Manfred Reehuis

The rate earth transition metal phosphide oxides LnFePO, LnRuPO and LnCoPO with ZrCuSiAs type structure

Abstract The compounds LnFePO (Ln = La−Nd, Sm, Gd), LnRuPO (Ln = La−Nd, Sm, Gd) and (Ln = La−Nd, Sm) crystallize with the tetragonal) ZrCuSiAs type structure (P4/nmm, Z = 2), which was refined from single-crystal X-ray data of PrFePO (a = 391.13(6) pm, c = 834.5(2) pm, R = 0.026) and CeRuPo (a = 402.6(1) pm, c = 825.6(2) pm, R = 0.018). The refinement of the occupancy parameters showed the oxygen position to be fully occupied in both compounds. The oxygen content of the samples was also proven by EDAX analyses. The structures of the compounds SmFePO and LaCoPO were refined by Rietveld analyses of X-ray powder data.

Structure and magnetic properties of the phosphides CaCo2P2 and LnT2P2 with ThCr2Si2 structure and LnTP with PbFCl structure (Ln = Lanthanoids, T = Fe, Co, Ni)†

Abstract The structures of the ThCr2Si2-type phosphides PrFe2P2 (a new compound) and EuFe2P2 were refined from single-crystal X-ray data to residuals of R = 0.011 (246F-values) and R = 0.027 (215 Fs). As was previously reported for the Ni atoms of the ThCr2Si2-type phosphides LnNi2P2, the Fe atoms in the series LnFe2P2 do not carry magnetic moments. In contrast, the magnetic behaviour of the corresponding Co-containing phosphides is frequently complex, due to the possibility for magnetic order in both the Ln and the Co sublattices. Antiferromagnetic order can be inferred for the Co sublattices of CeCo2P2 (TN = 440 K), PrCo2P2 (TN = 304 K), NdCo2P2 (TN = 309 K) and SmCo2P2 (TN = 302 K). At about 15–30 K the 1 χ vs T plots of PrFe2P2, PrCo2P2 and NdCo2P2 suggest a variety of different ordering phenomena for the magnetic moments of the lanthanoid atoms. The magnetic properties of CeFe2P2 and CeCo2P2 indicate valence fluctuations of the Ce atoms. At lower temperatures the magnetic susceptibility of SmCo2P2 reflects the Van Vleck paramagnetism of the Sm3+ species, while the magnetic behaviour of CaCo2P2 is suggestive of the itinerant exchange of a Fermi liquid. In the closely related (anti-) PbFCl-type structure of the new compound SmCoP the Co sublattice also shows antiferromagnetic order with a magnetic moment of μexp = 1.36 μB per Co atom. The isotypic compounds SmFeP, PrCoP and NdCoP are reported for the first time. The compounds LaFe2P2, CeFe2P2, CaNi2P2, LaNi2P2 and CeNi2P2 do not become superconducting down to 1.8 K.

Magnetic order and dynamics in an orbitally degenerate ferromagnetic insulator.

Neutron scattering was used to determine the spin structure and the magnon spectrum of the Mott-Hubbard insulator YTiO3. The magnetic structure is complex, comprising substantial G-type and A-type antiferromagnetic components in addition to the predominant ferromagnetic component. The magnon spectrum, on the other hand, is gapless and nearly isotropic. We show that these findings are inconsistent with the orbitally ordered states thus far proposed for YTiO3 and discuss general implications for a theoretical description of exchange interactions in orbitally degenerate systems.

Lattice Instability and Competing Spin Structures in the Double Perovskite Insulator Sr2FeOsO6

The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron-osmium layers along the tetragonal c axis. Neutron powder diffraction experiments, 57Fe Mössbauer spectra, and density functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example of a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.

Spin and orbital order in the vanadium spinel MgV2O4

octahedra and a low temperature space group (I4m2) that maintains the mirrorplane symmetry. The magnetic structure that develops below 42K consists of antiferromagneticchains with a strongly reduced moment while inelastic neutron scattering reveals one-dimensionalbehavior and a single band of excitations. The implications of these results are discussed in terms ofvarious orbital ordering scenarios. We conclude that although spin-orbit coupling must be signi cantto maintain the mirror plane symmetry, the trigonal distortion is large enough to mix the 3d levelsleading to a wave function of mixed real and complex orbitals.

Antiferromagnetic order in the ThCr2Si2 type phosphides CaCo2P2 and CeCo2P2

Abstract Magnetic susceptibility and neutron powder diffraction experiments reveal that the cobalt atoms in CaCo 2 P 2 and CeCo 2 P 2 order antiferromagnetically below the Neel temperatures T N =113(2) K and T N =440(5) K, respectively. In these tetragonal compounds with ThCr 2 Si 2 type structure ( I 4 /mmm ) the magnetic moments of the cobalt atoms are ordered ferromagnetically within the basal plane and antiferromagnetically along the c axis with the stacking sequence +−, +−, corresponding to a propagation vector k =(0, 0, 1). In the calcium compound the cobalt moments with μ exp =0.32(2) μ B are ordered perpendicular to the c axis, whereas in the cerium compound these moments with μ exp =0.94(2) μ B are aligned parallel to this axis. The magnetic properties of the previously investigated cobalt containing phosphides ACo 2 P 2 (A= Sr, La, Pr, Nd, Sm, Eu) are briefly reviewed. The magnetic ordering temperatures and the magnetic moments of the cobalt atoms correlate with the formal charge of the cobalt-phosphorus polyanion and with the Co–Co distances.

A neutron diffraction study of the magnetic order in the ThCr2Si2 type phosphides PrCo2P2 and NdCo2P2

Abstract The magnetic structures of the compounds LnCo 2 P 2 (Ln = Pr and Nd) were investigated at different temperatures by neutron powder diffractometry and refined by the Rietveld technique. In agreement with the previously determined magnetic susceptibility data the magnetic moments of both the Pr 3+ ions and the cobalt atoms of PrCo 2 P 2 were found to order antiferromagentically with different, well-defined Neel temperatures. The magnetic lattices of the two magnetic species have different propagation vectors of k = (0, 0, 1) and k = (0, 0, 1 2 ) for the Pr 3+ ions and the cobalt atoms, respectively. In contrast, in NdCo 2 P 2 only the magnetic moments of the cobalt atoms order at a well-defined Neel temperature with a propagation vector again of k = (0, 0, 1 2 ) , while the magnetic order of the Nd 3+ moments is induced gradually at a lower temperature with the same propagation vector through the magnetic order of the cobalt atoms. The magnetic moments of the cobalt atoms of both compounds are ordered ferromagnetically within the basal planes of the tetragonal magnetic structures and parallel to the c -axis with the stacking sequence + + − − , + + − − along this direction. For the moments of the Pr 3+ ions the order is again ferromagnetic within the basal planes and the stacking sequence is + − + − , + − + − .In the magnetic structure of the Nd 3+ sublattice no magnetic order (0) was found for every other basal plane; the stacking is of the type 0 − 0 + , 0 − 0 + . At low temperatures the magnetic moments of the cobalt atoms were found to be about μ exp = 0.8 μ B in both compounds. The experimental moments of the Ln 3+ ions were 3.08(2)μ B for Pr 3+ and 2.78(6) μ B for Nd 3+ , somewhat smaller than the theoretical values of μ S = 3.20 μ B and 3.27μ B , respectively, expected for the Ln 3+ ions.

Synthesis, crystal structure, and physical properties of Sr2FeOsO6.

In the exploration of new osmium based double perovskites, Sr2FeOsO6 is a new insertion in the existing family. The polycrystalline compound has been prepared by solid state synthesis from the respective binary oxides. Powder X-ray diffraction (PXRD) analysis shows the structure is pseudocubic at room temperature, whereas low-temperature synchrotron data refinements reveal the structure to be tetragonal, space group I4/m. Heat capacity and magnetic measurements of Sr2FeOsO6 indicated the presence of two magnetic phase transitions at T1 = 140 K and T2 = 67 K. Band structure calculations showed the compound as a narrow energy gap semiconductor, which supports the experimental results obtained from the resistivity measurements. The present study documents significant structural and electronic effects of substituting Fe(3+) for Cr(3+) ion in Sr2CrOsO6.

Magnetization and neutron diffraction studies of the magnetic order in the compounds Pr2Co12P7, Nd2Co12P7, Ho2Co12P7 and Lu2Co12P7

Abstract The magnetic order of the lanthanoid and cobalt moments in the hexagonal Zr 2 Fe 12 P 7 -type phosphides Ln 2 Co 12 P 7 (Ln=Pr, Nd, Ho and Lu) has been investigated by SQUID magnetization measurements and by neutron powder diffraction. The cobalt sublattices order ferromagnetically below the Curie temperatures of 136(2), 140(2), 152(2) and 150(2) K in the praseodymium, neodymium, holmium and lutetium compounds, respectively. The cobalt atoms with tetrahedral and square-pyramidal phosphorus coordination carry magnetic moments of 0.3(1) and 0.9(1) μ B respectively, and they are aligned parallel to the hexagonal axis. Below the second transition temperature T 1 = 15(1) K the praseodymium moments order ferromagnetically with an antiferromagnetic component relative to the cobalt sublattice and with an angle off the c axis. In contrast, the neodymium and holmium moments do not order at a well-defined transition temperature, instead the order is induced gradually by the ferromagnetic cobalt sublattice. In both compounds it sets in at about 65 K. The neodymium moments are antiparallel and the holmium moments are parallel to the cobalt moments, resulting in ferri- and ferromagnetism, respectively. The magnetic structures of the three compounds are confirmed by temperature and field-dependent SQUID magnetization measurements of powders; for Pr 2 Co 12 P 7 and Nd 2 Co 12 P 7 these measurements were also carried out with oriented single crystals. The experimentally obtained values t 1 , magnetic moments amount to only between 44 and 77% of the theoretical values and this is rationalized by partial disorder.

Magnetic ordering in the ternary phosphides Tb2Fe12P7, Tb2Co12P7, Tb2Ni12P7 and Ho2Ni12P7

Abstract The ordering of the magnetic moments of the lanthanide atoms in the hexagonal phosphides Tb 2 T 12 P 7 (T = Fe, Co, Ni) and Ho 2 Ni 12 P 7 has been investigated by SQUID measurements and neutron powder diffraction. In Tb 2 Co 12 P 7 the magnetic moments of both the Tb 3+ ions and the cobalt atoms are ferromagnetically ordered below T c = 150 K. In contrast to this compound, the transition metal atoms in Tb 2 Fe 12 P 7 , Tb 2 Ni 12 P 7 and Ho 2 Ni 12 P 7 carry virtually no magnetic moments. In Tb 2 Fe 12 P 7 the terbium moments show below T N = 4.5(5) K a trinangular spin arrangement within the ab -plane with the wave vector k = (⅓ ⅓ 0). For the lanthanide atoms in Tb 2 Ni 12 P 7 and Ho 2 Ni 12 P 7 a collinear antiferromagnetic order was found below T N = 12.5(5) K and T N = 10.0(5) K. reserved.