Aging in a Topological Spin Glass

Using a combination of static and time dependent magnetic susceptibility measurements we have examined the nonconventional spin glass phase of the 2-dimensional kagomé antiferromagnet (H3O)Fe3(SO4)2(OH)6. In contrast with 2-dimensional site-disordered spin glasses, ac susceptibility results show critical behaviour at finite temperatures. This transition agrees well with a predicted anisotropy-driven topological spin glass phase transition. Aging effects are found as in site-disordered spin glasses, but the dependence on temperature changes is remarkably weak. These glassy dynamics appear to be governed by the topology of this highly frustrated system. Geometrically frustrated magnetic materials, notably of the 'fully frustrated' kagomé and pyrochlore geome-tries, exhibit an astounding array of exotic ground states and properties: e.g spin fluid, spin glass, spin ice, cooperative paramagnetism, colossal magnetoresis-tance, heavy fermion and superconductor. 1 In all of these cases the ground state properties are directly related to the geometries of the lattices; the kagomé and the pyrochlore lattices are made up of corner sharing triangles and tetrahedra respectively. Perhaps the least well understood of all these are the spin glass-like states observed in materials such as the kagomé antiferromagnet the disordered pyrochlore slab SrCr x Ga 1−x O 19. 2–6 In some pyrochlores, the spin-glass character of the transition has been identified by a divergence of the non-linear susceptibility 3. In 2 dimensions, for conventional (site-disordered) spin glasses, no transition is expected at T = 0; but, in the case of the kagomé antiferromagnet, a vanishingly small amount of xy anisotropy can lead to a transition to a glassy state at finite temperature 7. Studies involving magnetic dilution demonstrate the complexity of these glassy states. While in the 3d Y 2 Mn 2 O 7 the effect of increasing disorder is to progressively restore conventional spin glass behaviour 9 , in (H 3 O)Fe 3 (SO 4) 2 (OH) 6 it is first found to increase the freezing temperature (T g), and then if sufficient, to induce long-range magnetic order 2. Discussion of SrCr x Ga 1−x O 19 is complicated significantly by the in-homogeneous Cr 3+ occupancy, as this can lead to either an increase or a decrease in T g with increasing dilution 6. Very little is known about the specific properties of the non-disordered systems themselves. In this letter, we study the glassy transition and slow dynamics of the S = 5/2 Heisenberg kagomé antiferromagnet (H 3 O)Fe 3 (SO 4) 2 (OH) 6 , …