Showing papers by "J. Paul Attfield published in 2004"

Magnetic frustration in (LaA)CoNbO6 (A = Ca, Sr, and Ba) double perovskites

Abstract: The crystal and magnetic structures of the $(\mathrm{La}A)\mathrm{Co}\mathrm{Nb}{\mathrm{O}}_{6}$ ($A=\mathrm{Ca}$, Sr, and Ba) double perovskites have been investigated. The $A=\mathrm{Ca}$ and Sr compounds crystallize with a monoclinic $P{2}_{1}∕n$ superstructure while the larger $A=\mathrm{Ba}$ gives a tetragonal $I4∕m$ superstructure. These materials have a rocksalt ordered arrangement of Co and Nb with almost no inversion $(l1%)$ for $A=\mathrm{Ca}$ and 4% inversion for $A=\mathrm{Sr}$ and Ba. Magnetic susceptibility measurements reveal antiferromagnetic ordering transitions with Neel temperatures of $17\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}(A=\mathrm{Ca})$, $16\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}(A=\mathrm{Sr})$, and $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}(A=\mathrm{Ba})$. The low temperature antiferromagnetic ordering is described by the magnetic propagation vector $\mathbf{k}=(\frac{1}{2}\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}\frac{1}{2})$ for all materials. The saturated cobalt magnetic moment decreases from $2.97(2)\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}2.52(3)\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}1.85(5)\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ for $A=\mathrm{Ca}$, Sr, and Ba, respectively. The decrease of ordering temperature and moment with increasing size of $A$ evidences magnetic frustration due to competition between 90\ifmmode^\circ\else\textdegree\fi{} superexchange pathways. This is shown to be a general feature in antiferromagnetic double perovskites.

Structural, magnetic, and transport properties of (La1+xSr1-x)CoRuO6 double perovskites

Abstract: The structures, magnetism, and electronic transport properties of (La1+xSr1-x)CoRuO6 (−0.50 ≤ x ≤ 0.25) double perovskites have been investigated. All samples are found to crystallize with the monoclinic P21/n superstructure. An increase of antisite disorder with |x| is found to be controlled by a reduction in the average charge and size difference between the Co and Ru states. Neutron diffraction results for (LaSr)CoRuO6 show the presence of a long-range, ordered antiferromagnetic state with propagation vector (1/2 0 1/2) below 85 K. Hole doping (x 0) results in a sharp decrease. Resistivity measurements showed all compositions to be variable-range-hopping semiconductors, where hole doping decreases the resistivity by 2 orders of magnitude, but electron doping has a much less pronounced effect.

Spin, charge, and orbital order in Mn2OBO3

Abstract: The magnetic ordering in ${\mathrm{Mn}}_{2}\mathrm{O}\mathrm{B}{\mathrm{O}}_{3}$ has been determined from neutron diffraction and magnetization measurements. Long range antiferromagnetic order occurs below ${T}_{N}=26\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, other apparent magnetic transitions are due to secondary phases. The magnetic order has a $(\frac{1}{2}0\frac{1}{2})$ propagation vector and the antiferromagnetic spin arrangement has been refined. The low ordering temperature and ordered moment $(1.0\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}})$ at the ${\mathrm{Mn}}^{3+}$ sites evidence magnetic frustration arising from the charge and orbital ordered arrangement of ${\mathrm{Mn}}^{2+}$ and Jahn-Teller ${\mathrm{Mn}}^{3+}$ states. This is supported by a qualitative assessment of the Mn-O-Mn superexchange interactions.

Control of antisite disorder, magnetism, and asymmetric doping effects in (La1+xCa1-x)CoRuO6 double perovskites

Abstract: The structures, magnetism, and electronic transport properties of the semiconducting (La 1 + x Ca 1 - x )CoRuO 6 double perovskites (-0.25 0) enhances the magnetization with little change to the resistivity, perhaps signaling the formation of magnetic clusters. LaCaCoRuO 6 orders antiferromagnetically at T N =96 K, accompanied by a magnetostrictive anomaly. The Co/Ru spin structure, with a magnetic vector k = (½,0,½), cannot be determined unambiguously from neutron powder diffraction data.

X-Ray and neutron powder diffraction studies of the crystal structure of vitamin K3

Abstract: The previously unknown structure of vitamin K3 (menadione, 2-methyl-1,4-naphthoquinone, C11H8O2), a synthetic compound from which it is possible to derive the naturally occurring K vitamins, has been solved from X-ray powder diffraction data. Solution was achieved using simulated annealing and restrained Rietveld refinement, using restraints derived from similar known structures. Further refinement has been carried out using neutron diffraction data; this confirmed the molecular packing and allowed further insight into the structural detail with regard to methyl group orientation, thus allowing the proposal of an extensive C–H⋯O hydrogen bonding scheme by reference to comparable known structures. The crystal structure consists of chains of molecules lying head-to-tail. This study may aid the understanding of the biological activity of vitamin K.

Structural and Magnetic Properties of the Double Perovskite LaCaMnNbO6

Abstract: The structural and magnetic properties of the double perovskite LaCaMnNbO6 have been investigated using neutron powder diffraction and SQUID magnetometry. LaCaMnNbO6 has a monoclinic P21/n superstructure (a = 5.5929(1) A, b = 5.7406(1) A, c = 7.9824(1) A and β = 89.897(1)°) with an almost fully ordered rocksalt arrangement of Mn and Nb over the perovskite B-sites. Magnetisation measurements reveal a sharp antiferromagnetic transition at TN = 9 K. The low temperature antiferromagnetic ordering is described by a magnetic propagation vector of (0 1/2 1/2). Two collinear solutions are found for the magnetic structure. Strukturelle und magnetische Eigenschaften des Doppelperowskits LaCaMnNbO6 Die strukturellen und magnetischen Eigenschaften des Doppelperowskits LaCaMnNbO6 wurden mittels Neutronenbeugung an pulverformigen Proben sowie magnetischen Messungen mit einem SQUID-Magnetometer untersucht. LaCaMnNbO6 hat eine monokline Uberstruktur (P21/n; a = 5.5929(1) A, b = 5.7406(1) A, c = 7.9824(1) A und β = 89.897(1)°) mit einer NaCl-Ordnung von Mn und Nb auf den Perowskit-B-Platzen. Magnetisierungsmessungen zeigten einen scharfen antiferromagnetischen Ubergang bei TN = 9 K. Die antiferromagnetische Ordnung bei tiefer Temperatur wird durch einen magnetischen Fortpflanzungsvektor von (0 1/2 1/2) beschrieben. Zwei kollinare Losungen werden fur die magnetische Struktur gefunden.

Magnetic properties ofDyB2C,HoB2C,andErB2C

Abstract: Magnetic properties of layered borocarbides $R{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Dy},$ Ho, and Er) have been studied by the magnetization and specific heat measurements at 1.8--300 K under the field up to 5 T. ${\mathrm{ErB}}_{2}\mathrm{C}$ has a basic two-sublattice antiferromagnetic order below ${T}_{\mathrm{N}}=16.3\mathrm{K}.$ ${\mathrm{HoB}}_{2}\mathrm{C}$ and ${\mathrm{DyB}}_{2}\mathrm{C}$ show the three-dimensional magnetic order below ${T}_{\mathrm{C}}=7.0\mathrm{K}$ and 8.5 K, respectively. However, they have still large magnetic fluctuations below ${T}_{\mathrm{C}}.$ The magnetic ordering temperatures of $R{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Dy},$ Ho, and Er) do not follow the de Gennes relation, where the ordering temperatures of ${\mathrm{HoB}}_{2}\mathrm{C}$ and ${\mathrm{DyB}}_{2}\mathrm{C}$ are suppressed. The suppressions of ordering temperature and the unconventional fluctuating ground states of ${\mathrm{HoB}}_{2}\mathrm{C}$ and ${\mathrm{DyB}}_{2}\mathrm{C}$ originate from the spin frustration effects. The instability of the unconventional magnetic phase associated with the frustration is significant in this series; the fluctuating ground state results from the complex spin--spin interactions or spin--quadrupole interactions.

Phase separation in manganites induced by orbital–ordering strains

Abstract: Doped manganite perovskites AMnO3 exhibit a rich variety of electronic properties, resulting from the interplay of charge (Mn3+/Mn4+), spin (Mn magnetic moment) and orbital (Mn3+ Jahn–Teller distortion) degrees of freedom. Magnetisation measurements and ESR spectra have been used to study a series of eight AMnO3 perovskites, in which the A cation sites are occupied by a distribution of 70% trivalent lanthanide and 30% divalent Ca, Sr or Ba ions. These all have a mean A cation radius of 1.20 A but different values of the cation size variance σ2. A change from orbital disorder to order (cooperative Jahn–Teller distortions) was previously found in the insulating regime at σ2 ≈ 0.005 A2. This work has shown that co-existence of the orbitally ordered and disordered phases is found in σ2 = 0.0016–0.0040 A2 samples, with a difference of 40 K between their Curie temperatures. This is ascribed to competition between orbital ordering and microstructural lattice strains. At larger σ2 > 0.005 A2, the orbital ordering strains are dominant and only this phase is observed. This intermediate temperature phase segregation is one of many strain-driven separation phenomena in manganites.

Magnetic properties of DyB 2 C , HoB 2 C , and ErB 2 C

Abstract: Magnetic properties of layered borocarbides $R{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Dy},$ Ho, and Er) have been studied by the magnetization and specific heat measurements at 18\char21{}300 K under the field up to 5 T ${\mathrm{ErB}}_{2}\mathrm{C}$ has a basic two-sublattice antiferromagnetic order below ${T}_{\mathrm{N}}=163\mathrm{K}$ ${\mathrm{HoB}}_{2}\mathrm{C}$ and ${\mathrm{DyB}}_{2}\mathrm{C}$ show the three-dimensional magnetic order below ${T}_{\mathrm{C}}=70\mathrm{K}$ and 85 K, respectively However, they have still large magnetic fluctuations below ${T}_{\mathrm{C}}$ The magnetic ordering temperatures of $R{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Dy},$ Ho, and Er) do not follow the de Gennes relation, where the ordering temperatures of ${\mathrm{HoB}}_{2}\mathrm{C}$ and ${\mathrm{DyB}}_{2}\mathrm{C}$ are suppressed The suppressions of ordering temperature and the unconventional fluctuating ground states of ${\mathrm{HoB}}_{2}\mathrm{C}$ and ${\mathrm{DyB}}_{2}\mathrm{C}$ originate from the spin frustration effects The instability of the unconventional magnetic phase associated with the frustration is significant in this series; the fluctuating ground state results from the complex spin\char21{}spin interactions or spin\char21{}quadrupole interactions

Structural, Magnetic, and Transport Properties of (La1+xSr1-x)CoRuO6 Double Perovskites.

Abstract: The structures, magnetism, and electronic transport properties of (La1+xSr1-x)CoRuO6 (−0.50 ≤ x ≤ 0.25) double perovskites have been investigated. All samples are found to crystallize with the monoclinic P21/n superstructure. An increase of antisite disorder with |x| is found to be controlled by a reduction in the average charge and size difference between the Co and Ru states. Neutron diffraction results for (LaSr)CoRuO6 show the presence of a long-range, ordered antiferromagnetic state with propagation vector (1/2 0 1/2) below 85 K. Hole doping (x 0) results in a sharp decrease. Resistivity measurements showed all compositions to be variable-range-hopping semiconductors, where hole doping decreases the resistivity by 2 orders of magnitude, but electron doping has a much less pronounced effect.