Europium and manganese magnetic ordering in EuMn2Ge2.

The antiferromagnetic structures of both the manganese and europium sublattices in EuMn2Ge2 have been determined using thermal neutron diffraction. T(N)(Mn) = 714(5) K with the 3.35(5) µ(B) (at 285 K) Mn moments ordering according to the I4'/m'm'm space group. The Eu order is incommensurate with the 6.1(2) µ(B) (at 3.6 K) Eu moments oriented parallel to the c-axis with a propagation vector of k = [0.153(2) 0 0]. Both neutron diffraction and (151)Eu Mössbauer spectroscopy reveal evidence of magnetic short-range ordering of the Eu sublattice around and above T(N)(Eu) ∼ 10 K. The ordering temperature of the Eu sublattice is strongly affected by the sample's thermal history and rapid quenching from the melting point may lead to a complete suppression of that ordering.

The magnetic structure of EuCu2Sb2.

Antiferromagnetic ordering of EuCu2Sb2 which forms in the tetragonal CaBe2Ge2-type structure (space group P4/nmm #129) has been studied using neutron powder diffraction and (151)Eu Mössbauer spectroscopy. The room temperature (151)Eu isomer shift of -12.8(1) mm s(-1) shows the Eu to be divalent, while the (151)Eu hyperfine magnetic field (B(hf)) reaches 28.7(2) T at 2.1 K, indicating a full Eu(2+) magnetic moment. B(hf)(T) follows a smooth S = 7/2 Brillouin function and yields an ordering temperature of 5.1(1) K. Refinement of the neutron diffraction data reveals a collinear A-type antiferromagnetic arrangement with the Eu moments perpendicular to the tetragonal c-axis. The refined Eu magnetic moment at 0.4 K is 7.08(15) µ(B) which is the full free-ion moment expected for the Eu(2+) ion with S = 7/2 and a spectroscopic splitting factor of g = 2.

Determination of the magnetic structure of Gd2Fe2Si2C by Mössbauer spectroscopy and neutron diffraction.

We have determined the magnetic structure of the intermetallic compound Gd2Fe2Si2C using neutron powder diffraction, (155)Gd and (57)Fe Mössbauer spectroscopy. This compound crystallizes in a monoclinic (C2/m) structure and its magnetic structure is characterized by antiferromagnetic order of the Gd sublattice along the b-axis, with cell-doubling along the c-axis. The propagation vector is k = [0 0 ½]. At 3.6 K the Gd moment reaches 6.2(2) µ(B). Finally, (57)Fe Mössbauer spectroscopy shows no evidence of magnetic ordering of the Fe sublattice.

Long-range magnetic order in the 5d(2) double perovskite Ba2CaOsO6: comparison with spin-disordered Ba2YReO6.

The B-site ordered double perovskite Ba2CaOsO6 was studied by dc magnetic susceptibility, powder neutron diffraction and muon spin relaxation methods. The lattice parameter is a = 8.3619(6) Å at 280 K and cubic symmetry [Formula: see text] is retained to 3.5 K with a = 8.3462(7) Å. Curie-Weiss susceptibility behaviour is observed for T > 100 K and the derived constants are C = 0.3361(3) emu K mol(-1) and ΘCW = -156.2(3) K, in excellent agreement with literature values. This Curie constant is much smaller than the spin-only value of 1.00 emu K mol(-1) for a 5d(2) Os(6+) configuration, indicating a major influence of spin-orbit coupling. Previous studies had detected both susceptibility and heat capacity anomalies near 50 K but no definitive conclusion was drawn concerning the nature of the ground state. While no ordered Os moment could be detected by powder neutron diffraction, muon spin relaxation (µSR) data show clear long-lived oscillations indicative of a continuous transition to long-range magnetic order below TC = 50 K. An estimate of the ordered moment on Os(6+) is ∼ 0.2 µB, based upon a comparison with µSR data for Ba2YRuO6 with a known ordered moment of 2.2 µB. These results are compared with those for isostructural Ba2YReO6 which contains Re(5+), also 5d(2), and has a nearly identical unit cell constant, a = 8.36278(2) Å-a structural doppelgänger. In contrast, Ba2YReO6 shows ΘCW = - 616 K, and a complex spin-disordered and, ultimately, spin-frozen ground state below 50 K, indicating a much higher level of geometric frustration than in Ba2CaOsO6. The results on these 5d(2) systems are compared to recent theory, which predicts a variety of ferromagnetic and antiferromagnetic ground states. In the case of Ba2CaOsO6, our data indicate that a complex four-sublattice magnetic structure is likely. This is in contrast to the spin-disordered ground state in Ba2YReO6, despite a lack of evidence for structural disorder, for which theory currently provides no clear explanation.

Phase diagram and magnetic structures of the Co-bearing dugganites Pb3TeCo3A2O14 (A = V, P).

Exhibiting rich magnetic behaviour and potentially multiferroic properties, the dugganites, a Te(6+) containing subgroup of the langasite series, are an attractive family of compounds for future study. It was recently shown that Pb-bearing members of the dugganite series undergo distortions away from the P321 symmetry that is characteristic of the langasites. Here, we detail the consequences these distortions have on the magnetism exhibited by Pb3TeCo3V2O14 and Pb3TeCo3P2O14, solving the magnetic structures of both compounds with respect to a new supercell. Using neutron scattering and magnetic susceptibility measurements, we show that small applied magnetic fields can seriously perturb the delicate magnetic states in both of these systems. This is further demonstrated by presenting how doping P(5+) onto the nonmagnetic V(5+) site completely changes the magnetic structure from either of the end series members. Finally, it is shown using inelastic neutron scattering and magnetic susceptibility measurements that Pb3TeCo3V2O14 can be characterized using a model for isosceles trimers, which do not exist in the previously reported P321 subcell.