Showing papers by "Colin Greaves published in 2002"

Structures and magnetic ordering in the brownmillerite phases, Sr2MnGaO5 and Ca2MnAlO5

Abstract: The crystal and magnetic structures of the two related phases, Sr2MnGaO5 and Ca2MnAlO5 are reported. Rietveld analysis of neutron powder diffraction has revealed that both phases adopt the brownmillerite structure. Subtle differences in structure lead to the structure of Sr2MnGaO5 being best described by the space group Icmm (a = 5.4888(2) A, b = 16.2256(6) A, c = 5.35450(2) A at 2 K) while that of Ca2MnAlO5 is best described by Ibm2 (a = 5.46258(9) A, b = 14.9532(3) A, c = 5.23135(8) A at 2 K). Low temperature neutron powder diffraction data show that both phases have a simple antiferromagnetic structure. However, magnetisation data suggest a more complex picture of the magnetic order within these phases.

Staged fluorine insertion into manganese oxides with Ruddlesden–Popper structures: LaSrMnO4F and La1.2Sr1.8Mn2O7F

Abstract: The synthesis, structures and magnetic properties of two fluorine-intercalated layered manganates, LaSrMnO4F and La1.2Sr1.8Mn2O7F, are reported. Rietveld structure refinement based on time of flight neutron powder diffraction data indicates that both compounds are tetragonal (space group P4/nmm) with cell parameters a = 3.77696(7) A, c = 14.1026(8) A (LaSrMnO4F) and a = 3.8103(1) A, c = 21.7220(2) A (La1.2Sr1.8Mn2O7F). Fluorine occupies interstitial sites in only one of the two rock salt layers available in the unit cell, leading to novel staged intercalation structures. The interstitial fluorine increases the spacing between the two adjacent rock salt layers and thereby causes a significant reduction in the Mn–O apical bonds directed towards the intercalated regions. The Mn valences of both compounds are close to 4+, and no long range magnetic order was observed in the title compounds down to 5 K.

Magnetic order of LiMnBO 3 : A new type of chiral magnetic ground state

Abstract: $\ensuremath{\beta}\ensuremath{-}{\mathrm{LiMnBO}}_{3}$ crystallizes in a hexagonal lattice and is built up of ${\mathrm{MnO}}_{5}$ chains linked by ${\mathrm{BO}}_{3}$ groups. Magnetic susceptibility shows a broad maximum at 47 K, above which the behavior is typical for a one-dimensional Heisenberg antiferromagnet with $J=5.4 \mathrm{K}.$ Magnetic neutron diffraction reveals a much lower ordering temperature (28.5 K) to give a novel chiral ground state with a magnetic unit cell of $\sqrt{3}{a}_{\mathrm{N}},$ ${2c}_{\mathrm{N}}$ consisting of a mixture of one ferromagnetic ${\mathrm{Mn}}_{3}$ triangle and two normal 120\ifmmode^\circ\else\textdegree\fi{} antiferromagnetic ${\mathrm{Mn}}_{3}$ triangles.

Magnetic Compton scattering study of colossal magnetoresistance materials Fe1-xCuxCr2S4

Abstract: The spin-polarized electron momentum distributions in the Cr-based chalcogenide spinels ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ $(x=00,05,10)$ have been measured using a magnetic Compton scattering technique at $T=10\mathrm{K}$ The experimental magnetic Compton profiles have been compared with the results from electronic structure calculations performed using the full potential linearized augmented plane-wave method The theoretical analysis includes a decomposition of the various orbital and atomic contributions For $x=00$ and 05 sample compositions, the results show that the magnetic moments of the tetrahedrally coordinated ${\mathrm{Fe}}^{2+}$ (for $x=00)$ and ${\mathrm{Fe}}^{3+}$ (for $x=05)$ with spin moments of $366{\ensuremath{\mu}}_{B}$ (for $x=00)$ per formula unit $({\mathrm{FU}}^{\ensuremath{-}1})$ and $16{\ensuremath{\mu}}_{B}$ ${\mathrm{FU}}^{\ensuremath{-}1}$ (for $x=05),$ are aligned antiparallel to those of the octahedrally coordinated ${\mathrm{Cr}}^{3+}$ with spin moments of $50{\ensuremath{\mu}}_{B}$ ${\mathrm{FU}}^{\ensuremath{-}1}$ (for $x=00)$ and $482{\ensuremath{\mu}}_{B}$ ${\mathrm{FU}}^{\ensuremath{-}1}$ (for $x=05)$ In the ferromagnetic system for $x=10,$ the Cr moment is found to be $528{\ensuremath{\mu}}_{B}$ ${\mathrm{FU}}^{\ensuremath{-}1}$

Magnetic interactions in Cu-based layered transition metal oxides

Abstract: The nuclear and magnetic structures of several layered mixed copper-transition-metal oxides have been determined from neutron powder diffraction. The nuclear structures of the compounds studied here all consist of apex-linked pyramidal $\mathrm{Cu}/M\ensuremath{-}{\mathrm{O}}_{5}$ double layers separated by other structural blocks. These compounds were found to order antiferromagnetically within the $\mathrm{Cu}/M\mathrm{O}$ planes, but along the apical direction ferromagnetic order occurs provided that the $\mathrm{Cu}/M$ ratio is close to unity. The magnetic order can be successfully explained by the electron configuration of the magnetic ions in the compounds: $\mathrm{Cu}{d}_{{z}^{2}}^{2}\ensuremath{-}\mathrm{O}{2p}_{z}\ensuremath{-}\mathrm{F}\mathrm{e}/\mathrm{C}\mathrm{o}{d}_{{z}^{2}}^{1}$ interaction favors ferromagnetic ordering along c while ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}^{1}\ensuremath{-}\mathrm{O}{p}_{x,y}\ensuremath{-}{d}_{{x}^{2}\ensuremath{-}{y}^{2}}^{1}$ superexchange results in antiferromagnetic order within the $\mathrm{MO}$ planes. In this context, it is found that the above explanation is consistent with the reported magnetic structures of all mixed Cu/Fe or Cu/Co compounds with layered structures.