Structure and dynamics of low dimensional and frustrated magnetic materials
In low dimensional and geometrically-frustrated, antiferromagnetic materials, quantum fluctuations are enhanced and cooperative quantum magnetic states arise. In the case of materials with either multiple atom super-exchange pathways or near 90° exchange bonds, the super-exchange processes are weak enough that the techniques of high field magnetism including neutron scattering in applied field can be utilized. This enables direct access to new quantum magnetic phases and determination of magnetic coupling parameters.
The natural mineral azurite, Cu3(CO3)2(OH)2, has been investigated as an ideal example of a one-dimensional (1D) diamond chain antiferromagnet. The magnetism arises in this material from the spin-½ Cu2+ ions, which couple into dimerised pairs, separated by single monomer spins. A combination of neutron diffraction and inelastic neutron scattering studies performed at the HZB (and other neutron scattering facilities) have revealed the magnetic moment structure, accurate low temperature structural parameters and defined the energy scales of the exchange interactions between neighbouring Cu –ions. (for further reading see Rule et al PRL 100 117202, and Gibson et al PRB in press).
Tb2Sn2O7 is a geometrically frustrated magnetic material which has been observed to display both antiferromagnetic exchange interactions and ferromagnetic dipolar coupling below 0.87K. The transformation from a frustrated spin liquid phase to an “ordered dipolar spin ice” phase is unusual in that it is unpredicted from theory.
Neutron scattering results, utilizing the techniques of polarized neutron scattering and neutron spin echo, have indicated that Tb2Sn2O7 enters a partially ordered ferromagnetic state. These data reveal a strong Q-dependence of the fluctuation rate for the dynamic moments of Tb2Sn2O7 which are due to the near neighbour spin-spin correlations. (for further reading see Rule et al PRB 76 212405, and Rule et al J Phys Cond Matt 21 486005).
Figure 1: First evidence of magnetic Bragg peaks in azurite as found on E2, the flat-cone diffractometer at HZB. Measurements below the ordering temperature indicated that the magnetic propagation vector for this magnetic structure was k = [½ ½ ½].
Figure 2: Inelastic neutron scattering results taken on FLEX at HZB. Dispersionless scattering of azurite along the H (left) and L (right) reciprocal lattice directions indicate the 1D nature of the magnetic interactions. The magnetic excitations along the Cu2+ diamond chains (centre) resembles the 1D Heisenberg antiferromagnetic spin chain model with spinon continuum (white dashed line).
Figure 3: Polarised neutron investigations of Tb2Sn2O7 indicating the onset of magnetic Bragg reflections below 0.9K. The depolarisation of the beam (upper right plot) is a strong indication of ferromagnetic ordering in this material. These measurements were taken on D7 at the ILL.
