Browsing Physics & Astronomy, Dept. of by Author "Aeppli, G."
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ItemStrong magnetic fluctuations in transition metal oxides(American Institute of Physics, 1996-04-15) Aeppli, G.; Lee, S.-H.; Broholm, C.; DiTusa, J. F.; Bao, W.Most magnets have long-range magnetic order when the thermal energy is less than the local magnetic exchange energy (T<|ΘCW|) effects such as reduced dimensionality and frustration, 1 however, can suppress the ordering transition and lead to unusual cooperative paramagnetic phases at low temperatures. We review neutron scattering experiments exploring such short-range-ordered phases in insulating transition metal oxides. We discuss (V1-xCrx)2O3, in which orbital fluctuations appear to limit spin correlations to within small “molecular” clusters, SrCr9pGa12-9pO19, in which geometrical frustration allows local antiferromagnetic constraints to be fulfilled without long-ranged order, and Y2BaNiO5, in which magnetic interactions occur only within chains of spins which are unable to order because of the Haldane effect. Emphasis is placed on the common features of exchange interactions in these oxides and the important role which magnetic neutron scattering has played in understanding the unusual magnetic phenomena. ItemA strongly fluctuating quasi-two-dimensional insulator(American Institute of Physics, 1991-04-15) Cooper, A.S.; Espinosa, G.P.; Broholm, C.; Aeppli, G.The authors describe neutron-scattering data for SrCr8-xGa4+xO19, a layered compound containing planes of S=3/2 Cr3+ ions which form Kagome lattices. Despite strong antiferromagnetic interactions (θCW≈-500 K), fluctuations account for more than 75% of the free ion moment at 1.5 K. The spectrum (averaged over reciprocal space) is gapless and resembles that of a 2D long-range-ordered antiferromagnet, as does the low-temperature specific heat of the compound. Even so, the static correlation length does not exceed 7±2 Å. Monte-Carlo simulations of the antiferromagnetic three-state Potts model on the Kagome lattice show that this model does not have a finite temperature phase-transition. Even at T=0, there does not seem to be true long-range antiferromagnetic-order. However, the magnetic correlations decay algebraically rather than exponentially. The implications of this result for the ground-state of SrCr8-xGa4+xO19 is discussed.