrm{TbO}}_{7}{\ensuremath{-}\mathrm{F}\mathrm{e}\mathrm{O}}_{6}{\ensuremath{-}\mathrm{T}\mathrm{b}\mathrm{O}}_{7}$- chains in the $c$ direction, forming relatively small angles with the $c$ axis. The coupling between parallel $\mathrm{ac}$ planes is antiferromagnetic along the $b$ direction. This model leads to a best fit of ${R}_{\mathrm{mag}}=3.02%.$ The thermal evolution of the magnetic moments suggests that below \ensuremath{\sim}20 K the faster increase of the ${\mathrm{Tb}}^{3+}$ moments is due to the stronger Fe-Tb interactions and crystal field effects. The maximum in $\ensuremath{\chi}(T)$ at 20 K does not correspond then to any phase transition, but is caused by the exchange interaction with the ordered iron subsystem.

Two-step metamagnetic phase transition induced by a magnetic field parallel to the b-axis in DyFeO 3

By means of magnetic and magneto-optic studies performed in the temperature range 1.7–6 K, the phase diagram of DyFeO 3 in the presence of a magnetic field in the crystallographic ( ab) plane was determined. Despite earlier theoretical predictions, a certain temperature interval was found in which the transition from the Γ 5(g x a y )Γ 1( G y A x C z ) to the Γ 3( f y c x )Γ 4( G x A y F z ) phase is a two-step transition even when the magnetic field is directed exactly along the b-axis. A qualitative explanation of the observed phenomena resulting from the appearance of the Γ 1234( F xF yF zG xG yG z ) phase in the iron subsystem, is proposed.