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Register a new userThe magnetic and crystal structure of azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$ as determined by neutron diffraction
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Here we present neutron diffraction results on the mineral azurite. We have found that the crystal structure of azurite can be described in the space group $P2_1$ which is the next lower symmetric group of $P2_1/c$ as found in earlier work. This small change in symmetry does not greatly influence the lattice parameters or atomic fractional coordinates which are presented here for single crystal diffraction refinements. The ordered magnetic moment structure of this material has been determined and is comprised of two inequivalent magnetic moments on copper sites of magnitude 0.68(1) and 0.25(1) $mu_{B}$. This result is discussed in terms of the anisotropic exchange and Dzyaloshinskii-Moriya interactions. It is found that the system is likely governed by one-dimensional behaviour despite the long-range ordered ground state. We also highlight the significance of strain in this material which is strongly coupled to the magnetism.
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X-ray diffraction with photon energies near the Ru L$_2$-absorption edge was used to detect resonant reflections characteristic of a G-type superstructure in RuSr$_2$GdCu$_2$O$_8$ single crystals. A polarization analysis confirms that these reflections are due to magnetic order of Ru moments, and the azimuthal-angle dependence of the scattering amplitude reveals that the moments lie along a low-symmetry axis with substantial components parallel and perpendicular to the RuO$_2$ layers. Complemented by susceptibility data and a symmetry analysis of the magnetic structure, these results reconcile many of the apparently contradictory findings reported in the literature.
Polarised neutron diffraction measurements have been made on HoFeO$_3$ single crystals magnetised in both the [001] and [100] directions ($Pbnm$ setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of H = 9 T parallel to [001] at T = 70 K and with the lower field H = 0.5 T parallel to [100] at T = 5, 15, 25~K. A Fourier projection of magnetization induced parallel to [001], made using the $hk0$ reflections measured in 9~T, indicates that almost all of it is due to alignment of Ho moments. Further analysis of the asymmetries of general reflections in these data showed that although, at 70~K, 9~T applied parallel to [001] hardly perturbs the antiferromagnetic order of the Fe sublattices, it induces significant antiferromagnetic order of the Ho sublattices in the $xmhyphen y$ plane, with the antiferromagnetic components of moment having the same order of magnitude as the induced ferromagnetic ones. Strong intensity asymmetries measured in the low temperature $Gamma_2$ structure with a lower field, 0.5 T $parallel$ [100] allowed the variation of the ordered components of the Ho and Fe moments to be followed. Their absolute orientations, in the 180degree domain stabilised by the field were determined relative to the distorted perovskite structure,. This relationship fixes the sign of the Dzyalshinski-Moriya (D-M) interaction which leads to the weak ferromagnetism. Our results indicate that the combination of strong y-axis anisotropy of the Ho moments and Ho-Fe exchange interactions breaks the centrosymmetry of the structure and could lead to ferroelectric polarization.
We have investigated the temperature evolution of the magnetic structures of HoFeO$_3$ by single crystal neutron diffraction. The three different magnetic structures found as a function of temperature for hfo are described by the magnetic groups Pb$$n$2_1$, Pbn$2_1$ and Pbn$2_1$ and are stable in the temperature ranges $approx$ 600-55~K, 55-37~K and 35$>T>2$~K respectively. In all three the fundamental coupling between the Fe sub-lattices remains the same and only their orientation and the degree of canting away from the ideal axial direction varies. The magnetic polarisation of the Ho sub-lattices in these two higher temperature regions, in which the major components of the Fe moment lie along $x$ and $y$, is very small. The canting of the moments from the axial directions is attributed to the antisymmetric interactions allowed by the crystal symmetry. They include contributions from single ion anisotropy as well as the Dzyaloshinski antisymmetric exchange. In the low temperature phase two further structural transitions are apparent in which the spontaneous magnetisation changes sign with respect to the underlying antiferromagnetic configuration. In this temperature range the antisymmetric exchange energy varies rapidly as the the Ho sub-lattices begin to order. So long as the ordered Ho moments are small the antisymmetric exchange is due only to Fe-Fe interactions, but as the degree of Ho order increases the Fe-Ho interactions take over whilst at the lowest temperatures, when the Ho moments approach saturation the Ho-Ho interactions dominate. The reversals of the spontaneous magnetisation found in this study suggest that in hfo the sums of the Fe-Fe and Ho-Ho antisymmetric interactions have the same sign as one another, but that of the Ho-Fe terms is opposite.
Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.
We report the magnetic diffraction pattern and spin wave excitations in (CD$_3$)$_2$ND$_2$[Mn(DCO$_2$)$_3$] measured using elastic and inelastic neutron scattering. The magnetic structure is shown to be a G-type antiferromagnet with moments pointing along the $b$ axis. By comparison with simulations based on linear spin wave theory, we have developed a model for the magnetic interactions in this multiferroic metal-organic framework material. The interactions form a three-dimensional network with antiferromagnetic nearest-neighbour interactions along three directions of $J_1=-0.103(8)$~meV, $J_2=-0.032(8)$~meV and $J_3=-0.035(8)$~meV.
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